Elucidating the Electronic Structure of High-Spin [MnIII(TPP)Cl] Using Magnetic Circular Dichroism Spectroscopy.

Manganese porphyrins are used as catalysts in the oxidation of olefins and nonactivated hydrocarbons. Key to these reactions are high-valent Mn-(di)oxo species, for which [Mn(Porph)(X)] serve as precursors. To elucidate their properties, it is crucial to understand the interaction of the Mn center with the porphyrin ligand. Our study focuses on simple high-spin [MnIII(TPP)X] (X = F, Cl, I, Br) complexes with emphasis on the spectroscopic properties of [MnIII(TPP)Cl], using variable-temperature variable-field magnetic circular dichroism spectroscopy and time-dependent density functional theory to help with band assignments. The optical properties of [MnIII(TPP)Cl] are complicated and unusual, with a Soret band showing a high-intensity feature at 21050 cm-1 and a broad band that spans 23200-31700 cm-1. The 15000-18500 cm-1 region shows the Cl(px/y) → dπ (CT(Cl,π)), Q band, and overlap-forbidden Cl(px/y)_dπ → dx2-y2 transitions that gain intensity from the strongly allowed π → π*(0) transition. The 20000-21000 cm-1 region displays the prominent pseudo A-type signal of the Soret band. The strongly absorbing features at 22500-28000 cm-1 exhibit A1u⟨79⟩/A2u⟨81⟩ → dπ, CT(Cl,π/σ), and symmetry-forbidden CT character, mixed with the π → π*(0) transition. The strong dx2-y2_B1g⟨80⟩ orbital interaction drives the ground-state MO mixing. Importantly, the splitting of the Soret band is explained by strong mixing of the porphyrin A2u(π)⟨81⟩ and the Cl(pz)_dz2 orbitals. Through this direct orbital pathway, the π → π*(0) transition acquires intrinsic metal-d → porphyrin CT character, where the π → π*(0) intensity is then transferred into the high-energy CT region of the optical spectrum. The heavier halide complexes support this conclusion and show enhanced orbital mixing and drastically increased Soret band splittings, where the 21050 cm-1 band shifts to lower energy and the high-energy features in the 23200-31700 cm-1 range increase further in intensity, compared to the chloro complex.

Effects of protein structure on iron-polypeptide vibrational dynamic coupling in cytochrome c.

Cytochrome c (Cyt c) has a heme covalently bound to the polypeptide via a Cys-X-X-Cys-His (CXXCH) linker that is located in the interface region for protein-protein interactions. To determine whether the polypeptide matrix influences iron vibrational dynamics, nuclear resonance vibrational spectroscopy (NRVS) measurements were performed on (57)Fe-labeled ferric Hydrogenobacter thermophilus cytochrome c-552, and variants M13V, M13V/K22M, and A7F, which have structural modifications that alter the composition or environment of the CXXCH pentapeptide loop. Simulations of the NRVS data indicate that the 150-325 cm(-1) region is dominated by NHis-Fe-SMet axial ligand and polypeptide motions, while the 325-400 cm(-1) region shows dominant contributions from ν(Fe-NPyr) (Pyr = pyrrole) and other heme-based modes. Diagnostic spectral signatures that directly relate to structural features of the heme active site are identified using a quantum chemistry-centered normal coordinate analysis (QCC-NCA). In particular, spectral features that directly correlate with CXXCH loop stiffness, the strength of the Fe-His interaction, and the degree of heme distortion are identified. Cumulative results from our investigation suggest that compared to the wild type (wt), variants M13V and M13V/K22M have a more rigid CXXCH pentapeptide segment, a stronger Fe-NHis interaction, and a more ruffled heme. Conversely, the A7F variant has a more planar heme and a weaker Fe-NHis bond. These results are correlated to the observed changes in reduction potential between wt protein and the variants studied here. Implications of these results for Cyt c biogenesis and electron transfer are also discussed.

Heme-protein vibrational couplings in cytochrome c provide a dynamic link that connects the heme-iron and the protein surface.

