Multidimensional spectroscopy of β-carotene: Vibrational cooling in the excited state

Pump-degenerate four wave mixing (Pump-DFWM) is used for investigating the vibrational dynamics in the excited state of β-carotene in solution. In this 2D technique, an initial pump pulse promotes the system to the excited state, which is then probed by the succeeding DFWM sequence. We focus particularly on the internal conversion between the S₂ and S₁ state with high temporal and spectral resolution. The frequency shift of the excited state vibrations is measured and is explained as mode-specific vibrational cooling. Our results suggest an internal conversion in a time range between 260 and 500 fs without any intermediate states.     

Femtosecond stimulated Raman spectroscopy of the dark S1 excited state of carotenoid in photosynthetic light harvesting complex

Vibrational dynamics of the excited state in the light-harvesting complex (LH1) have been investigated by femtosecond stimulated Raman spectroscopy (FSRS). The native and reconstituted LH1 complexes have same dynamics. The ν(1) (C=C stretching) vibrational mode of spirilloxanthin in LH1 shows ultrafast high-frequency shift in the S(1) excited state with a time constant of 0.3 ps. It is assigned to the vibrational relaxation of the S(1) state following the internal conversion from the photoexcited S(2) state.     

Intramolecular and intermolecular hydrogen-bonding effects on photophysical properties of 2'-aminoacetophenone and its derivatives in solution.

Effects of intra- and intermolecular hydrogen-bonds on the photophysical properties of 2'-aminoacetophenone derivatives (X-C6H4-COCH3) having a substituted amino group (X) with different hydrogen-bonding ability to the carbonyl oxygen (X: NH2(AAP), NHCH3(MAAP), N(CH3)2(DMAAP), NHCOCH3(AAAP), NHCOCF3(TFAAP)) are investigated by means of steady-state and time-resolved fluorescence spectroscopy and time-resolved thermal lensing. Based on the photophysical parameters obtained in aprotic solvents with different polarity and protic solvents with different hydrogen-bonding ability, the characteristic photophysical behavior of the 2'-aminoacetophenone derivatives is discussed in terms of hydrogen-bonding and n,pi*-pi,pi* vibronic coupling. The dominant deactivation process of AAP and MAAP in nonpolar aprotic solvents is the extremely fast internal conversion (k(ic)= 1.0 x 10(11) s(-1) for AAP and 3.9 x 10(10) s(-1) for MAAP in n-hexane). The internal conversion rates of both compounds decrease markedly with increasing solvent polarity, suggesting that vibronic interactions between close-lying S1(pi,pi*) and S2(n,pi*) states lead to the large increase in the non-radiative decay rate of the lowest excited singlet state. It is also suggested that for MAAP, which has a stronger hydrogen-bond as compared to AAP, an intramolecular hydrogen-bonding induced deactivation is involved in the dissipation of the S1 state. For DMAAP, which cannot possess an intramolecular hydrogen-bond, the primary relaxation mechanism of the S1 state in nonpolar aprotic solvents is the intersystem crossing to the triplet state, whereas in protic solvents very efficient internal conversion due to intermolecular hydrogen-bonding is induced. In contrast, the fluorescence spectra of AAAP and TFAAP, which have an amino group with a much stronger hydrogen-bonding ability, give strongly Stokes-shifted fluorescence, indicating that these compounds undergo excited-state intramolecular proton transfer reaction upon electronic excitation.     

Studies on chromophore coupling in isolated phycobiliproteins: III. Picosecond excited state kinetics and time-resolved fluorescence spectra of different allophycocyanins from Mastigocladus laminosus

