Time-resolved resonance Raman and density functional theory study of the deprotonation reaction of the triplet state of p-hydroxyacetophenone in water solution

[reaction; see text] Picosecond and nanosecond time-resolved resonance Raman (TR(3)) spectroscopy was employed to investigate the deprotonation/ionization reaction of p-hydroxyacetophenone (HA) after ultraviolet photolysis in water solution. The TR(3) spectra in conjunction with density functional theory (DFT) calculations were used to characterize the structure and dynamics of the excited-state HA deprotonation to form HA anions in near neutral water solvent. DFT calculations based on a solute-solvent intermolecular H-bonded complex model containing up to three water molecules were used to evaluate the H-bond interactions and their influence on the deprotonation reaction and the structures of the intermediates. The deprotonation reaction was found to occur on the triplet manifold with a planar H-bonded HA triplet complex as the precursor species. The HA triplet species is generated within several picoseconds and then decays with a approximately 10 ns time constant to produce the HA triplet anion species after 267 nm photolysis of HA in water solution. The triplet anion species was observed to decay with a time constant of about 90 ns into the ground-state anion species that was found to have a lifetime of about 200 ns. The DFT calculations on the H-bonded complexes of the anion triplet and ground-states species suggest that these anion species are H-bonded complexes with planar quinonoidal structures containing two water molecules H-bonded, respectively, with oxygen lone pairs of the carbonyl and deprotonated hydroxyl moieties. A deactivation scheme of the photoexcited HA in regard to the deprotonation reaction in neutral water solutions was proposed. With the above dynamic and structural information available, we briefly discuss the possible implications of the model HA photochemistry in water solutions for the photodeprotection reactions of related p-HP phototrigger compounds in aqueous solutions.     

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.     

Time-resolved resonance Raman and density functional theory study of hydrogen-bonding effects on the triplet state of p-methoxyacetophenone

Picosecond and nanosecond time-resolved resonance Raman spectroscopy combined with density functional theory calculations have been performed to characterize the structure, dynamics, and hydrogen-bonding effects on the triplet state of the phototrigger model compound p-methoxyacetophenone (MAP) in cyclohexane, MeCN, and 50% H2O/50% MeCN (v:v) mixed solvent. Analogous work has also been done to study the corresponding ground state properties. The ground and triplet states of MAP were both found to be associated strongly with the water solvent molecules in the 50% H2O/50% MeCN solvent system. A hydrogen-bond complex model involving one or two water molecules bonded with the oxygen atoms of the MAP carbonyl and methoxy moieties has been employed to explore the hydrogen-bond interactions and their influence on the geometric and electronic properties for the ground and triplet states of MAP. Among the various hydrogen-bond configurations examined, the carbonyl hydrogen-bond configuration involving one water molecule was calculated to lead to the most stable hydrogen-bond complex for both the ground and the triplet states with the strength of the hydrogen-bond interaction being stronger in the triplet state than the ground state. The increased carbonyl located hydrogen-bond strength in the triplet state results in substantial modification of both the electronic and the structural conformation so that the triplet of the hydrogen-bond complex can be considered as a distinct species from the free MAP triplet state. This provides a framework to interpret the differences observed in the TR3 spectral and triplet lifetime obtained in the neat MeCN solvent (attributed to the free MAP triplet state) and the 50% H2O/50% MeCN solvent (due to the triplet of the hydrogen-bond complex). Temporal evolution at early picosecond times indicates rapid ISC conversion, and subsequent relaxation of the excess energy of the initially formed energetic triplets occurs for both the free MAP and the hydrogen-bond complex. The triplet of the carbonyl hydrogen-bond complex appears to be generated directly from the corresponding ground state complex and it does not dissociate back to the free triplet state within the triplet state lifetime. We briefly discuss the influence of the carbonyl hydrogen-bond effect on the pi pi* triplet reactivity for MAP and closely related compounds.     

Ultrafast time-resolved study of photophysical processes involved in the photodeprotection of p-hydroxyphenacyl caged phototrigger compounds

A combined femtosecond Kerr gated time-resolved fluorescence (fs-KTRF) and picosecond Kerr gated time-resolved resonance Raman (ps-KTR(3)) study is reported for two p-hydroxyphenacyl (pHP) caged phototriggers, HPDP and HPA, in neat acetonitrile and water/acetonitrile (1:1 by volume) solvents. Fs-KTRF spectroscopy was employed to characterize the spectral properties and dynamics of the singlet excited states, and the ps-KTR(3) was used to monitor the formation and subsequent reaction of triplet state. These results provide important evidence for elucidation of the initial steps for the pHP deprotection mechanism. An improved fs-KTRF setup was developed to extend its detectable spectral range down to the 270 nm UV region while still covering the visible region up to 600 nm. This combined with the advantage of KTRF in directly monitoring the temporal evolution of the overall fluorescence profile enables the first time-resolved observation of dual fluorescence for pHP phototriggers upon 267 nm excitation. The two emitting components were assigned to originate from the (1)pipi (S(3)) and (1)npi (S(1)) states, respectively. This was based on the lifetime, the spectral location, and how these varied with the type of solvent. By correlating the dynamics of the singlet decay with the triplet formation, a direct (1)npi --> (3)pipi ISC mechanism was found for these compounds with the ISC rate estimated to be approximately 5 x 10(11) s(-)(1) in both solvent systems. These photophysical processes were found to be little affected by the kind of leaving group indicating the common local pHP chromophore is largely responsible for the fluorescence and relevant deactivation processes. The triplet lifetime was found to be approximately 420 and 2130 ps for HPDP and HPA, respectively, in the mixed solvent compared to 150 and 137 ns, respectively, in neat MeCN. The solvent and leaving group dependent quenching of the triplet is believed to be associated with the pHP deprotection photochemistry and indicates that the triplet is the reactive precursor for pHP photorelease reactions for the compounds examined in this study.     