The active site of cytochrome c (Cyt c) consists of a heme covalently linked to a pentapeptide segment (Cys-X-X-Cys-His), which provides a link between the heme and the protein surface, where the redox partners of Cyt c bind. To elucidate the vibrational properties of heme c, nuclear resonance vibrational spectroscopy (NRVS) measurements were performed on (57)Fe-labeled ferric Hydrogenobacter thermophilus cytochrome c(552), including (13)C(8)-heme-, (13)C(5)(15)N-Met-, and (13)C(15)N-polypeptide (pp)-labeled samples, revealing heme-based vibrational modes in the 200- to 450-cm(-1) spectral region. Simulations of the NRVS spectra of H. thermophilus cytochrome c(552) allowed for a complete assignment of the Fe vibrational spectrum of the protein-bound heme, as well as the quantitative determination of the amount of mixing between local heme vibrations and pp modes from the Cys-X-X-Cys-His motif. These results provide the basis to propose that heme-pp vibrational dynamic couplings play a role in electron transfer (ET) by coupling vibrations of the heme directly to vibrations of the pp at the protein-protein interface. This could allow for the direct transduction of the thermal (vibrational) energy from the protein surface to the heme that is released on protein/protein complex formation, or it could modulate the heme vibrations in the protein/protein complex to minimize reorganization energy. Both mechanisms lower energy barriers for ET. Notably, the conformation of the distal Met side chain is fine-tuned in the protein to localize heme-pp mixed vibrations within the 250- to 400-cm(-1) spectral region. These findings point to a particular orientation of the distal Met that maximizes ET.

Theoretical and spectroscopic analysis of N,N'-diphenylurea and N,N'-dimethyl-N,N'-diphenylurea conformations.

Structural organization of macromolecules is highly dependent on the conformational propensity of the monomer units. Our goal is to systematically quantify differences in the conformational propensities of aromatic oligourea foldamer units. Specifically, we investigate the conformational propensities of N,N'-diphenylurea and N,N'-dimethyl-N,N'-diphenylurea in different media using a combination of theoretical methods, and infrared and nuclear magnetic resonance spectroscopies. Our results show variation in the conformational behavior upon adding methyl substituents on N,N'-diphenylurea, and varying the environments surrounding the compounds. Our energetic analyses and conformational distributions in the gas phase show predominance of the cis-trans and trans-trans conformations for N,N'-diphenylurea, while cis-cis conformation is favored for N,N'-dimethyl-N,N'-diphenylurea. In solution, our results support the trans-trans conformer as the predominant conformer for N,N'-diphenylurea, whereas the cis-cis and cis-trans forms are favored in N,N'-dimethyl-N,N'-diphenylurea. N,N'-Dimethyl-N,N'-diphenylurea also exhibits a more dynamic conformational behavior in solution, with constant fluctuations between cis-cis and cis-trans conformations. Our detailed quantitative analyses are an important aspect in fine-tuning desired conformations and dynamic properties of this class of oligomers by providing a molecular basis for the behavior at the monomeric level.

Ligand recruitment and spin transitions in the solid-state photochemistry of Fe(III)TPPCl.

We report evidence for the formation of long-lived photoproducts following excitation of iron(III) tetraphenylporphyrin chloride (Fe(III)TPPCl) in a 1:1 glass of toluene and CH(2)Cl(2) at 77 K. The formation of these photoproducts is dependent on solvent environment and temperature, appearing only in the presence of toluene. No long-lived product is observed in neat CH(2)Cl(2) solvent. A 2-photon absorption model is proposed to account for the power-dependent photoproduct populations. The products are formed in a mixture of spin states of the central iron(III) metal atom. Metastable six-coordinate high-spin and low-spin complexes and a five-coordinate high-spin complex of iron(III) tetraphenylporphyrin are assigned using structure-sensitive vibrations in the resonance Raman spectrum. These species appear in conjunction with resonantly enhanced toluene solvent vibrations, indicating that the Fe(III) compound formed following photoexcitation recruits a toluene ligand from the surrounding environment. Low-temperature transient absorption (TA) measurements are used to explain the dependence of product formation on excitation frequency in this photochemical model. The six-coordinate photoproduct is initially formed in the high-spin Fe(III) state, but population relaxes into both high-spin and low-spin state at 77 K. This is the first demonstration of coupling between the optical and magnetic properties of an iron-centered porphyrin molecule.