The excited state kinetics of three different allophycocyanin (AP) complexes has been studied by picosecond fluorescence spectroscopy. Both the fluorescence kinetics and the decay-associated fluorescence spectra of the different complexes can be understood on the basis of a structural model for AP which uses (a) an analogy to the known x-ray determined structure of C-phycocyanin, (b) the biochemical analogies of AP and C-phycocyanin, and (c) the biochemical composition of AP-B (AP-681). A model is developed that describes the excited state kinetics as a mixture of internal conversion processes within a coupled exciton pair and energy transfer processes between exciton pairs. We found excited state relaxation times in the range of 13 ps (AP with linker peptide) up to 66 ps (AP-B). The trimeric aggregates AP 660 and AP 665 show one fast relaxation component each, as was expected on the basis of their symmetry properties. The lower symmetry of AP-B (AP-681) gives rise to two fast lifetime components (τ₁ = 23 ps and τ₂ = 66 ps) which are attributed to internal conversion and/or energy transfer between excitonic states formed by the coupling of symmetrically and spectrally nonequivalent chromophores. It is proposed that the internal conversion between exciton states of strongly coupled chromophores fulfills the requirements of the small energy gap limit. Thus, internal conversion rates in the order of tens of picoseconds are feasible. The influence of the interaction of the linker peptide on the properties of the AP trimer are manifested in the fluorescence kinetics. Lack of the linker peptide in AP 660 gives rise to a heterogeneity in the chromophore conformations and chromophore-chromophore interactions.     

Photoexcited triplet states of new UV absorbers, cinnamic acid 2-methylphenyl esters

Phosphorescence spectra of nonphosphorescent or very weakly phosphorescent new UV absorbers, 2-methylphenyl cinnamate (MePC), 2-methylphenyl 4-methoxycinnamate (MePMC) and 2-methylphenyl 4-ethoxycinnamate (MePEC) have been observed by using external heavy atom effects of ethyl iodide in ethanol at 77 K. The lowest excited triplet (T(1)) energies of these new UV absorbers are lower than those of a widely used UV-A absorber, 4-tert-butyl-4'-methoxydibenzoylmethane (BM-DBM), in both keto and enol forms. The intermolecular triplet-triplet energy transfer from photolabile BM-DBM to MePMC was observed by measuring the time-resolved phosphorescence spectra. Electron paramagnetic resonance spectra have been observed for the T(1) states of these new UV absorbers in ethanol at 77 K by using benzophenone as a triplet sensitizer. The observed T(1) lifetimes, zero-field splitting (ZFS) parameters and molecular orbital calculations of the ZFS parameters suggest that T(1) states of these new UV absorbers posses mainly (3)ππ* character. The deactivation processes of the lowest excited singlet (S(1)) states are predominantly fluorescence and internal conversion to the ground (G) states in MePMC and MePEC, while the main deactivation process of the S(1) state of MePC is internal conversion to the G state. The molar absorption coefficients of MePMC and MePEC in the UV-A and UV-B regions are larger than that of most widely used UV-B absorber, octyl methoxycinnamate.     

Solvent effects on excitation relaxation dynamics of a keto-carotenoid, siphonaxanthin

Solvent effects on relaxation dynamics of a keto-carotenoid, siphonaxanthin, were investigated by means of the femtosecond time-resolved fluorescence spectroscopy. After excitation to the S2 state of siphonaxanthin, the S2-->1(n, pi*) internal conversion occurred with a time constant of 30-35 fs, followed by the 1(n, pi*)-->S1 internal conversion in 180-200 fs. Solvent dependence of the internal conversions was small, however intensities of the S1 fluorescence with its lifetime of longer than 10 ps were enhanced in methanol. These were explained by displacement of the potential surfaces and interaction through the hydrogen-bond between the C=O group of siphonaxanthin and solvents.     

Effects of tunable excitation in carotenoids explained by the vibrational energy relaxation approach

Carotenoids are fundamental building blocks of natural light harvesters with convoluted and ultrafast energy deactivation networks. In order to disentangle such complex relaxation dynamics, several studies focused on transient absorption measurements and their dependence on the pump wavelength. However, such findings are inconclusive and sometimes contradictory. In this study, we compare internal conversion dynamics in \(\beta\)-carotene, pumped at the first, second, and third vibronic progression peak. Instead of employing data fitting algorithms based on global analysis of the transient absorption spectra, we apply a fully quantum mechanical model to treat the high-frequency symmetric carbon–carbon (C=C and C–C) stretching modes explicitly. This model successfully describes observed population dynamics as well as spectral line shapes in their time-dependence and allows us to reach two conclusions: Firstly, the broadening of the induced absorption upon excess excitation is an effect of vibrational cooling in the first excited state (\(S_{1}\)). Secondly, the internal conversion rate between the second excited state (\(S_{2}\)) and \(S_{1}\) crucially depends on the relative curve displacement. The latter point serves as a new perspective on solvent- and excitation wavelength-dependent experiments and lifts contradictions between several studies found in literature.     

