Unique interplay between electronic states and dihedral angle for the molecular rotor diphenyldiacetylene

A new analysis of the optical properties of the molecular rotor 1,4-diphenyl-1,3-butadiyne (diphenyl-diacetylene, DPDA) is presented, taking account of the conformational dynamics. The absorption spectra are interpreted in terms of simultaneous contributions from planar as well as non-planar rotamers, characterized by a temperature dependent equilibrium distribution. The investigation is based on IR Linear Dichroism and UV Synchrotron Radiation Linear Dichroism (SRLD) spectroscopy on oriented samples in stretched polyethylene (PE), and on variable temperature UV spectroscopy. The study is supported by the results of detailed quantum chemical Time Dependent Density Functional Theory (TD-DFT) calculations. The resulting analysis has profound implications for the understanding of the optical, photochemical, and photophysical characteristics of this and related chromophores, of importance in a variety of applications.