Crystallization-Enhanced Emission and Room-Temperature Phosphorescence of Cyclic Triimidazole-Monohexyl Thiophene Derivatives

The development of organic room-temperature phosphorescent (ORTP) materials represents an active field of research due to their significant advantages with respect to their organometallic counterparts. Two cyclic triimidazole (TT) derivatives bearing one and three hexyl-thiophene moieties, TT-HThio and TT-(HThio)3, have been prepared and characterized. Both compounds display enhanced quantum yields in their crystalline form with respect to those in a solution state, revealing crystallization-enhanced emissive (CEE) behavior. Importantly, while single fluorescence is observed in solution, crystalline powders also feature dual ORTP, whose respective molecular and aggregate origins have been disclosed through X-ray diffraction analysis and DFT/TDDFT calculations. The relation between the photophysical properties of TT-HThio and its crystallinity degree has been confirmed by a decrease in photoluminescent quantum yield (Φ) and loss of vibronic resolution when its crystals are ground in a mortar, revealing mechanochromic behavior and confirming CEE features.

In diluted DCM solutions (1 × 10 −5 M), TT-HThio displays a single fluorescence (FL with photoluminescent quantum yield, , equal to 11%; wavelength of the maximum peak (λem) = 370 nm and lifetime () = 0.81 ns at 298 K; and λem = 365 nm and  =1.71 ns a 77 K (Figures 1, S5 and S6). In PMMA-blended films (0.5 w% TT-HThio), one fluorescence (FL,  = 15%, λem = 365 nm,  = 0.72 ns at 298 K, Figures 1, S19) is observed in exactly the same position as that measured in DCM at 77 K, as expected for a molecular emission in rigidified media. TT-HThio and TT-(HThio) 3 display enhanced properties in the crystalline form with respect to their solutions revealing CEE (crystallization-enhanced emissive) behavior. Importantly, while single fluorescence is observed in solution, crystals are also characterized by dual phosphorescence, whose origin has been disclosed through X-ray diffraction analysis and DFT/TDDFT calculations.
Crystals of TT-HThio show, both at 298 (Φ = 26%) and 77 K, an excitation-dependent PL behavior comprising one fluorescence and two phosphorescences ( Figure 2). In particular, at 298 K, one fluorescence at λ em = 376 nm (FL, τ = 1.15 ns, Figure S7) is observed by exciting at 300 nm, one high-energy phosphorescence (HEP, with vibronic replica at λ em = 425 and 451 nm, τ = 5.91 ms, Figure S8) is visible by exciting at 376 nm and one low-energy phosphorescence (LEP, at λ em = 497, 530 and 578 nm, τ = 50.22 ms, Figure S9) appears when exciting at 450 nm. At 77 K, the three components are better resolved vibronically, in almost the same positions but with longer lifetimes with respect to those measured at room temperature (FL, λ em = 368 nm, τ = 1.48 ns; HEP, λ em = 420 and 448 nm, τ = 10.61 ms; LEP, λ em = 494, 533 and 577 nm, τ = 132.01 ms, Figures S10, S11 and S12, respectively). Intriguingly, the corresponding excitation profiles are characterized at low energy by three contributions recognizable at both 298 and 77 K. Specifically, one low-energy state (332, 354 nm at 298 K and 335, 347 at 77 K) assigned to singlet molecular excited state S 1 in agreement with structural and theoretical studies (see later), one triplet excited state of molecular origin (389 nm at 298 K and 390, 403 nm at 77 K), T mol , and one triplet excited state of aggregate origin (446 nm at 298 K and 448, 471 nm at 77 K), T ag , are observed. A mirroring relationship with the corresponding emission is clearly visible at 77 K.  Crystals of TT-HThio show, both at 298 ( = 26%) and 77 K, an excitation-dependent PL behavior comprising one fluorescence and two phosphorescences ( Figure 2). In particular, at 298 K, one fluorescence at λem = 376 nm (FL,  = 1.15 ns, Figure S7) is observed by exciting at 300 nm, one high-energy phosphorescence (HEP, with vibronic replica at λem = 425 and 451 nm,  = 5.91 ms, Figure S8) is visible by exciting at 376 nm and one low-energy phosphorescence (LEP, at λem = 497, 530 and 578 nm,  = 50.22 ms, Figure S9) appears when exciting at 450 nm. At 77 K, the three components are better resolved vibronically, in almost the same positions but with longer lifetimes with respect to those measured at room temperature (FL, λem = 368 nm,  = 1.48 ns; HEP, λem = 420 and 448 nm,  = 10.61 ms; LEP, λem = 494, 533 and 577 nm,  = 132.01 ms, Figures S10, S11 and S12, respectively). Intriguingly, the corresponding excitation profiles are characterized at low energy by three contributions recognizable at both 298 and 77 K. Specifically, one low-energy state (332, 354 nm at 298 K and 335, 347 at 77 K) assigned to singlet molecular excited state S1 in agreement with structural and theoretical studies (see later), one triplet excited state of molecular origin (389 nm at 298 K and 390, 403 nm at 77 K), Tmol, and one triplet excited state of aggregate origin (446 nm at 298 K and 448, 471 nm at 77 K), Tag, are observed. A mirroring relationship with the corresponding emission is clearly visible at 77 K. Based on the observation that  increases from 11%, in diluted DCM solution, to 26%, measured for TT-HThio crystals, we hypothesized possible aggregation-enhanced emissive (AEE) features; therefore, solvent (THF)/non-solvent (water) experiments have been performed. Addition of increasing water fractions to TT-HThio in THF (keeping the concentration equal to 1 × 10 −5 M) results, instead, in emission quenching (Figure 3), indicating Based on the observation that Φ increases from 11%, in diluted DCM solution, to 26%, measured for TT-HThio crystals, we hypothesized possible aggregation-enhanced emissive (AEE) features; therefore, solvent (THF)/non-solvent (water) experiments have been performed. Addition of increasing water fractions to TT-HThio in THF (keeping the concentration equal to 1 × 10 −5 M) results, instead, in emission quenching (Figure 3), indicating that aggregation itself is not sufficient to intensify luminescence, and that proper molecule organization (i.e., crystallization) is an indispensable condition to enhance emission from TT-HThio. Similarly, in TT itself, crystallization is a necessary condition to switch on luminescence [9].

