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We have prepared a green chromophore, 1,7-bis(pyrrolidin-1-yl)-3,4:9,10-perylene-bis(dicarboximide) (5PDI), that exhibits photophysical and redox properties similar to those of chlorophyll a (Chl a), yet unlike Chl a, it can be easily functionalized and incorporated into a wide variety of biomimetic electron donor−acceptor systems. The N,N‘-dicyclohexyl derivative (5PDI) absorbs strongly (ε = 46 000 M^-1 cm^-1) at 686 nm in toluene and fluoresces at 721 nm with a 35% quantum yield. Additionally, 5PDI is both oxidized and reduced in CH2Cl2 at 0.57 V and −0.76 V vs SCE, respectively, making it a facile electron donor or acceptor. Rodlike covalent electron donor−acceptor molecules 5PDI−PI, 5PDI−NI, and 5PDI−PDI were prepared by linking the imide group of the 5PDI donor to pyromellitimide (PI), 1,8:4,5-naphthalenebis(dicarboximide) (NI), and 1,7-bis(3,5-di-tert-butylphenoxy)-3,4:9,10-perylene-bis(dicarboximide) (PDI) acceptors via an N−N bond. Following femtosecond laser excitation of 5PDI, 5PDI−PI, 5PDI−NI, and 5PDI−PDI in both toluene and 2-methyltetrahydrofuran, the formation and decay of their excited and radical ion pair states were monitored directly by transient absorption spectroscopy. We also report steady state emission and spectro-electrochemistry data for these molecules, which aid in elucidation of the transient spectra and the mechanisms of photoinduced charge separation. In toluene, charge separation occurs with high yield only in 5PDI−NI and 5PDI−PDI, whereas for 5PDI−PI charge separation is slow relative to excited-state decay of ^1*5PDI−PI indicating that ΔGCS ≅ 0. This fact provides a means of estimating the ionic radii of the photogenerated ions, which for perylene chromophores 5PDI and PDI are 7.6 ± 0.5 Å, whereas those of the PI and NI electron acceptors are 5.6 ± 0.5 Å. These ionic radii are used in turn to determine the free energies of reaction of the remaining molecules with the series. Electroabsorption measurements are used to show that the change in dipole moment, Δμ that occurs upon formation of ^1*5PDI is 3.5 D. The rates of charge separation in 5PDI−NI, 5PDI−PI, and 5PDI−PDI are compared to those of related donor−acceptor molecules having a 9-(pyrrolidin-1-yl)-perylene-3,4-dicarboximide (5PMI) donor. The 5PMI donor with Δμ = 15.4 D has a lowest excited singlet state with significantly higher charge-transfer character than does 5PDI, and has greater electron density near the imide group to which the acceptor is attached. The rate constants for charge separation from ^1*5PMI are greater than those from ^1*5PDI, which suggests that the rates of electron transfer from donors with CT excited states to an attached acceptor depend on the charge distribution in the CT excited state.

The photorefractive effect was observed in room-temperature nematic liquid crystal composites consisting of a eutectic mixture of 65% 4‘-(n-pentyl)-4-cyanobiphenyl and 35% 4‘-(n-octyloxy)-4-cyanobiphenyl doped with a donor−acceptor dyad in which 9-(N-pyrrolidinyl)perylene-3,4-dicarboximide is directly linked to 1,8:4,5-naphthalenetetracarboxydiimide. This dyad has a broad, intense absorption band that extends from 500 to 750 nm. Measurements of two-beam asymmetric energy exchange (beam-coupling) and the diffraction efficiency of the refractive index grating produced in these liquid crystal thin films were carried out. A comparison is made between the covalent donor−acceptor dyad and the corresponding noncovalent donor−acceptor pair. In each case the covalent dyad outperforms the noncovalent donor−acceptor pair. The results show that aminoaryleneimides are promising new charge donating chromophores that are capable of extending photorefractive performance of nematic liquid crystal composites into the near-infrared region of the spectrum.