Influence of heme propionates on the nitrite reductase activity of myoglobin.

The heme propionates in myoglobin (Mb) form a H-bonding network among several residues within its second-sphere coordination, providing a key structural role towards Mb's functional properties. Our work aims to understand the role of the heme propionates on the nitrite reductase (NiR) activity (e.g. reduction of NO2- to NO) of this globin by studying an artificial dimethylester heme-substituted horse heart Mb (DME-Mb). The minor structural change brought about by esterification of the heme propionates causes the NiR rate to increase by more than over two-fold (5.6 ± 0.1 M-1 s-1) relative to wildtype (wt) Mb (2.3 ± 0.1 M-1 s-1). The lower pKa observed in DME-Mb may enhance the tendency of His64 towards protonation, therefore increasing the NiR rate. In addition, the nitrite binding constant (Knitrite) for DME-MbIII is greater than wt MbIII (350 M-1 versus 120 M-1). The disparity in the NiR activity correlates with the differences in electrostatic behavior, which influences the system's reactivity towards the approaching NO2- ion, and thus the formation of the FeII-NO2- intermediate.

Elucidating the role of the proximal cysteine hydrogen-bonding network in ferric cytochrome P450cam and corresponding mutants using magnetic circular dichroism spectroscopy.

Although extensive research has been performed on various cytochrome P450s, especially Cyt P450cam, there is much to be learned about the mechanism of how its functional unit, a heme b ligated by an axial cysteine, is finely tuned for catalysis by its second coordination sphere. Here we study how the hydrogen-bonding network affects the proximal cysteine and the Fe-S(Cys) bond in ferric Cyt P450cam. This is accomplished using low-temperature magnetic circular dichroism (MCD) spectroscopy on wild-type (wt) Cyt P450cam and on the mutants Q360P (pure ferric high-spin at low temperature) and L358P where the "Cys pocket" has been altered (by removing amino acids involved in the hydrogen-bonding network), and Y96W (pure ferric low-spin). The MCD spectrum of Q360P reveals fourteen electronic transitions between 15200 and 31050 cm(-1). Variable-temperature variable-field (VTVH) saturation curves were used to determine the polarizations of these electronic transitions with respect to in-plane (xy) and out-of-plane (z) polarization relative to the heme. The polarizations, oscillator strengths, and TD-DFT calculations were then used to assign the observed electronic transitions. In the lower energy region, prominent bands at 15909 and 16919 cm(-1) correspond to porphyrin (P) → Fe charge transfer (CT) transitions. The band at 17881 cm(-1) has distinct sulfur S(π) → Fe CT contributions. The Q band is observed as a pseudo A-term (derivative shape) at 18604 and 19539 cm(-1). In the case of the Soret band, the negative component of the expected pseudo A-term is split into two features due to mixing with another π → π* and potentially a P → Fe CT excited state. The resulting three features are observed at 23731, 24859, and 25618 cm(-1). Most importantly, the broad, prominent band at 28570 cm(-1) is assigned to the S(σ) → Fe CT transition, whose intensity is generated through a multitude of CT transitions with strong iron character. For wt, Q360P, and L358P, this band occurs at 28724, 28570, and 28620 cm(-1), respectively. The small shift of this feature upon altering the hydrogen bonds to the proximal cysteine indicates that the role of the Cys pocket is not primarily for electronic fine-tuning of the sulfur donor strength but is more for stabilizing the proximal thiolate against external reactants (NO, O(2), H(3)O(+)), and for properly positioning cysteine to coordinate to the iron center. This aspect is discussed in detail.

Mutation in the flavin mononucleotide domain modulates magnetic circular dichroism spectra of the iNOS ferric cyano complex in a substrate-specific manner.

