Potential-energy curves of the torsional mode of 1,1′-binaphthyl in the ground and lowest excited singlet states. A CS–INDO/CI study

The application of a recently developed theoretical method, CS–INDO/CI, to the study of the sections of the potential-energy surfaces of the ground and four lowest excited singlet states of 1,1′-binaphthyl corresponding to the coordinate of twisting around the inter-ring 1,1′-bond has proved to be successful in explaining the measured spectroscopic and photophysical properties of this molecule. All the changes observed in its spectroscopy and photophysics on going from low-temperature rigid solutions to room-temperature fluid solutions (strong bathochromic shift of the fluorescence maximum, change in the Snâ†�S1 absorption spectrum, shortening of the fluorescence lifetime by an order of magnitude) are qualitatively accounted for in terms of the calculated potential-energy curves. In particular, the S1 state experiences a drastic change in its electronic nature (from Lb to La) when the interplanar angle is allowed to relax from the near-orthogonal ground-state conformation to the trans twisted S1 equilibrium conformation (ϕ≈ 130 °). A comparison with the results of previous theoretical approaches stresses the reliability of the CS–INDO/CI method and the wealth of information on potential-energy surfaces which can be achieved through its application.