DMPC (dimyristoyl-l-α-phosphatidylcholine) liposomes are used as artificial photosynthetic media to study the behavior of carotenoids. 8‘-Apo-β-caroten-8‘-al (I) and β-carotene (II) degrade faster under irradiation in DMPC liposomes than in organic solvents, which is possibly because vibrational deactivation of carotenoid excited states is less efficient in rigid lipid membranes. The lifetime of the first excited singlet state (S1) of I in DMPC liposomes is 27.2 ps, very close to that in 3-methylpentane (26.4 ps), but longer than its lifetime in EtOH (17.1 ps) or CH2Cl2 (14.1 ps). The lifetime of the S1 state of I in DMPC liposomes is as expected for an alkane environment. The lifetime of the S1 state of II in DMPC liposomes is 10.3 ps, very close to its lifetimes in 3-methylpentane (8.1 ps), EtOH (9.2 ps), and CH2Cl2 (8.5 ps). This independence of the S1 state lifetime of II from the matrix agrees with earlier conclusions. Carotenoid I can suppress the photodegradation of chlorophyll a (Chl a) in liposomes, which shows the protection role of I on Chl a under strong irradiation. In liposomes, Chl a fluorescence quenching by I is observed when using either the Qy band or the Soret band of Chl a as the excitation line.

The effect of terminal groups, polyene chain length, and solvent on the first excited singlet states (S1) of carotenoids was studied by steady-state and transient optical absorption spectroscopy, and AM1 semiempirical molecular orbital calculations. The carotenoids studied were ethyl 8‘-apo-β-caroten-8‘-oate (I), ethyl 6‘-apo-β-caroten-6‘-oate (II), ethyl 4‘-apo-β-caroten-4‘-oate (III), 8‘-apo-β-caroten-8‘-nitrile (IV), 6‘-apo-β-caroten-6‘-nitrile (V), 4‘-apo-β-caroten-4‘-nitrile (VI), 8‘-apo-β-caroten-8‘-al (VII), and 6‘-apo-β-caroten-6‘-al (VIII). Solvents were 3-methylpentane (3-MP) and MeCN. The effect of solvent on the S1 absorption maxima is similar to that on the ground state (S0) absorption maxima, which suggests that both effects stem from the same type of interaction, i.e., the dispersive interaction between carotenoids and solvents. Carotenoids with terminal CHO groups have S1 absorption maxima at longer wavelenths than those with terminal CN or CO2Et groups. The S1 absorption maxima are red-shifted with increasing polyene chain length. In the nonpolar solvent 3-MP, the S1 lifetimes of carotenoids depend mainly on the polyene chain length. With a one C&dbd;C bond increase, the S1 lifetime decreases by a factor of ca. 2 (ca. 24 ps for I, IV, and VII; 12 ps for II, V, and VIII; and 7 ps for III and VI). Terminal groups have little effect on the S1 lifetimes in 3-MP. However, in the polar solvent MeCN, carotenoids with terminal CHO groups have decreased S1 lifetimes (ca. 8 ps for VII and 6 ps for VIII), while carotenoids with terminal CN and CO2Et groups have essentially unchanged S1 lifetimes. This observation, along with the data of β-carotene and 7‘-apo-7‘,7‘-dicyano-β-carotene, suggests that polar solvents could decrease the S1 lifetimes of carotenoids, when, and only when, there is considerable charge transfer character in their excited states.

The spectroscopic properties and dynamics of the lowest excited singlet states of peridinin, fucoxanthin, neoxanthin, uriolide acetate, spheroidene, and spheroidenone in several different solvents have been studied by steady-state absorption and fast-transient optical spectroscopic techniques. Peridinin, fucoxanthin, uriolide acetate, and spheroidenone, which contain carbonyl functional groups in conjugation with the carbon−carbon π-electron system, display broader absorption spectral features and are affected more by the solvent environment than neoxanthin and spheroidene, which do not contain carbonyl functional groups. The possible sources of the spectral broadening are explored by examining the absorption spectra at 77 K in glassy solvents. Also, carotenoids which contain carbonyls have complex transient absorption spectra and show a pronounced dependence of the excited singlet state lifetime on the solvent environment. It is postulated that these effects are related to the presence of an intramolecular charge transfer state strongly coupled to the S1 (2¹Ag) excited singlet state. Structural variations in the series of carotenoids studied here make it possible to focus on the general molecular features that control the spectroscopic and dynamic properties of carotenoids.

