Intramolecular interactions in the triplet excited states of benzophenone-thymine dyads

Time-resolved and product studies on the synthesized dyads 1 and 2 have provided evidence that the benzophenone-to-thymine orientation strongly influences intramolecular photophysical and photochemical processes. The prevailing reaction mechanism has been established as a Paterno-Büchi cycloaddition to give oxetanes 3-6; however, the ability of benzophenone to achieve a formal hydrogen abstraction from the methyl group of thymidine has also been evidenced by the formation of photoproducts 7 and 8. These processes have been observed only in the case of the cisoid dyad 1. Adiabatic photochemical cycloreversion of the oxetane ring is achieved upon direct photolysis to give the starting dyad 1 in its excited triplet state. The photobiological implications of the above results are discussed with respect to benzophenone-photosensitized damage of thymidine.     

Theoretical analysis of singlet and triplet excited states of nickel porphyrins

Local density and generalized gradient approximation time-dependent density functional methods have been used for calculation of the singlet and triplet excited states of nickel-porphine, Ni-tetraphenyloporphine, and Ni-octaethyloporphyrine. Special attention is paid to metal-ligand transitions and d-d transitions. It is shown that the lowest exited singlet states of the three compounds can be described as a transfer of an electron from the porphine ring to the d(x2-y2) orbital of the nickel atom. On the other hand, the lowest excited triplet state arises from promotion of an electron between two nickel d orbitals, an occupied d(z2) and an empty d(x2-y2). It is proposed that a rapid quenching of the excited singlet states is due to an ultrafast intersystem crossing between 1Eg)and 3Eg or 3B1g states.     

Theoretical study of the excited singlet and triplet states of alloxan

The possibility of excited‐state protomeric shifts in the biologically important molecule, alloxan, is investigated. We have focused on the S₁ and T₁ excited states of alloxan and its hydroxy tautomers. Modifications brought in by excitation on the relative stabilities, activation barriers, and optimized geometries, computed at the MNDO, AM1, and PM3 levels of approximation, have been discussed for both excited electronic states. The absorption and fluorescence spectra for the three tautomers are also discussed. Results show significant changes in the geometries on excitation, although the changes are similar for the singlet and triplet excited states. Though the relative stability orders do not change, the 2‐hydroxy tautomer is stabilized, while the 4‐hydroxy tautomer gets destabilized on excitation. The excited states are (n,π*) states, involving the promotion of a nonbonding oxygen lone pair from the CO? CO? CO moiety, which explains why the oxygens of this group become less basic and the 4‐hydroxy tautomer gets destabilized on excitation. However, the activation barriers do not reduce significantly on excitation, and this precludes the possibility of ground‐ or excited‐state proton transfer in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Dipole moment changes in the singlet and triplet excited states of 4-dimethylamino-4′-nitrostilbene detected by nanosecond time-resolved microwave conductivity

Dipole moment changes accompanying the formantion of singlet and triplet excited states of 4-dimethylamino-4′-nitro- stilbene on laser flash photolysis of solutions in n-hexane and benzene have been observed by measuring changes in the microwave conductivity of the medium with nanosecond time resolution.     

Radical cation generation from singlet and triplet excited states of all-trans-lycopene in chloroform

On direct photoexcitation, subpicosecond time-resolved absorption spectroscopy revealed that the 1B(u)-type singlet excited state of all-trans-lycopene in chloroform was about seven times more efficient than all-trans-beta-carotene in generating the radical cation. The time constant of radical cation generation from the 1B(u)-type state was found to be approximately 0.14 ps, a value that was comparable for the two carotenoids. On anthracene-sensitized triplet excitation, radical cation generation was found to be much less efficient for lycopene than for beta-carotene. A slow rising phase (20-30 micros) in the bleaching of ground-state absorption was common for both lycopene and beta-carotene in chloroform and was ascribed to an efficient secondary reaction with a solvent radical leading to the formation of carotenoid radical cations. The reverse ordering in the tendency of the excited states of different multiplicities for the two carotenoids to generate radical cations is discussed in relation to the two carotenoids as scavengers of free radicals.     

Photoproduct selectivity in reactions involving singlet and triplet excited states within bile salt micelles

Generally, photochemical reactions tend to give more than one product. For such reactions to be useful one should be able to control them to yield a single product. Of the many approaches used in this context, the use of reaction media with features different from those of isotropic solutions has been very effective. We provide results of our studies on four reactions within bile salt micelles (cholic acid and deoxycholic acid). These four reactions involve homolytic cleavage of a C-C or C-O bond to yield either a singlet or triplet radical pair. The bile salt micelles control the rotational and translational mobilities of the radical pair, resulting in photoproduct selectivity. The dynamic nature of the bile salt micelles results in differential effects on the singlet and triplet radical pairs.     

