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Article

Luminescent Arylalkynyltitanocenes: Effect of Modifying the Electron Density at the Arylalkyne Ligand, or Adding Steric Bulk or Constraint to the Cyclopentadienyl Ligand

by
Matilda Barker
1,
Samantha C. Walter
1,
Elizabeth A. McCallum
1,
River S. Golden
1,
John H. Zimmerman
1,
Jackson S. McCarthy
1,
Colin D. McMillen
2,* and
Paul S. Wagenknecht
1,*
1
Department of Chemistry, Furman University, Greenville, SC 29613, USA
2
Department of Chemistry, Clemson University, Clemson, SC 29634, USA
*
Authors to whom correspondence should be addressed.
Crystals 2025, 15(8), 745; https://doi.org/10.3390/cryst15080745
Submission received: 25 July 2025 / Revised: 13 August 2025 / Accepted: 18 August 2025 / Published: 21 August 2025
(This article belongs to the Special Issue Celebrating the 10th Anniversary of International Crystallography)
Browse Figures
Versions Notes

Abstract

Photocatalysis using complexes of d0 metals with ligand-to-metal charge-transfer (LMCT) excited states is an active area of research. Because titanium is the second most abundant transition metal in the earth’s crust, d0 complexes of TiIV are an appropriate target for this research. Recently, our group has demonstrated that the arylethynyltitanocene Cp2Ti(C2Ph)2CuBr is not emissive in room-temperature fluid solution, whereas the corresponding Cp* complex, Cp*2Ti(C2Ph)2CuBr, is emissive. The Cp* ligand is hypothesized to provide steric constraint that inhibits excited-state structural rearrangement. However, modifying the structure also changes the orbital character of the excited state. To investigate the impact of the excited-state orbital character on the photophysics, herein we characterize complexes similar to Cp*2Ti(C2Ph)2CuBr—but one with a more electron-rich arylethynyl ligand, ethynyldimethylaniline (C2DMA), and one with a more electron-poor arylethynyl ligand, ethynyl-α,α,α-trifluorotoluene. We have also prepared complexes with the C2DMA ligand but with different Cp ligands that adjust the steric bulk and constraint around the Ti, by replacing the Cp* ligands with either indenyl ligands or an ansa-cyclopentadienyl ligand where the two Cp ligands are bridged by a dimethylsilylene. All four target complexes have been characterized crystallographically and structure activity relationships are highlighted.

1. Introduction

The use of solar energy to drive uphill reactions for the synthesis of fuels [1,2,3] or other organic materials [4,5,6,7,8] requires photocatalysts that, upon absorption of light, have excited states with lifetimes sufficient to undergo bimolecular energy- or electron-transfer. Photocatalysts are also used in applications such as dye-sensitized solar cells [9,10,11], photon upconversion [12,13], and singlet oxygen generation [14]. Transition-metal complexes of ruthenium and iridium have dominated the field of photocatalysis, but recently there has been significant interest in developing complexes of earth-abundant metals, particularly of first-row transition metals [15,16,17,18,19]. One challenge with the use of first-row transition metals is that low-energy, metal-centered (MC) excited states, i.e., excited states resulting from electronic transitions between d orbitals, are typically accessible from the photoactive state [20]. Because of the very short lifetime of such MC states, access to these states significantly decreases the excited-state lifetimes and such complexes are either non-emissive or the emission has a very short lifetime.
Recently, there has been interest in overcoming this obstacle by using complexes of d0 metals with ligand-to-metal charge-transfer (LMCT) excited states [21]. The d0 electron configuration eliminates the presence of MC states. Recently several ZrIV complexes have been shown to be emissive in room-temperature (RT) fluid solution with lifetimes in the microsecond range, some having utility as photocatalysts [22,23,24,25,26,27,28]. However, the corresponding TiIV complexes have not been emissive [22,26,28]. In 2022, our group demonstrated that Ph[Ti] (Figure 1) is weakly emissive in RT fluid solution with a photoluminescent quantum yield, ΦPL = 2 × 10−4. However, this complex is extremely sensitive to photodecomposition with a photodecomposition quantum yield, Φdecomp = 0.65. Coordination of CuBr between the alkynes to give Ph[Ti]CuBr (Figure 1) improved the photostability (Φdecomp = 1.2 × 10−3), but resulted in a complex that is non-emissive [29]. In 2023, we demonstrated that replacement of the cyclopentadienyl ligand with pentamethylcyclopentadienyl, Cp*, to give Ph[Cp*Ti]CuBr (Figure 1) resulted in a complex that is reasonably photostable (Φdecomp = 0.015) and emissive in RT tetrahydrofuran (THF) solution (ΦPL = 1.3 × 10−3, τ = 0.18 μs). We hypothesized that the increased steric bulk of the Cp* ligand restricted excited-state distortion, thus decreasing the rate of nonradiative decay and that this was responsible for the improved emission characteristics [30]. To further test this hypothesis, we prepared a complex with ortho-methyl substituents on the phenyl ring to give the xylyl complex, xyl[Cp*Ti]CuBr (Figure 1). This xylyl complex is also reasonably photostable and has an emission quantum yield and excited-state lifetime approximately an order of magnitude greater than the phenyl analog (ΦPL = 1.2 × 10−2, τ = 1.5 μs). This complex was also shown to sensitize molecular photon upconversion and singlet oxygen formation [31].
Several open questions remain. Modifying the complex often changes the identity of the excited state. For example, whereas the excited state of the non-emissive Ph[Ti]CuBr is dominated by a combination of C2Ph-to-Ti LMCT and CuBr-to-Ti metal halide-to-metal CT (MXMCT), the Cp* version, Ph[Cp*Ti]CuBr, has significant Cp*-to-Ti LMCT character [30]. How does this shift in excited-state character change the excited-state behavior? And, are there other means to provide steric restriction that might give similar excited-state behavior as the Cp* ligand? To investigate these questions, we have prepared complexes similar to Ph[Cp*Ti]CuBr—but one with a more electron-rich arylethynyl ligand, ethynyldimethylaniline, DMA[Cp*Ti]CuBr, and one with a more electron-poor arylethynyl ligand, ethynyl-α,α,α-trifluorotoluene, PhCF3[Cp*Ti]CuBr (Figure 2). We have also prepared complexes with the ethynyldimethylaniline (C2DMA) ligand but with different Cp ligands that adjust the steric bulk and constraint around the Ti, namely the indenyl complex, DMA[IndTi]CuBr, and an ansa-titanocene DMA[ansa-CpTi]CuBr (Figure 2). All four complexes have been characterized by single-crystal X-ray diffraction.

2. Materials and Methods

2.1. General Methods

UV-Vis: Absorption spectra were collected using a Varian (Walnut Creek, CA, USA) Cary-50 UV–vis spectrophotometer.
Emission Spectra: Emission spectra were collected using a Horiba Scientific (Piscataway, NJ, USA) Fluorolog-3 spectrofluorometer equipped with a FL-1013 liquid nitrogen dewar assembly for 77 K measurements. Emission spectra at 77 K were performed in 2-methyltetrahydrofuran (2-Me-THF) glass. All emission spectra were corrected for the response factor of the R928 photomultiplier tube using the factory correction files. Emission spectra in room-temperature solution were additionally corrected with blank subtraction.
Excitation spectra: Excitation spectra were collected using a Horiba Scientific Fluorolog-3 sprectrofluorometer.
Emission Lifetime: Emission lifetimes were measured using a Photon Technology International (PTI, Lawrenceville, NJ, USA) GL-3300 pulsed nitrogen laser fed into a PTI GL-302 dye laser as the excitation source. The resulting data set was collected on an OLIS (Athens, GA, USA) SM-45 EM fluorescence lifetime measurement system using a Hamamatsu R928 photomultiplier tube fed through a variable feed-through terminator into a LeCroy (Chestnut Ridge, NY, USA) WaveJet 352A oscilloscope and analyzed using OLIS Spectral Works. Double exponential fits were performed using WaveMatrics (Lake Oswego, OR, USA) IGOR Pro 9.
NMR spectra: 1H NMR spectra were obtained using a JEOL (Peabody, MA, USA) JNM-ECZR (500 MHz) spectrometer.
Infrared spectra: Vibrational spectra were performed on a PerkinElmer (Shelton, CT, USA) Spectrum Two FT-IR spectrometer using a universal attenuated total reflection accessory.
Elemental analysis: Elemental analyses were performed by Midwest Microlabs (Indianapolis, IN, USA).
Quantum Yields of Photoluminescence (ΦPL): Relative solution-state photoluminescence quantum yields were determined in THF against a [Ru(bpy)3]2+ standard in air-saturated CH3CN (ΦPL = 0.018) [32]. The absorbance of the analyte and [Ru(bpy)3]2+ solutions were matched at the wavelength of excitation (450 nm), and the reported quantum yields are averages of at least three replicates.
Quantum Yields of Photodecomposition (Φdecomp): The quantum yields for photodecomposition were determined using the previously published method [31] and are detailed in the Supplementary Materials.

