Mechanism of the Intramolecular Charge Transfer State Formation in all-trans-β-Apo-8′-carotenal: Influence of Solvent Polarity and Polarizability

Abstract

In this work we analyzed the infrared and visible transient absorption spectra of all-trans-β-apo-8'-carotenal in several solvents, differing in both polarity and polarizability at different excitation wavelengths. We correlate the solvent dependence of the kinetics and the band shape changes in the infrared with that of the excited state absorption bands in the visible, and we show that the information obtained in the two spectral regions is complementary. All the collected time-resolved data can be interpreted in the frame of a recently proposed relaxation scheme, according to which the major contributor to the intramolecular charge transfer (ICT) state is the bright 1Bu(+) state, which, in polar solvents, is dynamically stabilized through molecular distortions and solvent relaxation. A careful investigation of the solvent effects on the visible and infrared excited state bands demonstrates that both solvent polarity and polarizability have to be considered in order to rationalize the excited state relaxation of trans-8'-apo-β-carotenal and clarify the role and the nature of the ICT state in this molecule. The experimental observations reported in this work can be interpreted by considering that at the Franck-Condon geometry the wave functions of the S1 and S2 excited states have a mixed ionic/covalent character. The degree of mixing depends on solvent polarity, but it can be dynamically modified by the effect of polarizability. Finally, the effect of different excitation wavelengths on the kinetics and spectral dynamics can be interpreted in terms of photoselection of a subpopulation of partially distorted molecules.

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

“…As already noticed, the character of the ICT state strongly depends on the ambient polarity; to get a better insight of its formation dynamics and evolution, we repeated the measurements in two solvents: the non-polar cyclohexane and the slightly polar chloroform. 27 Representative transient spectra at different pump-probe delays for all the excitation wavelengths are reported in Figure 3.…”

“…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.…”

“…Furthermore, we demonstrated that solvent polarizability has an important role on the dynamical stabilization of the ICT state. 27 The use of non-linear time-resolved spectroscopic methods in the mid-IR region and, in particular, of transient two-dimensional infrared spectroscopy in the excited state (T2D-IR), 28 was particularly informative for the characterization of the dynamics of the ICT state formation. Based on these previous results, here, we apply the same methods to peridinin, with the aim of clarifying the relation between the photophysical properties of carotenoids and peculiar structural properties, namely, chain length and nature of substituents.…”

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

“…For ω VIS < 19200 cm −1 , a broad negative transient dominates, attributed to S n ← S 1 excited state absorptions (ESAs). Our degenerate electronic pump−probe experiments offer a superior instrument response (27 fs) than previous TA studies of β-apo-8′-carotenal; 16,33 however, they do not stretch below ω VIS = 18200 cm −1 , where in polar solutions the ESAs from the ICT state are expected to peak at ∼15400 cm −1 (∼650 nm). 11,14,16 The broadband degenerate TA data could not be analyzed in a self-consistent manner by fitting the decay of individual probe frequencies− the ground state absorption stretches almost the entire probe window, and overlaps with the very broad S n ← S 1 ESAs.…”

“…The specific lifetimes for the different decay components are given in Table 1 and are in agreement with previous literature values of the S 2 and S 1 lifetimes. 29,32,33 c. Electronic-Vibrational Spectroscopy. i. Cyclohexane.…”