Theoretical investigations of absorption and fluorescence spectra of protonated pyrene

Abstract

The equilibrium geometry and 75 vibrational normal-mode frequencies of the ground and first excited states of protonated pyrene isomers were calculated and characterized in the adiabatic representation by using the complete active space self-consistent field (CASSCF) method. Electronic absorption spectra of solid neon matrixes in the wavelength range 495–415 nm were determined by Maier et al. and they were analyzed using time-dependent density functional theory calculations (TDDFT). CASSCF calculations and absorption and emission spectra simulations by one-photon excitation equations were used to optimize the excited and ground state structures of protonated pyrene isomers. The absorption band was attributed to the S0 → S1 electronic transition in 1H-Py⁺, and a band origin was used at 20580.96 cm⁻¹. The displaced harmonic oscillator approximation and Franck–Condon approximation were used to simulate the absorption spectrum of the (1) ¹A′ ← ¹A′ transition of 1H-Py⁺, and the main vibronic transitions were assigned for the first ππ* state. It shows that the vibronic structures were dominated by one of the eight active totally symmetric modes, with ν₁₅ being the most crucial. This indicates that the electronic transition of the S₁(¹A′) state calculated in the adiabatic representation effectively includes a contribution from the adiabatic vibronic coupling through Franck–Condon factors perturbed by harmonic oscillators. The present method can adequately reproduce experimental absorption and fluorescence spectra of a gas phase.