Mono-, Di-, Tri-Pyrene Substituted Cyclic Triimidazole: A Family of Highly Emissive and RTP Chromophores

Abstract

The search of new organic emitters is receiving a strong motivation by the development of ORTP materials. In the present study we report on the preparation, optical and photophysical characterization, by both steady state and time resolved techniques, of two pyrene-functionalized cyclic triimidazole derivatives. Together with the already reported mono-substituted derivative, the di- and tri-substituted members of the family have revealed as intriguing emitters characterized by impressive quantum yields in solution and RTP properties in the solid state. In particular, phosphorescence lifetimes increase from 5.19 to 20.54 and 40.62 ms for mono-, di- and trisubstituted compounds, respectively. Based on spectroscopical results and theoretical DFT/TDDFT calculations on the di-pyrene molecule, differences in photophysical performances of the three compounds have been assigned to intermolecular interactions increasing with the number of pyrene moieties appended to the cyclic triimidazole scaffold.

1. Introduction

Organic room temperature phosphorescent materials (ORTP), due to their increased biocompatibility and reduced cost with respect to their organometallic counterparts, are the subject of intense research in the field of functional materials, appearing promising for applications spanning from bioimaging [1,2] to anti-counterfeiting [3,4] and displays [5]. Many strategies have been developed to realize ORTP materials. Among them, π⋯π stacking interactions [6,7,8], host–guest systems [9,10,11], co-assembly based on macrocyclic compounds [12], crystallization [13,14,15], halogen bonding [16,17], and doping in a polymer matrix [18], can be mentioned. Triimidazo[1,2-a:1′,2′-c:1″,2″-e][1,3,5]triazine, TT [7], is the prototype of a family of ORTP materials, showing in the crystalline phase ultralong phosphorescence (up to 1 s) associated with the presence of strong π⋯π stacking interactions [6]. Introduction of one or multiple halogen atoms on the TT scaffold results in a complex excitation dependent photoluminescent behavior including dual fluorescence, molecular and supramolecular phosphorescences, together with ultralong components [8,19,20,21]. The introduction of one chromophoric fragment, namely 2-fluoropyridine, 2-pyridine and pyrene (TTPyr₁) has further enriched the photophysical behavior, preserving the solid state ultralong component [22,23,24]. In particular, the pyrene functionality was chosen due to its favorable emissive properties (i.e., high fluorescence quantum efficiency and good hole-transporting ability) which can be exploited in a large number of applications [25,26,27]. TTPyr₁ was isolated and characterized in two solvated phases and two not solvated polymorphs. In particular, the two solvated and one polymorph, belonging to centrosymmetric space groups, are obtained at room temperature, while a non-centrosymmetric polymorph can be irreversibly prepared by proper thermal treatment in air starting from the room temperature phases. While in solution the chromophore is characterized by a single fluorescence (quantum yield Φ = 90%, see

Table 1. Photophysical parameters of Pyrene, TTPyr₁, TTPyr₂ and TTPyr₃ at 298 K.

		Φ %	λem (nm)	τav	kr (107 s−1)	knr (107 s−1)
Pyrene	DMSO (2.5 × 10−6 M)	33.4	374	94.15 ns	0.355	0.707
TTPyr1	DMSO (10−5 M)	92	420	2.76 ns	33.3	2.90
	Ground Crystal (TTPyr RT)	54	475	2.15 ns	25.1	21.4
			514	5.19 ms
TTPyr2	DMSO (2.5 × 10−6 M)	78	419	9.22 ns	8.46	2.39
	Powders	40.2	490	4.64 ns	8.66	12.9
			528	20.54 ms
TTPyr3	DMSO (2.5 × 10−6 M)	74.4	422	11.16 ns	6.67	2.29
	Powders	36.9	476	5.12 ns	7.21	12.3
			522	40.62 ms

), in the solid state intermolecular interactions impart rigidification to the molecule favoring the appearance of a long-lived component whose lifetime and intensity strongly depend on the crystallinity of the sample. Moreover, differently from the room temperature phases, the non-centrosymmetric one displays not only second order nonlinear optical features (SHG ten times that of standard urea), but also an additional fluorescence associated with the presence of two possible almost isoenergetic conformers in its crystals. Additionally, the material in aggregate-state was tested for cellular and bacterial imaging in view of its applications in bioimaging [24].