Charge separation and energy transfer in a caroteno-C60 dyad: photoinduced electron transfer from the carotenoid excited states.

We have designed and synthesized a molecular dyad comprising a carotenoid pigment linked to a fullerene derivative (C-C(60)) in which the carotenoid acts both as an antenna for the fullerene and as an electron transfer partner. Ultrafast transient absorption spectroscopy was carried out on the dyad in order to investigate energy transfer and charge separation pathways and efficiencies upon excitation of the carotenoid moiety. When the dyad is dissolved in hexane energy transfer from the carotenoid S(2) state to the fullerene takes place on an ultrafast (sub 100 fs) timescale and no intramolecular electron transfer was detected. When the dyad is dissolved in toluene, the excited carotenoid decays from its excited states both by transferring energy to the fullerene and by forming a charge-separated C.+ -C(60).- . The charge-separated state is also formed from the excited fullerene following energy transfer from the carotenoid. These pathways lead to charge separation on the subpicosecond time scale (possibly from the S(2) state and the vibrationally excited S(1) state of the carotenoid), on the ps time scale (5.5 ps) from the relaxed S(1) state of the carotenoid, and from the excited state of C(60) in 23.5 ps. The charge-separated state lives for 1.3 ns and recombines to populate both the low-lying carotenoid triplet state and the dyad ground state.     

Towards a spin coupling model for the Mn4 cluster in Photosystem II

The X-band EPR spectra of the IR sensitive untreated PSII and of MeOH- and NH(3)-treated PSII from spinach in the S(2)-state are simulated with collinear and rhombic g- and Mn-hyperfine tensors. The obtained principal values indicate a 1Mn(III)3Mn(IV) composition for the Mn(4) cluster. The four isotropic components of the Mn-hyperfine tensors are found in good agreement with the previously published values determined from EPR and (55)Mn-ENDOR data. Assuming intrinsic isotropic components of the Mn-hyperfine interactions identical to those of the Mn-catalase, spin density values are calculated. A Y-shape 4J-coupling scheme is explored to reproduce the spin densities for the untreated PSII. All the required criteria such as a S=1/2 ground state with a low lying excited spin state (30 cm(-1)) and an easy conversion to a S=5/2 system responsible for the g=4.1 EPR signal are shown to be satisfied with four antiferromagnetic interactions lying between -290 and -130 cm(-1).     

The rate of Q(x)→Q(y) relaxation in bacteriochlorophylls of reaction centers from Rhodobacter sphaeroides determined by kinetics of the ultrafast carotenoid bandshift

Transient absorption changes induced by excitation of isolated reaction centers (RCs) from Rhodobacter sphaeroides with 600nm laser pulses of 20fs (full width at half maximum) were monitored in the wavelength region of 420-560nm. The spectral features of the spectrum obtained are characteristic for an electrochromic band shift of the single carotenoid (Car) molecule spheroidene, which is an integral constituent of these RCs. This effect is assigned to an electrochromic bandshift of Car due to the local electric field of the dipole moment formed by electronic excitation of bacteriochlorophyll (BChl) molecule(s) in the neighborhood of Car. Based on the known distances between the pigments, the monomeric BChl (B(B)) in the inactive B-branch is inferred to dominate this effect. The excitation of B(B) at 600nm leads to a transition into the S(2) state (Q(x) band), which is followed by rapid internal conversion to the S(1) state (Q(y) band), thus leading to a change of strength and orientation of the dipole moment, i.e., of the electric field acting on the Car molecule. Therefore, the time course of the electrochromic bandshift reflects the rate of the internal conversion from S(2) to S(1) of B(B). The evaluation of the kinetics leads to a value of 30fs for this relaxation process. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.     

