PHOTOPHYSICS AND DYNAMICS OF THE LOWEST EXCITED SINGLET-STATE IN LONG SUBSTITUTED POLYENES WITH IMPLICATIONS TO THE VERY LONG-CHAIN LIMIT

In this paper we explore the intramolecular relaxation processes within two long carotenoids, namely decapreno-beta-carotene (M15) and dodecapreno-beta-carotene (M19) with 15 and 19 conjugated double bonds (N), respectively. Amplified 200 fs pulses at 590 nm were used to excite the optically allowed S-0--> S-2 (1 (1)A(g) --> 1 B-1(u)) transition of the two carotenoids. The excited state dynamics were probed by continuum light between 400-890 nm in solvents with different polarizabilities. The transient absorption spectra consist of a bleaching region, due to loss of ground state absorption, and of an excited state absorption region at longer wavelengths, due to the S-1--> S-n transition. The S-n state was assigned to an n B-1(u) state. The overall wavelength dependence of the measured kinetics could be well described by introducing three decay time constants. One reflects the S-1 lifetime (tau(1)) and was determined to 1.1 and 0.5 ps for M15 and M19, respectively. A second lifetime, between 5 and 15 ps, was attributed to vibrational cooling in the ground state. A third decay time was in the subpicosecond range, and was ascribed to the vibrational redistribution and relaxation of the S-1 potential surface after being populated by the subpicosecond S-2-S-1 internal conversion. No significant change of the decay constants was observed for M15 embedded in a 77 K matrix. This shows that the relaxation rates are only influenced by intramolecular processes. The S-2 lifetime was shorter than the pulse duration and was estimated to be in the order of 100 fs. The S-0-->S-2 transition of M15 in the liquid phase exhibits a 0.39 anisotropy at short times, while the S-1--> S-n transition has an initial value of only 0.31. This corresponds to an angle of 23 degrees between the transition dipoles. The measured S-1 rate constants were analyzed, together with decay constants of shorter carotenes, in terms of the energy gap law. When going from the shortest (N = 5) to the longest (N = 19) polyene, tau(1) decreases about 6000 times, i.e., from 3 ns to 0.5 ps. By using an empirical form of the energy gap law the 0-0 transition of S-1(2 (1)A(g))--> S-0 was estimated to be located at 11 300 and 10 200+/- 1 000 cm(-1) for M15 and M19, respectively. By fitting the excitation energies of all carotenes in the series (3 less than or equal to N less than or equal to 19) with a truncated two or three term expansion of a power series in 1/N the long-chain limit values were extrapolated to be 11 000 and 3 500 cm(-1) for the 1 B-1(u) and 2 (')A(g) state, respectively. The implication of these limit values on the electronic structure of polyacetylene are discussed. (C) 1995 American Institute of Physics.