Investigations on the Photoreactions of Phenothiazine and Phenoxazine in Presence of 9-cyanoanthracene by Using Steady State and Time Resolved Spectroscopic Techniques

By using electrochemical, steady state and time resolved (fluorescence lifetime and transient absorption) spectroscopic techniques, detailed investigations were made to reveal the mechanisms of charge separation or forward electron transfer reactions within the electron donor phenothiazine (PTZH) or phenoxazine (PXZH) and well known electron acceptor 9-cyanoanthracene (CNA). The transient absorption spectra suggest that the charge separated species formed in the excited singlet state resulted from intermolecular photoinduced electron transfer reactions within the donor PTZH (or PXZH) and CNA acceptor relaxes to the corresponding triplet state. Though alternative mechanisms of via formations of contact neutral radical by H-transfer reaction have been proposed but the observed results obtained from the time resolved measurements indicate that the regeneration of ground state reactants is primarily responsible due to direct recombination of triplet contact ion-pair (CIP) or solvent-separated ion-pair (SSIP).     

Fluorescence quenching of 9-cyanoanthracene in presence of zinc tetraphenylporphyrin in a polar liquid medium

Steady-state and time-resolved techniques are used to study photoinduced electron and/or excitational energy transfer processes involved within a novel donor (zinc tetraphenylporphyrin)-acceptor (9-cyanoanthracene) system in a polar liquid medium (acetonitrile) at the ambient temperature (300 K). After photoexcitation of 9-cyanoanthracene, its fluorescence emission as well as lifetime are found to be quenched in presence of zinc tetraphenylporphyrin. The fluorescence quenching is ascribed to be due to the combined effect of electron transfer from zinc tetraphenylporphyrin to 9-cyanoanthracene and energy transfer (radiative as well as non-radiative) from 9-cyanoanthracene to zinc tetraphenylporphyrin. The highly exergonic values of Gibbs free energy change for both forward electron transfer reaction (−1.15 eV) and charge recombination reaction (−1.94 eV) indicate the possibilities of occurrences of these two processes in the Marcus inverted region. The fluorescence quenching rate due to photoinduced electron transfer reaction is found to be close to the diffusion-controlled limit within the present donor-acceptor system upon excitation of the acceptor molecules.     

An investigation of the structure of free-base porphycene by time-resolved electron nuclear double resonance and density functional theory on the photoexcited triplet state

The photoexcited triplet state of free-base porphycene has been investigated by time-resolved electron nuclear double resonance spectroscopy at 10 K. In order to determine whether porphycene is cis or trans configured, experimentally determined A_zz hyperfine coupling tensor components are compared with those predicted from density functional theory. The structures of cis and trans porphycene, in both the singlet ground state and lowest excited triplet state were calculated using the Becke3-Lee-Yang-Parr composite exchange correlation functional and the 6–31G* basis set. Hyperfine couplings for these structures were then calculated using the same functional but with the extended EPR-II basis set. From the results it can be concluded that porphycene has a trans configuration in both the ground singlet and lowest excited triplet state. Additionally, the significant differences in structure between the singlet and triplet states are reflected in the calculated hyperfine couplings, with those for the trans conformation in the triplet state in reasonable agreement with the experimental values.     

Spectroscopic investigations of a novel push-pull azo compound embedded in rigid polymer

A new heterocyclic push-pull azo compound-in-poly(methymethacrylate) (PMMA) film has been made by means of the spin-coating method. The spectroscopic properties of the films have been investigated with the steady-state absorption spectra, and steady-state fluorescence and femtosecond time-resolved fluorescence spectra in the first time, which is an important characteristic for the application of the film. The excited singlet (S₁) state lifetimes for trans and cis isomers of the film at room temperature have been measured. The excited triplet (T₁) state lifetime of cis isomer of the film has been obtained. The electronic structure of the film has been explained. The results show that the aggregate state of the azo molecules greatly influences its absorption spectra.     