We have obtained low-temperature magnetic circular dichroism (MCD) spectra for ferric cyano complexes of the wild type and E546N mutant of a human inducible nitric oxide synthase (iNOS) oxygenase/flavin mononucleotide (oxyFMN) construct. The mutation at the FMN domain has previously been shown to modulate the MCD spectra of the l-arginine-bound ferric iNOS heme (Sempombe, J.; et al. J. Am. Chem. Soc. 2009, 131, 6940-6941). The addition of l-arginine to the wild-type protein causes notable changes in the CN(-)-adduct MCD spectrum, while the E546N mutant spectrum is not perturbed. Moreover, the MCD spectral perturbation observed with l-arginine is absent in the CN(-) complexes incubated with N-hydroxy-L-arginine, which is the substrate for the second step of NOS catalysis. These results indicate that interdomain FMN-heme interactions exert a long-range effect on key heme axial ligand-substrate interactions that determine substrate oxidation pathways of NOS.

Favorable Protonation of the (µ-edt)[Fe(2)(PMe(3))(4)(CO)(2)(H-terminal)](+) Hydrogenase Model Complex Over Its Bridging µ-H Counterpart: A Spectroscopic and DFT Study.

The mechanism of hydrogen production in [FeFe] hydrogenase remains elusive. However, a species featuring a terminal hydride bound to the distal Fe is thought to be the key intermediate leading to hydrogen production. In this study, density functional theory (DFT) calculations on the terminal (H-term) and bridging (µ-H) hydride isomers of (µ-edt)-[Fe(2)(PMe(3))(4)(CO)(2)H](+) are presented in order to understand the factors affecting their propensity for protonation. Relative to H-term, µ-H is 12.7 kcal/mol more stable, which contributes to its decreased reactivity towards an acid. Potential energy surface (PES) calculations for the reaction of the H-term isomer with 4-nitropyridinium, a proton source, further reveal a lower activation energy barrier (14.5 kcal/mol) for H-term than for µ-H (29 kcal/mol). Besides these energetic considerations, the H-term isomer displays a key molecular orbital (MO <139>) that has a relatively strong hydride (1s) contribution (23%), which is not present in the µ-H isomer. This indicates a potential orbital control of the reaction of the hydride complexes with acid. The lower activation energy barrier and this key MO together control the overall catalytic activity of (µ-edt)[Fe(2)(PMe(3))(4)(CO)(2)(H-term)](+). Lastly, Raman and IR spectroscopy were performed in order to probe the ν(Fe-H) stretching mode of the two isomers and their deuterated counterparts. A ν(Fe-H) stretching mode was observed for the µ-H complex at 1220 cm(-1). However, the corresponding mode is not observed for the less stable H-term isomer.

Vibrational analysis of the model complex (mu-edt)[Fe(CO)(3)](2) and comparison to iron-only hydrogenase: the activation scale of hydrogenase model systems.