2.2. Computational Methods

DFT and TDDFT calculations used Gaussian 16 [33] and were performed according to the literature [31], using GaussView 6.32 [34] for images of orbitals, and GaussSum 3.0 [35] for Mulliken population analysis. The computational model for DFT and TDDFT used B3LYP [36,37] and 6-311+G(d) [38,39,40,41,42] including a Tomasi polarizable continuum (PCM) model using the dielectric constant for THF [43].

2.3. Single-Crystal X-Ray Diffraction

Crystals of DMA[Cp*Ti]CuBr and PhCF3[Cp*Ti]CuBr were grown by slow evaporation of THF. Crystals of DMA[IndTi]CuBr were grown by slow evaporation of dichloromethane. Crystals of DMA[ansa-CpTi]CuBr were grown by dissolving the complex in THF and allowing diethylether to diffuse the solution (vapor-phase). In each case, triethylamine (5% v/v) was added to the solvent to protect against acid hydrolysis. Single-crystal X-ray diffraction data were collected at 100 K using a Bruker (Madison, WI, USA) D8 Quest diffractometer. The data were collected using phi and omega scans (0.5° frame width) with a Mo Kα (λ = 0.71073 Å) microfocus source and a Photon 3 detector. Data were processed (SAINT) and corrected for absorption (multi-scan, SADABS), using the Bruker Apex4 suite [44]. The structures were solved by intrinsic phasing (SHELXT) and subsequently refined by full matrix least squares on F2(SHELXL) [45,46]. Reflections obscured by the beamstop were omitted from the refinements. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms attached to carbon atoms were refined in calculated positions using riding models where Ueq(H) = 1.2Ueq(C) for aromatic C–H and Ueq(H) = 1.5Ueq(C) for methyl C–H. The structure of PhCF3[Cp*Ti]CuBr was found to be a THF solvate, and THF was included in the structural model without restraints. In DMA[IndTi]CuBr and DMA[ansa-CpTi]CuBr the bromine site was found to support mixed occupancy of Br and OH. These were refined in separate, nearby positions with the site occupancies refined as free variables. In DMA[IndTi]CuBr this resulted in a ratio of 0.814(4):0.186(4) for Br:OH, and in DMA[ansa-CpTi]CuBr this resulted in a ratio of 0.673(5):0.327(5) for Br:OH. The structure of DMA[ansa-CpTi]CuBr was refined in a noncentrosymmetric space group (Pna21) with a corresponding absolute structure parameter of 0.041(18). Crystallographic data are summarized in Table 1. Crystallographic data for the complex DMA[IndTi]CuBr0.29(OH)0.71, differing from DMA[IndTi]CuBr in the degree of Br/OH substitution, are reported in the Supplementary Information, Table S1. CCDC 2475385-2475389 contains the complete supplementary crystallographic data for this paper and can be obtained from the Cambridge Crystallographic Data Centre.

