Picosecond laser studies of the effects of reactants on intramolecular energy relaxation of diphenylcarbene: Reaction of diphenylcarbene with alcohols

Picosecond laser induced fluorescence measurements provide for the first time the direct measurement of the intramolecular and intermolecular energy decay dynamics of singlet diphenylcarbene (¹ DPC) in the presence of reactive molecules. As exemplified by the reaction of ¹DPC with alcohols it is found that reactive molecules provide ¹DPC with not only a chemical decay channel but also an intramolecular decay channel which is due to a solvent polarity effect. These chemical and physical effects can act in opposite directions leading to novel results such as a significant increase in the singlet state lifetime upon addition of reacting molecules. The absolute reaction rate constants of ¹DPC with alcohols, in different solvents, obtained by direct measurements are also reported.     

Picosecond laser studies of electron solvation in alcohols

We report the direct measurement of electron solvation times in methanol and ethanol. A picosecond technique utilizing a laser-triggered flash lamp and a streak camera is used to monitor the solvation process. The relationship of the solvation dynamics to dielectric relaxation, local heating, and pulse-radiolysis results are presented     

Picosecond spectroscopic investigations of diphenylcarbene protonation

The details of the picosecond kinetic investigation of diphenylcarbene (Ph₂C:) protonation with H₂O, MeOH, EtOH and 2-propanol by optical absorption technique are presented. The protonation reactions were monitored by detection of the diphenylcarbenium ion (Ph₂C⁺H). Evidence of solvent induced alteration of the singlet-triplet energy level of Ph₂C:, and involvement of an excited carbene singlet state in the protonation of Ph₂C: diphenylcarbene with H₂O are presented.     

Localized excited triplet states in mixed charge-transfer single crystals: formation of excited multicomplexes

Single crystals of charge-transfer (CT) complexes between tetracyanobenzene as acceptor and different aromatic donors were doped with guest donors. The molecular arrangements of the guest CT complexes forming triple energy traps in the host crystal were determined from the triplet ESR spectra of the traps. A method for the determination of relative charge-transfer triplet energies is proposed. Extended electron delocalization over more than one donor-acceptor pair has been found.     

Long-distance electron transfer from a triplet excited state

Electron transfer from the triplet excited state of N,N,N',N'-tetramethylphenylene diamine to phthalic anhydride has been monitored by phosphorescence emission decay. The kinetics of the transfer process were observed directly and the rate constant depends exponentially on the reacting distance, k(r) = 1 × 10⁴ exp(−0.58r) s⁻¹. The electron transfer rate has been found to be invariant over the temperature interval 77–143 K.     

Enhanced triplet formation by twisted intramolecular charge-transfer excited states in conjugated oligomers and polymers

The triplet yield and intersystem crossing rate of a set of conjugated oligomers and polymers that, in polar solvents, form a charge-transfer state with a twisted conformation has been investigated. It was observed that in these dibenzothiophene-fluorene oligomers a greater than 10-fold increase on the triplet yield is achieved by simply changing the medium polarity to favor the formation of the twisted charge-transfer state, while the fluorescence lifetime is only slightly increased. The increase in the intersystem crossing rate is attributed to the improved mixing between the singlet and triplet states in the twisted excited state. In analogous polymers, the intersystem crossing rate does not show the same increase, most likely because of the greater energetic and conformational disorder increasing the intersystem crossing rate at all times, regardless of the formation of the twisted charge-transfer state or not.     

Picosecond study of an electron transfer in the first excited state of dodci

Electron transfer from the first excited singlet state of a polymethine cyanine dye. DODCI, to various electron acceptors (p-benzoquinone,p-dinitrobenzene and methylviologen) was investigated using picosecond fluorescence and absorption spectroscopy. The electron transfer to methylviologen was confirmed by conventional nanosecond laser spectroscopy. Its efficiency, as expressed by the ratio k₅/(k₄ + k₅) = 0.07. can be explained by coulombic repulsion in the initial radical pair. On the other hand, although fluorescence quenching by p-BQ and p-DNB is very efficient, no electron transfer was observed.     

Photoinduced electron transfer from electron donor to bis-carbocyanine dye in excited triplet state

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Temperature dependence of the reactions of singlet and triplet diphenylcarbene. : Evidence for reversible ylide formation in the reaction with alcohols

The temperature dependences of the reaction of singlet diphenylcarbene are consistent with reversible intermediate formation.     

