Investigation of the S₁/ICT → S₀ Internal Conversion Lifetime of 4‘-apo-β-caroten-4‘-al and 8‘-apo-β-caroten-8‘-al:  Dependence on Conjugation Length and Solvent Polarity

Abstract

The ultrafast internal conversion (IC) dynamics of aldehyde-substituted apocarotenoids (n'-apo-beta-caroten-n'-als with n=4, 8 and 12) have been investigated in a systematic variation of conjugation length and solvent polarity using time-resolved femtosecond transient absorption spectroscopy. After excitation to the S2 state with different excess energies, the subsequent intramolecular dynamics were investigated at several probe wavelengths covering the S0-->S2 and S1/ICT-->Sn absorption bands. Time constants tau1 for the internal conversion process S1/ICT-->S0 of 4'-apo-beta-caroten-4'-al and 8'-apo-beta-caroten-8'-al have been newly measured. We compared these results with our earlier measurements for 12'-apo-beta-caroten-12'-al (D.A. Wild, K. Winkler, S. Stalke, K. Oum, T. Lenzer Phys. Chem. Chem. Phys. 2006, 8, 2499). In the case of the aldehyde with the longest conjugation (4'-apo-beta-caroten-4'-al), tau1 is almost independent of solvent polarity (4-5 ps), whereas a significant reduction of tau1 from 22.7 to 8.6 ps for the shorter 8'-apo-beta-caroten-8'-al and an even more pronounced reduction from 220 to 8.0 ps for 12'-apo-beta-caroten-12'-al were observed when the solvent medium was changed from n-hexane to methanol, respectively. In n-hexane, tau1 of the apocarotenals is strongly dependent on the conjugation length and this can be well understood in terms of an energy gap law description where the S1-S0 energy differences were estimated from their steady-state fluorescence spectra. In highly polar solvents, the IC to S0 is very fast, irrespective of the conjugation length. This is probably due to the stabilization of an intramolecular charge transfer (ICT) state in 12'-apo-beta-caroten-12'-al and 8'-apo-beta-caroten-8'-al. In the case of 4'-apo-beta-caroten-4'-al, such an influence of an ICT state is presumably less important than for the other two apocarotenals.

An Approach Based on Synthetic Organic Chemistry Toward Elucidation of Highly Efficient Energy Transfer Ability of Peridinin in Photosynthesis

“…These experiments on peridinin and its derivatives revealed an increasing solvent effect with the decreasing -electron chain length. This result agrees with the experimental results carried out on conjugated apo-carotenoids (Ehlers et al, 2007). The lifetime of the lowest excited singlet state of C33 peridinin derivative 5 is the one most strongly dependent on the solvent polarity.…”

Dielectric relaxation and ultrafast transient absorption spectroscopy of [C6mim]+[Tf2N]−/acetonitrile mixtures

“…This decay directly provides the S 1 /ICT lifetime t 1 , which is summarized in Table 2 for all mixtures. 16 This type of behavior is also well-known for other carbonyl-substituted carotenoids, e.g., peridinin or fucoxanthin. 44 Such a dependence can be described in terms of an energy-gap-law approach: 16,45 ln(k 1 /s…”

“…[15][16][17][18][19] Interestingly, this correlation was not followed by a large range of ILs. 20 The lifetime of the S 1 /ICT state in these ILs was comparable to that in solvents such as ethanol or methanol, and thus the probe's local environment appears to be considerably more polar than the dielectric constants from DRS suggest.…”

Excited-State Dynamics of Carotenoids Studied by Femtosecond Transient Absorption Spectroscopy

“…All of these bands decay with 19.4 ps lifetime, 19 which was confirmed as the S 1 lifetime by visible transient absorption measurements. 13,15,24,25 The ν(C=C) bands of 7',7'-dicyano-7'-apo-β-carotene in the range of 1450-1700 cm Figure 1. Steady-state absorption spectra of 8'-apo-β-caroten-8'-al in cyclohexane (solid) and chloroform (dashed) and 7',7'-dicyano-7'-apo-β-carotene in chloroform (dotted).…”