2.4. Syntheses

Diethylether and THF were dried and degassed using an Innovative Technology (Newburyport, MA, USA) PureSolv solvent purification system before use. All other materials were used as received. All reactions were performed under an inert atmosphere using standard Schlenk techniques. Dichlorobis(indenyl) titanium(IV), copper(I)bromide, n-butyllithium, 4-ethynyl-α,α,α-trifluorotoluene, and 4-ethynyl-N,N-dimethylanaline were purchased from Sigma Aldrich (St. Louis, MO, USA). Bis-(pentamethylcyclopentadienyl)titanium dichloride was purchased from Strem Chemicals (Newburyport, MA, USA). [Me2Si(η5-C5H4)2TiCl2], ansa-Cp2TiCl2, was prepared according to the literature method [47]. DMA[Cp*Ti] was prepared by a modification of the literature procedure [48] where THF was substituted for diethylether (Supplementary Materials).
General synthetic method for the parent titanocenes: An oven-dried, two-necked round-bottom flask was purged with argon and then charged with dried, degassed solvent and the appropriate alkyne. The solution was stirred and chilled to −78 °C (dry ice/acetone) for 10 min. Next, n-butyllithium (2.5 M in hexanes) was added via syringe and the solution was stirred for an additional 10 min at −78 °C. The cooling bath was removed and the solution was stirred for an additional 10 min. The appropriate titanocene was then added and the mixture was stirred at RT for 4 hrs in the dark. Following filtration, the product was purified as detailed for each complex.
General method for coordination of CuBr: An oven-dried, two-necked round-bottom flask was purged with argon and charged with the parent titanocene and CuBr. Following injection of dried, degassed solvent, the reaction mixture was stirred in the dark at RT for 3–4 h and then purified as detailed for each complex.
DMA[Cp*Ti]CuBr: The general method for coordination of CuBr was followed using DMA[Cp*Ti] (139 mg, 0.23 mmol, 1.0 equiv), CuBr (70 mg, 0.49 mol, 2.1 equiv) and THF (16 mL) Following a 3h reaction period the mixture was filtered (removing CuBr). The solvent was evaporated from the deeply colored filtrate. THF (2 mL) was added to the dried product, and the flask was sonicated in a cleaning bath and cooled at −20 °C for 1.5 h. The solid was filtered, rinsed with hexanes, and dried in vacuo (75 mg, 44%). UV–Vis (THF) λmax (ε); 546 (9770), 360 sh (18,800), 287 (47,500). 1H NMR (500 MHz, CDCl3) δ 7.52 (d, 4H), 6.67 (d, 4H), 2.97 (s, 12H), 1.97 (s, 30H); Anal. Calcd (found) for C40H50N2TiCuBr•½H2O: C, 63.28 (63.19); H, 6.77 (6.76); N, 3.69 (3.50). IR (neat) νC≡C = 1985, 1969 cm−1, νO-H (from water) = 3427, 3519 cm−1 (very weak).
PhCF3[Cp*Ti]: The general procedure for the parent titanocenes was followed using THF (8 mL), 4-ethynyl-α, α, α-trifluorotoluene (0.25 mL, 1.5 mmol, 4 equiv), n-butyllithium (2.5 M, 0.68 mL, 1.7 mmol, 4.4 equiv) and Cp*2TiCl2 (150 mg, 0.385 mmol, 1.0 equiv). After the 4 h reaction period, impurities were removed via vacuum filtration, and the solvent was removed from the deep-red filtrate using rotary evaporation. The resulting solid was loaded onto a silica gel column (1 cm × 13 cm) and eluted using 5% (v/v) mixture of triethylamine in dichloromethane. A red band was eluted into a round-bottom flask, and the solvent was evaporated. Hexanes (4 mL) were then added to the flask. Following sonication of the flask in a cleaning bath and cooling (dry ice) the solid was filtered and dried in vacuo (147 mg, 57%). UV-Vis (THF) λmax (ε); 538 (1410), 476 (2140), 373 (7980), 346 (10,600), 301 (32,700). 1H NMR (500 MHz, CDCl3) δ 7.47 (d, 4H), 7.36 (d, 4H), 2.06 (s, 30H). Anal. Calcd (found) for C38H38TiF6: C, 69.51 (69.67); H, 5.83 (6.16). IR (neat) νC≡C = 2075 cm−1.
PhCF3[Cp*Ti]CuBr: The general method for coordination of CuBr was followed using PhCF3[Cp*Ti] (50 mg, 0.076 mmol, 1.0 equiv), CuBr (22 mg, 0.15 mmol, 2.0 equiv), and CH2Cl2 (6 mL). After a 4 h reaction period, the mixture was filtered through Celite (to remove CuBr) and the filtrate collected. The solvent was removed using rotary evaporation. Dichloromethane (0.5 mL) was added to partially dissolve the product which was then precipitated by the addition of hexanes (8 mL). The mixture was cooled in dry ice for 10 min, and the solid was filtered and dried in vacuo (42 mg, 70%). UV-Vis (THF) λmax (ε); 460 (4680), 375 (9250). 1H NMR (500 MHz, CDCl3) δ 7.81 (d, 4H), 7.60 (d, 4H), 1.97 (s, 30H) Anal. Calcd (found) for C38H38TiF6CuBr•3H2O: C, 53.44 (53.11); H, 5.19 (4.87). IR (neat) νC≡C = 1960 cm−1, νO-H (from water) = 3426, 3519 cm−1.
DMA[ansa-CpTi]: The general procedure for the parent titanocenes was followed using Et2O (8 mL), 4-ethynyl-N,N-dimethylaniline (169 mg, 1.17 mmol, 2.5 equiv), n-butyllithium (2.5 M, 0.47 mL, 1.2 mmol, 2.5 equiv) and [Me2Si(η5-C5H4)2TiCl2] (150 mg, 0.469 mmol, 1.0 equiv). After the 4 h reaction period, the reaction mixture was chilled to −20 °C for 30 min and then immediately filtered through a Hirsch funnel. The precipitate was loaded onto an alumina (neutral, Grade I, ~60 mesh) column (2 cm × 15 cm) and eluted using a 5% (v/v) mixture of triethylamine in CH2Cl2. The dark purple band was eluted into a round-bottom flask and the solvent was evaporated. THF (1 mL) was added to partially dissolve the product and hexanes (25 mL) was added to precipitate the product. Following sonication of the flask in a cleaning bath and then cooling to −20 °C for 1 h, the product was filtered and dried in vacuo (145 mg, 58%). UV-Vis (THF) λmax (ε); 548 (20,100), 450 sh (11,700). 1H NMR (500 MHz, CDCl3) δ 7.48 (t, 4H), 7.21 (d, 4H), 6.58 (d, 4H), 5.72 (t, 4H), 2.94 (s, 12H), 0.56 (s, 6H). Anal. Calcd (found) for C32H34N2SiTi: C, 73.55 (73.39); H, 6.56 (6.76); N 5.36 (5.49). IR (neat) νC≡C = 2044, 2024 cm−1.
DMA[ansa-CpTi]CuBr: The general method for coordination of CuBr was followed using DMA[ansa-CpTi] (100 mg, 0.186 mmol, 1.0 equiv), CuBr (67 mg, 0.47 mmol, 2.5 equiv) and THF (10 mL). After a 3 h reaction period, the solvent was removed using rotary evaporation. The resulting solid was then purified on a silica gel column (2 cm × 15 cm) with 2% (v/v) triethylamine in CH2Cl2 as the eluent. The dark-blue band was eluted into a round-bottom flask and the solvent was evaporated. Hexanes (10 mL) was added, and the flask was sonicated in a cleaning bath, and cooled to −20 °C for 30 min. The solid was filtered and dried in vacuo (89 mg, 71%). UV-Vis (THF) λmax (ε); 583 (11,200), 400 (19,000), 383 sh (18,700). 1H NMR (500 MHz, CDCl3) δ 7.43 (d, 4H), 6.64 (d, 4H), 6.50 (t, 4H), 5.80 (t, 4H), 2.99 (s, 12H), 0.65 (s, 6H). Anal. Calcd (found) for C32H34N2SiTiCuBr0.67(OH)0.33: C, 59.56 (59.87); H, 5.36 (5.56); N 4.34 (4.50). Calcd for C32H34N2SiTiCuBr•½C6H14: C, 59.28; H, 5.83; N 3.95. IR (neat) νC≡C = 1996, 1940 cm−1.
DMA[IndTi]: The general procedure for the parent titanocenes was followed using Et2O (8 mL), 4-ethynyl-N,N-dimethylaniline (137 mg, 0.946 mmol, 2.2 equiv), n-butyllithium (2.5 M, 0.38 mL, 0.95 mmol, 2.2 equiv), and Ind2TiCl2 (150 mg, 0.430 mmol, 1.0 equiv). After the 4 h reaction period, the solid was collected via vacuum filtration and was loaded onto a silica gel column (2 cm × 15 cm) and eluted using a 5% (v/v) mixture of triethylamine in CH2Cl2. The purple band was eluted into a round-bottom flask, and the solvent was evaporated. CH2Cl2 (1 mL) was added to partially dissolve the product which was then precipitated by the addition of hexanes (20 mL). The flask was briefly sonicated in a cleaning bath and the solid was filtered and dried in vacuo (123 mg, 49.2%). UV-Vis (THF) λmax (ε); 558 (16,000), 448 (12,000), 285 (64,300). 1H NMR (500 MHz, C6D6) δ 7.57 (m, 4H), 7.50 (d, 4H), 6.98 (m, 4H), 6.48 (d, 4H), 6.39 (t, 2H), 6.28 (d, 4H), 2.42 (s, 12H). Anal. Calcd (found) for C38H34N2Ti•H2O: C, 78.08 (78.12); H, 6.21 (6.22); N, 4.79 (5.17). IR (neat) νC≡C = 2046, 2025 cm−1, νO-H (from water) = 3390 cm−1 (broad).
DMA[IndTi]CuBr: The general method for coordination of CuBr was followed using DMA[IndTi] (100 mg, 0.177 mmol, 1.0 equiv), CuBr (51 mg, 0.35 mmol, 2.0 equiv), and THF (10 mL). After a 3 h reaction period, the solvent was evaporated, and the residue was purified on a silica gel column (2 cm × 15 cm) using 5% (v/v) triethylamine in CH2Cl2 as the eluent. The green band was eluted into a round-bottom flask, and the solvent was evaporated. CH2Cl2 (2 mL) was added to partially dissolve the product which was then precipitated by the addition of hexanes (20 mL). The mixture was cooled in dry ice and the solid was filtered and dried in vacuo (94 mg, 75%). UV-Vis (THF) λmax (ε); 592 (9000), 401 (18,200), 291 (41,200). 1H NMR (500 MHz, CDCl3) δ 7.48 (m, 4H), 7.44 (d, 4H), 7.21 (m, 4H), 6.66 (d, 4H), 6.03 (d, 4H), 5.81 (t, 2H), 3.00 (s, 12H). Anal. Calcd (found) for C38H34N2TiCuBr: C, 64.28 (63.88); H, 4.83 (5.03); N, 3.95 (4.04). IR (neat) νC≡C = 1981, 1969 cm−1.

3. Results and Discussion

3.1. Syntheses and Spectroscopic Characterization

The syntheses were performed using the methods published previously for similar complexes [30,31,48]. Namely, to make the parent arylethynyltitanocenes, the appropriate arylacetylene was deprotonated with n-BuLi followed by addition of the appropriate titanocene dichloride (Figure 3). The syntheses of DMA[IndTi] and DMA[ansa-CpTi] were performed in Et2O as has been employed for many related titanocenes [48]. However, the Cp* complex syntheses were performed in THF. Attempts in Et2O resulted in poorer yields. Purification of these arylalkynyltitanocenes was performed by chromatography using an eluent of 5% Et3N in CH2Cl2 (v/v). The addition of Et3N follows previous reports and is necessary to prevent acid hydrolysis during column purification [29,49]. Silica gel was used as the stationary phase for all complexes except for DMA[ansa-CpTi] which decomposed on that support. Replacing silica gel with alumina resulted in a pure product. Following chromatography, each complex was recrystallized, resulting in analytically pure products. In all cases, the 1H NMR spectra support the proposed structure of the parent compound (Supplementary Materials Figures S2, S4 and S6). The arylalkynyl complexes are air-stable but light-sensitive and can be handled on the bench top under reduced light conditions.
The coordination of CuBr between the alkynes followed the procedure for related arylalkynyltitanocenes where excess CuBr was added to the appropriate parent complex (Figure 3) [29]. Upon stirring for 3–4 h, the sample was filtered and DMA[ansa-CpTi]CuBr and DMA[IndTi]CuBr were purified by chromatography on silica gel using an eluent of 2–5% (v/v) Et3N in CH2Cl2, followed by recrystallization. The Cp* complexes were purified only by recrystallization similar to other Cp* complexes of this type [30,31]. In all cases, the 1H NMR spectra of the complexes with CuBr coordinated between the alkynes have the same peaks and splittings as the parent complexes, but with slight shifts in the resonances (Supplementary Materials Figures S1–S7). Additional evidence of CuBr coordination is provided by the shift in the C≡C stretching frequency in the IR spectra (Supplementary Materials Figure S8). For all four complex pairs, there is a shift to lower energy of 50 to 115 cm−1 upon coordination of CuBr, consistent with the alkynes engaging in η2 coordination with the metal [29,50,51].
For DMA[ansa-CpTi]CuBr, the crystal structure revealed a substitution of 33% of the Br by OH (vide infra) and this ratio resulted in a reasonable agreement between the predicted and experimental values for the elemental analysis. However, solvation by ½ hexane molecule per formula unit also gives reasonable agreement (both are listed in the experimental) and is qualitatively supported by the presence of a hexanes peak at 0.88 ppm in the 1H NMR spectrum. Furthermore, the 1H NMR spectrum of DMA[ansa-CpTi]CuBr is very sharp and does not reveal a second set of peaks attributable to the OH analog introduced by this substitution, suggesting that substitution occurred during crystal growth. Though we have not previously reported this substitution, we had observed such substitution previously but were able to obtain unsubstituted crystals by changing the crystal growth conditions. In those cases, we only published the unsubstituted structure, something we were unable to obtain herein. We believe that this substitution can also occur during the column purification, as an impurity was observed for Ph[Cp*Ti]CuBr when purified by column chromatography [30] that we believe to be the OH analog. However, for Ph[Cp*Ti]CuBr there was 1H NMR evidence for the impurity. It is worth noting that in the case of Ph[Cp*Ti]CuBr, the presence of the impurity did not impact the photobehavior [30].