Characterization of the singlet and triplet excited states of 3-chloro-4-methylumbelliferone

An extensive photophysical characterization of 3-chloro-4-methylumbelliferone (3Cl4MU) in the ground-state, S(0), first excited singlet state, S(1), and lowest triplet state, T(1), was undertaken in water, neutral ethanol, acidified ethanol, and basified ethanol. Quantitative measurements of quantum yields (fluorescence, phosphorescence, intersystem crossing, internal conversion, and singlet oxygen formation) together with lifetimes were obtained at room and low temperature in water, dioxane/water mixtures, and alcohols. The different transient species were assigned and a general kinetic scheme is presented, summarizing the excited-state multiequilibria of 3Cl4MU. In water, the equilibrium is restricted to neutral (N*) and anionic (A*) species, both in the ground (pK(a) = 7.2) and first excited singlet states (pK(a)* = 0.5). In dioxane/water mixtures (pH ca. 6), substantial changes of the kinetics of the S(1) state were observed with the appearance of an additional tautomeric T* species. In low water content mixtures (mixture 9:1 v:v), only the neutral (N*) and tautomeric (T*) forms of 3Cl4MU are observed, whereas at higher water content mixtures (water mole fraction superior to 0.45), all three species N*, T*, and A* coexist in the excited state. In the triplet state, in the nonprotic and nonpolar solvent dioxane, the observed transient signals were assigned as the triplet-triplet transition of the neutral form, N*(T(1)) → N*(T(n)). In water, two transient species were observed and are assigned as the triplets of the neutral N*(T(1)) and the anionic form, A*(T(1)) (also obtained in basified ethanol). The phosphorescence spectra and decays of 3Cl4MU, in neutral, acidified, and basified solutions, demonstrate that only these two species N*(T(1)) and A*(T(1)) exist in the lowest lying triplet state, T(1). The radiative channel was found dominant for the deactivation of the anionic species, whereas with the neutral the S(1) ? S(0) internal conversion competes with fluorescence. For both N* and A* the intersystem crossing yield represents a minor deactivation channel for S(1).     

Radio Frequency induced magnetic resonance transitions in excited molecular triplet states as detected via optical nuclear polarization

Optical nuclear polarization (ONP) has been used to detect magnetic resonance transitions in the rf region of both the excited and the ground state of aromatic molecules in a crystalline environment. The possibilities of the method are demonstrated and the kind of independent information available on molecular parameters is pointed out.     

Photophysics of monodisperse platinum-acetylide oligomers: delocalization in the singlet and triplet excited states

A series of monodisperse Pt-acetylide polymers that contain the [-CC-(p-C6H4)-CC-(t-Pt(PBu3)2)-]n repeat unit has been prepared for n = 1, 2, 3, 4, 5, and 7. The photophysical properties of the series provide information concerning the relationship between the oligomer length and delocalization in the singlet and triplet excited states of the pi-conjugated electron system. The results imply that the singlet excited state is delocalized over approximately 6 repeat units; however, the triplet state is considerably more localized. The triplet energy is almost invariant with oligomer length, but the phosphorescence spectra and triplet nonradiative decay rates indicate that the electron-vibrational coupling in the triplet state decreases with increasing oligomer length.     

Photophysical properties of the lowest excited singlet and triplet states of thio- and seleno-psoralens

The decay processes of the lowest excited singlet and triplet states of five heteropsoralens (HPS) were investigated by steady-state and shift-phase fluorometry and by laser-flash photolysis in different solvents. The emission spectra of HPS are detectable only in trifluoroethanol (TFE), where fluorescence lifetimes (τ_F) and quantum yields (φ_F) were measured. The triplet lifetimes (τ_T), triplet (φ_T) and singlet-oxygen production (φ_Δ) quantum yields were determined in benzene, ethanol and TFE by laser-flash photolysis. Semiempirical (INDO/1-CI) calculations allowed the nature of the lowest excited singlet and triplet states and transition probabilities to be obtained. Theoretical and experimental results indicate that the two lowest excited singlet states S₁ and S₂ of HPS are close-lying and different in nature (π,π* and n,π*). The "proximity effect" between these two states controls the photophysical properties of HPS as it does for the other furocoumarins. However, HPS have a peculiar behavior with respect to the related compounds because they are fluorescent and have, in three cases, detectable intersystem crossing only in TFE. This behavior can be tentatively explained by a different energy gap and/or order between the S₁ and S₂ states.     

