Combination of Transient 2D-IR Experiments and Ab Initio Computations Sheds Light on the Formation of the Charge-Transfer State in Photoexcited Carbonyl Carotenoids

Abstract

The excited state dynamics of carbonyl carotenoids is very complex because of the coupling of single- and doubly excited states and the possible involvement of intramolecular charge-transfer (ICT) states. In this contribution we employ ultrafast infrared spectroscopy and theoretical computations to investigate the relaxation dynamics of trans-8'-apo-β-carotenal occurring on the picosecond time scale, after excitation in the S2 state. In a (slightly) polar solvent like chloroform, one-dimensional (T1D-IR) and two-dimensional (T2D-IR) transient infrared spectroscopy reveal spectral components with characteristic frequencies and lifetimes that are not observed in nonpolar solvents (cyclohexane). Combining experimental evidence with an analysis of CASPT2//CASSCF ground and excited state minima and energy profiles, complemented with TDDFT calculations in gas phase and in solvent, we propose a photochemical decay mechanism for this system where only the bright single-excited 1Bu(+) and the dark double-excited 2Ag(-) states are involved. Specifically, the initially populated 1Bu(+) relaxes toward 2Ag(-) in 200 fs. In a nonpolar solvent 2Ag(-) decays to the ground state (GS) in 25 ps. In polar solvents, distortions along twisting modes of the chain promote a repopulation of the 1Bu(+) state which then quickly relaxes to the GS (18 ps in chloroform). The 1Bu(+) state has a high electric dipole and is the main contributor to the charge-transfer state involved in the dynamics in polar solvents. The 2Ag(-) → 1Bu(+) population transfer is evidenced by a cross peak on the T2D-IR map revealing that the motions along the same stretching of the conjugated chain on the 2Ag(-) and 1Bu(+) states are coupled.

Femtosecond transient infrared and stimulated Raman spectroscopy shed light on the relaxation mechanisms of photo-excited peridinin

“…Since the spectrum refers to a delay of 10 ps, the observed excited state bands can be attributed to the S 1 state. We tentatively assign the two bands to the symmetric C= =C stretching (1700 cm −1 ), which, according to previous measurements on different carotenoids and polyenes, upshifts upon excitation, 26,39 and to the lactone C= =O (1750 cm −1 ), whose vibration acquires some Raman activity because of the conjugation with the polyene chain, and is expected to downshift in the excited state. 34 The strong negative peaks in the Raman spectrum in Fig.…”

“…Femtosecond time resolved infrared spectra were previously recorded for peridinin 40 and several band assignments have been proposed based on quantum chemical calculations. 35 Recently, we measured the T1D-IR and T2D-IR spectra of all-trans-β-apo-8 ′ -carotenal 26,27 and proposed an assignment for the excited state bands in the 1600-1800 cm −1 region based on quantum chemical calculations and on transient Raman spectra. 36,39,41 It is expected that in peridinin, as well as in other polar carotenoids, the vibrational modes mostly contributing to the excited state absorption in the 1600-1800 cm −1 region arise from carbonyl groups and polyene chain vibrations.…”

“…In the case of all-trans-β-apo-8 ′ -carotenal, we showed that the analysis of non-equilibrium 2D-IR spectra can reveal interesting information on the molecular excited state evolution. 26 In fact, the narrow band infrared pump pulses used to measure twodimensional pump-probe spectra, unveil multiple bands, often hidden under broad envelopes in the linear spectra. Figure 8 shows the T2D-IR maps recorded for peridinin in chloroform, upon 400 nm visible excitation, at four different Vis pump /IR probe delays and at the fixed IR pump /IR probe delay of 500 fs.…”

“…18,25 These studies revealed that the formation of the ICT state is related to the chain length and is enhanced in short chain analogues; in addition, the amount of polyene conjugation also regulates the energy difference between S 2 and S 1 states. In recent papers, 26,27 we analyzed the deactivation pathway of a different and structurally simpler carbonyl carotenoid, trans-β-apo-8 ′ -carotenal, by means of visible pump/Mid-IR probe spectroscopy, transient absorption spectroscopy, and transient 2DIR spectroscopy. Initial data were recorded in two solvents with different polarities, the nonpolar cyclohexane and the slightly polar chloroform, and the measurements were subsequently extended to a number of solvents, differing both in polarity and polarizability.…”

Following Coupled Electronic-Nuclear Motion through Conical Intersections in the Ultrafast Relaxation of β-Apo-8′-carotenal

“…S 0 ), based on complete active space self-consistent field (CASSCF) calculations of a smaller chain apocarotenal. 32 CASSCF does not treat dynamic electron correlation and therefore is expected to poorly describe the electronic structure of the S 1 , and perhaps S 2 , states. More recent studies by the Righini group used FSRS 35 and 2D transient exchange IR 34 techniques.…”