3.2. Structural Characterization

3.2.1. General Structural and Packing Discussion

The four complexes with CuBr coordinated between the alkynes have been characterized by single-crystal X-ray diffraction (Figure 4, Figure 5, Figure 6 and Figure 7, Table 1 and Table 2 and Supplementary Materials Figures S9–S14 and Tables S1 and S2). Structural metrics for Ph[Cp*Ti]CuBr have been previously published [30] and are included in Table 2 for comparison. In general, the structures of the complexes investigated herein are quite similar to a range of arylalkynyltitanocenes with CuBr coordinated between the alkynes [29]. The packing structure and unique features of each structure are discussed below.
The PhCF3[Cp*Ti]CuBr (Figure 4) complex crystallized as a THF solvate, with one fully unique complex and one THF molecule in the asymmetric unit. The two aryl substituents of PhCF3[Cp*Ti]CuBr have rotamer orientations quite different from one another, with one much more in plane with the alkyne-Ti-alkyne plane (dihedral angle of 19.6(2)°) than the other (64.90(11)°). The length of the Cu–Br bond is 2.3148(7) Å, which is typical for these complexes [29,30]. The Cu–Br bond is angled slightly away from the in-plane-rotated aryl group (Figure 4, right), perhaps to provide steric accommodation for the in-plane PhCF3 ring. Neighboring complexes display C–H···F short contacts between CF3 fluorine atoms and Cp* methyl hydrogen atoms and PhCF3 hydrogen atoms. The THF molecules maintain C–H···O contacts to the PhCF3 group that displays more out-of-plane rotation, while also interacting with an additional complex via C–H···Br short contacts. The packing of molecules is shown in the Supplementary Materials, Figure S9.
For DMA[Cp*Ti]CuBr (Figure 5), there are two unique complexes in the asymmetric unit. They display slightly different rotamer orientations of the aryl planes relative to the alkyne-Ti-alkyne plane (Table 2, dihedral angles). A more noticeable distinction between the unique complexes is their different CuBr attachment into the alkynyl pocket—one is mostly in the alkyne-Ti-alkyne plane (Ti–Cu–Br = 176.17(3)°) and the other has the Cu–Br bond pointing slightly out of plane (Ti–Cu–Br = 166.10(3)°), while both complexes maintain similar Cu–Br distances of 2.2951(6) Å and 2.3203(6) Å (Supplementary Materials Figure S13). The long-range structure features double-stranded chains of complexes propagating along the b-axis enabled by C–H···Br contacts originating from the DMA methyl carbon atoms (Supplementary Materials Figure S10). Given the only slightly different dihedral angles of both DMA ligands to the alkyne-Ti-alkyne plane occurring in both unique complexes, it seems the out-of-plane deviation in Cu–Br may be a result of these packing interactions.
The structure of DMA[IndTi]CuBr (Figure 6) has one fully unique complex in the asymmetric unit, and the refinement indicated substitution of 19% of the Br sites as OH in a substitutional disorder (Cu–Br = 2.3011(11) Å). Given that the elemental analysis for this complex is consistent with the CuBr complex, the substitution likely occurs during crystallization. A crystal of a second sample with lower purity (and using different crystal growth conditions) was dominated by OH substitution (71% OH and 29% Br). However, minimal structural perturbation is introduced by Br/OH exchange in this system. Namely, both DMA[IndTi]CuBr samples crystallize in the same space group and effectively have the same lattice parameters (accounting for some contraction in the OH-majority crystal, Supplementary Materials Table S1 and Figure S14). The key bond angles and interatomic distances are also nearly identical for the two samples (Supplementary Materials Table S2). Using the majority-Br-occupied sample as the representative example here, the dihedral angles between the DMA planes and the alkyne-Ti-alkyne plane indicate approximately perpendicular orientations of similar magnitude (84.75(13)° and 78.55(15)°). The packing extends into a three-dimensional framework via multiple C–H···Br short contacts (Supplementary Materials Figure S11).
In the structure of DMA[ansa-CpTi]CuBr (Figure 7) there is one fully unique complex in the asymmetric unit, and also a partial substitution of OH for Br in the crystal structure (33% OH and 67% Br; Cu–Br = 2.290(3) Å). Given the lack of variation in the structural metrics upon OH/Br substitution for DMA[IndTi]CuBr, the majority-Br-occupied structure is likely representative of DMA[ansa-CpTi]CuBr. Rotations of the DMA ligands in this complex are similar to what occurs in PhCF3[Cp*Ti]CuBr where one ligand remains mostly in-plane (dihedral angle of 22.01(12)°) and the second ligand is approximately perpendicular (82.36(18)°) relative to the alkyne-Ti-alkyne plane. This again results in CuBr attachment that bends away from the co-planar ligand (Figure 7, right). The bromine atom interacts with three neighboring complexes via C–H···Br short contacts to create a three-dimensional framework. (Supplementary Materials Figure S12).

