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

Abstract

We report the first photoinduced single-crystal-to-single-crystal (SCSC) phase transition of a gold complex that involves the shortening of intermolecular aurophilic bonds. This is also the first solid state photochromism of a gold complex. Upon UV irradiation, the blue-emitting crystals of the gold(i) isocyanide complex 1 (1B) transform into the weakly yellow-emitting polymorph 1Y. X-ray diffraction analyses reveal that this phase transition proceeds in an SCSC manner. After phase transition from 1B to 1Y, the intermolecular Au···Au separation decreases from 3.5041(14) to 3.2955(6) Å, resulting in a red-shifted emission. The photoinduced shortening of the aurophilic bond in the excited state initiates the change in the crystal structure from 1B to 1Y. Moreover, the crystal 1B showed a photosalient effect: the 1B crystals jump upon irradiation with strong UV light owing to the phase transition into 1Y. The aurophilic bond formation in the crystal generates the mechanical movement of the crystal.

Fig. 1. (a) The structure of complex 1. (b) Photographs of 1B and 1Y under ambient light. (c) A series of photographs of the SCSC phase transition from 1B to 1Y induced by strong photoirradiation (367 nm, approx. 100 mW cm^–2) taken under excitation at 365 nm.

Scheme 1. Synthesis of 1.

Fig. 2. Normalized excitation (blue dashed line, detected at 450 nm) and emission (blue solid line, λ _ex = 370 nm) spectra of 1B. Normalized excitation (greenish yellow dashed line, detected at 590 nm) and emission (greenish yellow solid line, λ _ex = 390 nm) spectra of 1Y. The inset shows a magnified emission spectrum of 1Y.

Fig. 3. The single-crystal structure of 1B. (a) ORTEP representation of a dimer unit. (b) ORTEP and (c) space-filling representations of the packing structure viewed along the directions roughly parallel (in b) and perpendicular (in c) to the aurophilic bonding axis. A dimer unit in (c) is shown as an ORTEP diagram to aid visualization. The crystallographic axes a, b, and c are depicted by arrows.

Fig. 4. The single-crystal structure of 1Y. (a) ORTEP representation of a dimer unit. (b) ORTEP and (c) space-filling representations of the packing structure viewed along the directions roughly parallel (in b) and perpendicular (in c) to the aurophilic bonding axis. A dimer unit in (c) is shown as an ORTEP diagram to aid visualization. The crystallographic axes a, b, and c are depicted by arrows.

Fig. 5. (a) DSC profiles of 1B (blue line) and 1Y (greenish yellow line) at heating and cooling rates of 10 and 2 °C min^–1, respectively. (b) Photographs of the crystal 1B before and after strong photoirradiation at 435 nm for 5 min (approx. 200 mW cm^–2). These photographs were taken under weak UV light at 365 nm (approx. 1 mW cm^–2). The scale bars are 0.1 mm. These photographs show that the emission color is unchanged after irradiation with visible light. (c) Photograph of a crystal 1B obtained after photoirradiation (367 nm, approx. 100 mW cm^–2) at a local area taken under UV light. The mark “*” denotes the irradiation region of the crystal. The scale bars are 0.1 mm.

Fig. 6. (a) A schematic representation of aurophilic bonds and their simplified orbital levels. An aurophilic bond produces d _{z ²} σ and d _{z ²} σ* orbitals and shortens upon photoexcitation. (b) A schematic representation of the relaxation pathway upon photoirradiation of [Au(CN)₂ ^–] _n complexes. See ref. 15 for details. (c) HOMO of the dimer derived from the single-crystal structure of 1B (PBEPBE/SDD). (d) The structure of 1T_Opt obtained using triplet excited-state optimization of a 1B dimer in a vacuum (PBEPBE/SDD).

Fig. 7. A series of photographs of the photosalient effect of 1B through the transformation into 1Y induced by strong photoirradiation (367 nm, approx. 400 mW cm^–2). Photographs from (a) to (f) and (g) to (l) were cropped from the ESI Movies S1 and S2, respectively. The arrows in (d) and (k) indicate the crystals just before the jump. The irradiation times are shown.