Equilibration Dependence of Fucoxanthin S₁ and ICT Signatures on Polarity, Proticity, and Temperature by Multipulse Femtosecond Absorption Spectroscopy

Abstract

To demonstrate the value of the multipulse method in revealing the nature of coupling between excited states and explore the environmental dependencies of lowest excited singlet state (S) and intramolecular charge transfer (ICT) state equilibration, we performed ultrafast transient absorption pump-dump-probe and pump-repump-probe spectroscopies on fucoxanthin in various solvent conditions. The effects of polarity, proticity, and temperature were tested in solvents methanol at 293 and 190 K, acetonitrile, and isopropanol. We show that manipulation of the kinetic traces can produce one trace reflecting the equilibration kinetics of the states, which reveals that lower polarity, proticity, and temperature delay S/ICT equilibration. On the basis of a two-state model representing the S and ICT states on the same S/ICT potential energy surface, we were able to show that the kinetics are strictly dependent on the initial relative populations of the states as well as the decay of the ICT state to the ground state. Informed by global analysis, a systematic method for target analysis based on this model allowed us to quantify the population transfer rates throughout the life of the S/ICT state as well as separate the S and ICT spectral signatures. The results are consistent with the concept that the S and ICT states are part of one potential energy surface.

Excited‐State Evolution of Keto‐Carotenoids after Excess Energy Excitation in the UV Region

“…The carotenoid S 1 lifetime follows the energy gap law, [24] except for keto‐carotenoids whose S 1 lifetime depends on solvent polarity. The S 1 ‐S 0 relaxation in keto‐carotenoids with N <10 is governed by coupling of the S 1 state to an intramolecular charge transfer (ICT) state [15,25,26] . This coupling increases with solvent polarity and in most polar solvents such as methanol or acetonitrile the lifetime of the coupled S 1 /ICT can be order of magnitude faster than in a non‐polar solvent [27,28] …”

“…The S 1 -S 0 relaxation in ketocarotenoids with N < 10 is governed by coupling of the S 1 state to an intramolecular charge transfer (ICT) state. [15,25,26] This coupling increases with solvent polarity and in most polar solvents such as methanol or acetonitrile the lifetime of the coupled S 1 /ICT can be order of magnitude faster than in a nonpolar solvent. [27,28] The short excited state lifetimes of carotenoids imply fast and efficient dissipation of absorbed energy both to molecular vibrations, generating hot states, and eventually to the environment.…”

Solvent-Dependent Characterization of Fucoxanthin through 2D Electronic Spectroscopy Reveals New Details on the Intramolecular Charge-Transfer State Dynamics

“…In these conditions, the ICT can be effectively populated via internal conversion from S 2 . Extensive experimental and computational studies have not succeeded yet to fully clarify the nature and the properties of the ICT state, whether it is coupled with the S 1 state, in which case the two states would represent two minima of the same potential energy surface, − or it is a distinct electronic state. ,,,, Several models have been suggested, but none of them fully explains all the experimental evidence collected to date. ,, …”

“…The S 1 -ESA band instead is characterized by a rising component, derived from the internal conversion from the S 2 state. This new experimental evidence could only be captured with a high temporal resolution, and it goes together with recent studies that suggest that S 1 and ICT are indeed two distinct electronic states, as the temporal evolution of the relative ESA signals is different. , The presence of an ESA from the ICT state already at t 2 = 0 suggests an instant population of that state. This evidence has led to the hypothesis that the ICT state could be directly coupled to the S 2 state, an idea already presented in the (S 1 +S 2 )/ICT state model invoked previously.…”

“…This model refers to peridinin, whose ESA bands are indistinguishable, and it should also be tested for fucoxanthin; but the idea of a strong mixing between the various states of the system could explain this and other controversial features. Moreover, it is important to stress that, even though the S 1 and the ICT states are two distinct electronic states, an interplay between the two states is still possible, as recent pump-dump-probe studies have proposed. , In this picture, more polar solvents, besides stabilizing the ICT and lowering its energy, would also promote a better mixing with S 2 and thus a lower barrier of potential. Another interesting insight of this model is the interpretation of the asymmetrical increase of the red tail of the linear absorption spectrum in polar solvents: if the ICT state, coupled with the S 2 state, can be populated instantly, there has to be a trace of this process also in the linear absorption, and this could be indeed related to the increased absorbance in the red tail.…”