A theory of nonvertical triplet energy transfer in terms of accurate potential energy surfaces: the transfer reaction from pi,pi* triplet donors to 1,3,5,7-cyclooctatetraene

Triplet energy transfer (TET) from aromatic donors to 1,3,5,7-cyclooctatetraene (COT) is an extreme case of "nonvertical" behavior, where the transfer rate for low-energy donors is considerably faster than that predicted for a thermally activated (Arrhenius) process. To explain the anomalous TET of COT and other molecules, a new theoretical model based on transition state theory for nonadiabatic processes is proposed here, which makes use of the adiabatic potential energy surfaces (PES) of reactants and products, as computed from high-level quantum mechanical methods, and a nonadiabatic transfer rate constant. It is shown that the rate of transfer depends on a geometrical distortion parameter gamma=(2g(2)/kappa(1))(1/2) in which g stands for the norm of the energy gradient in the PES of the acceptor triplet state and kappa(1) is a combination of vibrational force constants of the ground-state acceptor in the gradient direction. The application of the model to existing experimental data for the triplet energy transfer reaction to COT from a series of pi,pi(*) triplet donors, provides a detailed interpretation of the parameters that determine the transfer rate constant. In addition, the model shows that the observed decrease of the acceptor electronic excitation energy is due to thermal activation of C=C bond stretchings and C-C bond torsions, which collectively change the ground-state COT bent conformation (D(2d)) toward a planar triplet state (D(8h)).     

Influence of triplet—triplet excitation transfer on the decay function of donor luminescence

The triplet—triplet electronic excitation transfer from benzophenone to naphthalene in rigid solutions has been investigated by measuring the non-exponential decay curve of benzophenone photphorescence using a laser pulse for excitation. The results of measurements were satisfactorily reproduced by Inokuti—Hirayama's formula based on Dexter's theory of exchange interaction.     

Ab inito study on triplet excitation energy transfer in photosynthetic light-harvesting complexes

We have studied the triplet energy transfer (TET) for photosynthetic light-harvesting complexes, the bacterial light-harvesting complex II (LH2) of Rhodospirillum molischianum and Rhodopseudomonas acidophila, and the peridinin-chlorophyll a protein (PCP) from Amphidinium carterae. The electronic coupling factor was calculated with the recently developed fragment spin difference scheme (You and Hsu, J. Chem. Phys. 2010, 133, 074105), which is a general computational scheme that yields the overall coupling under the Hamiltonian employed. The TET rates were estimated based on the couplings obtained. For all light-harvesting complexes studied, there exist nanosecond triplet energy transfer from the chlorophylls to the carotenoids. This result supports a direct triplet quenching mechanism for the photoprotection function of carotenoids. The TET rates are similar for a broad range of carotenoid triplet state energy, which implies a general and robust TET quenching role for carotenoids in photosynthesis. This result is also consistent with the weak dependence of TET kinetics on the type or the number of π conjugation lengths in the carotenoids and their analogues reported in the literature. We have also explored the possibility of forming triplet excitons in these complexes. In B850 of LH2 or the peridinin cluster in PCP, it is unlikely to have triplet exciton since the energy differences of any two neighboring molecules are likely to be much larger than their TET couplings. Our results provide theoretical limits to the possible photophysics in the light-harvesting complexes.     