Discussion
As previously observed for a series of TT derivatives bearing one to three chromophoric substituents (i.e., 2-pyridine [25], fluoropyridine [24] or pyrene [26,27]) on the TT scaffold, the presence of the chromophore switches on fluorescence which is otherwise absent in TT itself in solution. From DFT/TDDFT calculations, it was inferred that TT lacks low-energy singlet states with non-null oscillator strength (f) due to the high symmetry of its electronic system. Analogously, "silent" low-energy singlet states are present in trihalogenated TT derivatives [21,23]. However, in monosubstituted TT derivatives, the disruption of the molecular symmetry results in S1 and upper levels with medium-to-high oscillator strength depending on the nature of the substituent itself. This is also noted for TT derivatives functionalized with three equal chromophoric fragments owing to the adopted non-planar conformation [27]. For TT-HThio, the S1 level is computed at 258 nm (f = 0.49), quite similar to the mono-pyridine TT derivative for which S1 is computed at 256 nm with f = 0.48. In both cases, S1 is mainly (88%) a HOMO→LUMO transition of (π,π*) type and very weak CT character (in the present case from thiophene to TT) ( Figure  8 and Table S2). Moving to TT-(HThio)3 ( Figure 9 and Table S3), it should be noted that the presence of three, rather than one, chromophoric substituents does not significantly affect the absorption and emission properties of the compound, as also observed for the mono-, di-and three-pyrene series of TT derivatives [27]. The absence of inter-chromophoric communication in multiple-substituted TT derivatives, evidenced also by electro- (b) a fragment of crystal packing with shorter distances between triazinic geometrical centroids (green spheres) and C· · · C, C· · · S and C-H· · · π intermolecular contacts shorter than the sum of vdW radii (light grey-dashed lines); (c) the hydrogen bonded ribbon formed by A molecules and the π-stacks of B molecules. Ellipsoids at 20% probability.