EPR studies of photoinduced electron transfer in triad model compounds of photosynthesis

Time-resolved EPR spectra are reported for porphyrin-quinone-quinone and porphyrin-porphyrin-quinone triads obtained after photoexcitation in the nematic and soft glass phase of liquid crystals. Spin-polarized EPR spectra were observed for the triplet states of the porphyrin created by spin-selective intersystem crossing (ISC) from the excited singlet state and those of the charge-separated radical pair states (RP) generated by electron transfer (ET) processes. The EPR polarization patterns of the RP are discussed in terms of the favored decay channel of the photoexcited singlet state of the porphyrin donor. The decay pathway may either be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character or triplet ET after ISC from the porphyrin singlet to the triplet state, or a superposition of both pathways. It is demonstrated that the nature of the linking bridge between donor and acceptor, i.e., aliphatic cyclohexylene or aromatic phenylene, significantly influences the ET mechanism and thus the polarization patterns of the RP spectra. Using liquid crystals, information about the orientation of the guest molecules in the liquid crystal matrix with respect to the long axes of the liquid crystal molecules can be obtained. In the porphyrin-porphyrin-quinone triads the energy and ET processes strongly depend on the type of metallation of the porphyrins, specifically, whether the distal, the vicinal or both porphyrins bear a zinc atom.     

四苯基卟啉—锌化合物的微微秒瞬态吸收和荧光动力学研究

用微微秒瞬态吸收和荧光动力学测量方法,研究了四苯基卟啉-锌的三重态建立过程和单重态的衰减过德;测量了样品的系际交叉速率、单重态的寿命及其对532nm波长的吸收截面.     

Transient absorption spectroscopy of a monofullerene C<Subscript>60</Subscript>-bis-(pyropheophorbide <Emphasis Type="Italic">a</Emphasis>) molecular system in polar and nonpolar environments

The population dynamics of the excited and ground states of the monofullerene-bis (pyropheophorbide a) complex (FP1) were studied in polar (DMF) and nonpolar (toluene) solvents using picosecond transient absorption techniques. A strong quenching of the fluorescence signal of FP1 was observed in both solvents, in comparison to the fluorescence of bis (pyropheophorbide a) (P2). This quenching is due to an intramolecular photoinduced electron transfer from the pyropheophorbide a (pyroPheo) moiety to the fullerene C₆₀ monoadduct. In DMF the charge-separated (CS) state of FP1 has a lifetime of 0.32 ns and undergoes a direct transition to the ground state, resulting in a very low value of photosensitised singlet oxygen generation. In toluene, energy transfer from the first excited triplet state of pyroPheo, which has been populated via relaxation of the CS state, generates a considerable amount of singlet oxygen. The lifetime of the CS state in the nonpolar solvent was estimated to be 0.29 ns. It was also shown that in both DMF and toluene the first excited singlet state as well as the triplet state of the fullerene moiety in FP1 are not occupied. PACS 31.70.Dk; 31.70.Hq; 33.50.-j; 34.70.+e     

Effect of halogen atom on electron spin polarization studied by CIDEP,using halogen substituted aromatic acyl compounds

The reaction and spin dynamics of the photocleavage reaction of 2-chloro-2′-acetylnaphthalene were studied by time-resolved FT-EPR and transient absorption (TA) spectroscopy. The photocleavage reaction from both singlet and triplet states was observed by TA and EPR experiments, although the radical cleavage reaction in the excited triplet state is energetically unfavourable. This feature has been explained by the ionic cleavage reaction due to the electro-negativity of the chlorine atoms. The time-resolved FT-EPR spectra were similar to those observed in the bromine substituted compound, 2-BAN, reported in a previous paper. The origin of the electron spin polarization was assigned to the radical triplet pair mechanism (RTPM) and free radical pair mechanism (F-pair RPM) from analysis of the time profiles of the spin polarization.     