Here, we report the synthesis, characterization and photophysical studies of the analogue derivatives with two and three pyrene moieties on the TT scaffold, TTPyr₂ and TTPyr₃, respectively. The photophysical behavior of the solid compounds, comprising one fluorescence and one phosphorescence, is similar to that of TTPyr₁ in the room temperature phases. However, a systematic increase of the phosphorescence lifetime is observed by increasing the number of pyrene groups. Unfortunately, due to the impossibility to get single crystals suitable for X-ray diffraction analysis, an explanation of this phenomenon can be given only at a qualitative level.

2. Materials and Methods

All reagents were purchased from chemical suppliers and used without further purification. Pyrene was crystallized from hot toluene solution before photoluminescence measurements. Triimidazo[1,2-a:1′,2′-c:1″,2″-e][1,3,5]triazine (TT) [28], its dibromo- and tribromo-derivatives (namely 3,7-dibromotriimidazo[1,2-a:1′,2′-c:1″,2″-e][1,3,5]triazine, TTBr₂, and 3,7,11-tribromotriimidazo[1,2-a:1′,2′-c:1″,2″-e][1,3,5]triazine, TTBr₃) [19] and TTPyr₁ [24] were prepared according to literature procedures.

¹H and ¹³C 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 (DMSO, ¹H 2.50 ppm, ¹³C 39.5 ppm; CH₂Cl₂, ¹H 5.32 ppm, ¹³C 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; m, multiplet.

Mass spectra were recorded on a Thermo Fisher (Thermo Fisher Scientific, Waltham, MA USA) LCQ Fleet Ion Trap Mass Spectrometer equipped with UltiMate™ 3000 HPLC system. UV-Visible spectra were collected by a Shimadzu UV3600 spectrophotometer (Shimadzu Italia S.r.l., Milan, Italy).

Absolute photoluminescence quantum yields were measured using a C11347 (Hamamatsu Photonics K.K). A description of the experimental setup and measurement method can be found in the article of K. Suzuki et al. [29]. For any fixed excitation wavelength, the fluorescence quantum yield Φ is given by:

$$\mathsf{\Phi }=\frac{\mathrm{PN}\left(\mathrm{Em}\right)}{\mathrm{PN}\left(\mathrm{Abs}\right)}=\frac{\int \frac{\mathsf{\lambda }}{\mathrm{hc}}\left[{\mathrm{I}}_{\mathrm{em}}^{\mathrm{sample}}\left(\mathsf{\lambda }\right)-{\mathrm{I}}_{\mathrm{em}}^{\mathrm{reference}}\left(\mathsf{\lambda }\right)\right]\mathrm{d}\mathsf{\lambda }}{\int \frac{\mathsf{\lambda }}{\mathrm{hc}}\left[{\mathrm{I}}_{\mathrm{ex}}^{\mathrm{reference}}\left(\mathsf{\lambda }\right)-{\mathrm{I}}_{\mathrm{ex}}^{\mathrm{sample}}\left(\mathsf{\lambda }\right)\right]\mathrm{d}\mathsf{\lambda }}$$

where PN(Em) is the number of photons emitted from a sample and PN(Abs) is the number of photons absorbed by a sample, λ is the wavelength, h is Planck’s constant, c is the velocity of light, ${\mathrm{I}}_{\mathrm{em}}^{\mathrm{sample}}\left(\mathsf{\lambda }\right)$ and ${\mathrm{I}}_{\mathrm{em}}^{\mathrm{reference}}\left(\mathsf{\lambda }\right)$ are the photoluminescence intensities with and without a sample, respectively, ${\mathrm{I}}_{\mathrm{ex}}^{\mathrm{sample}}\left(\mathsf{\lambda }\right)$ and ${\mathrm{I}}_{\mathrm{ex}}^{\mathrm{reference}}\left(\mathsf{\lambda }\right)$ are the integrated intensities of the excitation light with and without a sample, respectively. PN(Em) is calculated in the wavelength interval [λ_i, λ_f], where λ_i is taken 10 nm below the excitation wavelength, while λ_f is the upper end wavelength in the emission spectrum. The error made was estimated at around 5%.