Electron spin-lattice relaxation of the S0 state of the oxygen-evolving complex in photosystem II and of dinuclear manganese model complexes

The temperature dependence of the electron spin-lattice relaxation time T1 was measured for the S0 state of the oxygen-evolving complex (OEC) in photosystem II and for two dinuclear manganese model complexes by pulse EPR using the inversion-recovery method. For [Mn(III)Mn(IV)(mu-O)2 bipy4]ClO4, the Raman relaxation process dominates at temperatures below 50 K. In contrast, Orbach type relaxation was found for [Mn(II)Mn(III)(mu-OH)(mu-piv)2(Me3 tacn)2](ClO4)2 between 4.3 and 9 K. For the latter complex, an energy separation of 24.7-28.0 cm(-1) between the ground and the first excited electronic state was determined. In the S0 state of photosystem II, the T1 relaxation times were measured in the range of 4.3-6.5 K. A comparison with the relaxation data (rate and pre-exponential factor) of the two model complexes and of the S2 state of photosystem II indicates that the Orbach relaxation process is dominant for the S0 state and that its first excited state lies 21.7 +/- 0.4 cm(-1) above its ground state. The results are discussed with respect to the structure of the OEC in photosystem II.     

Photophysics of pyrene-labelled compounds of biophysical interest.

The effects of the chemical constitution and structure of the substituent on the excited state dynamics of several model fluorescent pyrene-labelled molecules of biophysical interest have been examined. Nine new 1-substituted pyrenyl compounds, Py-NH-CO-C2H5, Py-NH-CO-Leu-Boc, Py-CH2-NH-CO-C2H5, Py-CH2-NH-CO-Leu-Boc, Py-CO-NH-C3H7, Py-CO-NH-Leu-OMe, Py-CH2-CO-NH-C3H7, Py-CH2-CO-NH-Leu-OMe and Py-C3H6-CO-NH-Leu-OMe, have been synthesized and their electronic spectra, fluorescence quantum yields and excited state lifetimes measured. These data have been used to calculate the radiative, kr, and non-radiative decay constants of their S1 states and the values of these constants correlated with the structures of the tethers. Non-radiative S1 decay rates (mainly intersystem crossing to T1) vary in parallel with the radiative rates so that the excited state lifetimes and radiative rate constants change considerably with the structure of the substituent whereas the quantum yields of fluorescence do not. An excellent correlation between [epsilon]max of the S1-S0 transition and either kr or the excited state lifetime is observed as long as no additional intermolecular or intramolecular excited state decay process of significant rate competes with the 'normal' radiative and non-radiative (ISC) decay processes of the pyrenyl chromophore. This correlation may have predictive value. Rates of bimolecular quenching of the S1 states of these molecules by molecular oxygen have been measured. The quenching process is diffusion-controlled with a spin statistical factor of 1, indicating that the S1-T1 electronic energy spacings of all the derivatives exceed the O2(1Deltag-3Sigmag-) electronic excitation energy of ca. 1 eV.     

Molecular modeling of two-photon absorption and third-order nonlinearities of polymethine dyes for all-optical switching

Stimulated by a recent experimental report [Hales JM et al. (2010) Science 327:1485-1488], two-photon absorption and third-order optical nonlinearities of selenopyrylium- and bis(dioxaborine)-terminated polymethine dyes (called SE-7C and DOB-9C) used for all-optical switching were investigated theoretically with time-dependent DFT (TD-DFT) and response theory as well as visualized real-space analysis. The calculated results for the first hyperpolarizability and second hyperpolarizability demonstrated that the two molecules both have large third-order optical nonlinearities. Using real-space analysis, we were able to visually determine that in the one-photon absorption (OPA) process, the first singlet excited state of SE-7C and DOB-9C is an intramolecular charge transfer (ICT) excited state with strong absorption, while the second excited state of these dyes (also termed the "ICT state") shows weak absorption. However, in the two-photon absorption (TPA) process, a larger TPA absorption cross-section was predicted for the second excited state. In this paper, we describe the properties of the S2 excited state, incorporating charge transfer and the transition moment, via real-space analysis, which was very important for understanding the TPA characteristics of the S(2) state.     