Research on simple [FeFe] hydrogenase model systems of type (mu-S(2)R)[Fe(CO)(3)](2) (R = C(2)H(4) (edt), C(3)H(6) (pdt)) which have been shown to function as robust electrocatalysts for proton reduction, provides a reference to understand the electronic and vibrational properties of the active site of [FeFe] hydrogenases and of more sophisticated model systems. In this study, the solution and solid state Raman spectra of (mu-edt)[Fe(CO)(3)](2) and of the corresponding (13)CO-labeled complex are presented and analyzed in detail, with focus on the nu(C=O) and nu(Fe-CO)/delta(Fe-C=O) vibrational regions. These regions are specifically important as vibrations involving CO ligands serve as probes for the "electron richness" of low-valent transition metal centers and the geometric structures of the complexes. The obtained vibrational spectra have been completely assigned in terms of the nu(C=O), nu(Fe-CO), and delta(Fe-C=O) modes, and the force constants of the important C=O and Fe-CO bonds have been determined using our Quantum Chemistry Centered Normal Coordinate Analysis (QCC-NCA). In the 400-650 cm(-1) region, fifteen mixed nu(Fe-CO)/delta(Fe-C=O) modes have been identified. The most prominent Raman peaks at 454, 456, and 483 cm(-1) correspond to a combination of nu(Fe-CO) stretching and delta(Fe-C=O) linear bending modes. The less intense peaks at 416 cm(-1) and 419 cm(-1) correspond to pure delta(Fe-C=O) linear bends. In the nu(C=O) region, the nu(C=O) normal modes at lower energy (1968 and 1964 cm(-1)) are almost pure equatorial (eq) nu(C=O)(eq) stretching vibrations, whereas the remaining four nu(C=O) normal modes show dominant (C=O)(eq) (2070 and 1961 cm(-1)) and (C=O)(ax) (2005 and 1979 cm(-1); ax = axial) contributions. Importantly, an inverse correlation between the f(C=O)(ax/eq) and f(Fe-CO)(ax/eq) force constants is obtained, in agreement with the idea that the Fe(I)-CO bond in these types of complexes is dominated by pi-backdonation. Compared to the reduced form of [FeFe] hydrogenase (H(red)), the nu(C=O) vibrational frequencies of (mu-edt)[Fe(CO)(3)](2) are higher in energy, indicating that the dinuclear iron core in (mu-edt)[Fe(CO)(3)](2) is less electron rich compared to H(red) in the actual enzyme. Finally, quantum yields for the photodecomposition of (mu-edt)[Fe(CO)(3)](2) have been determined.

Nuclear resonance vibrational spectroscopy applied to [Fe(OEP)(NO)]: the vibrational assignments of five-coordinate ferrous heme-nitrosyls and implications for electronic structure.

This study presents Nuclear Resonance Vibrational Spectroscopy (NRVS) data on the five-coordinate (5C) ferrous heme-nitrosyl complex [Fe(OEP)(NO)] (1, OEP(2-) = octaethylporphyrinato dianion) and the corresponding (15)N(18)O labeled complex. The obtained spectra identify two isotope sensitive features at 522 and 388 cm(-1), which shift to 508 and 381 cm(-1), respectively, upon isotope labeling. These features are assigned to the Fe-NO stretch nu(Fe-NO) and the in-plane Fe-N-O bending mode delta(ip)(Fe-N-O), the latter has been unambiguously assigned for the first time for 1. The obtained NRVS data were simulated using our quantum chemistry centered normal coordinate analysis (QCC-NCA). Since complex 1 can potentially exist in 12 different conformations involving the FeNO and peripheral ethyl orientations, extended density functional theory (DFT) calculations and QCC-NCA simulations were performed to determine how these conformations affect the NRVS properties of [Fe(OEP)NO]. These results show that the properties and force constants of the FeNO unit are hardly affected by the conformational changes involving the ethyl substituents. On the other hand, the NRVS-active porphyrin-based vibrations around 340-360, 300-320, and 250-270 cm(-1) are sensitive to the conformational changes. The spectroscopic changes observed in these regions are due to selective mechanical couplings of one component of E(u)-type (in ideal D(4h) symmetry) porphyrin-based vibrations with the in-plane Fe-N-O bending mode. This leads to the observed variations in Fe(OEP) core mode energies and NRVS intensities without affecting the properties of the FeNO unit. The QCC-NCA simulated NRVS spectra of 1 show excellent agreement with experiment, and indicate that conformer F is likely present in the samples of this complex investigated here. The observed porphyrin-based vibrations in the NRVS spectra of 1 are also assigned based on the QCC-NCA results. The obtained force constants of the Fe-NO and N-O bonds are 2.83-2.94 (based on the DFT functional applied) and about 12.15 mdyn/A, respectively. The electronic structures of 5C ferrous heme-nitrosyls in different model complexes are then analyzed, and variations in their properties based on different porphyrin substituents are explained. Finally, the shortcomings of different DFT functionals in describing the axial FeNO subunit in heme-nitrosyls are elucidated.

Energies of low-lying excited states of linear polyenes.