Redox-active tyrosine residues: role for the peptide bond in electron transfer

Photosystem II (PSII) catalyzes the light-driven oxidation of water and reduction of plastoquinone. In PSII, redox-active tyrosine Z conducts electrons between the primary chlorophyll donor and the manganese cluster, which is the catalytic site. In this report, difference FT-IR spectroscopy is used to show that oxidation of redox-active tyrosine Z causes perturbations of the peptide bond. PSII data were acquired on control samples, as well as samples in which tyrosine was 2H4 (ring)-labeled. Comparison to model compound data, acquired both from tyrosinate and its 2H4 isotopomer, was performed. The PSII FT-IR spectrum exhibited vibrational bands that are assignable to imide and amide vibrational modes. In previous work, we have shown that oxidation of tyrosinate perturbs the terminal amino group of tyrosinate (Ayala, I.; Range, K.; York, D.; Barry, B. A. J. Am. Chem. Soc. 2002, 124, 5496-5505). Density functional calculations on tyrosinate supported the interpretation that the perturbation is due to spin delocalization onto the amino group. In tyrosine-containing dipeptides, perturbations of the peptide bond were observed. Therefore, the imide and amide perturbations observed here are attributed to spin delocalization into the peptide bond in PSII. Migration of the electron hole in PSII may be consistent with peptide bond involvement in tyrosyl radical-based electron-transfer reactions.     

Expanding excitation wavelengths for azobenzene photoswitching into the near-infrared range via endothermic triplet energy transfer

Developing azobenzene photoswitches capable of selective and efficient photoisomerization by long-wavelength excitation is an enduring challenge. Herein, rapid isomerization from the Z- to E-state of two ortho-functionalized bistable azobenzenes with near-unity photoconversion efficiency was driven by triplet energy transfer upon red and near-infrared (up to 770 nm) excitation of porphyrin photosensitizers in catalytic micromolar concentrations. We show that the process of triplet-sensitized isomerization is efficient even when the sensitizer triplet energy is substantially lower (>200 meV) than that of the azobenzene used. This makes the approach applicable for a wide variety of sensitizer-azobenzene combinations and enables the expansion of excitation wavelengths into the near-infrared spectral range. Therefore, indirect excitation via endothermic triplet energy transfer provides efficient and precise means for photoswitching upon 770 nm near-infared light illumination with no chemical modification of the azobenzene chromophore, a desirable feature in photocontrollable biomaterials.

Triplet energy transfer enables efficient Z-to-E photoswitching of azobenzenes even with near-infrared light. Ultrafast intersystem crossing of azobenzene makes the process entropy-driven and enables the use of endothermic sensitizer-azobenzene pairs.     

Decisive role of π conjugation in the central bond length shortening of butadiene

The influence of π conjugation and hyperconjugation in the shortening of the central C-C bond in butadiene with respect to a C_sp³-C_sp³ bond in alkanes is theoretically investigated by a direct analysis. As expected from simple π models it is demonstrated that the origin of this shortening is mainly due to π conjugation in the planar s-trans conformation while hyperconjugation largely compensates the lack of π conjugation in the perpendicular form and leads to a similar shortening of the central bond, These results contradict one of the conclusions of a recent ab initio study.     

Regioselective bond cleavage in the dissociative electron transfer to benzyl thiocyanates: the role of radical/ion pair formation

Important aspects of the electrochemical reduction of a series of substituted benzyl thiocyanates were investigated. A striking change in the reductive cleavage mechanism as a function of the substituent on the aryl ring of the benzyl thiocyanate was observed, and more importantly, a regioselective bond cleavage was encountered. A reductive alpha-cleavage (CH(2)-S bond) was seen for cyano and nitro-substituted benzyl thiocyanates leading to the formation of the corresponding nitro-substituted dibenzyls. With other substituents (CH(3)O, CH(3), H, Cl, and F), both the alpha (CH(2)-S) and the beta (S-CN) bonds could be cleaved as a result of an electrochemical reduction leading to the formation of the corresponding substituted monosulfides, disulfides, and toluenes. These final products are generated through either a protonation or a nucleophilic reaction of the two-electron reduction-produced anion on the parent molecule. The dissociative electron transfer theory and its extension to the formation/dissociation of radical anions, as well as its extension to the case of strong in-cage interactions between the produced fragments ("sticky" dissociative electron transfer (ET)), along with the theoretical calculation results helped rationalize (i) the observed change in the ET mechanism, (ii) the dissociation of the radical anion intermediates formed during the electrochemical reduction of the nitro-substituted benzyl thiocyanates, and more importantly (iii) the regioselective reductive bond cleavage.     

