The lifetime of the lowest excited singlet (S 1 ) state of peridinin and many other carbonyl-containing carotenoids and polyenes has been reported depend on the polarity of the solvent. This effect has been attributed to the presence of an intramolecular charge transfer (ICT) state in the manifold of excited states for these molecules. The nature of this ICT state has yet to be elucidated. In the present work, steady-state and ultrafast time-resolved optical spectroscopy have been performed on peridinin and three synthetic analogues, C 33 -peridinin, C 35 -peridinin, and C 39 -peridinin which have different numbers of conjugated carbon-carbon double bonds. Otherwise, the molecules are structurally similar in that they posses the same functional groups. The trends in the positions of the steady-state and transient spectral profiles for this systematic series of molecules allow an assignment of the spectral features to transitions involving the S 0 , S 1 , S 2 and ICT states. A kinetics analysis reveals the lifetimes of the excited states and the dynamics of their excited state deactivation pathways. The most striking observation in the data is that the lifetime of the ICT state converges to the same value of 10.0 ± 2.0 ps in the polar solvent, methanol, for all the peridinin analogues regardless of the extent of π-electron conjugation. This suggests that the ICT state is highly localized on the lactone ring which is a common structural feature in all the molecules. The data further suggest that the S 1 and ICT states behave independently and that the ICT state is populated both from both S 1 and S 2 , the rate and efficiency from S 1 being dependent on the length of the π-electron chain of the carotenoid and the solvent polarity. Keywords peridinin; peridinin analogue; carotenoid; ICT state; excited state; kinetics analysis *Author to whom correspondence should be addressed: Harry A. Frank, Department of Chemistry, 55 North Eagleville Road, University of Connecticut, Storrs,. harry.frank@uconn.edu. Supporting Information available: Overlay of the fluorescence spectra of C 33 -peridinin, C 35 -peridinin and peridinin taken at room temperature in carbon disulfide and n-hexane, NIR transient spectra of all four analogues taken at room temperature in methanol, and steady-state absorption spectra of C 33 -peridinin, C 35 -peridinin, peridinin and C 39 -peridinin taken at room temperature in methanol, extended to high energy to show the "cis-peak" region between 27,000 and 40,000 cm −1 . This material is available free of charge via the Internet at

The lifetime of the lowest excited singlet state of carbonyl-containing carotenoids typically depends on the polarity of the solvent, an effect that has been attributed to the presence of an intramolecular charge transfer (ICT) state. The nature of this ICT state has yet to be clarified. In the present work, steady-state and ultrafast time-resolved optical spectroscopic experiments have been performed on peridinin and three synthetic analogues, C33-peridinin, C35-peridinin, and C39-peridinin, which have different extents of π-electron conjugation. Steady-state absorption at cryogenic temperatures revealed new absorption bands on the long-wavelength side of the strongly allowed S0 (1(1)Ag(-)) → S2 (1(1)Bu(+)) transition that can be assigned to S0 (1(1)Ag(-)) → S1 (2(1)Ag(-)) absorption. Analysis of the time-resolved absorption and fluorescence data sets revealed that the influence of polarity of the solvent on the excited state lifetime is unique for each molecule, leading to subtle differences in the values in highly polar solvents. Measurements in the most polar solvent, acetonitrile, demonstrated that the ICT state lifetime is shortest at 6.4 ps for C39-peridinin and gradually increases as the extent of π-electron conjugation decreases, becoming 10.6 ps for C33-peridinin. This suggests that the energy of the ICT state is dependent on the number of conjugated carbon-carbon double bonds.

The spectroscopic properties and dynamics of the excited states of two different synthetic analogues of peridinin were investigated as a function of solvent polarity using steady-state absorption, fluorescence, and ultrafast time-resolved optical spectroscopy. The analogues are denoted S-1- and S-2-peridinin and differ from naturally-occurring peridinin in the location of the lactone ring and its associated carbonyl group, known to be obligatory for the observation of a solvent dependence of the lifetime of the S1 state of carotenoids. Relative to peridinin, S-1- and S-2-peridinin have their lactone rings two and four carbons more toward the center of the π-electron system of conjugated carbon-carbon double bonds, respectively. The present experimental results show that as the polarity of the solvent increases, the steady-state spectra of the molecules broaden, and the lowest excited state lifetime of S-1-peridinin changes from ~155 ps to ~17 ps which is similar to the magnitude of the effect reported for peridinin. The solvent-induced change in the lowest excited state lifetime of S-2-peridinin is much smaller and changes only from ~90 ps to ~67 ps as the solvent polarity is increased. These results are interpreted in terms of an intramolecular charge transfer (ICT) state that is formed readily in peridinin and S-1-peridinin, but not in S-2-peridinin. Quantum mechanical computations reveal the critical factors required for the formation of the ICT state and the associated solvent-modulated effects on the spectra and dynamics of these molecules and other carbonyl-containing carotenoids and polyenes. The factors are the magnitude and orientation of the ground and excited state dipole moments which must be suitable to generate sufficient mixing of the lowest two excited singlet states.

