Photophysics and dynamics of the lowest excited singlet state in long substituted polyenes with implications to the very long-chain limit

Abstract

Articles you may be interested inThe photophysical behavior of 3-chloro-7-methoxy-4-methylcoumarin related to the energy separation of the two lowest-lying singlet excited states Lowest energy excited singlet states of isomers of alkyl substituted hexatrienes J. Chem. Phys. 94, 4691 (1991); 10.1063/1.460581Theoretical study of the force field of the lowest singlet electronic states of long polyenes J. Chem. Phys. 91, 6215 (1989); 10.1063/1.457388Polyene spectroscopy: The lowest energy excited singlet state of diphenyloctatetraene and other linear polyenes In this paper we explore the intramolecular relaxation processes within two long carotenoids, namely decapreno-␤-carotene ͑M15͒ and dodecapreno-␤-carotene ͑M19͒ with 15 and 19 conjugated double bonds ͑N͒, respectively. Amplified 200 fs pulses at 590 nm were used to excite the optically allowed S 0 →S 2 ͑1 1 A g →1 1 B 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 1 B 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 ͑ 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°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 ͑Nϭ5͒ to the longest ͑Nϭ19͒ polyene, 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 200Ϯ1 000 cm Ϫ1 for M15 and M19, respectively. By fitting the excitation energies of all carotenes in the series ͑3рNр19͒ 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 Ϫ1 for the 1 1 B u and 2 1 A g state, respectively. The implication o...

An unusual pathway of excitation energy deactivation in carotenoids: Singlet-to-triplet conversion on an ultrafast timescale in a photosynthetic antenna

“…2 A, dotted line), but it lacks the ESA feature at 542 nm. Its overall shape agrees well with the S 1 spectra reported earlier (14), and around 490 nm it is almost identical to the S 2 spectrum, as expected for ground-state bleaching. The S* spectrum is clearly distinct from that of S 1 and exhibits positive peaks at 542 and 564 nm and a bleaching at 520 nm.…”

“…Moreover, the lifetime associated with the second SADS, 1.4 ps, closely agrees with what one should expect from a Car of this length (2). A possible partition of the Car S 1 states into ''fast'' and ''slow'' decaying is discarded by the shape of the ''slow'' spectrum, unusual when compared to literature data (1,12,14). It thus appears extremely likely that S* is a hitherto uncharacterized electronic state of Spx, lying somewhere below S 2 , and being one-photon forbidden.…”

“…A ''hot'' ground state can, in principle, cause the absorption features at 542 and 564 nm (14), but then one would also expect a bleaching at 490 nm. Cooling of a ''hot'' S 1 excited state would produce a blue shift and a spectral narrowing, which we observe on going from the second to the third SADS in Fig.…”

“…The ESA at 590 nm is related to the presence of the optically forbidden S 1 state. It is well known that S 1 is formed from the S 2 state via IC on a femtosecond timescale, and that the spectroscopic signature of this dark state in Cars is a large absorption cross section to higher excited states, which is red-shifted with respect to the ground-state absorption (1,14). We conclude that the lifetime of the Spx S 1 state is 1.4 ps, which is in agreement with the value inferred by extrapolating kinetic data obtained for shorter Cars (2).…”

Vibrational Relaxation in β-Carotene Probed by Picosecond Stokes and Anti-Stokes Resonance Raman Spectroscopy

“…If the cooling rate were slower than the rate of internal conversion then there would be a build-up of anti-Stokes signal at long times and the decay of the anti-Stokes signal would exhibit biexponential kinetics. The relaxation time of the C-CH 3 mode (12 ps) is consistent with the 5-15 ps cooling time observed in longer carotenoids 4 and with our condition that the cooling must occur in <13 ps, so it is quite likely that the cooling time of the C-CH 3 mode is equivalent to the overall rate of energy dissipation to the solvent.…”