3.2.2. Comparison of Key Structural Metrics

For all complexes, the arylethynyl ligands linked to Ti create a binding pocket for CuBr. A comparison of the C–Ti–C bond angle for the Cp* and Ind complexes shows that this angle appears dependent on the aryl substituent. Namely, this angle is approximately 88°–89° for the Ph and PhCF3 substituents but increases to approximately 91.5°–93° for the dimethylaniline (DMA) substituent (Table 2). A comparison of the dihedral angle between the aryl planes and the alkyne-Ti-alkyne plane also indicates that complexes with the DMA substituent behave differently (particularly for the Cp* and Ind derivatives—the ansa-Cp derivative will be discussed separately below). Namely, for the DMA substituent, this dihedral angle is closer to 90° (the aryl planes being nearly perpendicular to the alkyne-Ti-alkyne plane), whereas for the Ph and PhCF3 substituents, at least one of the two dihedral angles in each molecule is <32° and the largest dihedral angle is only ~65° (Table 2 and Figure 4, Figure 5, Figure 6 and Figure 7). It is noteworthy that the corresponding Cp complexes without CuBr bound between the alkynes, Ph[Ti] (Figure 1), and DMA[Ti] have been carefully investigated regarding such aryl rotamers. It was found that there was very little barrier to rotation of the Ph substituent, but a much larger barrier for DMA rotation, and the perpendicular orientation of the DMA substituents was the most thermodynamically favored [49]. The reason given was that conjugation involving the lone pair on the N atom results in a cumulene-like structure, Ti=C=C=Caryl, as one resonance structure. It is possible that because such conjugation involves orbitals on Ti, that the perpendicular orientation is the one that most favors such conjugation. This may also be occurring in the CuBr complexes investigated herein. Such conjugation could also explain the slightly larger C-Ti-C bond angle for the DMA complexes, because electron-electron repulsion from the partial double bond character might slightly increase that angle.
A comparison of the angle formed between the cyclopentadienyl centroids and the central Ti for each complex is also worthwhile. This is because one hypothesis given for the observation that the Cp* complex, Ph[Cp*Ti]CuBr, phosphoresces in RT solution (whereas the Cp derivative, Ph[Ti]CuBr, does not) was that the steric bulk provided by the Cp* ligand limits excited-state structural rearrangement through steric congestion [30]. Excited-state structural rearrangement is known to increase the rate constant for non-radiative decay, thus competitively diminishing radiative decay and shortening the overall excited-state lifetime [17]. Evidence cited for steric congestion involved the centroid-Ti-centroid angle, which for Ph[Ti]CuBr is 134.2°, and for Ph[Cp*Ti]CuBr is 141.6°. The expansion of this angle for the Cp* complex was attributed to increased repulsion between the Me substituents on the Cp* ligands [30]. The bond angle is described as being due to a balance between methyl-methyl contacts and contacts between the aryl hydrogens and the same methyl groups. Further evidence cited for such congestion is that the methyl groups on Ph[Cp*Ti]CuBr bend away from each other, deviating from the cyclopentadienyl plane by 0.20 Å.
The two Cp* complexes reported herein have nearly identical centroid-Ti-centroid angles to that of Ph[Cp*Ti]CuBr (Table 2). Furthermore, the average deviation of the methyl groups from the cyclopentadienyl plane for PhCF3[Cp*Ti]CuBr (0.22 Å) and DMA[Cp*Ti]CuBr (0.20 Å) compare well with that of Ph[Cp*Ti]CuBr (0.20 Å). As will be shown below, both of these Cp* complexes are emissive in RT fluid solutions. The indenyl complex, DMA[IndTi]CuBr, on the other hand, has a centroid-Ti-centroid angle (135.7(2)°) much closer to that of Ph[Ti]CuBr and that may indicate that the fused aryl ring on the indenyl complex is better able to avoid congestion through a conformation where the two fused aryl rings are offset, both avoiding each other and avoiding interaction with the arylethynyl ligand. As will be discussed below, DMA[IndTi]CuBr, is not emissive in RT fluid solution.
Lastly, for DMA[ansa-CpTi]CuBr, the centroid-Ti-centroid angle (131.5(3)°) is particularly acute and this likely results from the short SiMe2 bridge between the two cyclopentadienyl rings. Though this bridge likely inhibits rotational movement of the Cp rings, this pinning back of the Cp rings likely minimizes steric congestion between the Cp rings and the arylethynyl substituents. As will be discussed below, DMA[ansa-CpTi]CuBr, is not emissive in RT fluid solution. One notable difference between DMA[ansa-CpTi]CuBr and the corresponding Ind and Cp* complexes with C2DMA ligands is the dihedral angle between the aryl rings and the alkyne-Ti-alkyne plane. Whereas for the DMA[IndTi]CuBr and DMA[Cp*Ti]CuBr complexes, both aryl rings were nearly perpendicular to the alkyne-Ti-alkyne plane, for DMA[ansa-CpTi]CuBr, one is close to perpendicular (82.36(18)°) and the other is closer to coplanar (22.01(12)°). It is unclear what causes this difference, but the more acute centroid–Ti–centroid angle due to the SiMe2 bridge for this complex is a likely candidate—and this may result in different packing thermodynamics.

3.3. Photochemical and Photophysical Characterization

3.3.1. Comparison of the Cp* Complexes

As discussed above, the enhanced phosphorescence of Ph[Cp*Ti]CuBr vs. Ph[Ti]CuBr was attributed to increased steric bulk of the Cp* ligand vs. the Cp ligand [30]. However, the excited state character also changed. Though for Ph[Ti]CuBr the lowest-energy CT transition was dominated by CuBr-to-Ti CT and phenylethynyl-to-Ti LMCT, the dominant transitions involved for Ph[Cp*Ti]CuBr changed to Cp*-to-Ti LMCT and CuBr-to-Ti CT, with very little phenylethynyl-to-Ti LMCT. Perhaps the greater emission for Ph[Cp*Ti]CuBr can be attributed to the presence of Cp*-to-Ti LMCT orbital character in the excited state. To test this, we have prepared the corresponding Cp* complex with an ethynyldimethylaniline (C2DMA) ligand, DMA[Cp*Ti]CuBr. We hypothesize that the more electron-rich DMA substituent (compared to phenyl) will lead to increased arylethynyl-to-Ti LMCT character. Furthermore, we have also prepared a complex with a more electron-deficient substituent than phenyl, namely PhCF3[Cp*Ti]CuBr, that we hypothesize will be dominated by Cp*-to-Ti LMCT due to the electron-poor nature of the arylethynyl ligand.
DMA[Cp*Ti]CuBr is phosphorescent both in 77 K 2-methyltetrahydrofuran (2-Me-THF) glass and in fluid solution at RT (Figure 8). The observation that the excitation spectra are a good match with the absorbance spectrum is a good indication that emission originates from the analyte rather than an impurity (Supplementary Materials Figure S15). As is typical for these complexes [29], the 77 K emission spectrum of the CuBr coordinated complex is red-shifted compared to the parent DMA[Cp*Ti] (Supplementary Materials, Figure S16). The photoluminescent quantum yield for DMA[Cp*Ti]CuBr (ΦPL = 3.7 × 10−3) and the phosphorescence lifetime (τ = 0.16 μs) compare favorably with the corresponding phenyl complex, Ph[Cp*Ti]CuBr (Table 3).
To further investigate the orbital involvement in this emissive excited state, density functional theory (DFT) and time-dependent DFT (TDDFT) were performed using B3LYP/6-311+G(d) functional that was shown to effectively model the excited-state behavior of Ph[Cp*Ti]CuBr and xyl[Cp*Ti]CuBr [31]. Mulliken population analysis was also performed to estimate the charge distribution and quantity of charge transfer for the one-electron transitions of interest (Table 4). The lowest-energy triplet excited state for DMA[Cp*Ti]CuBr is 93% HOMO-to-LUMO in character, being dominated by C2DMA-to-Ti LMCT (Figure 9, Table 4) with no Cp*-to-Ti LMCT character. This suggests that the presence of Cp*-to-Ti LMCT character is not a prerequisite for the increased lifetime observed in these complexes.
PhCF3[Cp*Ti]CuBr also phosphoresces in both 77 K 2-Me-THF glass and in RT fluid solution (Figure 10). Again, the observation that the excitation spectra are a good match with the absorbance spectrum is a good indication that emission originates from the analyte rather than an impurity (Supplementary Materials, Figure S19). It is noteworthy that the RT solution phosphorescence quantum yield and lifetime are significantly smaller than those for Ph[Cp*Ti]CuBr and DMA[Cp*Ti]CuBr (Table 3). Furthermore, whereas the coordination of CuBr consistently results in a redshift in the emission band of >1600 cm−1 relative to the parent complex [29], for PhCF3[Cp*Ti]CuBr, there is a blueshift of 400 cm−1 (Supplementary Materials, Figure S20). In addition, coordination of CuBr typically results in emission lifetimes at 77 K in the μs range [29]. However, the excited-state lifetime for PhCF3[Cp*Ti]CuBr is in the ms range (Table 3). To further investigate the excited-state behavior, DFT and TDDFT were performed using B3LYP/6-311+G(d) functional employed for DMA[Cp*Ti]CuBr. For this complex, the HOMO has very little electron density on the C2PhCF3 ligand and the transition is mainly Cp*-to-Ti LMCT and includes a small amount of Cp*-to-C2PhCF3 ligand-to-ligand CT (LL’CT) (Figure 9, Table 4). Thus, the CT transition does not appear to involve Cu and this may lower the degree of spin–orbit coupling induced by the heavier Cu atom, resulting in longer phosphorescence lifetimes at 77 K due to decreased rate constants for triplet to singlet transitions. It is also likely that diminished spin–orbit coupling decreases the rate of intersystem crossing (ISC) to the emissive triplet state so that in RT solution, direct nonradiative decay from the initially formed singlet is competitive with ISC, and this would diminish the overall phosphorescence quantum yield and lifetime. It is also worth noting that the charge-transfer components for all complexes only account for about half of the overall one-electron transition, with the remainder largely being of π-to-π* character localized on the arylethynyl ligand (Table 4).