The effect of dephasing and relaxation on electron transfer in bridged molecular systems is investigated using a simple molecular model. The interaction between the molecular system and the thermal environment is described on the level of the Redfield theory, modified when needed for the description of steady-state situations. Noting that transient as well as steady-state measurements are possible in such system, we discuss the relationship between the rates obtained from these different types of experiments and, in particular, the conditions under which these rates are the same. Also, a formal relation between the steady-state rate for electron transfer across a molecular bridge and the conductance of this bridge when placed between two metal contacts is established. The effect of dephasing and relaxation on the electron transfer is investigated, and new observations are made with regard to the transition from the superexchange to the thermal (hopping through bridge) regime of the transfer process. In particular, the rate is temperature-independent in the superexchange regime, and its dependence on the bridge length (N) is exponential, exp(−βN). The rate behaves like (α1 + α2N)^-1 exp(−ΔE/kBT) beyond a crossover value of N, where ΔE is the energy gap between the donor/acceptor and the bridge levels, and where α1 and α2 are characteristic times for activation onto the bridge and diffusion in the bridge, respectively. We find that, in typical cases, α1 ≫ α2, and therefore, a region of very weak N dependence is expected before the Ohmic behavior, N^-1, is established for large enough N. In addition, a relatively weak exponential dependence, exp(−αN), is expected for long bridges if competing processes capture electrons away from the bridge sites. Finally, we consider ways to distinguish experimentally between the thermal and the tunneling routes.

The effects of metal ion chelation to the 2,2‘-bipyridine (bpy) groups on the photophysics and exciton dynamics of two conjugated polymers 1 and 2 in solution are investigated. The structures of polymers 1 and 2 have 2,2‘-bipyridyl-5-vinylene units that alternate with one and three 2,5-bis(n-decyloxy)-1,4-phenylenevinylene monomer units, respectively. The photophysics and exciton dynamics of metalated polymers 1 and 2 are compared to those of the metal-free polymers (Chen et al. J. Phys. Chem. A 1999, 103, 4341−4351). The origins of ionochromic effects due the metal ion chelation were studied using both steady-state and transient optical spectroscopy, and the results indicate that both conformational flattening and participation of π electrons from the metal in the π-conjugation of the polymer backbone play important roles in metal ion binding induced red shifts in absorption and photoluminescence spectra. The photoluminescence properties of the metalated polymers are determined by the metal ion electronic structures, where the closed shell Zn²⁺-bound polymer 2 has an increased photoluminescence quantum yield and the corresponding open shell Ni²⁺- or Fe³⁺-bound polymers have quenched photoluminescence due to spin−orbit coupling. The dual character of metalated polymer 2 as a conjugated polymer and as a metal−bpy complex is discussed. In addition, the structures of metal ion binding sites are studied via X-ray absorption fine structure (XAFS) and are related to the photophysical properties of the metalated polymers.

The branched donor−acceptor triad 4 and tetrad 5 were synthesized to study the possibility of controlling the direction of electron transfer in a divergent array of electron acceptors using ultrafast laser pulses. Compounds 4 and 5 employ 1,3,5-triaminobenzene as the central branch point. In 4, a 4-(N-piperidinyl)-1,8-naphthaleneimide electron donor (ANI) was attached to the 1 position and two electron acceptors, 1,8:4,5-naphthalenediimide, NI, and pyromellitimide, PI, were attached to the 3 and 5 positions of the central benzene ring. In 5, the terminal end of the PI acceptor is functionalized with an additional NI molecule. Selective excitation of ANI in 4 and 5 with 400 nm, 130 fs laser pulses results in exclusive formation of NI^-−ANI⁺−PI and NI^-−ANI⁺−PI−NI, respectively, with τ = 115 ps, and a quantum yield of 0.99. Excitation of NI^- with 480 nm, 130 fs laser pulses produces the excited doublet state *NI^-, which transfers an electron to PI on the second branch of the benzene ring with a time constant of τ = 600 fs in 4 and τ = 750 fs in 5. The overall quantum yields for the two step process are 0.44 and 0.36 in 4 and 5, respectively. The resultant state NI−ANI⁺−PI^- in 4 undergoes a charge shift reaction returning to the initial ion pair state NI^-−ANI⁺−PI with τ = 400 ps, while the corresponding state NI−ANI⁺−PI^-−NI in 5 undergoes a charge shift with τ = 200 ps to yield NI−ANI⁺−PI−NI^-, which in turn exhibits a 2000 ps lifetime. These results show that once the electron has been switched to the branch containing the thermodynamically uphill PI acceptor, the electron will cascade down that branch to other acceptors that are more easily reduced. Model compounds 1−3, which were synthesized to aid in characterization of the switching dynamics, are also discussed. Photochemical control of directional charge transport may make it possible to design networks of organic molecules for information processing.