A possible contribution to the population of triplet states in electron impact measurements of total cross sections for triplet state excitation

It is proposed that low energy secondary electrons produced in medium energy electron impact experiments may play an important role in the excitation of triplet states even at low sample gas densities. A model calculation is carried out which shows that the population of the 4³S, S³P and 4³D triplet states in He from secondary electron excitation can be comparable to the population of these states by direct excitation at an incident electron energy of 1000 eV and sample gas pressures as low as 10^?3 torr. The model calculations show that the secondary electron mechanism becomes more important as the incident energy increases and that the produced populations are of a similar magnitude for the 3³P and 4³D states which in turn are about a factor of 4 larger than the population found for the 4³S state. The results indicate that the effect of secondary electron excitation in careful experimental measurements of electron impact total triplet state cross sections may have to be considered when incident electron excitation energies in the range of 1 keV or higher are used.     

Saturable absorption dynamics in the triplet system and triplet excitation induced singlet fluorescence of some organic molecules

The triplet saturable absorption behaviour of the xanthene dyes eosin Y, erythrosin B, and rose bengal and of the fullerene molecule C₇₀ is studied. The molecules are excited to the S₁-state by intense picosecond pulses (wavelength λ_P=527 nm). They relax dominantly to the triplet system by intersystem crossing. The triplet–triplet saturable absorption is investigated with time-delayed intense picosecond pulses (wavelength λ_L=1054 nm) in the transparency region of the molecules in the singlet ground state. Higher excited-state triplet absorption cross-sections and higher excited-state triplet relaxation times are determined by numerical simulation of the experimental results. Time-resolved fluorescence measurements reveal higher excited-state triplet to singlet back-intersystem-crossing and multi-step triplet photoionization. Additionally the two-photon absorption cross-sections at λ_L=1054 nm are determined by measurement of the fundamental pulse two-photon induced fluorescence relative to the second-harmonic pulse single-photon induced fluorescence.     

Spectroscopic evidence for triplet excitation energy transfer among carotenoids in the LH2 complex from photosynthetic bacterium Rhodopseudomonas palustris

The LH2 complex from Rhodopsudomonas (Rps.) palustris is unique in the heterogeneous carotenoid compositions. The dynamics of triplet excited state Carotenoids (3Car*) has been investigated by means of sub-microsecond time-resolved absorption spectroscopy both at physiological temperature (295 K) and at cryogenic temperature (77 K). Broad and asymmetric Tn←T-1 transient absorption was observed at room temperature following the photo-excitation of Car at 532 nm, which suggests the contribution from various carotenoid compositions having different numbers of conjugated C=C double bonds (Nc=c). The triplet absorption bands of different carotenoids, which superimposed at room temperature, could be clearly distinguished upon decreasing the temperature down to 77 K. At room temperature the shorter-wavelength side of the main Tn←T1 absorption band decayed rapidly to reach a spectral equilibration with a characteristic time constant of-1 μs, the same spectral dynamics, however, was not observed at 77 K. The     

Rapid cleavage of the naphthylmethyl-oxygen bond in higher triplet excited states

Rapid cleavage of the naphthylmethyl-oxygen bond of 1- and 2-[(4-benzoylphenoxy)methyl]naphthalenes in higher triplet excited states occurred within a laser flash of 5 ns to give 1- and 2-naphthylmethyl radicals with formation quantum yields of 0.042 +/- 0.004 and 0.020 +/- 0.002, respectively, during two-colour two-laser flash photolysis.     