3.3.2. Replacing Cp* with Indenyl or ansa-Cp

Previous research on this class of titanocenes has suggested that steric constraint can be used to increase the lifetimes of the emissive excited state [30,31]. It is important to recognize the rationale for why steric constraint might improve these lifetimes. In fluid solution, the lowest-energy triplet excited state (T1) for a flexible molecule will necessarily undergo geometric rearrangement relative to the ground state (S0). Thus, the T1 potential well is shifted along the nuclear coordinate (relative to S0) setting up a low barrier for nonradiative decay via T1 to S0 potential well surface crossing (Figure 11). A rigid molecule will not undergo the same degree of geometric rearrangement and thus the T1 potential well will be more “nested” with the ground state potential well, increasing the barrier for nonradiative decay through surface crossing. This should increase the overall excited-state lifetime.
Given that the Cp* complex with the DMA ligand is luminescent in solution at RT, it is reasonable to study other forms of steric bulk and constraint at the Cp ligand. We chose to prepare both the DMA[IndTi]CuBr and DMA[ansa-CpTi]CuBr complexes (Figure 2) to compare directly to DMA[Cp*Ti]CuBr. The indenyl ligand was chosen because the increased steric bulk compared to cyclopentadienyl might have a similar effect as the Cp* ligand. The ansa-Cp ligand was chosen because of the constraint imposed by the Me2Si bridge between the two Cp ligands. Yet as discussed in the structural discussion, there was evidence that neither of these ligands resulted in the same degree of steric congestion that the Cp* ligand provided. Thus, it is perhaps unsurprising that neither of these complexes showed emission in RT THF solution. However, both DMA[ansa-CpTi]CuBr and DMA[IndTi]CuBr are emissive in 77 K 2-Me-THF glass (Figure 12). Again, the excitation spectra are a good match with the absorption spectra (Supplementary Materials, Figures S21 and S23). Upon coordination of CuBr, there is a significant redshift in the emission relative to the corresponding parent complexes as is common for this class of compounds (Table 5 and Supplementary Materials, Figures S22 and S24).
It is worthwhile to note that the 77 K emission maximum for both DMA[ansa-CpTi]CuBr (770 nm) and DMA[IndTi]CuBr (793 nm) are significantly redshifted relative to the 77 K emission maxima for the three Cp* compounds that show phosphorescence in RT fluid solution, namely DMA[Cp*Ti]CuBr (679 nm), Ph[Cp*Ti]CuBr (619 nm), and xyl[Cp*Ti]CuBr (692 nm). Note that if the T1 potential well is lowered in energy (Figure 11), this would also reduce the activation barrier for T1 to S0 potential well surface crossing. Furthermore, lowering the energy of the T1 state may also increase the rate of nonradiative decay through energy-gap law behavior [53]. Thus, the lack of RT emission from either DMA[ansa-CpTi]CuBr or DMA[IndTi]CuBr cannot be attributed unequivocally to the ansa-Cp ligand and the indenyl ligand not providing sufficient steric constraint.

3.3.3. Impact of CuBr Coordination on Photostability

Previous reports on arylethynyltitanocenes have demonstrated that coordination of CuBr both redshifts the emission and decreases the quantum yield for photodecomposition, Φdecomp. For a series of homoleptic cyclopentadienyl complexes where only the aryl substituent and the metal halide bound between the alkynes were varied, the data was explained using a mechanism where photodecomposition occurred out of the lowest energy triplet excited state, and that the decomposition rate constant was dependent on the energy of the excited state [29]. This hypothesis can be explained by the assumption that decomposition requires breaking Ti-C bonds and thus there is a threshold of excited-state energy necessary. Largely, the data presented here are consistent with that hypothesis. For example, for DMA[ansa-CpTi], DMA[IndTi], and DMA[Cp*Ti] there is a significant redshift in the 77 K emission upon coordination of CuBr and a concomitant lowering of Φdecomp (Table 5). The one complex that deviates from this trend is PhCF3[Cp*Ti]CuBr, which as discussed earlier, shows a blue-shift in the emission relative to the parent PhCF3[Cp*Ti]. Yet there is still a lowering of Φdecomp. One possible explanation is that this complex is the first for which the excited state does not involve charge transfer to Ti from either CuBr or the arylethynyl ligand—and thus the decomposition mechanism may differ from that proposed for previously investigated complexes [29].

4. Conclusions

Several different methods of providing steric constraint to arylethynyltitanocenes as a means to increase the excited-state lifetimes were investigated. Chiefly, comparison was made between complexes of three different cyclopentadienyl analogs, Cp*, Ind, and ansa-Cp. Whereas DMA[Cp*Ti]CuBr is emissive in RT fluid solution, neither DMA[IndTi]CuBr, nor DMA[ansa-CpTi]CuBr are emissive under those conditions. The structures of the complexes suggest that the Cp* complex shows significant steric congestion involving Me-Me contacts between the Cp* ligands and contacts between the Me groups and the aryl ring of the arylethynyl ligand. Similar steric constraint does not appear to exist for the corresponding Ind and ansa-Cp complexes. Furthermore, the fact that the ansa-Cp derivative is not emissive suggests that simply hindering rotational motion of the Cp ligands does not provide sufficient constraint to result in emission. However, it was also noted that both DMA[IndTi]CuBr and DMA[ansa-CpTi]CuBr have lower-energy excited states than DMA[Cp*Ti]CuBr, so the lack of emission cannot be unequivocally attributed to a lack of steric constraint. Additionally, the relationship between the orbital character of the excited state and the observation of emission in RT fluid solution was examined. It does not appear that Cp*-to-Ti LMCT character is required for such behavior. However, the results suggest that the coordinated CuBr plays a role in spin–orbit coupling, enhancing the rates of intersystem crossing from the initially formed singlet state to the phosphorescent triplet state and radiative decay from that triplet state. Such a role for coordinated CuX (X = Br, Cl) has been proposed previously for structurally similar ethynylferrocene (C2Fc) complexes, Cp2Ti(C2Fc)2CuX [54].

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/cryst15080745/s1, Figures S1–S7: 1H NMR spectra, Figure S8: Infrared spectra, Figures S9–S12: Crystal packing diagrams for crystallographically characterized complexes, Figure S13: Diagrams of two unique molecules for DMA[Cp*Ti]CuBr unit cell, Table S1: Crystallographic data and refinement details for DMA[IndTi]CuBr0.29(OH)0.71, Figure S14: Crystal structure of DMA[IndTi]CuBr0.29(OH)0.71, Table S2: Structural metrics for DMA[IndTi]CuBr0.29(OH)0.71, Figure S15: Absorption and, RT and 77 K excitation spectra for DMA[Cp*Ti] and DMA[Cp*Ti]CuBr, Figure S16: 77 K emission spectra for DMA[Cp*Ti] and DMA[Cp*Ti]CuBr, Figure S17: Luminescence decay trace for DMA[Cp*Ti]CuBr and PhCF3[Cp*Ti]CuBr in 77 K 2-MeTHF, Figure S18: Luminescence decay trace for DMA[Cp*Ti]CuBr in THF solution, Figure S19: Absorption and, RT and 77 K excitation spectra for PhCF3[Cp*Ti] and PhCF3[Cp*Ti]CuBr, Figure S20: 77 K emission spectra for PhCF3[Cp*Ti] and PhCF3[Cp*Ti]CuBr, Figure S21: Absorption and, RT and 77 K excitation spectra for DMA[ansa-CpTi] and DMA[ansa CpTi]CuBr, Figure S22: 77 K emission spectra for DMA[ansa-CpTi] and DMA[ansa-CpTi]CuBr, Figure S23: Absorption and, RT and 77 K excitation spectra for DMA[IndTi] and DMA[IndTi]CuBr, Figure S24: 77 K emission spectra for DMA[IndTi] and DMA[IndTi]CuBr.

Author Contributions

Conceptualization, P.S.W.; methodology, M.B., C.D.M. and P.S.W.; validation, M.B., S.C.W., E.A.M., J.H.Z. and C.D.M.; formal analysis, M.B., S.C.W., E.A.M., J.H.Z., R.S.G. and C.D.M.; investigation, M.B., S.C.W., E.A.M., J.H.Z., R.S.G., J.S.M. and C.D.M.; resources, P.S.W. and C.D.M.; data curation, M.B., C.D.M. and P.S.W.; writing—original draft preparation, M.B., S.C.W., E.A.M., R.S.G., C.D.M. and P.S.W.; writing—review and editing, M.B., S.C.W., E.A.M., J.H.Z., R.S.G., J.S.M., C.D.M. and P.S.W.; visualization, M.B., S.C.W., E.A.M., J.H.Z. and C.D.M.; supervision, P.S.W.; project administration, P.S.W.; funding acquisition, P.S.W. and C.D.M. All authors have read and agreed to the published version of the manuscript.