Room temperature absorption and emission spectra of the all-trans isomers of decatetraene, dodecapentaene, tetradecahexaene, and hexadecaheptaene have been obtained in a series of nonpolar solvents. The resolved vibronic features in the optical spectra of these model systems allow the accurate determination of S(0) (1(1)A(g)(-)) --> S(2) (1(1)B(u)(+)) and S(1) (2(1)A(g)(-)) --> S(0) (1(1)A(g)(-)) electronic origins as a function of solvent polarizability. These data can be extrapolated to predict the transition energies in the absence of solvent perturbations. The effects of the terminal methyl substituents on the transition energies also can be estimated. Franck-Condon maxima in the absorption and emission spectra were used to estimate differences between S(0) (1(1)A(g)(-)) --> S(1) (2(1)A(g)(-)) and S(0) (1(1)A(g)(-)) --> S(2) (1(1)B(u)(+)) electronic origins and "vertical" transition energies. Experimental estimates of the vertical transition energies of unsubstituted, all-trans polyenes in vacuum as a function of conjugation length are compared with long-standing multireference configuration interaction (MRCI) treatments and with more recent ab initio calculations of the energies of the 2(1)A(g)(-) (S(1)) and 1(1)B(u)(+) (S(2)) states.

Modulating the nitrite reductase activity of globins by varying the heme substituents: Utilizing myoglobin as a model system.

Globins, such as hemoglobin (Hb) and myoglobin (Mb), have gained attention for their ability to reduce nitrite (NO2(-)) to nitric oxide (NO). The molecular interactions that regulate this chemistry are not fully elucidated, therefore we address this issue by investigating one part of the active site that may control this reaction. Here, the effects of the 2,4-heme substituents on the nitrite reductase (NiR) reaction, and on the structures and energies of the ferrous nitrite intermediates, are investigated using Mb as a model system. This is accomplished by studying Mbs with hemes that have different 2,4-R groups, namely diacetyldeuteroMb (-acetyl), protoMb (wild-type (wt) Mb, -vinyl), deuteroMb (-H), and mesoMb (-ethyl). While trends on the natural charge on Fe and O-atom of bound nitrite are observed among the series of Mbs, the Fe(II)-NPyr (Pyr=pyrrole) and Fe(II)-NHis93 (His=histidine) bond lengths do not significantly change. Kinetic analysis shows increasing NiR activity as follows: diacetyldeuteroMb

Binding interaction of hypocrellin B to myoglobin: a spectroscopic and computational study.

Hypocrellin B (Hyp B), a perylenequinone naturally present in Hypocrella bambusae, is commonly used to treat a variety of diseases. Its versatile role in different biomedical applications necessitates a thorough investigation of its interaction with different biomolecules, particularly enzymes. To address this need, the binding mode of Hyp B to myoglobin (Mb) was studied using UV-visible absorption, emission, and synchronous fluorescence spectroscopies, as well as flexible docking simulations. Analyses of the absorbance and fluorescence data establish that Hyp B quenches tyrosine (Tyr) and tryptophan (Trp) fluorescence via the formation of two unique ground-state complexes on the surface of Mb, with one site being more energetically preferred than the other (the fraction of fluorophores accessible by Hyp B is 0.32). Molecular modeling simulations demonstrate preferential Hyp B binding at the Tyr103 site first, followed by the Trp7 site. In both cases, a ground-state complex is generated through H-bonding interaction between Hyp B and the respective residues, with the Tyr103 complex being more stable than that of the Trp7 complex. Synchronous fluorescence measurements indicate that the microenvironment surrounding Trp7 becomes more hydrophilic upon Hyp B interaction. This is evidenced by a red-shift of the band associated with this residue, while that of Tyr103 remains the same. Electrostatic potential surfaces reveal a more pronounced shift in electron density of Trp7 upon Hyp B binding compared to Tyr103. The binding constant of Hyp B to Mb is 1.21×10(5)M(-1), suggesting a relatively strong interaction between the ligand and enzyme.

Cation radicals of xanthophylls.