A theoretical investigation of p-hydroxyphenacyl caged phototrigger compounds: an examination of the excited state photochemistry of p-hydroxyphenacyl acetate

Ab initio and density functional theory methods were employed to study the excited states and potential energy surfaces of the p-hydoxyphenacyl acetate (HPA) phototrigger compound. Complete active space (CAS) ab initio calculations predicted adiabatic electronic transition energies for the HPA-T(1)((3)npi), HPA-T(2)((3)pipi), HPA-S(1)((1)npi), HPA-T(3)((3)npi), HPA-S(2)((1)npi), HPA-S(3)((1)pipi) <-- HPA-S(0) transitions that were similar to and in agreement with those found experimentally for closely related aromatic ketones such as p-hydroxyacetophenone and results from similar calculations for other related aromatic carbonyl systems. The alpha or beta bond cleavage reactions from the S(1) excited state were both found to have relatively high barriers to reaction, and the S(1), T(1), and T(2) states are close in energy with the three S(1)((1)npi), T(1)((3)npi), and T(2)((3)pipi) surfaces intersecting at the same region. The calculations suggest that intersystem crossing (ISC) can occur very fast from the S(1) state to the nearby triplet states. This is consistent with results from ultrafast spectroscopy experiments that observe the S(1) state ISC occurs within about 1-2 ps to produce a triplet state for HPA and related pHP compounds. The alpha and beta bond cleavage reactions for the T(1) state of HPA are both predicted to have fairly high barriers and compete with one another. However, this is not completely consistent with experiments that observe the photodeprotection reactions (e.g. the beta bond cleavage) of HPA and some other pHP phototriggers in largely water containing solvents are predominant and occur very fast to release the leaving group. Comparison of the computational results with experimental results for HPA and related pHP compounds suggests that water molecules likely play an important part in changing the triplet state beta bond cleavage so that it becomes the predominant pathway and occurs very fast to give an efficient deprotection reaction. The results reported here provide new insight into the photophysics, reaction pathways, and photochemistry of the p-hydoxyphenacyl acetate and related pHP caged phototrigger compounds and also provide a benchmark for further and more sophisticated investigations in the future.     

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 ESR study of the lowest excited triplet states of para-quinones in glassy matrices at 77 k

The lowest excited nπ* triplet of 9.10-anthraquinone, 1.4-naphthoquinone and 1,4-benzoquinone were studied in glassy matrices at 77 K using a time-resolved ESR method. The D value of the triplet state of 9,10-anthraquinone varied from ?0.351 cm^?1 in a polar solvent to ?0.318 cm^?1 in a non-polar solvent. Both 1,4-naphthoquinone and 1,4-benzoquinone in polar solvents showed triplet state spectra with a D value of ?0.330 cm^?1. A computer simulation revealed the existence of widely distributed zero-field splitting parameters in the glassy condition. These data are compared with an analysis of CIDEP results of para-quinones.     

Resonance Raman scattering from triplet states of acridine and anthracene

By preparing a population of triplet states through excitation with a laser pulse of 308 nm, we could determine the resonance Raman spectra of acridine and of anthracene in their lowest (π π*) state, T₀. Thirty Raman lines in the region between 200 and 2000 cm^?1 are reported for acridine in the T₀ state. These are attributed to either a₁ or b₂ vibrational modes. Nineteen Raman lines between 500 and 2000 cm^?1 are reported for anthracene in the T₀ state.     

Time-resolved resonance Raman study of the reaction of the 2-fluorenylnitrenium ion with 2-fluorenylazide

A time-resolved resonance Raman investigation of the reaction of the 2-fluorenylnitrenium ion with 2-fluorenylazide in a mixed aqueous solvent is presented. The reaction of the 2-fluorenylnitrenium ion with 2-fluorenylazide in the mixed aqueous solution generates two new species on the microsecond time scale. One of these species is identified as 2,2'-azobisfluorene, and the other species is tentatively assigned to a 1,4-bis-(2,2'-fluorenyl)-tetrazadiene cation intermediate. The structure and properties of these two species are briefly discussed. The reaction of the 2-fluorenylnitrenium ion with 2-fluorenylazide is also briefly compared to that of the 2-fluorenylnitrenium ion reactions with guanosine and water.     

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.     