Funding

Support is acknowledged from the National Science Foundation under Grant No. 2055326. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect those of the National Science Foundation.

Data Availability Statement

CCDC 2475385-2475389 contain the supplementary crystallographic data for this paper. These data can be obtained from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033.

Acknowledgments

The authors thank George C. Shields for providing computational infrastructure [55], in part through a Research Corporation for Science Advancement Cottrell Instrumentation Supplements Award #27446.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
2-Me-THF2-methyltetrahydrofuran
ansa-CpMe2Si(η5-C5H4)2
CpCyclopentadienyl
Cp*Pentamethylcyclopentadienyl
DMADimethylaniline
ΦdecompQuantum yield for photodecomposition
ΦPLQuantum yield for photoluminescence
IndIndenyl
LL’CTLigand-to-ligand charge transfer
LMCTLigand-to-metal charge transfer
MCMetal-centered
MXMCTMetal halide-to-metal charge transfer
RTRoom temperature

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Crystals 15 00745 g001
Figure 1. Previous arylethynyltitanocenes investigated along with the abbreviations used herein and their photoluminescent quantum yields (ΦPL), emission lifetimes (τ), and photodecomposition quantum yields (Φdecomp) in deaerated THF. Figure adapted from [31].
Figure 1. Previous arylethynyltitanocenes investigated along with the abbreviations used herein and their photoluminescent quantum yields (ΦPL), emission lifetimes (τ), and photodecomposition quantum yields (Φdecomp) in deaerated THF. Figure adapted from [31].
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Crystals 15 00745 g002
Figure 2. New titanocenes prepared herein along with the abbreviations used in the discussion. The superscripted DMA indicates the dimethylaniline substituent on the alkynes; the superscripted PhCF3 indicates the α,α,α-trifluorotoluene substituent on the alkynes. Ind indicates the indenyl ligand, Cp* the pentamethylcyclopentadienyl ligand, and ansa-Cp the Me2Si(η5-C5H4)2 ligand.
Figure 2. New titanocenes prepared herein along with the abbreviations used in the discussion. The superscripted DMA indicates the dimethylaniline substituent on the alkynes; the superscripted PhCF3 indicates the α,α,α-trifluorotoluene substituent on the alkynes. Ind indicates the indenyl ligand, Cp* the pentamethylcyclopentadienyl ligand, and ansa-Cp the Me2Si(η5-C5H4)2 ligand.
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Crystals 15 00745 g003
Figure 3. Synthetic scheme for the synthesis of arylethynyltitanocenes and the corresponding CuBr coordinated complexes. RCp = Cp*, or indenyl, or ansa-Cp. R2 = CF3 or NMe2.
Figure 3. Synthetic scheme for the synthesis of arylethynyltitanocenes and the corresponding CuBr coordinated complexes. RCp = Cp*, or indenyl, or ansa-Cp. R2 = CF3 or NMe2.
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Crystals 15 00745 g004
Figure 4. Crystal structure of PhCF3[Cp*Ti]CuBr shown as 50% probability ellipsoids (left) and as an overhead wire frame view (right). Color code: Ti (orange); Cu (turquoise); Br (red); F (light green).
Figure 4. Crystal structure of PhCF3[Cp*Ti]CuBr shown as 50% probability ellipsoids (left) and as an overhead wire frame view (right). Color code: Ti (orange); Cu (turquoise); Br (red); F (light green).
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Crystals 15 00745 g005
Figure 5. Crystal structure of DMA[Cp*Ti]CuBr shown as 50% probability ellipsoids (left) and as an overhead wire frame view (right). Color code: Ti (orange); Cu (turquoise), Br (red).
Figure 5. Crystal structure of DMA[Cp*Ti]CuBr shown as 50% probability ellipsoids (left) and as an overhead wire frame view (right). Color code: Ti (orange); Cu (turquoise), Br (red).
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Crystals 15 00745 g006
Figure 6. Crystal structure of DMA[IndTi]CuBr shown as 50% probability ellipsoids (left) and as an overhead wire frame view (right). Color code: Ti (orange); Cu (turquoise); Br (red).
Figure 6. Crystal structure of DMA[IndTi]CuBr shown as 50% probability ellipsoids (left) and as an overhead wire frame view (right). Color code: Ti (orange); Cu (turquoise); Br (red).
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Crystals 15 00745 g007
Figure 7. Crystal structure of DMA[ansa-CpTi]CuBr shown as 50% probability ellipsoids (left) and as an overhead wire frame view (right). Color code: Ti (orange); Cu (turquoise); Br (red); Si (yellow).
Figure 7. Crystal structure of DMA[ansa-CpTi]CuBr shown as 50% probability ellipsoids (left) and as an overhead wire frame view (right). Color code: Ti (orange); Cu (turquoise); Br (red); Si (yellow).
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Crystals 15 00745 g008
Figure 8. Absorption spectrum of DMA[Cp*Ti]CuBr in RT THF, and emission spectra in RT THF solution (λex = 365 nm) and 77 K 2-MeTHF glass (λex = 360 nm).
Figure 8. Absorption spectrum of DMA[Cp*Ti]CuBr in RT THF, and emission spectra in RT THF solution (λex = 365 nm) and 77 K 2-MeTHF glass (λex = 360 nm).
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Crystals 15 00745 g009
Figure 9. Frontier orbitals (isovalue = 0.02) for DMA[Cp*Ti]CuBr, Ph[Cp*Ti]CuBr, and PhCF3[Cp*Ti]CuBr. The data for Ph[Cp*Ti]CuBr is from [31].
Figure 9. Frontier orbitals (isovalue = 0.02) for DMA[Cp*Ti]CuBr, Ph[Cp*Ti]CuBr, and PhCF3[Cp*Ti]CuBr. The data for Ph[Cp*Ti]CuBr is from [31].
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Crystals 15 00745 g010
Figure 10. Absorption spectrum of PhCF3[Cp*Ti]CuBr in RT THF, and emission spectra in RT THF solution (λex = 450 nm) and 77 K 2-Me-THF glass (λex = 377 nm).
Figure 10. Absorption spectrum of PhCF3[Cp*Ti]CuBr in RT THF, and emission spectra in RT THF solution (λex = 450 nm) and 77 K 2-Me-THF glass (λex = 377 nm).
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Crystals 15 00745 g011
Figure 11. Potential-well diagram for a system with a singlet (S0) ground state and a triplet (T1) excited state shown for both a flexible geometry (red dashed line), and a rigid geometry (blue).
Figure 11. Potential-well diagram for a system with a singlet (S0) ground state and a triplet (T1) excited state shown for both a flexible geometry (red dashed line), and a rigid geometry (blue).
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Crystals 15 00745 g012
Figure 12. (a) DMA[ansa-CpTi]CuBr UV-Vis spectrum in RT THF solution, and emission spectrum in 77 K 2-MeTHF glass (λex = 400 nm). (b) DMA[IndTi]CuBr UV-Vis spectrum in RT THF solution, and emission spectrum in 77 K 2-MeTHF glass (λex = 550 nm).
Figure 12. (a) DMA[ansa-CpTi]CuBr UV-Vis spectrum in RT THF solution, and emission spectrum in 77 K 2-MeTHF glass (λex = 400 nm). (b) DMA[IndTi]CuBr UV-Vis spectrum in RT THF solution, and emission spectrum in 77 K 2-MeTHF glass (λex = 550 nm).
Crystals 15 00745 g012
Table 1. Crystallographic data and refinement details.
Table 1. Crystallographic data and refinement details.
PhCF3[Cp*Ti]CuBr
THF		DMA[Cp*Ti]CuBrDMA[IndTi]CuBrDMA[ansa-CpTi]CuBr
empirical formulaC42H46BrCuF6OTiC40H50BrCuN2TiC38H34.19Br0.81CuN2O0.19TiC32H34.33Br0.67CuN2O0.33SiTi
formula wt. (g/mol)872.14750.17698.07645.29
crystal systemtriclinicmonoclinicmonoclinicorthorhombic
space group, ZP-1P21/c, 8P21/c, 4Pna21, 4
temperature (K)100(2)100(2)100(2)100(2)
a (Å)9.4166(4)15.5719(8)6.6123(5)22.7878(15)
b (Å)13.8760(6)16.9073(9)23.8625(17)9.3747(6)
c (Å)15.7364(6)27.5471(14)19.5013(14)13.6177(9)
α (°)75.7461(18)909090
β (°)74.2782(18)103.5177(17)97.482(2)90
γ (°)74.9301(19)909090
volume (Å3)1876.72(14)7051.7(6)3050.8(4)2090.1(3)
Dcalc (g/cm3)1.5431.4131.5201.473
crystal size (mm)0.04 × 0.08 × 0.100.07 × 0.08 × 0.100.02 × 0.10 × 0.200.07 × 0.08 × 0.10
abs. coeff. (mm−1)1.9071.9932.0511.992
F(000)892312014281326
Tmax, Tmin1.000, 0.9231.000, 0.9511.000, 0.8581.000, 0.510
Θ range for data (°)3.14 to 25.731.94 to 26.032.27 to 25.703.81 to 25.70
reflections coll.79,466207,80753,51141,647
data/restr./param.7140/0/47913,887/0/8395768/2/3995502/2/354
R(int)0.14870.13110.18610.0947
final R [I > 2σ(I)] R1, wR20.0544, 0.10210.0465, 0.10240.0603, 0.12190.0471, 0.0985
final R (all data) R1, wR20.0797, 0.12200.0954, 0.12530.1223, 0.15100.0632, 0.1075
goodness-of-fit on F21.0671.0151.0151.033
larg. diff. peak, hole (eÅ−3)0.780, −0.6411.243, −1.1030.862, −0.7800.738, −0.320
CCDC Deposition No.2475385247538624753872475388
Table 2. Selected structural metrics.
Table 2. Selected structural metrics.
Ph[Cp*Ti]CuBr [30]PhCF3[Cp*Ti]CuBrDMA[Cp*Ti]CuBr aDMA[IndTi]CuBrDMA[ansa-CpTi]CuBr
(1)(2)
∠ Cp–Ti–Cp b141.6°141.23(19)°141.24(15)°141.46(15)°135.7(2)°131.5(3)°
∠ C1–Ti–C488.0(4)°88.93(16)°92.89(15)°91.48(15)°92.6(2)°92.5(3)°
∠ Ti–CαC172.0(7)°167.8(4)°,
169.6(4)°		168.8(3)°,
167.5(3)°		167.9(3)°, 169.3(3)°171.9(5)°,
165.0(5)°		176.6(7)°,
165.3(6)°
∠ C≡C–C160.1(9)°158.1(4)°,
165.5 (4)°		173.9(4)°, 170.1(4)°172.3(4)°, 168.8(4)°167.3(6)°,
168.1(6)°		172.8(8)°,
162.6(7)°
∠ Ti–Cu–Br180°173.77(3)°176.17(3)°166.10(3)°177.07(5)°168.91(12)°
Ti–Cp c (Å)2.04(2)2.099(4),
2.095(4)		2.099(4),
2.104(4)		2.104(4), 2.100(4)2.083(6),
2.072(6)		2.054(7),
2.065(7)
Ti–CC≡C (Å)2.080(8)2.084(4),
2.093(4)		2.090(4),
2.090(4)		2.085(4), 2.082(4)2.081(6),
2.081(6)		2.069(8),
2.090(8)
Dihedral d31.1(7)°64.90(11)°,
19.6(2)°		70.41(11)°, 83.18(12)°68.42(12)°, 88.12(12)°84.75(13)°,
78.55(15)°		82.36(18)°,
22.01(12)°
Me/Cp dev e (Å)0.200.220.200.20--
a The unit cell contains two unique molecules. Both are included in this column. b Angle between the cyclopentadienyl centroids and Ti. c Titanium to cyclopentadienyl centroid distance. d Dihedral angle between aryl plane and the plane formed by the Ti and the alkynyl carbons. e Average deviation of the methyl carbons from the plane of the cyclopentadienyl ring of the Cp* ligand.
Table 3. Photophysical Data for a Series of R[Cp*Ti]CuBr complexes.
Table 3. Photophysical Data for a Series of R[Cp*Ti]CuBr complexes.
λmax 77 Kτ 77 K (μs)λmax RTτ RT (μs)ΦPL RT a
DMA[Cp*Ti]CuBr679 nm124 b744 nm0.16 c3.7 × 10−3
Ph[Cp*Ti]CuBr d619 nm655693 nm0.181.3 × 10−3
PhCF3[Cp*Ti]CuBr629 nm1130 b635 nm– e4.5 × 10−4
a Values reported in air-saturated solution and appear independent to the presence of O2 (i.e., insensitive to purging with Ar). b Weighted average from double-exponential fit of the emission decay trace (Supplementary Materials Figure S17). Weighted average performed according to [52]. c Lifetime obtained from single-exponential fit of the emission decay trace (Supplementary Materials Figure S18). d Data from [30,31]. e Below detection limits of our instrument (<120 ns).
Table 4. Orbital contribution and population analysis for lowest-energy triplet states a.
Table 4. Orbital contribution and population analysis for lowest-energy triplet states a.
Complex (% HOMO-LUMO)λ(nm) bTiCp*C2ArylCuBr
DMA[Cp*Ti]CuBr (93%)6871 → 49 (48)3 → 8 (5)83 → 38 (−45)14 → 5 (−9)
Ph[Cp*Ti]CuBr c (80%)5773 → 50 (47)27 → 8 (−19)39 → 35 (−4)31 → 7 (−24)
PhCF3[Cp*Ti]CuBr (88%)59110 → 49 (39)56 → 8 (−48)26 → 34 (8)8 → 8 (0)
a Mulliken population analysis was performed using GaussSum 3.0 [35]. The numbers in parentheses in the body of the table indicate the increase or decrease in electron density due to the one-electron transition. Positive numbers indicate an increase, and negative numbers indicate a decrease. b TDDFT predicted wavelength of the lowest-energy triplet state. c Data from [31] included for comparison.
Table 5. Effect of CuBr coordination on photostability.
Table 5. Effect of CuBr coordination on photostability.
ParentCuBr Complex
λmax 77 KΦdecomp RT aλmax 77 KΦdecomp RT a
DMA[ansa-CpTi]675 nm0.15770 nm<1 × 10−4
DMA[IndTi]705 nm0.24793 nm3.6 × 10−4
DMA[Cp*Ti]633 nm0.36679 nm5.1 × 10−4
PhCF3[Cp*Ti]644 nm0.48626 nm6.1 × 10−3
a Measurement appears independent to conditions of air-saturation vs. Ar-saturation.
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Barker, M.; Walter, S.C.; McCallum, E.A.; Golden, R.S.; Zimmerman, J.H.; McCarthy, J.S.; McMillen, C.D.; Wagenknecht, P.S. Luminescent Arylalkynyltitanocenes: Effect of Modifying the Electron Density at the Arylalkyne Ligand, or Adding Steric Bulk or Constraint to the Cyclopentadienyl Ligand. Crystals 2025, 15, 745. https://doi.org/10.3390/cryst15080745

