Deconstructing the Excited-State Dynamics of β-Carotene in Solution

Abstract

The femtosecond to nanosecond dynamics of the all-trans β-carotene carotenoid dissolved in 3-methylpentane is characterized and dissected with excitation-wavelength and temperature-dependent ultrafast dispersed transient absorption signals. The kinetics measured after red-edge (490 nm) and blue-edge (400 nm) excitation were contrasted under fluid solvent (298 K) and rigid glass (77 K) conditions. In all four measured data sets, the S* population kinetics was resolved prompting the development of a modified multicompartment model. The temperature-dependent and excitation wavelength-dependent S* quantum yield is ascribed to a competition of population surmounting a weak (55 cm(-1)) energy barrier on the S(2) state to favor S(1) generation and rapid internal conversion that favors S* generation. When cooled from room temperature to 77 K, the S* decay time scale shifted significantly from 30 to 400 ps, which is ascribed to small-scale structural relaxation with a 115 cm(-1) energy barrier. For the first time under low-energy excitation conditions, the triplet state is observed and confirmed to not originate from S* or S(1), but from S(2). The interconnectivity of the S* and S(1) populations is discussed, and no observed population flow is resolved between S* and S(1). Comparison of samples obtained from different laboratories with different purity levels demonstrates that sample contamination is not the primary origin of the S* state.

Excited-State Dynamics of Diindenoperylene in Liquid Solution and in Solid Films

“…The behavior is similar to that of well-studied polyene systems in which the emission also takes place from a low-lying state, close in energy to the strongly allowed 1 B u state but possessing a much smaller transition dipole moment. 36,37 When DIP is deposited as a solid film on glass, its spectroscopic behavior changes significantly. On strongly interacting substrates like metal surfaces, DIP molecules tend to adopt a "lying down" configuration.…”

Ultrafast Triplet Generation and its Sensitization Drives Efficient Photoisomerization of Tetra-cis-lycopene to All-trans-lycopene

“…The remnant excited population possibly arises from the postulated S * state which then yields another blue-shifted non-decaying population centered at ~470 nm corresponding to the triplet manifold. [31][32][33][34]61,62 The triplet state persists in our measurements till >1 ns ( Figure S15a) although its decay kinetics is limited by our detection sensitivity (~0.1 mOD) and the extent of our pump-probe delays (2 ns). It is therefore evident from the time-resolved absorption of prolycopene in solution that a modest fraction of the excited population can yield triplet which is the reactive state.…”

“…Additionally, it has been identified as a precursor state to facilitate 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ultrafast singlet fission to the triplet manifold. 33,61,[63][64][65] In order to confirm the assignment of the S * state, we also fitted the data at 495 nm ( Figure S14) which was away from the S 1 although capturing the blue edge of the S * state. Fitting the data with four exponents provided a 280 fs rise time and three long decay times of: 15.8 ps, ~300 ps and a non-decaying triplet component.…”

“…Our assignment of the kinetic pathway is consistent with the recently published report by Larsen and co-workers who propose similar scheme for β-carotene dynamics. 61 Page 20 of 37…”

Excited-state dynamics of astaxanthin aggregates

“…Then, the S 1 state can break down to two triplets localized at individual molecules in the aggregate [15]. But since the carotenoid S 2 state was also proposed to be the parent state [12,38], we cannot reliably determine the parent state for singlet homofission in astaxanthin aggregates.…”