Optical properties of para‐oligophenylenes: A case study of electronic absorption spectrum and relaxation dynamics of terphenyl

We present a detailed quantitative description of vibronic‐coupling effects on the singlet photophysics of para‐oligophenylenes. Four energetically close‐lying singlet states (S1, S2, S3, and S4) are taken into consideration to interpret the first UV/Vis absorption band of these molecules. As a representative example, the energetics of those four states are evaluated for para‐terphenyl by performing extensive electronic structure calculations within the linear vibronic coupling framework. We obtain absorption spectral profiles by wavepacket propagation technique using the multi‐configuration time‐dependent Hartree method. Computed spectral profiles are in accord with the experiment. A minor electronic population transfer to upper states on a timescale of 10 fs is observed, suggesting the excited‐state nonadiabatic behavior of the molecule. Our findings necessitate the experimental validation of the role of higher states in the spectroscopy and dynamics of photoexcited para‐oligophenylenes. Near degenerate S1, S2, S3, and S4 electronic states are included in examining the excited‐state structural relaxation dynamics and broad absorption spectrum of terphenyl. Potential energy surfaces constructed within the linear vibronic coupling model display vibronic mixing among the excited‐states. Quantum wavepacket dynamics simulations predict population transfer from S1 to S4 state, establishing the nonadiabatic behavior of terphenyl. These findings necessitate including higher excited states in interpretation of size‐dependent photophysical properties of paraoligophenylenes.