Optical Nature of Non-Substituted Triphenylmethyl Cation: Crystalline State Emission, Thermochromism, and Phosphorescence

Since the discovery of the triphenylmethyl (trityl) cation 120 years ago, a variety of aromatic cations having various colors and luminescence properties have been rigorously studied. Many, differently substituted trityl cations have been synthesized and their optical properties have been elucidated. However, the optical properties of the parent, non-substituted and highly reactive trityl cation, which was observed to be very weakly luminescent, have not been subjected to detailed investigation. In the effort described herein, we explored the optical nature of non-substituted trityl hexafluorophosphate (PF6) in the crystalline state. Trityl PF6 was found to exist as two crystal polymorphs including a yellow (Y) and an orange (O) form. Moreover, we observed that these crystalline forms display crystalline-state emission with different colors. The results of X-ray crystallographic analysis showed that the two polymorphs have totally different molecular packing arrangements. Furthermore, an investigation of their optical properties revealed that the O-crystal undergoes a distinct color change to yellow upon cooling as a consequence of a change in the nature of the charge transfer interaction between the cation and PF6 anion, and that both the Y and O crystal exhibit phosphorescence.


Synthesis of triphenylmethyl hexafluorophosphate (trityl PF6)
To a solution of triphenylmethylalchol (100 mg, 0.38 mmol) in acetic anhydride (2 ml) was slowly added hexafluorophosphoric acid solution (60%, 0.5 ml) at room temperature (Caution! This reaction is exothermic and the solution is splashed.). For collecting yellow crystal (Y-crystal), the reaction vial was putted in a fridge to keep 4~5 °C for several hours whereas, for collecting orange crystal (O-crystal), the reaction vial was left at room temperature from several days to over a week with absence of light. Y-or O-crystal was collected by filtration and washed by acetic anhydride and dried diethyl ether. The obtained yield was 115 mg (0.29 mmol, 78%) or 122 mg (0.31 mmol, 83%), respectively.

S4
‧X-ray crystallographic analysis     ‧Variable-temperature X-ray analyses in O-crystal  Due to having C3 symmetry, three inter-atomic distance C1···F1, C1···F2, and C1···F3 are equal. The charge transfer interactions E between C1···F1 are weaker than those observed in O-crystal owing to its longer C1···F1 distance. Figure S9. Natural bond orbital analysis of trityl PF6 face-to-face dimer (MP2/6-311+G**) for evaluating charge transfer interaction between trityl cation and PF6 anion. Compared with those in monomers ( Figure S7), the charge transfer interaction energies E at both 273 K and 83 K are increased probably for reducing the cation-cation repulsive energy in the face-to-face cation dimer structure. Table S3. Hirshfeld atomic population analysis of (a) trityl PF6 monomer and (b) trityl PF6 face-toface dimer (MP2/6-311+G**) in O-crystal at both 273 and 83 K, and (c) trityl PF6 monomer in Y-crystal at 170K. X-ray structures at both temperatures were used for the calculations as shown in Figure S7 and S8. The absolute value of total net charges both trityl cation and PF6 anion are decreased upon cooling and formation of dimeric structure. This is because that the shortening C1···F1 distance increases the charge transfer from PF6 anion to trityl cation, and the cation-cation repulsive energy in the face-to-face cation dimer structure increases the charge transfer as well, as discussed in Figure S8.
Evaluation of six-fold phenyl embrace interaction

‧TD-DFT calculations
Trityl PF6 (face-to-face dimer) in O-crystal. Table S4. TD-DFT calculation results of face-to-face dimer in O-crystal at 273 K. Grayed characters indicated a forbidden transitions (allowed transition wrote as black characters). A yellow highlighted transition as shown in allowed transitions indicated a major contribution for its transition.
The values for  and  indicate the contribution of HF exchange in short-range and long-range, respectively. (For example, in long-range correction (LC) scheme, the parameters of  and are0 and 1.0, respectively.) The parameter  determines the balance of DFT to HF exchange depending on the range distance. S14 Figure S13.  of face-to-face trityl PF6 dimer at 273 K (upper) and its energy diagram (lower). The calculation was conducted by CAM-B3LYP/6-311+G(d,p) level with tuned parameters ( = 0.15, = 0.079,  = 0.921). Table S5. TD-DFT calculation results of face-to-face dimer in O-crystal at 83 K. Grayed characters indicated a forbidden transitions (allowed transition wrote as black characters). A yellow highlighted transition as shown in allowed transitions indicated a major contribution for its transition.        S24 Figure S22. Simulated UV-vis spectra of lateral trityl PF6 dimers (a) 273 K, and (b) 83 K. Red bars in the spectra indicated a charge-transfer transition from PF6 anion to trityl cation. Table S9. TD-DFT calculation results of trityl cation monomer and dimer without PF6 anion in O-crystal at both 27 K and 83 K. The calculation was conducted by CAM-B3LYP/6-311+G(d,p) level with tuned parameters (= 0.15,  = 0.079,  = 0.921). Comparison with each conditions (temperature as well as monomer and dimer), no significant difference is observed, indicating that only the trityl-trityl interaction did not affect its optical properties.