Two-Photon and Fluorescence Spectroscopy and the Effect of Environment on the Photochemical Properties of Peridinin in Solution and in the Peridinin-Chlorophyll-Protein from Amphidinium carterae

Abstract

The ground and excited-state properties of peridinin in solution and in the peridinin-chlorophyll-protein (PCP) complex are studied by using one-photon and two-photon spectroscopy, solvent effects, and quantum theory. Two-photon excitation spectra, two-photon polarization data, and fluorescence spectra in CS2 reveal three low-lying excited singlet states in peridinin: a lowest-excited1Ag*--like state with a system origin at ∼16 200 cm-1, a 1Bu*+-like S2 state with a system origin at ∼19 300 cm-1, and a 1Bu*--like S3 state with a system origin at ∼22 000 cm-1. The 1Bu*+-like S2 state dominates the two-photon excitation spectrum of peridinin in solution and in PCP because of type II enhancement associated with a large oscillator strength (f ≈ 1.6) coupled with a change in dipole moment upon excitation (Δμ ≈ 3 D). Thus, the two-photon spectrum looks very much like the one-photon spectrum although weak vibronic bands of the 1Ag*--like state are observed at ∼17, ∼18.1, and ∼19.2 kK in the two-photon spectrum ...