Discussion
As previously observed for a series of TT derivatives bearing one to three chromophoric substituents (i.e., 2-pyridine [25], fluoropyridine [24] or pyrene [26,27]) on the TT scaffold, the presence of the chromophore switches on fluorescence which is otherwise absent in TT itself in solution. From DFT/TDDFT calculations, it was inferred that TT lacks low-energy singlet states with non-null oscillator strength (f ) due to the high symmetry of its electronic system. Analogously, "silent" low-energy singlet states are present in trihalogenated TT derivatives [21,23]. However, in monosubstituted TT derivatives, the disruption of the molecular symmetry results in S 1 and upper levels with medium-to-high oscillator strength depending on the nature of the substituent itself. This is also noted for TT derivatives functionalized with three equal chromophoric fragments owing to the adopted non-planar conformation [27]. For TT-HThio, the S 1 level is computed at 258 nm (f = 0.49), quite similar to the mono-pyridine TT derivative for which S 1 is computed at 256 nm with f = 0.48. In both cases, S1 is mainly (88%) a HOMO→LUMO transition of (π,π*) type and very weak CT character (in the present case from thiophene to TT) ( Figure 8 and Table S2). Moving to TT-(HThio) 3 (Figure 9 and Table S3), it should be noted that the presence of three, rather than one, chromophoric substituents does not significantly affect the absorption and emission properties of the compound, as also observed for the mono-, di-and three-pyrene series of TT derivatives [27]. The absence of inter-chromophoric communication in multiple-substituted TT derivatives, evidenced also by electrochemical studies on TT and its halogenated derivatives [31], is associated with a splitting of the MO levels of the mono-derivative in two or three almost degenerate levels, originating excited S n and T n levels nearly isoenergetic with those of the mono-derivative (Table S3). For example, the first singlet levels of TT-(HThio) 3 are computed at 258 (S 1 , f = 0.005), 257 (S 2 , f = 0.86) and 257 nm (S 3 , f = 0.86), all of them of (π,π*) type with very weak CT character.
The observation of HEP in the solid state for both TT-HThio and TT-(HThio) 3 can be explained by examining the singlet-triplet energy gap, ∆E ST , separating S 1 and the closer, lower-energy triplet state (T n ) and the nature of this level. In the DFT freely optimized geometry of TT-HThio, starting from its X-ray molecular structure, a rather high (0.53 eV) energy gap is obtained. However, compared with S 1 , the closer triplet state, T 5 (a 50% HOMO-2→LUMO and 20% HOMO-2→LUMO+9 (π,π*) transition) has a much more marked CT character, as deduced by looking at the involved HOMO-2 ( Figure 8), which is essentially localized on thiophene only. The corresponding excited state dipoles, in fact, are 1.90 D (S 1 ) and 3.37 D (T 5 ). The different character of S 1 and T 5 suggests an easy intersystem crossing (ISC) which explains the HEP observed in crystals. Moreover, it should be noted that the optimized structure is characterized by much larger twisting than that observed in the X-ray structure. In fact, the C1-C2-C10-S1 torsion angle (ω, see Figure 7a for the atom numbering scheme) measures 38.3 • in the optimized geometry, to be compared with the corresponding experimental torsions, 11.1(3) • and 7.4(3) • for molecules A and B, respectively, of the asymmetric unit. By fixing ω to the values assumed in the crystal structure, ∆E ST decreases from 0.53 to 0.28 (ω = 11.1 • ) and 0.27 eV (ω = 7.4 • ), making the singlet-to-triplet ISC even easier. HEP is therefore associated with radiative decay from T 5 itself or a lower triplet state T mol after internal conversion (IC) from T 5 . It is visible in crystals thanks to the rigidifying effect and protection from oxygen quenching ascribable to intermolecular interactions. The observation of HEP in the solid state for both TT-HThio and TT-(HThio)3 can be explained by examining the singlet-triplet energy gap, EST, separating S1 and the closer, lower-energy triplet state (Tn) and the nature of this level. In the DFT freely optimized geometry of TT-HThio, starting from its X-ray molecular structure, a rather high  The additional phosphorescent emission, LEP, can be associated with the presence of H-aggregates in the crystal structure, in agreement with previous findings on compounds with the same triazinic scaffold [9,10,[21][22][23][24][25][26][27]. The schematic representation of the photophysical processes involved in the emissive behavior of both TT-HThio and TT-(HThio)3 is reported in the Jablonski diagram below (Figure 10). It is interesting to compare the relative simplicity of this diagram with that, much more complicated, depicted for the analogous derivative with 2-pyridine, TT-Py [25]. Though the thiophene and pyridine chromophoric groups share rather similar electronic features, reflected for example in the similar emissive properties of their TT derivatives in DCM solution ( = 11%, λ em = 370 nm for TT-HThio;  = 17%, λ em = 351 nm for TT-Py), they are responsible for a different emissive behavior in their solid state. While crystals of both compounds display a highenergy phosphorescence of molecular origin (at 425, 451 nm for TT-HThio and at 408-418 nm, according to the crystalline phase, for TT-Py), associated with emission from a 3 (π,π*) low-energy triplet level, TT-Py shows an additional long-lived molecular component, almost overlapped with the fluorescence. This latter emission was ascribed to radiative decay from a triplet state of mixed 3 (/π,π*) symmetry, which is responsible for the observed dual anti-Kasha phosphorescence [32]. Such triplet states are absent in TT-HThio, whose frontier MOs have all π symmetry. Moreover, crystals of TT-Py display an additional lowenergy fluorescence, associated with the higher mobility of the chromophoric pendant The additional phosphorescent emission, LEP, can be associated with the presence of H-aggregates in the crystal structure, in agreement with previous findings on compounds with the same triazinic scaffold [9,10,[21][22][23][24][25][26][27]. The schematic representation of the photophysical processes involved in the emissive behavior of both TT-HThio and TT-(HThio) 3 is reported in the Jablonski diagram below (Figure 10). It is interesting to compare the relative simplicity of this diagram with that, much more complicated, depicted for the analogous derivative with 2-pyridine, TT-Py [25]. Though the thiophene and pyridine chromophoric groups share rather similar electronic features, reflected for example in the similar emissive properties of their TT derivatives in DCM solution (Φ = 11%, λ em = 370 nm for TT-HThio; Φ = 17%, λ em = 351 nm for TT-Py), they are responsible for a different emissive behavior in their solid state. While crystals of both compounds display a high-energy phosphorescence of molecular origin (at 425, 451 nm for TT-HThio and at 408-418 nm, according to the crystalline phase, for TT-Py), associated with emission from a 3 (π,π*) low-energy triplet level, TT-Py shows an additional long-lived molecular component, almost overlapped with the fluorescence. This latter emission was ascribed to radiative decay from a triplet state of mixed 3 (σ/π,π*) symmetry, which is responsible for the observed dual anti-Kasha phosphorescence [32]. Such triplet states are absent in TT-HThio, whose frontier MOs have all π symmetry. Moreover, crystals of TT-Py display an additional low-energy fluorescence, associated with the higher mobility of the chromophoric pendant compared with the hexylthiophene group.
In summary, two new members of the photophysically intriguing TT family have been isolated and characterized. The compounds, bearing one or three hexylthiophene fragments, display CEE behavior comprising fluorescence and heavy-atom-free dual phosphorescence, associated with molecular and supramolecular features. This work adds a new building block to the knowledge of this family and to RTP organic phenomena in general. From these results, the preparation of new multicomponent emitters can be envisaged.