Delayed-fluorescence ODMR and EPR studies of triplet excitons in charge-transfer molecular crystals

Triplet excitons in electron donor—acceptor charge-transfer (CT) molecular crystals are generated through the intersystem crossing process by excitation in the CT visible band and give rise to delayed fluorescence. Delayed-fluorescence optically detected magnetic resonance (DF ODMR) in magnetic field is analyzed in terms of microwave-induced transitions between energy levels of either the isolated triplet excitons or the annihilating triplet exciton pair. The spin polarization of the triplet excitons plays an important role in the described phenomena. A comparison between DF ODMR and EPR spectra of the anthracene—tetracyanobenzene and biphenyl—tetracyanobenzene systems is presented. In the former case the microwave transitions occurring between free exciton sublevels are predominantly responsible of the DF ODMR signal, whereas the transitions between energy levels of the exciton pair are the most important for biphenyl—TCNB.     

Intermolecular electron transfer from naphthalene derivatives in the higher triplet excited states

Intermolecular electron transfer (ELT) from a series of naphthalene derivatives (NpD) in the higher triplet excited states (T(n)) to carbon tetrachloride (CCl(4)) in Ar-saturated acetonitrile was observed using the two-color two-laser flash photolysis method. The ELT efficiency depended on the driving force of ELT. Since the ELT from the T(n) state occurred competitively with the internal conversion (IC, T(n) --> T(1)) and the triplet energy transfer (ENT), the ELT became apparent only when sufficient free energy change of ELT was attained. On the other hand, ELT from the T(1) state was not observed, although ELT from the T(1) state with sufficiently long lifetime has a slightly exothermic driving force. The fast ELT from the T(n) state and lack of the reactivity of the T(1) state were explained well by the "sticky" dissociative electron-transfer model based on one-electron reductive attachment to CCl(4) leading to the C-Cl bond cleavage.     

Time-resolved electron paramagnetic resonance of the lowest excited triplet state of phenazinium cation

Time-resolved and steady-state electron paramagnetic resonance (EPR) spectra have been observed for the lowest excited triplet (T(1)) states of phenazine (Phz) and its singly protonated cation (phenazinium) in sulfuric acid-ethanol mixtures at 77K. The single protonation appears to have little effect on the anisotropic sublevel populating rates of the T(1) state of phenazine. However, the zero-field splitting (ZFS) parameter D decreases on the protonation, reflecting the increase of delocalization of the two unpaired electrons. The sublevel preferentially populated by intersystem crossing (ISC) is T(y) in both phenazine and phenazinium (the y-axis is parallel to the in-plane long axis). From the analysis of the observed anisotropy in the ISC rates and the semi empirical molecular orbital calculations of the ZFS parameters, we concluded that the T(1) state of phenazinium is the (3)A(1)(pipi*) state.     

Positive photocatalysis of a Diels-Alder reaction by quenching of excited naphthalene-indole charge-transfer complex with cyclohexadiene

[reaction: see text] Naphthalene photo-catalyzes formation of cyclohexadiene-indole cycloadducts in a wavelength-dependent process. Steady-state irradiation and time-resolved fluorescence studies agree well with NP-InH ground-state charge transfer (CT) complexes as the key species responsible for the photo-catalyzed process.     

Time-resolved resonance Raman studies on proton-induced electron-transfer reaction from triplet excited state of 2-methoxynaphthalene to decafluorobenzophenone

In this paper, time-resolved resonance Raman (TR3) spectra of intermediates generated by proton-induced electron-transfer reaction between triplet 2-methoxynaphthalene ((3)ROMe) and decafluorobenzophenone (DFBP) are presented. The TR3 vibrational spectra and structure of 2-methoxynaphthalene cation radical (ROMe(?+)) have been analyzed by density functional theory (DFT) calculation. It is observed that the structure of naphthalene ring of ROMe(?+) deviates from the structure of cation radical of naphthalene.     

Time-resolved resonance Raman spectroscopic studies on the triplet excited state of thioxanthone

Thioxanthone has been investigated extensively owing to its unique photochemical and photophysical applications and its solvatochromic behavior. Here, we report the time-resolved resonance Raman studies on the structure of the lowest triplet excited state of thioxanthone in carbon tetrachloride. In addition, FT-IR and FT-Raman techniques have been used to study the vibrational structure in the ground state. To corroborate the experimental findings, density functional theory calculations have been carried out. Isotopic calculations and normal coordinate analysis have been used to help in assigning the observed bands to Raman vibrational modes. Structural information derived from this study is expected to help in better understanding the triplet state photochemistry of thioxanthone.     