EPR-detected photoinduced electron transfer in three structurally related molecular triads

A comparative electron paramagnetic resonance (EPR) study has been performed on a series of structurally related molecular triads which undergo photoinduced electron transfer and differ one from the other in terms of the acceptor or donor moieties. The molecular triads, C-P-C₆₀, TTF-P-C₆₀ and C-P-P_F, share the same free-base, tetraarylporphyrin (P) as the primary electron donor, which after light excitation initiates the electron transfer process, but differ either in terms of the electron acceptor (fullerene derivative, C₆₀, versus fluorinated free-base porphyrin, P_F), or in terms of the final electron donor (carotenoid polyene, C, versus tetrathiafulvalene, TTF). All these molecular triads can be considered artificial photosynthetic reaction centers in their ability to mimic several key properties of the reaction center primary photochemistry. Photoinduced charge separation and recombination have been followed by time-resolved EPR in a glass of 2-methyltetrahydrofuran and in the nematic phase of the uniaxial liquid crystal E-7. All the triads undergo photoinduced electron transfer, with the generation of charge-separated states in both the low-dielectric environment of the 2-methyl-tetrahydrofuran glass and in anisotropic E-7 medium. Different photochemical pathways have been recognized depending on the specific donor and acceptor moieties constituting the molecular triads. In the presence of the tetrathiafulvalene electron donor singlet- and triplet-initiated electron transfer routes are concurrently active. Recombination to the low-lying carotenoid triplet state occurs in the carotene-based triads, while singlet recombination is the only active route for the TTF-P-C₆₀ triad, where a low-lying triplet state is lacking. Long-lived charge separation has been observed in the case of TTF-P-C₆₀: about 8 μs for the singlet-born radical pair in the glassy isotropic matrix and about 7 μs for the triplet-born radical pair in the nematic phase of E-7. For all the molecular triads, a weak exchange interaction (J?1 G) between the electrons in the final spin-correlated radical pair has been evaluated by simulation of the EPR spectra, providing evidence for superexchange electronic interactions mediated by the tetraarylporphyrin bridge.     

Structural changes in excited states and photoisomers determined by time-resolved Raman scattering

Molecules in the excited singlet and triplet state and photoproducts occuring after photoisomerization or proton transfer are investigated by time-resolved resonance Raman and resonance CARS spectroscopy with nanosecond time resolution. From the time-resolved spectra information on structures and bond strengths of these short-lived species and on their kinetics are obtained.     

Novel intramolecular electron transfer in axial bis(terpyridoxy)phosphorus(V) porphyrin studied by time-resolved EPR spectroscopy

Laser flash-induced spin-polarized transient electron paramagnetic resonance (TREPR) spectra for bis(terpyridoxy)phosphorus(V) porphyrin in a nematic liquid crystal isotropic and in frozen solution are presented. At room temperature, two sequential spin-polarized TREPR spectra are observed. The first is consistent with the triplet state of a radical pair, while the later is assigned to the triplet state of the porphyrin formed by charge recombination. On the basis of the spectroscopic and redox properties of the terpyridine and porphyrin moieties it is proposed that electron transfer from the terpyridine to the excited phosphorus(V) porphyrin occurs. The lifetime of the radical pair is estimated to be of about 175 ns. At low temperature, the radical pair spectrumis no longer observed and the spin polarization pattern of the porphyrin triplet is dramatically different. This behavior is explained by postulating that the electron transfer is inhibited at low temperature because molecular motion is required to stabilize the radical pair. It is proposed that in the absence of this stabilization, the porphyrin triplet state is populated via spin-orbit coupling-mediated intersystem crossing from the excited singlet state.     