General Information
All reagents were purchased from chemical suppliers and used without further purification unless otherwise stated. TTBr and TTBr3 were prepared according to published procedures [10,21]. Tributyl(4-hexylthiophen-2-yl)stannane was prepared according to a published procedure [33]. 1

Synthesis of TT-HThio
TT-HThio was prepared by Stille coupling between TTBr and tributyl(4-hexylthiophen-2-yl)stannane. In a typical reaction, TTBr (0.580 g; 2.10 mmol), tributyl(4-hexylthiophen-2-yl)stannane (0.959 g, 2.10 mmol), LiCl (1.000 g, 23.60 mmol), Pd(PPh3)2Cl2 (0.160 g, 0.210 mmol) and dry toluene (10 mL) are transferred inside a 100 mL dry Schlenk flask equipped with a magnetic stirrer. The mixture is de-aerated by means of three freezepump-thaw cycles. The system is heated under a static nitrogen atmosphere at 120 °C for 16 h. The reaction is then cooled to room temperature, filtered on Buchner and the solvent removed in vacuum. The crude reaction mixture is purified by flash chromatography on SiO2 with DCM/ACN as eluents (Rf=0.53 in DCM/ACN=8/2). The product is further purified by precipitation from DCM with hexane to give TT-HThio as a white solid (0.535 g, 1.47 mmol, Yield: 70%). Crystals suitable for X-ray diffraction analysis were obtained by slow evaporation from a MeOH solution. In summary, two new members of the photophysically intriguing TT family have been isolated and characterized. The compounds, bearing one or three hexylthiophene fragments, display CEE behavior comprising fluorescence and heavy-atom-free dual phosphorescence, associated with molecular and supramolecular features. This work adds a new building block to the knowledge of this family and to RTP organic phenomena in general. From these results, the preparation of new multicomponent emitters can be envisaged.