Radiationless transitions between excited singlet states of biacetyl

Emission processes from lower excited states S₁ (fluorescence) and T₁ (phosphorescence) have been studied in the gas and liquid phases when biacetyl is excited into the second singlet state S₂. (In agreement with Kasha's rule no fluorescence is observed from the S₂ state.) In the liquid phase, when biacetyl is excited into the singlet states S₁ and S₂, no difference is observed between these emission processes. This phenomenon certainly results from an efficient nonradiative transition between the second excited singlet state S₂ and the first excited state S₁ with practically no excess vibrational energy. The quantum yield of this transition is almost unity and does not depend on the nature of the solvent. In the gas phase no emission processes are observed when biacetyl is excited into the S₂ state at low pressure (less than 10 mm Hg). High pressure of inert gas is necessary in order to observe these processes. As for excitation into the S₁ state with vibrational energy, loss of vibrational energy through collisions occurs from the S₂ state. The quantum yield of the S₂ → S₁ transition by excitation at 290 nm is estimated around 0.5–0.6 at 6 atm of inert gas (ethane, ethylene, or carbon dioxide).     

Theoretical study of photoinduced singlet and triplet excited states of 4-dimethylaminobenzonitrile and its derivatives

Singlet and triplet low-lying states of the 4-dimethylaminobenzonitrile and its derivatives have been studied by the density functional theory and ab initio methodologies. Calculations reveal that the existence of the methyl groups in the phenyl ring and the amino twisting significantly modify properties of their excited states. A twisted singlet intramolecular charge-transfer state can be accessed through decay of the second planar singlet excited state with charge-transfer character along the amino twisting coordinate or by an intramolecular charge-transfer reaction involved with a locally first excited singlet state. Plausible charge-transfer triplet states and intersystem crossing processes among singlet and triplet states have been explored by spin-orbit coupling calculations. The intersystem crossing process was predicted to be the dominant deactivation channel of the photoexcited 4-dimethylaminobenzonitrile.     

Thermally activated delayed fluorescence as a cycling process between excited singlet and triplet states: application to the fullerenes

In efficient thermally activated delayed fluorescence (TADF) the excited chromophore alternates randomly between the singlet and triplet manifolds a large number of times before emission occurs. In this work, the average number of cycles n is obtained and is shown to have a simple experimental meaning: n+1 is the intensification factor of the prompt fluorescence intensity, owing to the occurrence of TADF. A new method of data analysis for the determination of the quantum yield of triplet formation, combining steady-state and time-resolved data in a single plot, is also presented. Application of the theoretical results to the TADF of [70]fullerenes shows a general good agreement between different methods of fluorescence analysis and allows the determination of several photophysical parameters.     

Exciplexes in highly excited triplet states

A mechanism of energy transfer from highly excited triplet aromatic molecules has been developed, which involves a stage of formation of an exciplex between a highly excited energy-donor molecule and an unexcited energy-acceptor molecule. Interpretation of the experimental data on the shape and the intensity of triplet-triplet absorption bands and the energy transfer probability is presented. In this interpretation, the results of quantum-chemical calculations of the energies of highly excited triplet states of toluene and benzene molecules are used.     

Characteristic luminescence of the anilinium ion and the pKa values of excited singlet and triplet states

Contrary to earlier observations a characteristic luminescence due to the C₆H₅NH⁺₃ ion was observed. The ion at 80 K, gives a fluorescence similar to that of benzene and a phosphorescence similar to that of aniline. The benzene type fluorescence changes to the aniline type on warming. It is suggested that an excited C₆H₅NH⁺₃ ion giving rise to the fluorescence becomes dissociated in the excited state. Computations for a change in the pKa values for excited singlet and triplet states of C₆H₅NH⁺₃ ion also suggest that is a larger probability of losing the proton in the excited state.     

Low-lying singlet and triplet electronic states of RhB

The low-lying XSigma+, a3Delta, A1Delta, b3Sigma+, B1Pi, c3Pi, C1Phi, D1Sigma+, E1Pi, d3Phi, and e3Pi electronic states of RhB have been investigated at the ab initio level, using the multistate multiconfigurational second-order perturbation (MS-CASPT2) theory, with extended atomic basis sets and inclusion of scalar relativistic effects. Among the eleven electronic states included in this work, only three (the X1Sigma+, D1Sigma+, and E1Pi states) have been investigated experimentally. Potential energy curves, spectroscopic constants, dipole moments, binding energies, and chemical bonding aspects are presented for all electronic states.