Energy transfer by singlet and triplet excitons in carbazole-containing polymers

Quantum yields of pyrene fluorescence and bis[2-(2′-benzothienyl)-pyridinato-N,C^3′](acetylacetonate)iridium [Btp₂Ir(acac)] phosphorescence upon excitation via a matrix of poly-N-epoxypropylcarbazole (PEPC) or poly-N-epoxypropyl-3,6-dibromocarbazole (DBrPEPC), respectively, were found to be lower than those for the compounds directly excited in a polystyrene (PS) matrix. It was established that the energy in PEPC was transferred to an acceptor by both singlet excitons (by migration and long-range dipole–dipole interaction) and triplet excitons (through migration and short-range exchange electron interaction); however, only by triplet excitons in DBrPEPC, which did not show any fluorescence. The energy-transfer efficiency in PEPC by singlet excitons was higher than by triplet excitons. The observed effects were explained by the fact that energy transfer to the acceptor competed with such processes as localization of the excitons in the “tail” energy states, dissociation of singlet excitons into geminal electron–hole pairs (EHP), and capture of triplet excitons by polymer oxidation products.     

Magnetic field effect on the pairing state competition in quasi-one-dimensional organic superconductors (TMTSF)2X

We study the effect of the magnetic field on the pairing state competition in organic conductors (TMTSF)₂X by applying random phase approximation to a quasi-one-dimensional extended Hubbard model. We show that the singlet pairing, triplet pairing and the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superconducting states may compete when charge fluctuations coexist with spin fluctuations. This rises a possibility of a consecutive transition from singlet pairing to FFLO state and further to S_z = 1 triplet pairing upon increasing the magnetic field. We also show that the singlet and S_z = 0 triplet components of the gap function in the FFLO state have “d-wave” and “f-wave” forms, respectively, which are strongly mixed.     

Interaction between the end groups and the main chain of conjugated polymers by time-resolved EPR and fluorescence spectroscopy

ABSTRACT

Interaction between a zinc porphyrin (ZnPor) as the end-group and poly(9,9-di-n-octylfluorene-2,7-vinylene) (PFV) as the main chain in a porphyrin end-modified fluorescent conjugated polymer, ZnPFV, was studied by time-resolved electron paramagnetic resonance (EPR) and fluorescence spectroscopy. While fluorescence from the PFV part of ZnPFV showed a spectral profile almost identical to that of a PFV oligomer without end-modification, the emission spectrum of the ZnPor part exhibited a much broader profile compared to that of the reference zinc porphyrin monomer. Based on the analysis of lifetimes and quantum yields, it was found that radiative rate constant of the ZnPor part was enhanced by nearly three times. The observed unusual enhancement in the radiative rate constant was rationalised in terms of a partial π-conjugation between the end group and the main chain, as a result of co-planarisation in fluid solution. On the other hand, the time-resolved EPR spectrum of ZnPFV at 100?K basically showed a similar spectral pattern to that of the reference zinc porphyrin, but with significant differences in zero-field spitting parameters and initial population ratios. The π-system of the excited triplet state is deduced to deviate from D_4h symmetry in the end zinc porphyrin groups. The obtained results show that interaction of the porphyrin end group with the main chain of the polymer significantly influences the excited singlet state properties of the porphyrin, while its triplet state properties were affected to a lesser extent.     

Photoinduced Gas-Phase Electron Transfer Reactions

The electron transfer quenching process, when a reactive excited state is singlet or triplet, for gas-phase systems (benzophenone and anthraquinone with amines and pyridine as well as carbazole with halomethanes) was systematically investigated using time-resolved fluorescence. Bimolecular rate constants were obtained. Variable-temperature measurements were performed for eight donor-acceptor pairs. It was found that under solvent-free conditions various quenchers differing in photochemical reactivity led to change in quenching rates by almost three orders of magnitude. Positive and negative temperature dependences for the electron transfer rate constants were observed. The data were analyzed in terms of the Marcus-Jortner theory.     

