Photoinduced single-crystal-to-single-crystal phase transition and photosalient effect of a gold(i) isocyanide complex with shortening of intermolecular aurophilic bonds

We report the first photoinduced single-crystal-to-single-crystal phase transition of a gold complex that involves shortening of intermolecular aurophilic bonds. The gold(i) isocyanide complex also shows a photosalient effect.


Synthesis
To chloro(phenyl isocyanide)gold(I) (3, 0.159 g, 0.5 mmol), THF (0.5 ml) was added under nitrogen atmosphere. After cooling to 0 °C, organozinc iodide reagent 2 1 in THF (1.0 ml, 0.75 mmol, 0.727 M) was added dropwise with stirring. After 1 h stirring, the reaction was quenched by the addition of a phosphate buffer solution and then extracted with CH 2 Cl 2 three times and washed with H 2 O and brine. The organic layers were collected and dried over Na 2 SO 4 . After filtration, the solvent was removed in vacuo. Further purification by flash column chromatography (SiO 2 , CH 2 Cl 2 /hexane = 3:1) gave a white solid. The resulting solid was dissolved in CH 2 Cl 2 in a vial and hexane was carefully layered for crystallization and allowed to stand at room temperature to give analytically pure crystals of 1 (0.2126 g, 0.488 mmol, 98 %). 1 H NMR (400 MHz, CDCl 3 , δ): 3.88 (s, 3H), 5H),2H),7.90 (d,J = 8.4 Hz,2H). 13  Preparation of 1B: The polymorph 1B is readily obtained by crystallization. Typically, 1 (30 mg) is dissolved in 2 mL of CH 2 Cl 2 in a vial and hexane (8 mL) was carefully layered. After standing at −25 ºC for a few days, colorless, blue-emitting crystals 1B are formed. For the preparation of 1B with an enough quality for single crystal X-ray diffraction analysis, aforementioned crystallization should be conducted under dark conditions.

Preparation of 1Y:
The polymorph 1Y is obtained by photoirradiation of 1B. Typically, an Olympus BX51 fluorescence microscope equipped with an Ushio 100 W ultrahigh-pressure mercury lamp USH-1030L and an Olympus fluorescence mirror unit U-MWU2 ( max = 367 nm) without any neutral density filters is used as a strong UV light (approx. 100 mW·cm −2 ) for photoexcitation.
Photoirradiation of 1B for 60 s under ambient condition, phase transition into 1Y phase occurs with emission color change. Longer irradiation time sometimes results in the decomposition of 1.

Fig. S2
Concentration-dependent UV/vis absorption spectra of 1 in CH 2 Cl 2 at room temperature.

Data for Single Crystal X-ray Structural Analyses and Low-Temperature Luminescence
Single crystal X-ray structural analyses were carried out on a Rigaku R-AXIS RAPID diffractometer using graphite monochromated Mo-K  radiation. The structure was solved by direct methods and expanded using Fourier techniques. Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the riding model. All calculations were performed using the CrystalStructure crystallographic software package except for refinement, which was performed using SHELXL-97. 2 Confirmation that Solvent was Not Included: We checked the maximum residual electron density in 1B and 1Y. The maximum and minimum peaks in the final differential maps were 2.44 eand -3.78 e -[Å -3 ], respectively, for 1B; 5.16 eand -2.80 e -[Å -3 ], respectively, for 1Y. These values are within the range of ±0.075 e -× Z max = ±5.9 [Å -3 ] for the complex 1, where Z max denotes the maximum atomic number in the lattice. This indicated that no residual electron density that could be assigned to other molecules, such as solvent, was present in the crystal structure. This is a standard analysis to identify small molecule inclusion in crystal structures.  (2) 6.0552 (5) b / Å 11.755 (2) 7.0297 (6) c / Å 15.940 (3) 15.969 (2)  / ° 102.912 (5)

DFT Calculations
All calculations were performed using the Gaussian 09W (revision C.01) and Gaussian 09 program package. 3 In the calculations, the SDD basis set with an effective core potential was used for Au and other atoms. PBEPBE functionals were used because other functionals such as B3LYP, CAM-BLYP, LC-BLYP and M06 did not well reproduce the experimental results. [4][5][6][7] The geometry of the dimeric structures of 1B and 1Y were calculated using coordinates of C, N, and Au taken from the corresponding X-ray structures. The positions of heavy atoms were fixed and only the positions of H atoms were optimized using the Spartan '10 MMFF force-field calculation. 8 Molecular orbitals were drawn using the Avogadro 1.1.0 program. 9 The triplet-state structure optimizations of the dimers taken from the crystal structures of 1B and 1Y were carried out in vacuum, resulting in the same structure, 1T opt ( Figure S13). The structure of 1T opt is more similar to that of 1Y than that of 1B. Because the optimization was carried out in vacuum, these results indicate that the photoexcited molecules in the 1B crystals are forced to transform to the excited state-related structure 1Y. This also suggests that the triplet 1T opt structure is close to the excited state structure of 1Y. A change in molecular structure induced by photoexcitation does not generally occur readily in crystals. Thus, the structure of the photoexcited molecules in a 1B crystal is similar to the ground-state structure of 1B. In contrast, the structure of the photoexcited molecules in 1Y crystals is similar to the ground-state structures of 1Y and 1T opt .
The excitation spectra for 1B ( max = 371 nm) and 1Y ( max = 394 nm) crystals were reproduced qualitatively by TDDFT calculations using the dimer structures derived from the X-ray structures of 1B and 1Y ( Figure S15, Table S5 and S6). The lowest singlet excited state (1B-S 1 , Table   S5)  The trend in the phosphorescence spectra of 1B and 1Y could be qualitatively reproduced by DFT calculations. The phosphorescence spectrum of 1B ( max = 448 nm) matches the vertical excitation energy (Table S7, 1B-T 1 ,  = 491 nm) calculated by TDDFT of the dimer structure taken from the 1B crystal-structure analysis. The lowest triplet excited state includes antibonding HOMO and bonding LUMO, indicating that the aurophilic bond tends to be strong in the triplet excited state compared with in the ground state. Although the phosphorescence of 1Y ( max = 580 nm) is different from the calculated vertical excitation energy (Table S7, 1Y-T 1 ,  = 463 nm), the red shift from the S17 phosphorescence of 1B qualitatively corresponds to the T 1 energy (Table S7, 1ST-T 1 ,  = 664 nm) of the TDDFT calculation of the singlet model structure (1ST opt ) with 1T opt geometry. These results support the supposition that the structure of photoexcited molecules in a 1B crystal is similar to the ground-state structure of 1B, while the structure of the photoexcited molecules in 1Y crystals is similar to the ground-state structures of 1Y and 1T opt .