Steady state emission and excitation spectra and photoluminescence lifetimes were obtained using a FLS 980 (Edinburgh Instrument Ltd., Livingston, United Kingdom) spectrofluorimeter. The steady state measurements were recorded by a 450 W Xenon arc lamp. Photoluminescence lifetime measurements were performed using: Edinburgh Picosecond Pulsed Diode Laser EPL-375, EPLED-300, (Edinburg Instrument Ltd.) and microsecond flash Xe-lamp (60 W, 0.1 ÷ 100 Hz) with data acquisition devices time correlated single-photon counting (TCSPC) and multi-channel scaling (MCS) methods, respectively. Average lifetimes are obtained as:

$${\mathsf{\tau }}_{\mathrm{av}}\equiv \frac{{\sum }_{n=1}^m{\alpha }_n{\tau }_n^2}{{\sum }_{n=1}^m{\alpha }_n{\tau }_n},$$

where m is the multi-exponential decay number of the fit.

2.1. Synthesis of 3,7-Di(pyren-1-yl)triimidazo[1,2-a:1′,2′-c:1″,2″-e][1,3,5]triazine (TT-Pyr₂)

TTPyr₂ was prepared by Suzuki coupling between TTBr₂ and pyrene-1-boronic acid (see Scheme 1). In a typical reaction, TTBr₂ (595 mg; 1.67 mmol), pyren-1-ylboronic acid (970 mg, 3.94 mmol), potassium carbonate (1.3 g, 9.42 mmol), Pd(PPh₃)₂Cl₂ (170 mg, 0.24 mmol), water (3 mL) and DMF (25 mL) were transferred inside a 100 mL Schlenk flask equipped with a magnetic stirrer. The system was heated under static nitrogen atmosphere at 130 °C for 12 h. The reaction was then cooled to room temperature, precipitated with water (200 mL) and filtered on a Büchner. The solid crude reaction mixture was further purified by automated flash chromatography on SiO₂ with DCM/CH₃CN as eluents to give the TTPyr₂ product as a yellow solid (724 mg; Yield 72%; R_f = 0.5 in DCM/CH₃CN = 95/5).

NMR data (9.4 T, DMSO-d₆, 298 K, δ, ppm): ¹H NMR 8.46–8.07 (18H, m), 8.01 (1H, d, J = 1.5 Hz), 7.53 (1H, s), 6.98 (1H, s), 6.88 (1H, d, J = 1.5 Hz); ¹³C NMR 136.7, 136.6, 135.9, 131.4, 131.3, 130.9, 130.7, 130.4, 130.3, 129.8, 129.5, 128.8, 128.4, 128.2, 128.1, 128.0, 127.7, 127.5, 127.3, 127.2, 126.4, 125.7, 125.6, 125.4, 124.3, 124.2, 124.1, 123.7, 123.6, 123.5, 110.9.

MS (ESI-positive ion mode): m/z: 599 [M + H]⁺.

2.2. Synthesis of 3,7,11-Tri(pyren-1-yl)triimidazo[1,2-a:1′,2′-c:1″,2″-e][1,3,5]triazine (TT-Pyr₃)

TTPyr₃ was prepared by Suzuki coupling between TTBr₃ and pyrene-1-boronic acid (see Scheme 2). In a typical reaction, TTBr₃ (400 mg; 0.91 mmol), pyren-1-ylboronic acid (702 mg, 2.85 mmol), potassium carbonate (1.3 g, 9.19 mmol), Pd(PPh₃)₂Cl₂ (64 mg, 0.09 mmol), water (2 mL) and DMF (10 mL) were transferred inside a 100 mL Schlenk flask equipped with a magnetic stirrer. The system was heated under static nitrogen atmosphere at 130 °C for 12 h. The reaction was then cooled to room temperature, precipitated with water (200 mL) and filtered on a Büchner. The solid crude reaction mixture was further purified by automated flash chromatography on SiO₂ with Hexane/Et₂O/DCM as eluents to give the TTPyr₃ product as a pale yellow solid (435 mg; Yield 60%; R_f = 0.5 in Hexane/Et₂O/DCM = 60/20/20).

