Femtosecond Heterodyne Transient Grating Studies of Nonradiative Deactivation of the S2 (11Bu+) State of Peridinin: Detection and Spectroscopic Assignment of an Intermediate in the Decay Pathway

Femtosecond heterodyne transient grating spectroscopy was employed to investigate the nonradiative decay pathway from the S-2 (1(1)B(u)(+)) state to the S-1 (2(1)A(g)(-)) state of peridinin in methanol solution. Just as previously observed by this laboratory for beta-carotene in benzonitrile, the real (absorption) and imaginary (dispersion) components of the transient grating signal obtained CD with Fourier transform spectral interferometry from peridinin exhibit ultrafast responses indicating that S-2 state decays in 12 fs to produce an intermediate state, S-x. The excited state absorption spectrum from the S-x state of peridinin, however, is found to be markedly blue-shifted from that of beta-carotene because it makes a substantial contribution to the signal observed with 40 fs, 520 nm pulses. The results of a global target analysis and numerical simulations using nonlinear response functions and the multimode Brownian oscillator model support the assignment of Sx to a displaced conformation of the S-2 state rather than to a vibrationally excited (or hot) S-1 state. The S-x state in peridinin is assigned to a structure with a distorted conjugated polyene backbone moving past an activation energy barrier between planar and twisted structures on the S-2 potential surface. The lengthened lifetime of the S-x state of peridinin in methanol, 900 +/- 100 fs, much longer than that typically observed for carotenoids lacking carbonyl substituents, similar to 150 fs, can be attributed to the slowing of torsional motions by solvent friction. In peridinin, the system-bath coupling is significantly enhanced over that in beta-carotene solution most likely due to the intrinsic intramolecular charge transfer character it derives from the electron withdrawing nature of the carbonyl substituent. An important additional implication is that the S-x state, and the distorted structures reached subsequently along the torsional gradient on the S-2 potential surface, may serve as the principal excitation energy transfer donors to chlorophyll a in the peridinin-chlorophyll a protein from dinoflagellates.