Carotenes and xanthophylls are well known to act as electron donors in redox processes. This ability is thought to be associated with the inhibition of oxidative reactions in reaction centers and light-harvesting pigment-protein complexes of photosystem II (PSII). In this work, cation radicals of neoxanthin, violaxanthin, lutein, zeaxanthin, beta-cryptoxanthin, beta-carotene, and lycopene were generated in solution using ferric chloride as an oxidant and then studied by absorption spectroscopy. The investigation provides a view toward understanding the molecular features that determine the spectral properties of cation radicals of carotenoids. The absorption spectral data reveal a shift to longer wavelength with increasing pi-chain length. However, zeaxanthin and beta-cryptoxanthin exhibit cation radical spectra blue-shifted compared to that of beta-carotene, despite all of these molecules having 11 conjugated carbon-carbon double bonds. CIS molecular orbital theory quantum computations interpret this effect as due to the hydroxyl groups in the terminal rings selectively stabilizing the highest occupied molecular orbitals of preferentially populated s-trans-isomers. The data are expected to be useful in the analysis of spectral results from PSII pigment-protein complexes seeking to understand the role of carotene and xanthophyll cation radicals in regulating excited state energy flow, in protecting PSII reaction centers against photoinhibition, and in dissipating excess light energy absorbed by photosynthetic organisms but not used for photosynthesis.

Symmetry control of radiative decay in linear polyenes: low barriers for isomerization in the S1 state of hexadecaheptaene.

The room temperature absorption and emission spectra of the 4-cis and all-trans isomers of 2,4,6,8,10,12,14-hexadecaheptaene are almost identical, exhibiting the characteristic dual emissions S1-->S0 (21Ag- --> 11Ag-) and S2-->S0 (11Bu+ --> 11Ag-) noted in previous studies of intermediate length polyenes and carotenoids. The ratio of the S1-->S0 and S2-->S0 emission yields for the cis isomer increases by a factor of approximately 15 upon cooling to 77 K in n-pentadecane. In contrast, for the trans isomer this ratio shows a 2-fold decrease with decreasing temperature. These results suggest a low barrier for conversion between the 4-cis and all-trans isomers in the S1 state. At 77 K, the cis isomer cannot convert to the more stable all-trans isomer in the 21Ag- state, resulting in the striking increase in its S1-->S0 fluorescence. These experiments imply that the S1 states of longer polyenes have local energy minima, corresponding to a range of conformations and isomers, separated by relatively low (2-4 kcal) barriers. Steady state and time-resolved optical measurements on the S1 states in solution thus may sample a distribution of conformers and geometric isomers, even for samples represented by a single, dominant ground state structure. Complex S1 potential energy surfaces may help explain the complicated S2-->S1 relaxation kinetics of many carotenoids. The finding that fluorescence from linear polyenes is so strongly dependent on molecular symmetry requires a reevaluation of the literature on the radiative properties of all-trans polyenes and carotenoids.

Linear polyenes: models for the spectroscopy and photophysics of carotenoids.

We have developed procedures for synthesizing dimethyl polyenes using living polymerization techniques and have initiated investigations of the spectroscopic properties of these molecules. Purification using high-performance liquid chromatography (HPLC) of the polyene mixtures resulting from the syntheses promises to provide all-trans polyenes with a wide range in the number of conjugated double bonds. Low temperature optical measurements on these model systems, both in glasses and in n-alkane mixed crystals, yield absorption and fluorescence spectra with considerably higher vibronic resolution than the spectra currently available for carotenoids with comparable conjugation lengths. The dimethyl polyenes thus allow a more precise exploration of the electronic properties of long, linearly conjugated systems. These studies can be used to verify the existence of low-lying singlet states predicted by theory and recently invoked to explain low-resolution fluorescence, Raman excitation spectra, and the transient absorption spectroscopy of carotenoids. Steady state and time-resolved optical studies of the dimethyl series will be used to better understand the energies and dynamics of the low energy electronic states relevant to the photochemistry and photobiology of all linearly conjugated systems.