The pheophytin a Qx spectral region of the isolated photosystem II reaction center was investigated at 7 K using femtosecond transient absorption spectroscopy. At this temperature, uphill energy transfer, which greatly complicates the interpretation of the kinetics at or near room temperature, should be essentially shut off. Low-energy (~100 nJ) pulses at 661 and 683 nm were used to excite the short-wavelength and long-wavelength sides of the composite Qy band, providing preferential excitation of the accessory pigment pool and P680, respectively. The data analysis uses a background subtraction technique developed earlier (Greenfield et al. J. Phys. Chem. B 1997, 101, 2251−2255) to remove the kinetic components of the data that are due to the large time-dependent changes in the background that are present in this spectral region. The instantaneous amplitude of the bleach of the pheophytin a Qx band with 683 nm excitation is roughly two-thirds of its final amplitude, providing strong evidence of a multimer description of the reaction center core. The subsequent growth of the bleach shows biphasic kinetics, similar to our earlier results at 278 K. The rate constant of the faster component is (5 ps)^-1 for 683 nm excitation (a factor of almost two faster than at 278 K), and represents the intrinsic rate constant for charge separation. The bleach growth with 661 nm excitation is also biphasic; however, the faster component appears to be a composite of a (5 ps)^-1 component corresponding to charge separation following subpicosecond energy transfer to the long-wavelength pigments and a roughly (22 ps)^-1 component corresponding to charge separation limited by slow energy transfer. The combined quantum yield for these two energy transfer processes is near unity. For both excitation wavelengths, there is also a roughly (100 ps)^-1 component to the bleach growth. Exposure to high excitation energies (≥1 μJ) at 683 nm results in a substantial permanent loss of ground-state absorption at 680 nm. The transient behavior of these degraded samples is also examined and is consistent with the (5 ps)^-1 rate constant for charge separation. Our results are compared to other low-temperature transient absorption and hole burning studies, as well as to our 278 K results.

The spectroscopic properties and dynamic behavior of peridinin in several different solvents were studied by steady-state absorption, fluorescence, and transient optical spectroscopy. The lifetime of the lowest excited singlet state of peridinin is found to be strongly dependent on solvent polarity and ranges from 7 ps in the strongly polar solvent trifluoroethanol to 172 ps in the nonpolar solvents cyclohexane and benzene. The lifetimes show no obvious correlation with solvent polarizability, and hydrogen bonding of the solvent molecules to peridinin is not an important factor in determining the dynamic behavior of the lowest excited singlet state. The wavelengths of emission maxima, the quantum yields of fluorescence, and the transient absorption spectra are also affected by the solvent environment. A model consistent with the data and supported by preliminary semiempirical calculations invokes the presence of a charge transfer state in the excited state manifold of peridinin to account for the observations. The charge transfer state most probably results from the presence of the lactone ring in the π-electron conjugation of peridinin analogous to previous findings on aminocoumarins and related compounds. The behavior of peridinin reported here is highly unusual for carotenoids, which generally show little dependence of the spectral properties and lifetimes of the lowest excited singlet state on the solvent environment.

Time domain energy dispersive X-ray absorption spectroscopy provides a powerful probe of changes in molecular structure that occur during photoinduced electron transfer reactions. In this study a diporphyrin model for electron transfer within modified heme proteins is examined using this technique. Transient optical absorption experiments show that photoinduced electron transfer from the lowest excited triplet state of a Zn porphyrin attached at a fixed 25 Å distance by means of a long spacer molecule to an Fe(III) porphyrin occurs with a 6 ms time constant at 77 K. Time domain X-ray absorption measurements are consistent with transient reduction of Fe(III) to Fe(II) that is accompanied by substantial weakening of the bond between the Fe atom and a pyridine molecule ligated to it. Coupling of ligand loss to reduction of Fe(III) to Fe(II) provides a means of stabilizing the reduced intermediate.

Transient absorption studies of intramolecular photoinduced charge separation and thermal charge recombination were carried out on a molecule consisting of a 4-(N-pyrrolidinyl)naphthalene-1,8-imide donor (PyrNI) covalently attached to a pyromellitimide acceptor (PI) dissolved in the liquid crystal N-(4-methoxybenzylidene)-4-butylaniline (MBBA). In the isotropic phase, just above the nematic to isotropic phase transition, ordered microdomains exist that give rise to solvation dynamics that consist of collective reorientational motions of the microdomains and relatively faster reorientations of molecules within the domains. The different temperature dependence of these responses permits the assignment of collective or molecular solvent motions as mediators of the charge separation and recombination reactions. We find for PyrNI−PI in MBBA that charge separation is adiabatic and controlled by the collective microdomain reorientations, whereas the charge recombination is nonadiabatic and controlled by the faster molecular motions within the domains. The data are compared to results for charge separation and recombination within PyrNI−PI dissolved in 4‘-pentyl-4-cyanobiphenyl (5CB) under similar conditions. In 5CB, the opposite behavior was observed, i.e., the charge separation is nonadiabatic while the charge recombination is adiabatic. The results are interpreted as arising from the different signs of the dielectric anisotropies of the two liquid crystals, where that of MBBA is negative while that of 5CB is positive.