Peridinin exhibits an anomalous solvent dependence of its S 1 excited state lifetime attributed to the presence of an intramolecular charge transfer (ICT) state. The nature of this state has yet to be elucidated. Ultrafast time-resolved optical spectroscopy has been performed on a synthetic analog, C 35 -peridinin, having one less conjugated double bond than peridinin. The data reveal the lifetime decreases from 1.5 ns in n-hexane to 9.2 ps in methanol, an order of magnitude larger than peridinin. This is the strongest solvent dependence on the lifetime of an S 1 state of a carotenoid yet reported. The data support the view that the S 1 and ICT states are strongly coupled.

Stark absorption spectra of peridinin (Per) and five allene-modified analogues and their angular dependence as a function of an externally applied electric field were measured in methyl methacrylate polymer at 77K. In all cases, the energetically lowest absorption band has a significant change of static dipole moment upon photoexcitation (Δμ). In particular, Per has the largest value of |Δμ|. The angles between Δμ and the transition dipole moment of all the analogues were determined. It is suggested that the allene group in Per plays a key role as the electron donor in the charge transfer process following photoexcitation. The results of MNDO-PSDCI calculations support this idea.

The peridinin-chlorophyll a-protein (PCP) is a light-harvesting pigment-protein complex found in many species of marine algae. It contains the highly substituted carotenoid peridinin and chlorophyll a, which together facilitate the transfer of absorbed solar energy to the photosynthetic reaction center. Photoexcited peridinin exhibits unorthodox spectroscopic and kinetic behavior for a carotenoid, including a strong dependence of the S(1) excited singlet state lifetime on solvent environment. This effect has been attributed to the presence of an intramolecular charge transfer (ICT) state in the molecule. The present work explores the effect of changing the extent of π-electron conjugation and attached functional groups on the nature of the ICT state of peridinin and how these factors affect the excited singlet and triplet state spectra and kinetics of the carotenoid. In this investigation three peridinin analogues denoted C-1-R-peridinin, C-1-peridinin, and D-1-peridinin were synthesized and studied using steady-state absorption and fluorescence techniques and ultrafast time-resolved transient absorption spectroscopy. The study explores the effect on the singlet and triplet state spectra and dynamics of removing the allene group from the peridinin structure and either replacing it with a rigid furanoid ring, replacing it with an epoxide group, or extending the polyene chain into the β-ionylidine ring.

Fucoxanthin is an essential pigment for the highly efficient light-harvesting function of marine algal photosynthesis. It exhibits excited state properties attributed to intramolecular charge transfer (ICT) in polar environments due to the presence of the carbonyl group in its polyene backbone. This report describes the excited state properties of fucoxanthin homologues with four to eight conjugated double bonds in various solvents using the femtosecond pump-probe technique. The results clarified that fucoxanthin homologues with longer polyene chains did not possess pronounced ICT spectroscopic signatures, while the shorter fucoxanthin homologues had a strong ICT character, even in a nonpolar solvent. On the basis of the observations, we quantitatively correlated the ICT character in the excited state to the conjugated polyene chain lengths of fucoxanthin molecules.

The spectroscopic properties and dynamics of the lowest excited singlet states of peridinin and two derivatives have been studied by steady-state absorption and fast-transient optical spectroscopic techniques. One derivative denoted PerOlEs, possesses a double bond and a methyl ester group instead of the r-ylid-enebutenolide of peridinin. Another derivative denoted PerAcEs, is the biosynthetic precursor of peridinin and possesses a triple bond and a methyl ester group corresponding to the r-ylidenbutenolide function. Ultrafast time-resolved spectroscopic experiments in the visible and near-infrared regions were performed on the molecules and reveal the energies and regarding the structural features and interactions responsible for the unusual solvent-induced changes in the steady-state and transient absorption spectra and dynamics of dynamics of the excited electronic states. The data also provide information peridinin.