Visible luminescence spectroscopy of free-base and zinc phthalocyanines isolated in cryogenic matrices

Abstract

The absorption, emission and excitation spectra of ZnPc and H₂Pc trapped in Ne, N₂, Ar, Kr and Xe matrices have been recorded in the region of the Q states. A comparison of the matrix fluorescence spectra with Raman spectra recorded in KBr pellets reveals very strong similarities. This is entirely consistent with the selection rules and points to the occurrence of only fundamental vibrational transitions in the emission spectra. Based on this behaviour, the vibronic modes in emission have been assigned using results obtained recently on the ground state with large basis-set DFT calculations [Murray et al. PCCP, 12, 10406 (2010)]. Furthermore, the very strong mirror symmetry between excitation and emission has allowed these assignments to be extended to the excitation (absorption) bands. While this approach works well for ZnPc, coupling between the band origin of the S₂(Q_Y) state and vibrationally excited levels of S₁(Q_X), limits the range of its application in H₂Pc. The Q_X/Q_Y state coupling is analysed from data obtained from site-selective excitation spectra, revealing pronounced matrix and site effects. From this analysis, the splitting of the Q_X and Q_Y states has been determined more accurately than in any previous attempts.