For a series of alanine-based peptides having 1−3 amino acid residues as spacers, the chromophore, pyrenesulfonyl (Pyr), has been attached at the N-terminus and an electron donor, dimethyl-1,4-benzenediamine (DMPD), covalently bound at the C-terminus. Evidence for an intramolecular charge-transfer interaction involving the electron donor and acceptor groups has been obtained from absorption spectra. Intramolecular electron transfer involving the end groups, Pyr (electron acceptor) and DMPD (electron donor) has been confirmed by ultrafast pump−probe methods. The radical-ion pair states that are generated on Ti/sapphire laser excitation at 400 nm decay in the picosecond to nanosecond time domain and generally show multiexponential decay kinetics. These rates of charge recombination are among the fastest yet observed involving electron transfer between terminal groups for peptide oligomers. The falloff of rate constants for ion pair recombination is irregular in terms of the through-bond distance that separates Pyr and DMPD groups for the various peptide links; i.e., back electron transfer remains fast for the tripeptide, Pyr-Ala-Ala-Ala-DMPD, despite an average through-bond distance between photoactive groups that reaches 18 Å. Molecular modeling studies show that the peptides are free to adopt conformations in essentially random fashion, without showing evidence for long range ordering of the peptide chain.

Transient absorption measurements of the pheophytin a anion band and Qx band bleach region using preferential excitation of P680 are performed on isolated photosystem II reaction centers to determine the effective rate constant for charge separation. A novel analysis of the Qx band bleach region explicitly takes the changing background into account in order to directly measure the rate of growth of the bleach. Both spectral regions reveal biphasic kinetics, with a ca. (8 ps)^-1 rate constant for the faster component, and a ca. (50 ps)^-1 rate constant for the slower component. We propose that the faster component corresponds to the effective rate constant for charge separation from within the equilibrated reaction center core and provides a lower limit for the intrinsic rate constant for charge separation. The slower component corresponds to charge separation that is limited by slow energy transfer from a long-wavelength accessory chlorophyll a.

Spheroidene and a series of spheroidene analogues with extents of π-electron conjugation ranging from 7 to 13 carbon−carbon double bonds were incorporated into the B850 light-harvesting complex of Rhodobacter sphaeroides R-26.1. The structures and spectroscopic properties of the carotenoids and the dynamics of energy transfer from the carotenoid to bacteriochlorophyll (BChl) in the B850 complex were studied by using steady-state absorption, fluorescence, fluorescence excitation, resonance Raman, and time-resolved absorption spectroscopy. The spheroidene analogues used in this study were 5‘,6‘-dihydro-7‘,8‘-didehydrospheroidene, 7‘,8‘-didehydrospheroidene, and 1‘,2‘-dihydro-3‘,4‘,7‘,8‘-tetradehydrospheroidene. These data, taken together with results from 3,4,7,8-tetrahydrospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-dihydrospheroidene, and spheroidene already published (Frank, H. A.; Farhoosh, R.; Aldema, M. L.; DeCoster, B.; Christensen, R. L.; Gebhard, R.; Lugtenburg, J. Photochem. Photobiol. 1993, 57, 49. Farhoosh, R.; Chynwat, V.; Gebhard, R.; Lugtenburg, J.; Frank, H. A. Photosynth. Res. 1994, 42, 157), provide a systematic series of molecules for understanding the molecular features that determine the mechanism of energy transfer from carotenoids to BChl in photosynthetic bacterial light-harvesting complexes. The data support the hypothesis that only carotenoids having 10 or less carbon−carbon double bonds transfer energy via their 2¹Ag (S1) states to BChl to any significant degree. Energy transfer via the 1¹Bu (S2) state of the carotenoid becomes more important than the S1 route as the number of conjugated carbon−carbon double bonds increases. The results also suggest that the S2 state associated with the Qx transition of the B850 BChl is the most likely acceptor state for energy transfer originating from both the 2¹Ag (S1) and 1¹Bu (S2) states of all carotenoids.