Ultraviolet resonance Raman study of the pyrene S4, S3, and S2 excited electronic states

Abstract

UV resonance Raman excitation within the S₂, S₃, and S₄ π→π* electronic transitions of pyrene results in strong enhancement of totally symmetric ring vibrations. The Raman modes most strongly enhanced within these electronic transitions derive from vibrations exhibiting Franck–Condon vibronic structure in the absorption spectrum. The 1597 cm⁻¹ (b_3g) mode shows selective enhancement between the S₃ and S₄ transitions, and between the S₂ and S₃ transitions due to Herzberg–Teller coupling between these symmetry‐allowed states. The experimentally observed S₄ resonance Raman excitation profiles of the totally symmetric pyrene fundamentals agree closely with those calculated using resonance Raman transform theory if non‐Condon contributions are included. We see an increased non‐Condon contribution with an increasing vibrational frequency, indicating nonadiabatic vibronic interactions. The high incident energy fluxes of the Nd‐YAG laser‐based excitation source cause saturation of the pyrene Raman intensities since molecular absorption depletes the ground state population. The long‐lived S₁ singlet excited state bottlenecks relaxation back to the ground electronic state. Formation of pyrene phototransients is also observed with high incident energy fluxes at particular excitation wavelengths.