Solvent polarity effect on excited-state lifetime of carotenoids and some dyes

The theory demonstrating the role of medium at the fluorescence quenching of polar compounds in solutions is briefly presented. It has been shown, that the rate of S(1) --> X(n) nonradiative conversion between the intramolecular charge transfer states depends on the permanent dipole moments in the ground (S(0)) and excited (S(1), X(n)) states as well as on solvent polarity. A relation for the rate of nonradiative excited-state energy conversion has been obtained and employed to test the known literature data for solvent effect on the S(1)-state lifetime of some biologically significant carotenoids and dyes (phthalimides). For phthalimides, the solvent isotope effect on the S(1)-state energy conversion, when hydrogen is replaced by deuterium in the OH groups of alcohols and water, has been analyzed. Based on the data for fluorescence quenching in solvents of different polarity, the dipole moments in the intermolecular charge transfer S(1) state have been obtained for carotenoids (peridinin, fucoxanthin, uriolide acetate) and for hydrogen-bonding complexes, which are formed by 4-amino-, 4-methylamino-, and 4-dimethylamino-N-methylphthalimides in alcohols and water.     

Electronic spectra, excited state structures and interactions of nucleic acid bases and base assemblies: a review

A comprehensive review of recent theoretical and experimental advances in the singlet electronic transitions, excited state structures and dynamics of nucleic acid bases (NABs) and base assemblies are presented. It is well known that NABs absorb ultraviolet radiation, but the absorbed energy is efficiently dissipated in the form of ultrafast internal conversion processes believed to occur in the subpicosecond time scale and, therefore, enabling NABs highly photostable. It is not known how much evolutionary role was played in evolving these molecules and the ultimate selection by nature as genetic materials, but it is well accepted that survival-of-fittest prevails. Recently, significant efforts have been continuously paid to understand the mechanism of electronic excitation deactivation, but universally acceptable mechanism is still elusive. However, recent investigations reveal that electronic excited state geometries of DNA bases are usually nonplanar and this structural nonplanarity may facilitate nonradiative deactivation. Investigation of excited state structures is challenging and, therefore, it is not surprising that despite the impressive theoretical and computational advances, this research area is still hampered by the methodological and computational limitations. Further, stacking has significant influence on the emission properties of molecules. The 2-aminopurine, a fluorescent adenine derivative frequently used in studying DNA dynamics, shows significant attenuations in fluorescence quantum yield when incorporated in the DNA. Theoretical and computational bottlenecks limit a thorough theoretical understanding of effect of stacking interactions on the excited state dynamics of NABs. Despite these limitations the investigations of excited state properties are progressing in the right direction and our better understanding of excited state structure and dynamics of NABs and nucleic acids may help to design preventive strategy for radiation induced illness and photostable materials.     

On the photophysics of metallophthalocyanine-based photothermal sensitizers: synergism between theory and experiment

A comprehensive understanding of the factors governing the efficiency of metallophthalocyanine-based photothermal sensitizers requires the knowledge of their excited-state dynamics. This can only be properly gained when the nature and energy of the excited states (often spectroscopically silent) lying between the photogenerated state and the ground state are known. Here the excited state deactivation mechanism of two very promising metallophthalocyanine-based photothermal sensitizers, NiPc(OBu)(8) and NiNc(OBu)(8), is reviewed. It is shown that time dependent density functional theory (TDDFT) methods are capable to provide reliable information on the nature and energies of the low-lying excited states along the relaxation pathways. TDDFT calculations and ultrafast experiments consistently show that benzoannulation of the Pc ring modifies the photodeactivation mechanism of the photogenerated S(1)(pi,pi*) state by inducing substantial changes in the relative energies of the excited states lying between the S(1)(pi,pi*) state and the ground state.     

Kasha or state selective behavior in the photochemistry of ortho-nitrobenzaldehyde?