NMR data (9.4 T, CD₂Cl₂, 300 K, δ, ppm): ¹H NMR 8.34–8.06 (27H, m), 7.04 (3H, s); ¹³C NMR 136.9, 132.7, 131.9, 131.4, 130.3, 129.0, 128.8, 127.9, 126.9, 126.3, 126.1, 125.6, 125.1, 124.7, 123.7.

MS (ESI-positive ion mode): m/z: 799 [M + H]⁺.

2.3. Computational Details

Geometry optimization of TTPyr₂ has been performed at the ωB97X/6-311++G(d,p) level of theory [30], in agreement with the computational protocol previously developed to study TT [7] and its derivatives, in particular TTPyr₁ [24]. The adopted functional was in fact demonstrated to be able to accurately describe ground and excited states properties of TT and its derivatives, besides dispersive intermolecular interactions which are essential to interpret the multi-faceted properties of this family of compounds. Calculations on the tri-pyrene derivative have not be performed due to the required high computational costs. All calculations have been performed with Gaussian 16 [31].

3. Results

3.1. Synthesis and Molecular Structures

TTPyr₂ and TTPyr₃ were prepared by Suzuki coupling between pyrene-1-boronic acid and the corresponding di- and tri-brominated TT precursors (see Scheme 1 and Scheme 2). The new pyrene derivatives were characterized by ¹H and ¹³C NMR spectroscopy (Figures S1–S5) and mass spectrometry (Figures S3 and S6), samples with the purity grade required for photophysical characterization were obtained by repeated crystallizations.

In order to get insight on the structural features of the investigated compounds, DFT calculations were performed on the smaller derivative, TTPyr₂ (see optimized structures in Figure 1). Owing to the lack of the crystal structure of this compound, suitable models were built up starting from structural information previously obtained on TTPyr₁ by both X-ray diffraction analysis and theoretical calculations [24]. For this latter compound, crystallizing in different phases and polymorphs, two possible conformations were detected, which correspond to different orientation of pyrene with respect to the TT moiety as denoted by the (H)C1-C2-C10-C11(H) torsion angle τ (C2-C10 being the bond connecting TT with pyrene). In particular, the room temperature phases display τ values equal to about 129° (RT conformation), while the polymorph obtained at high temperature shows τ equal to 49° (HT conformation) [24]. DFT geometry optimization of the two conformations leads to different almost isoenergetic minima (having τ = 110.3 and 67.4°, respectively) in a very flat region. The HT conformation is only 0.6 kcal/mol more stable than the RT one, suggesting that both conformations are expected to be present in solution. To build up the TTPyr₂ model, an additional pyrene moiety was added, in the proper position, to both the RT and HT optimized geometries of TTPyr₁ imposing the same RT or HT conformation. The resulting optimized structures (see Figure 1), denoted respectively as ‘RT/RT’ and ‘HT/HT’ conformations, display C1–C2–C10–C11 and C1′–C2′–C10′–C11′ torsion angles equal to 112.3 and −67.2° (RT/RT) and 67.4 and 67.3° (HT/HT conformation), the latter being more stable by 0.54 kcal/mol. Of course, also the mixed ‘RT/HT’ conformation is envisaged to be present in solution, but no significant differences in the electronic levels are expected with respect to those computed by TDDFT for the RT/RT and HT/HT conformations, which are themselves very similar (see Table S1).

Owing to the twisted arrangement of TT and pyrene in the investigated systems, a Quantum Theory of Atoms in Molecules (QTAIM) topological analysis [32] on the TTPyr₁ and TTPyr₂ electron density was performed to detect the possible presence of intramolecular interactions between the two moieties. In all cases, a bond critical point (bcp) between a TT nitrogen atom (N3) and the pyrene C10 carbon atom, located at about 3.18 Å from each other, has been found (ρ_bcp ∼ 0.05 e Å⁻³, see Figures S7 and S8 for the TTPyr₁ and TTPyr₂ molecular graphs). Owing to the nature of the involved atoms, such N⋯C bond paths cannot be considered as classical non-covalent interactions [33], though contributing to stabilize the molecule due to the privileged electron-exchange channel associated with the bond path [34].