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.     

Time-resolved resonance Raman spectroscopy and density functional study of 2-fluorenylnitrene and its dehydroazepine products

We report time-resolved resonance Raman spectra for 2-fluorenylnitrene and its dehydroazepine products acquired after photolysis of 2-fluorenylnitrene in acetonitrile. The experimental Raman band frequencies exhibit good agreement with the calculated vibrational frequencies from UBPW91/cc-PVDZ density functional calculations for the singlet and triplet states of the 2-fluorenylnitrene as well as BPW91/cc-PVDZ calculations for the two dehydroazepine ring-expansion product species. The decay of the 2-fluorenylnitrene Raman signal and the appearance of the dehydroazepine products suggest the presence of an intermediate species (probably an azirine) that does not absorb very much at the 416 nm probe wavelength used in the time-resolved resonance Raman experiments. Comparison of the singlet 2-fluorenylnitrene species with the singlet 2-fluorenylnitrenium ion species indicates that protonation of the nitrene to give the nitrenium ion leads to a significant enhancement of the cyclohexadienyl character of the phenyl rings without much change of the C-N bond length.     

A resonance Raman study of the higher-lying electronic states of styrene vapor

Resonance Raman spectra of styrene vapor excited in the S₂ /S₃ and S₄ absorption systems are reported. The lack of double bond torsional scattering activity is taken as evidence of high barriers to rotation in all the excited states examined. Consequences for styrene photoisomerization are discussed. Excitation to S₂/S₃ is delocalized in nature, while S₄ excitation is benzene-like. The observed scattering intensities are largely in agreement with the jet absorption analysis and the results of Franck-Condon calculations.     

Time-resolved resonance Raman observation of the dimerization of didehydroazepines in solution

Time-resolved resonance Raman (TR3) studies of the photochemistry of phenyl azide, 3-hyroxyphenyl azide, 3-methoxyphenyl azide and 3-nitrophenyl azide in acetonitrile:water solutions is reported. After photolysis of these four aryl azides in room temperature solutions, only one species was observed in the TR3 spectra for each azide, respectively at the probe wavelengths employed in the TR3 experiments. The species observed after photolysis of 3-nitrophenyl azide was assigned to 3,3'-dinitroazobenzene, an azo compound formed from the dimerization reaction of triplet 3-nitrophenylnitrene. In contrast, the species observed after photolysis of phenyl azide, 3-hydroxyphenyl azide and 3-methoxyphenyl azide were tentatively assigned to intermediates formed from the dimerization of didehydroazepines that are produced from the ring expansion reaction of the respective singlet arylnitrene. To our knowledge, this is the first time-resolved vibrational spectroscopic observation of the dimerization reaction of didehydroazepines in solution. In addition, these are the first resonance Raman spectra reported for dimers formed from didehydroazepines. We briefly discuss the structures, properties and chemical reactivity of the dimer species observed in the TR3 spectra and possible implications for the photochemistry of aryl azides.     

Time-resolved resonance raman spectra and decay kinetics of triplet anthracene in fluid media

Reported here are the time-resolved resonance Raman spectra and decay kinetics of the lowest triplet state (³B_2u⁺) of anthracene-h₁₀ and anthracene-d₁₀ molecules in fluid media at room temperature. The triplet population (≈3 × 10^?5 M) is observed to decay at microsecond times by triplet—triplet annihilation. Vibrational assignments for the observed Raman bands are proposed.     

Magnetic resonance investigations of photoexcited triplet states in naphthalene-tetracyanobenzene single crystals

Photoexcited triplet states in naphthalene-tetracyanobenzene single crystals were investigated at 4.2 K by zero-field optically detected magnetic resonance and at room temperature by conventional high-field electron paramagnetic resonance. The low-temperature experiments revealed at least three triplet-state traps with distinct zero-field splittings, phosphorescence spectra, or excitation dependence.     

Theoretical study of low-lying triplet states of aniline

Multireference complete active space self-consistent-field CASSCF(10,12)/ANO and second-order perturbation theory MS-CASPT2 calculations were performed to determine the vertical low-lying singlet and triplet states of aniline. The sequence of the seven lower lying triplet states is T1(1(3)A'), T2(1(3)A' '), T3(2(3)A'), T4(3(3)A'), T5(2(3)A' '), T6(4(3)A'), and T7(3(3)A' '). The 3(3)A', 4(3)A', and 3(3)A' ' states are assigned as 3s, 3py, and 3pz Rydberg states, respectively, while other states correspond to pi <-- pi excitations. Both the T1 and T2 states are found to be below at the lowest-lying singlet S1 (1(1)A' ') state. Geometry, vibrational modes, and electron distribution of the lowest lying T1 state were determined using UB3LYP calculations. The vertical and adiabatic singlet-triplet energy gaps DeltaE(S0-T1) amount to 3.7 and 3.5 +/- 0.2 eV, respectively. In clear contrast with the S0 state, the triplet aniline is no longer aromatic, and its protonation occurs preferentially at the ring meta-carbon site, with a proton affinity PA = 243 +/- 3 kcal/mol.