Effectiveness of charge separation from the long-lived second excited state of donors

In molecular zinc-porphyrin-based donor–acceptor systems, the electron transfer from the second singlet excited state S₂ is accompanied by ultrafast recombination into the first excited state, resulting in a low quantum yield of the thermalized charge-separated state (20%). It is demonstrated that the quantum yield of ultrafast charge separation in donor–acceptor triads D–A₁–A₂ can be close to 100% in molecular systems with lifetimes of the S₂ state longer than 150 ps. As prototypes of such systems, donor–acceptor diads D–A₁ and triads D–A₁–A₂ are considered, wherein the xanthione molecule plays the role of a donor. The ranges of the model parameters are determined in which the efficiency of charge separation is high. The twostage photoinduced charge transfer is studied within the framework of a multichannel stochastic model that takes into account the reorganization of a polar solvent and a high-frequency intramolecular vibrational mode.     

EPR identification of the triplet ground state and photoinduced population inversion for a Si-C divacancy in silicon carbide

It is shown that intrinsic defects responsible for the semi-insulating properties of SiC represent Si-C divacancies in a neutral state (V_Si-V_C)⁰, which have the triplet ground state. The energy level scheme and the mechanism of creating the photoinduced population inversion of the triplet sublevels of the divacancy ground state are determined. It is concluded that there is a singlet excited state through which spin polarization is accomplished, and this fact opens the possibility of detecting magnetic resonance on single divacancies.     

Two-laser two-color time-resolved EPR study on higher-excited-state triplet-singlet intersystem crossing of porphyrins and phthalocyanines

Photoinduced effects on the electron spin polarization (ESP) in the lowest excited triplet (T₁) states of porphyrins (PORs) and phthalocyanines (PCs) have been observed with a two-color time-resolved (TR) electron paramagnetic resonance (EPR) technique in a glassy matrix at low temperatures. On single-color excitation with the wavelengths of the ground state absorptions of PORs and PCs, polarized EPR spectra due to the corresponding T₁ state were observed. The polarization patterns match well with interpretation as anisotropic intersystem crossing (ISC) induced by the spinorbit coupling between the singlet excited (S₁) and the triplet states. In contrast, two-color excitation led to a change of the phase of the T₁ state polarization pattern to the opposite. The observed ESP in the T₁ state resulting from the excitation to the upper triplet state (T _n ) was interpreted in terms of anisotropic ISC between the T _n and S₁ states. From the analysis of the ESP, changes in the quantum yields of the reverse ISC processes were determined at different temperatures. The results could be best interpreted by the existence of thermal pathways with small activation energy in the relaxation processes.     

On Electron Spin Polarization Created in the Excited Triplet State of Accessory Chlorophyll via Photoinduced Charge-Recombination of the Photosystem II Reaction Center

We present a theoretical approach to investigate the electron spin polarization (ESP) of the excited triplet state that has been detected using the time-resolved electron paramagnetic resonance (TREPR) method in the photosystem II center of the plants. We show, using the stochastic Liouville equation, that the ESP pattern created in the accessory chlorophyll (Chl_accD1) which reside near the P_D1 chlorophyll of the active branch is explained by one-step, concerted double electron transfer model, initiating from the singlet–triplet conversion of the light-induced charge-separated state composed of P_D1 radical cation and pheophytin radical anion. We also considered the sequential ESP transfer model via the triplet charge-recombination (CR) and the triplet–triplet energy transfer processes. It has been clearly shown that the ESP created in the ³Chl_accD1* is dependent on the rate constant (k _TT) of the triplet–triplet energy transfer from the intermediate triplet state created by the CR. Also we show that the relative orientation of the principal axes of the spin dipolar interaction in the intermediate triplet state (³P_D1*, as an example) may play a role in the ESP pattern, when the k _TT is smaller than the angular frequency of the Zeeman energy. We have theoretically shown that the TREPR measurement of the ESP is very powerful to investigate the primary chemical process and to characterize the intermediate as a signature of the stepwise ESP transfer.