Interestingly, the molecular graphs substantially change when considering the optimized geometries of TTPyr₁ and TTPyr₂ in the excited state (see Figures S9 and S10). The relaxed S₁ geometries are characterized by lower molecular twisting than the ground state one. In particular, for TTPyr₁ τ varies from 110.3 to 138.5° (RT conformation) and from 67.4 to 41.5° (HT conformation), respectively, with a concomitant shortening of the C2-C10 bond (from 1.478 to 1.435 Å), implying a greater conjugation between the TT and pyrene moieties compared with the ground state [24]. For TTPyr₂, on the other hand, only one pyrene unit undergoes a substantial relaxation with respect to TT, τ varying from 112.3 to 139.0° (RT/RT conformation) and from 67.3 to 40.8° (HT/HT conformation). The corresponding C2–C10 bond shortens from 1.477 to 1.422 and 1.433 Å (RT/RT and HT/HT conformation, respectively). For the other pyrene unit, τ shows only slight variations (from −67.2 to −64.6° in the RT/RT conformation and from 67.4 to 64.0° in the HT/HT conformation) and the C2′–C10′ bond remains almost unvaried. These results indicate that, as determined for TTPyr₁, also TTPyr₂ shows in the excited state a greater conjugation, which is however extended on TT and only one pyrene unit. The associated molecular graphs (see Figures S9 and S10) reveal, for both conformations of TTPyr₁ and TTPyr₂, the formation of an intramolecular C–H⋯N hydrogen bond between the TT and pyrene moieties. This interaction stabilizes the two conformations in the excited state allowing to reduce the molecular twisting.

3.2. Photophysical Studies

TTPyr₂ and TTPyr₃ have been photophysically characterized both in solution and in solid state by steady state and time resolved spectroscopy. The results are reported in Table 1 together with those previously obtained for the room temperature non-solvated species of TTPyr₁ and those of pyrene molecule in the same conditions.

Diluted DMSO solutions of the three compounds show two absorptions with peaks at 257, 268, 279 nm (ES₂) and 332, 347 nm (ES₁) respectively, with molar absorption coefficients, ε, computed at 347 nm, equal to 34,894, 46,408 and 84,443 M⁻¹ cm⁻¹ for TTPyr₁, TTPyr₂ and TTPyr₃, respectively. Such bands, even though strongly red shifted and broadened with respect to the parent pyrene moiety (see Figure 2), are ascribed to transitions of main pyrene character, as previously suggested based on the similarity with the absorption spectrum of pyrene itself in the same conditions and theoretical calculations [24]. Looking at the HOMO and LUMO energies of pyrene, TTPyr₁, and TTPyr₂, it is found that, while the HOMO energy is almost constant along the series, the LUMO one significantly decreases (by 0.17 or 0.19 eV according to the conformation) from pyrene to TTPyr₁, and remains almost unvaried going to TTPyr₂.

For the three compounds, perfectly overlapped emission spectra, consisting in an intense very broad single fluorescent emission at about 420 nm, are measured (see Figure 3). The three compounds display impressively high quantum yields when compared with pyrene itself (Φ = 33.4%) in the same conditions (see Table 1), suggesting that the TT moiety successfully suppresses the ACQ (aggregation caused quenching) phenomena affecting pyrene fluorescence [35,36,37]. Specifically, Φ equal to 92, 78 and 74% have been measured for TTPyr₁, TTPyr₂, TTPyr₃, respectively, with lifetime, τ, equal to 2.76 [24], 9.22 and 11.16 ns (Figures S11 and S14), resulting in k_r values 33.3, 8.46 and 6.67 × 10⁷ s⁻¹ and k_nr 2.90, 2.39 and 2.29 × 10⁷ s⁻¹, respectively (see Table 1). The observed quantum yields are similar or higher than those reported in the literature for other mono-, di- and tri-pyrene substituted families. For example, DCM solutions of di- and tri-pyrene derivatives of carbazole and mono-, di- and tri-pyrene functionalized adamantane display Φ equal to 94 and 72% [32] and 17, 19 and 19% [38] respectively.