The photochemistry of ortho-nitrobenzaldehyde dissolved in tetrahydrofuran was studied by means of femtosecond UV/Vis and IR spectroscopy. Comparison was made of the spectral and temporal signatures for ~400 nm and ~260 nm excitation. The 400 nm excitation promotes NBA to its lowest excited singlet state of nπ* character whereas for 260 nm an upper excited state of ππ* character is addressed. On the picosecond time scale, the molecule undergoes hydrogen transfer, yielding a ketene intermediate, internal conversion recovering the starting material, and intersystem crossing. Time constants and yields of these processes are virtually not affected by the excitation wavelength. For 400 nm excitation a ~100 fs decay component seen in the 260 nm experiment is absent, indicating that this component is due to a ππ* → nπ* internal conversion. In contrast to its formation, the decay of the ketene intermediate is influenced by the excitation wavelength. This can be attributed to different amounts of vibrational excitation.     

Excitation of micelle-solubilized chlorophyll during the peroxidase-catalyzed aerobic oxidation of isonicotinic acid hydrazide

Addition of micelle (hexadecyl-trimethylammoniumbromide)-solubilized chlorophyll alpha to the isoniazid/peroxidase/Mn2+/O2 system promotes light emission, identified as chlorophyll fluorescence. Based on O2 consumption, the quantum yield of chlorophyll excitation to the S1 state exceeds 6 X 10(-6). At least part of the excitation has its origin in the conversion of an intermediate--presumably a diazene--to pyridine-4-carboxaldehyde. On the basis of the present and earlier results [K. Zinner, C. C. C. Vidigal, N. Durán, and G. Cilento (1977) Arch. Biochem. Biophys. 180, 452-458], it is inferred that isoniazid, an important chemotherapeutic and also a carcinogenic agent, can lead to a substantial generation of electronically excited states.     

Mechanism of photoacid generation for an arylcycloalkylsulfonium salt by ring opening and aryl cleavage.

The photochemistry of 1-(4-tert-butylphenyl)-tetrahydro-thiopyranium triflate (1), an arylcycloalkylsulfonium salt, was investigated in acetonitrile and methanol at low conversion in order to understand the reaction mechanism and its efficiency as photoacid generator. Both types of C-S bond in 1 are cleaved from the excited state. The heterolytic cleavage of the methylene C-S bond produces 4-t-BuC(6)H(4)S(CH(2))(4)CH(2)(+) by ring opening. The carbocation generates acid and arylalkenylsufides by elimination or 1,2 hydride shift and elimination. The predominantly homolytic cleavage of the aryl C-S bond yields 4-t-BuC(6)H(4)* and c-C(5)H(10)S(+)* as the fragmentation products. The radicals react with the solvent forming acid, pentamethylene sulfide and tert-butylbenzene. In methanol, the formation of 4-tert-butylanisole indicates a contribution of solvolysis in the excited state of 1 or a competing formation of free aryl cation by heterolytic fragmentation. The acid generation efficiency of 1 in solution (acetonitrile or methanol) is lower than that corresponding to triphenylsulfonium triflate (TPS OTf) under the same conditions. This suggests a pathway for the regeneration of 1 after photocleavage. The photochemistry of 1 is discussed in terms of the contribution of fragmentation and ring opening reaction paths to its overall acid generation efficiency, a key property in terms of its applications in resist formulations.     

Efficient energy transfer from the carotenoid S(2) state in a photosynthetic light-harvesting complex

Previously, the spatial arrangement of the carotenoid and bacteriochlorophyll molecules in the peripheral light-harvesting (LH2) complex from Rhodopseudomonas acidophila strain 10050 has been determined at high resolution. Here, we have time resolved the energy transfer steps that occur between the carotenoid's initial excited state and the lowest energy group of bacteriochlorophyll molecules in LH2. These kinetic data, together with the existing structural information, lay the foundation for understanding the detailed mechanisms of energy transfer involved in this fundamental, early reaction in photosynthesis. Remarkably, energy transfer from the rhodopin glucoside S(2) state, which has an intrinsic lifetime of approximately 120 fs, is by far the dominant pathway, with only a minor contribution from the longer-lived S(1) state.