Zero-field-splitting of some aromatic charge-transfer complexes in their lowest excited triplet states—an ESR study

The ESR spectra of some aromatic charge-transfer complexes in the lowest excited triplet states were measured. From the extracted zero-field-splitting parameters it was found that in each case the lowest excited triplet state was localized on the donor, with a variable contribution of charge-transfer configuration.     

Stepwise laser photolysis studies of beta-bond cleavage in highly excited triplet states of biphenyl derivatives having C-O bonds

Photochemical profiles of beta-bond dissociation in highly excited triplet states (Tn) of biphenyl derivatives having C-O bonds were investigated in solution, using stepwise laser photolysis techniques. The lowest triplet states (T1) were produced by triplet sensitization of acetone (Ac) upon 308-nm laser photolysis. The molar absorption coefficients of the T1 states were determined using triplet sensitization techniques. Any photochemical reactions were absent in the T1 states. Upon 355-nm laser flash photolysis of the T1 states, they underwent fragmentation, because of homolysis of the C-O bond in the Tn states from the observations of the transient absorption of the corresponding radicals. The quantum yields (Phidec) for the decomposition of the T1 states upon the second 355-nm laser excitation were determined. Based on the Phidec values and the bond dissociation energies (BDEs) for the C-O bond fission, the state energies (ERT) of the reactive highly excited triplet states (TR) were determined. It was revealed that (i) the Phidec was related to the energy difference (DeltaE) between the BDE and the ERT, and (ii) the rate (kdis) of beta-cleavage in the TR state was formulated as being simply proportional to DeltaE. The reaction mechanism for beta-bond cleavage in the TR states was discussed.     

Coherent averaging of electron spins by rotary echoes in excited triplet states

A theoretical treatment of rotary echoes derived from average hamiltonian formalism in photoexcited triplet states is presented. From a general relaxation matrix, it is shown that relaxation fields perpendicular to the driving field H_eff are completely averaged, whereas relaxation fields parallel to H_eff are not. Rotary echoes are found to differ from other coherence techniques only in the geometry of the experiment and not in the averaging properties per se. Off-resonance driving field conditions are shown to impair the efficiency of the averaging and to cause beats in the observed echo decay. Finally, inhomogenous line broadening is shown to produce a damped echo decay with beats even when the field is on-resonance.     

Vibronic coupling in excited charge-transfer states of solid-state charge-transfer complexes

Symmetry properties of CT excited states of some weak donor-acceptor complexes are discussed in the context of vibronic coupling with intermolecular vibrations. The results are applied to the analysis of electroabsorption spectra of anthracene-PMDA.     

Time-resolved electron spin resonance of gallium and germanium porphyrins in the excited triplet state

Gallium and germanium porphyrin complexes in the lowest excited triplet (T1) state have been studied by time-resolved electron spin resonance (TRESR). It is found that for Ge(TPP)(OH)2 (TPP = dianion of tetraphenylporphyrin) intersystem crossing (ISC) from the lowest excited singlet (S1) state to the T1x and T1y sublevels is faster than that to the T1z sublevel (T1x, T1y, and T1z are sublevels of the T1 state), while the ISC of ZnTPP and Ga(TPP)(OH) is selective to the T1z sublevel. This is interpreted by a weak interaction between the dpi orbital of germanium and LUMO (eg) of the porphyrin ligand, resulting in small spin-orbit coupling (SOC). The interpretation is supported by molecular orbital calculations. The ISC of Ge(OEP)(OH)2 (OEP = dianion of octaethylporphyrin) and Ge(Pc)(OH)2 (Pc = dianion of tetra-tert-butylphthalocyanine) is found to be selective to the T1z sublevel in contrast to Ge(TPP)(OH)2. This dependence on the porphyrin ligand is reasonably explained by a difference between the 3(a(1u)eg) (the OEP and Pc complexes) and 3(a(2u)eg) (the TPP complex) configurations. This is the first observation of a difference in selective ISC between the 3(a(1u)eg) and 3(a(2u)eg) configurations. The TRESR spectrum of Ge(TPP)Br2 is different from those of Ge(TPP)Cl2 and Ge(TPP)(OH)2, and is interpreted by SOC between the T1 and T2 states. From ESR parameters the square of the coefficient of the eg orbital on bromine is evaluated as 0.018 in the T1 state.