To justify the slight decrease of quantum yield for TTPyr₂ and TTPyr₃ with respect to TTPyr₁, close inspection of optical properties has been performed. When better focusing on the 347 nm absorption of the three compounds (ES₁, see inset of Figure 2), besides the already mentioned macroscopic red shift and broadening with respect to pyrene, a slight red shift and widening of the band from TTPyr₁ to TTPyr₂ and TTPyr₃ can also be recognized. Based on DFT/TDDFT calculations, such band results from an envelope of more transitions (see Table S1 and Figures S20 and S21 for the simulated absorption spectra). In particular, two transitions are computed at low energy for pyrene (at 301 and 294 nm with oscillator strength f = 0.0003 and 0.347, respectively) and TTPyr₁ (at 304 and 302 nm with f = 0.060 and 0.498, respectively), with a bathochromic displacement of the stronger transition and intensification of the low energy one, justifying the experimental red shift and broadening of TTPyr₁ ES₁. Going to TTPyr₂, four transitions are computed in the same region (at 306, 304, 303 and 301 nm with f = 0.380, 0.101, 0.066 and 0.445, respectively, see Table S1 and Figures S20 and S21), implying further broadening of the associated band with very minor red shift with respect to TTPyr₁. It is as well to be taken into account that more conformations of similar energy are expected in solution, due to the flatness of the potential energy surface associated with the rotation of pyrene with respect to TT [24]. As reported in Table S1, such conformations are characterized by the same position of the transitions but slightly different oscillator strengths, further contributing to the band broadening. Similar results are expected for the tri-pyrene derivative, which was not submitted to theoretical calculations owing to the computational costs required by the further increased number of conformational degrees of freedom. Geometry optimization of S₁ for both conformations of TTPyr₂ leads, as determined for TTPyr₁, to a reduced molecular twisting (see Figures S9 and S10) with a large energy separation from the higher excited states and approximately the same energy position (λ = 372 nm), oscillator strength (0.693) and orbital composition (HOMO-LUMO with 93% weight) as found for TTPyr₁ (λ = 370 nm, f = 0.698, HOMO-LUMO with weight = 94%). Based on the strict similarity between TTPyr₁ and TTPyr₂ at molecular level, minor ACQ phenomena should be taken into account for TTPyr₂ and TTPyr₃. A similar result was previously obtained for pyrene-functionalized carbazole derivatives, with fluorescence quantum yield decreasing by increasing the number of pyrene moieties [39].

The three compounds are good emitters also in the solid state (see Table 1), where the emission is implemented by the presence, as already reported for TTPyr₁ [24], of one phosphorescent component in addition to the fluorescent band (at 475, 490 and 476 nm for TTPyr₁, TTPyr₂, TTPyr₃, respectively, see Figure 4). The long lived component (at 514, 528 and 522 nm, with lifetimes equal to 5.19 [24], 20.54 and 40.62 ms for TTPyr₁, TTPyr₂, TTPyr₃ respectively, see Figures S13 and S16) can be selectively activated by exciting at low energy (480 nm). Based on the conclusions drawn for TTPyr₁ [24], even for TTPyr₂ and TTPyr₃ such long-lived emission could be explained by (i) easy singlet-to-triplet intersystem crossing due to almost overlapped singlet and triplet energy levels (see Figure S20), and (ii) interchromophoric interactions in the solid state which inhibit the molecular motions reducing the thermal non-radiative dissipation. Unfortunately, differently from TTPyr₁ where such interactions have been deeply analyzed for the different isolated and crystallized phases, for TTPyr₂ and TTPyr₃ no single crystals suitable for X-ray diffraction analysis could be obtained. However, based on the longer lifetimes of TTPyr₃, a high number of interactions are expected for this latter with respect to the other members of the family. Such interactions are not efficacious for producing crystals due to the large number of conformational degrees of freedom (as expected, even though at lower extent, for TTPyr₂).

4. Conclusions

In conclusion, two new intriguing chromophores, TTPyr₂ and TTPyr₃, have been prepared and characterized. Their photophysical behavior fully analyzed in solution and in solid state has been compared with that of parent TTPyr₁ revealing a trend depending on the number of pyrene moieties appended to the TT scaffold. In particular, the fluorescence quantum yield in solution (much higher than that of pyrene itself) slightly decreases by increasing the number of pyrene units, while, in the solid state, the phosphorescent component, visible for all compounds at room temperature, possesses lifetimes increasing in the same order due to stabilizing interchromophoric interactions.