General Information
All reagents were purchased from chemical suppliers and used without further purification unless otherwise stated. TTBr and TTBr 3 were prepared according to published procedures [10,21]. Tributyl(4-hexylthiophen-2-yl)stannane was prepared according to a published procedure [33]. 1 H and 13 C NMR spectra were recorded on a Bruker AVANCE-400 instrument (400 MHz). Chemical shifts are reported in parts per million (ppm) and are referenced to the residual solvent peak (CH 2 Cl 2 , 1 H: δ = 5.32 ppm, 13 C: δ = 54.0 ppm); coupling constants (J) are given in hertz (Hz) and are quoted to the nearest 0.5 Hz. Peak multiplicities are described in the following way: s, singlet; d, doublet; t, triplet; p, pentet; m, multiplet.

Synthesis of TT-HThio
TT-HThio was prepared by Stille coupling between TTBr and tributyl(4-hexylthiophen-2-yl)stannane. In a typical reaction, TTBr (0.580 g; 2.10 mmol), tributyl(4-hexylthiophen-2-yl)stannane (0.959 g, 2.10 mmol), LiCl (1.000 g, 23.60 mmol), Pd(PPh 3 ) 2 Cl 2 (0.160 g, 0.210 mmol) and dry toluene (10 mL) are transferred inside a 100 mL dry Schlenk flask equipped with a magnetic stirrer. The mixture is de-aerated by means of three freezepump-thaw cycles. The system is heated under a static nitrogen atmosphere at 120 • C for 16 h. The reaction is then cooled to room temperature, filtered on Buchner and the solvent removed in vacuum. The crude reaction mixture is purified by flash chromatography on SiO 2 with DCM/ACN as eluents (R f = 0.53 in DCM/ACN = 8/2). The product is further purified by precipitation from DCM with hexane to give TT-HThio as a white solid (0.535 g, 1.47 mmol, Yield: 70%). Crystals suitable for X-ray diffraction analysis were obtained by slow evaporation from a MeOH solution.

Single-Crystal X-ray Studies
X-ray data of TT-HThio were collected on a Bruker Apex II diffractometer (Bruker AXS Inc., Madison, WI, US) using MoKα radiation [34]. The structure was solved using direct methods and refined with SHELXL-14 [35] using a full-matrix least-squares procedure based on F 2 using all data. Hydrogen atoms were placed at geometrically estimated positions. Details relating to the crystal and the structural refinement are presented in Table S1. Long bars of TT-HThio were grown at room temperature in a methanol solution of the compound. Full details of crystal data and structure refinement, in CIF format, are available as Supplementary Information. CCDC reference number: 1913040.

Computational Details
DFT and TDDFT calculations on TT-HThio and TT-(HThio) 3 were performed with the Gaussian 16 program (Revision A.03) [36] using the 6-311++G(d,p) basis set. The geometry of TT-HThio was optimized starting from the experimental molecular structure, as derived from X-ray studies. For comparison purposes, we adopted the same functional ωB97X [37] used for calculations of the previously reported parent cyclic triimidazole and its halogenated derivatives.

Photophysical Characterization
Photoluminescence quantum yields were measured using a C11347 Quantaurus-Absolute Photoluminescence Quantum Yield Spectrometer (Hamamatsu Photonics K.K, Shizuoka, Japan) equipped with a 150 W Xenon lamp, an integrating sphere and a multichannel detector. Steady-state emission, excitation spectra and photoluminescence lifetimes were obtained using an FLS 980 (Edinburgh Instrument Ltd., Livingston, United Kingdom) spectrofluorimeter. The steady-state measurements were recorded with a 450 W Xenon arc lamp. Photoluminescence lifetime measurements were performed using an EPLED-300 (Ed-inburgh Instrument Ltd.) and microsecond flash Xe-lamp (60W, 0.1 ÷ 100 Hz) with the data acquisition devices time-correlated single-photon counting (TCSPC) and multi-channel scaling (MCS) methods, respectively. Average lifetimes were obtained as τ av = ∑ A i τ 2 i ∑ A i τ i from bi-exponential or three-exponential fits. Low-temperature measurements were performed by immersion of the sample in a liquid N 2 quartz dewar.