trans–cis Photoisomerization of the Styrylpyridine Ligand in [Re(CO)3(2,2′-bipyridine)(t-4-styrylpyridine)]+: Role of the Metal-to-Ligand Charge-Transfer Excited States

Abstract

The trans-cis isomerization of the styrylpyridine carbon–carbon double bond induced by visible light irradiation in fac-[Re(CO)₃(bpy)(stpy)]⁺ (bpy = 2,2′-bipyridine; stpy = t-4-styrylpyridine) has been investigated by means of quantum-chemical methods. The structures of the various cis and trans conformers of [Re(CO)₃(bpy)(stpy)]⁺ have been optimized at the density functional theory (DFT) level. Three rotational conformers for the most stable trans isomer lie within 2.3 kJ mol⁻¹ each other. The energy difference between the cis and trans isomers is 27.0 kJ mol⁻¹. The electronic spectroscopy of the most stable conformers has been investigated by time-dependent DFT (TD-DFT) and complete active space self-consistent field/CAS second order perturbation theory (CASSCF/CASPT2) calculations. The lowest absorption bands are dominated by metal-to-ligand charge-transfer (MLCT, d_Re→π*_bpy) transitions calculated at about 25 000 cm⁻¹ and by a strong intraligand ¹IL (π_stpy→π*_stpy) transition in the near UV region. On the basis of CASSCF potential energy curves (PECs) calculated as a function of the torsion angle of the CC bond of the styrylpyridine ligand, it is shown that the role of the low-lying MLCT states is important in the photoisomerization mechanism. In contrast to the free organic ligand, in which the singlet mechanism is operational via the ¹IL (S₁) and electronic ground (S₀) states, coordination to the rhenium steers the isomerization to the triplet PEC corresponding to the ³IL state. From the ³IL_t (t = trans) the system evolves to the perpendicular intermediate ³IL_p (p = perpendicular) following a 90° rotation around the styrylpyridine CC bond. The metal center acts as a photosensitizer because of the presence of photoactive MLCT states under visible irradiation. The position of the crossing between the ³IL and electronic ground state PEC determines the quantum yield of the isomerization process.