AMA Style

Barker M, Walter SC, McCallum EA, Golden RS, Zimmerman JH, McCarthy JS, McMillen CD, Wagenknecht PS. Luminescent Arylalkynyltitanocenes: Effect of Modifying the Electron Density at the Arylalkyne Ligand, or Adding Steric Bulk or Constraint to the Cyclopentadienyl Ligand. Crystals. 2025; 15(8):745. https://doi.org/10.3390/cryst15080745

Chicago/Turabian Style

Barker, Matilda, Samantha C. Walter, Elizabeth A. McCallum, River S. Golden, John H. Zimmerman, Jackson S. McCarthy, Colin D. McMillen, and Paul S. Wagenknecht. 2025. "Luminescent Arylalkynyltitanocenes: Effect of Modifying the Electron Density at the Arylalkyne Ligand, or Adding Steric Bulk or Constraint to the Cyclopentadienyl Ligand" Crystals 15, no. 8: 745. https://doi.org/10.3390/cryst15080745

APA Style

Barker, M., Walter, S. C., McCallum, E. A., Golden, R. S., Zimmerman, J. H., McCarthy, J. S., McMillen, C. D., & Wagenknecht, P. S. (2025). Luminescent Arylalkynyltitanocenes: Effect of Modifying the Electron Density at the Arylalkyne Ligand, or Adding Steric Bulk or Constraint to the Cyclopentadienyl Ligand. Crystals, 15(8), 745. https://doi.org/10.3390/cryst15080745

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