The role of charge transfer in the hydrogen bond cooperative effect of cis-N-methylformamide oligomers

Two accumulating molecular systems have been designed to investigate the cooperative effect of hydrogen bonding in theory. The first system included a series of linear oligomers of cis-N-methylformamide (c-NMF) molecules. Substantial cooperative effect has been confirmed in the electronic structures and energies of the hydrogen bonds in them as shown by the results obtained using the B3LYP method at the level of cc-pVTZ basis sets. Such a cooperative effect gradually increases with the growth of the c-NMF oligomer. The second system included a series of modified c-NMF trimers whose central c-NMF molecules contained insertion fragments of varying structural and electrical compositions. On the basis of an examination of the structures and charge populations of the c-NMF oligomers in these two systems, a mechanism of the cooperative effect of hydrogen bonding in these systems based on charge flow in the c-NMF molecules is proposed. The results from the second system of c-NMF trimers were particularly instrumental in formulating this mechanism, because the charge flows between the C=O and N-H groups in the modified c-NMF molecule of these trimers were dampened by the various molecular insertions. A clear correlation between the degree of charge flow dampening from each inserted fragment and the magnitude of the cooperative effect of hydrogen bonding was observed. On the basis of an analysis of the electronic structural characteristics of the molecular fragments, we conclude that the charge flow between the hydrogen bond donor and acceptor groups in the c-NMF molecule is the most important factor inducing the cooperative effect of hydrogen bonding.     

Quantum dynamics study of fulvene double bond photoisomerization: the role of intramolecular vibrational energy redistribution and excitation energy

The double bond photoisomerization of fulvene has been studied with quantum dynamics calculations using the multi-configuration time-dependent Hartree method. Fulvene is a test case to develop optical control strategies based on the knowledge of the excited state decay mechanism. The decay takes place on a time scale of several hundred femtoseconds, and the potential energy surface is centered around a conical intersection seam between the ground and excited state. The competition between unreactive decay and photoisomerization depends on the region of the seam accessed during the decay. The dynamics are carried out on a four-dimensional model surface, parametrized from complete active space self-consistent field calculations, that captures the main features of the seam (energy and locus of the seam and associated branching space vectors). Wave packet propagations initiated by single laser pulses of 5-25 fs duration and 1.85-4 eV excitation energy show the principal characteristics of the first 150 fs of the photodynamics. Initially, the excitation energy is transferred to a bond stretching mode that leads the wave packet to the seam, inducing the regeneration of the reactant. The photoisomerization starts after the vibrational energy has flowed from the bond stretching to the torsional mode. In our propagations, intramolecular energy redistribution (IVR) is accelerated for higher excess energies along the bond stretch mode. Thus, the competition between unreactive decay and isomerization depends on the rate of IVR between the bond stretch and torsion coordinates, which in turn depends on the excitation energy. These results set the ground for the development of future optical control strategies.     

Theoretical study of singlet and triplet excitation energies in oligothiophenes

We have analyzed singlet and triplet excitation energies in oligothiophenes (up to five rings) using time-dependent density-functional theory (TD-DFT) with different exchange-correlation functionals and compared them with results from the approximate coupled-cluster singles and doubles model (CC2) and experimental data. The excitation energies have been calculated in geometries obtained by TD-DFT optimization of the lowest excited singlet state and in the ground-state geometries of the neutral and anionic systems. TD-DFT methods underestimate photoluminescence energies but the energy difference between singlet and triplet states shows trends with the chain-length similar to CC2. We find that the second triplet excited state is below the first singlet excited state for long oligomers in contrast with the previous assignment of Rentsch et al. (Phys.Chem. Chem. Phys. 1999, 1, 1707). Their photodetachment photoelectron spectroscopy measurements are better described by considering higher triplet excited states.     

Singlet sensitization of the norbornadiene rearrangement: excitation energy cascade potentially involving triplet exciplexes

Valence isomerization of dimethyl bicyclo[2.2.1]hepta-2,5-dien-2,3-dicarboxylate is induced on quenching the fluorescence of a variety of sensitizers. Quantum efficiencies for rearrangement depend on the triplet energy of of the sensitizer.     

On the role of vibrational excitation in electron transfer reactions with large negative free energies

Apparent disagreement of results of fluorescence quenching with Marcus' theory for outer-sphere electron transfer in polar media is explained, still within the general framework of Marcus' theory, by including vibrational transitions.