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 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.     

The resonance Raman spectrum of tetramethylbutadiene in its lowest excited triplet state

The resonance Raman spectrum of tetramethylbutadiene in its lowest excited triplet state, with a reported lifetime of 80 ns, is presented. The triplet state was produced by laser flash photolysis using acetone as sensituer in acetonitrile. Transient Raman bands were seen at 1620, 1395, 1353, 1275, 1239, 1048, and 521 cm⁻¹.     

Time resolved absorption and resonance Raman spectroscopic studies of the oxidation of benzidine in aqueous solution

We report a time resolved resonance Raman study of transient radicals produced in the pulse radiolytic oxidation of benzidine in aqueous solution. The intense and structured transient absorption in the 400–470 nm region, observed at microsecond times in the acidic medium, is attributed to the benzidine radical cation. The Raman spectrum, observed by excitation in resonance with this absorption, exhibits eight prominent bands which are assigned to planar phenyl vibrations. The ring breathing mode (v₁) at 844 cm^-1 is most highly resonance enhanced, indicating an overall expansion of the ring CC bonds in the excited state. The interring CC bond, with partial double bond character, is characterized by an intense (v₁₃) Raman band at 1335 cm^-1. The frequency of the in-phase v_7a CN stretching vibration is 1540 cm^-1. These frequencies and the presence of weak bands attributable to non-planar phenyl vibrations indicate the radical to be slightly non-planar. The pK_a for the proton loss from the radical cation is 10.87, four units higher than for the aniline radical cation. At high pH the observed transient has a broad and structureless absorption at ∽ 380 nm. It is identified from its resonance Raman features as the 4(4′aminophenyl)anilino radical formed by proton loss from the radical cation. The interring CC bond is characterized by a Raman band at 1292 cm^-1, indicating it to be a single bond. The structure of this neutral radical is highly nonplanar, with little conjugation between the two ring systems so that electronic excitation is primarily confined to the anilino moiety. The acidic and basic forms of the radical react rapidly in second order processes to produce products which absorb strongly at, respectively, 360 and 410 nm.     

Time resolved laser spectroscopy of excited state processes in photoinduced polymerization reactions

During the last years, the use of photoinduced radical chain polymerization of unsatured monomers has increased considerably because of the vide applications of these processes in photoactive polymer-based systems. The present paper is mainly concerned with an investigation of the processes involved. For this purpose, time resolved laser spectroscopy (which appears to be a very convenient method for investigating directly the experimental behaviour of selectively excited chromophores) has been used. The utility of such a technique for the investigation of the primary processes which occur in the excited states of the photoinitiators immediately after the absorption of photons is demonstrated. As examples, main outlines corresponding to the classical processes observed in radical photoinitiated polymerization are considered and discussed: excited state processes in photoinitiators; mechanism of excitation transfer in combination of photosensitive systems; visible laser light induced polymerization; holographic recording.     

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.     

Intermolecular hydrogen atom abstraction from the fluoranil triplet excited state: a Raman study

Intermolecular hydrogen abstraction reaction mechanisms in photoexcited ketones have traditionally been studied using time resolved absorption spectroscopy. Another approach is presented involving time resolved resonance Raman spectroscopy to study such reactions, using the fluoranil/isopropanol system as an example. It has been shown that vibrational spectra can be recorded starting from the triplet excited state to the product state (radical anion) via the intermediate state, which is the ketyl radical. Thus, it is demonstrated that following the reaction evolution in terms of structural (vibrational modes) details would prove to be useful not only for mechanistic investigation but also for structure-reactivity correlations in photoexcited systems.     

Rotationally resolved IR-diode laser studies of ground-state CO2 excited by collisions with vibrationally excited pyridine

Relaxation of highly vibrationally excited pyridine (C5NH5) by collisions with carbon dioxide has been investigated using diode laser transient absorption spectroscopy. Vibrationally hot pyridine (E' = 40,660 cm(-1)) was prepared by 248 nm excimer laser excitation followed by rapid radiationless relaxation to the ground electronic state. Pyridine then collides with CO2, populating the high rotational CO2 states with large amounts of translational energy. The CO2 nascent rotational population distribution of the high-J (J = 58-80) tail of the 00(0)0 state was probed at short times following the excimer laser pulse to measure rate constants and probabilities for collisions populating these CO2 rotational states. Doppler spectroscopy was used to measure the CO2 recoil velocity distribution for J = 58-80 of the 00(0)0 state. The energy-transfer distribution function, P(E,E'), from E' - E approximately 1300-7000 cm(-1) was obtained by re-sorting the state-indexed energy-transfer probabilities as a function of DeltaE. P(E,E') is fit to an exponential or biexponential function to determine the average energy transferred in a single collision between pyridine and CO2. Also obtained are fit parameters that can be compared to previously studied systems (pyrazine, C6F6, methylpyrazine, and pyrimidine/CO2). Although the rotational and translational temperatures that describe pyridine/CO2 energy transfer are similar to previous systems, the energy-transfer probabilities are much smaller. P(E,E') fit parameters for pyridine/CO2 and the four previously studied systems are compared to various donor molecular properties. Finally, P(E,E') is analyzed in the context of two models, one indicating that P(E,E') shape is primarily determined by the low-frequency out-of-plane donor vibrational modes, and the other that indicates that P(E,E') shape can be determined from how the donor molecule final density of states changes with DeltaE.     

Emission spectroscopic and ODMR studies of the lowest excited triplet state of dichlorophenylborane

The triplet state T₁ of dichlorophenylborane (PhBCl₂) has been investigated by optical emission and ODMR spectroscopic methods in order to study the influence of substituents with mesomeric and inductive effects. The zero-field splitting (ZFS) parameters D and E, the selective kinetic rates of radiative and non-radiative deactivation of the triplet sublevels and the phosphorescence spectrum were measured. From the small value of D = 0.1201 cm^?1 a considerable charge transfer admixture to the ³L_a state of benzene has to be assumed. The ratio of the radiative rates shows a distortion of the molecule. Further a heavy atom effect of the chlorine atoms on the in-plane rates of the deactivation of T₁ can be observed.     

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.     

Naphthalene in the higher triplet excited state

Naphthalene in the higher triplet excited state Np(Tn) was generated from the two-step excitation method using two-colour two-laser flash photolysis technique and the lifetime of Np(Tn) was estimated to be 4.5 ps from the triplet energy quenching by quenchers such as p-dichlorobenzene, o-dicyanobenzene and carbon tetrachloride.     

Accessing the triplet excited state in perylenediimides

Here, we present a strategy designed to permit access to the PDI triplet manifold that preserves the desirable colorfastness and visible light-absorption properties associated with these dyes. To this end, three new Pt(II) complexes each bearing two PDI moieties tethered to the metal center via acetylide linkages emanating from one of the PDI bay positions have been synthesized, structurally characterized, and thoroughly examined by nanosecond laser flash photolysis. Upon ligation, the bright singlet-state fluorescence of the PDI chromophore is quantitatively quenched, and no long wavelength photoluminescence is observed from the Pt(II)-PDI complexes in deaerated solutions. In each of the Pt-PDI chromophores, quantitatively similar transient absorption difference spectra were obtained; the only distinguishing characteristic is in their single-exponential lifetimes (tau = 246 ns, 1.0 micros, and 710 ns). Triplet-state sensitization experiments of "free" PDI-CCH using thioxanthone confirmed the PDI triplet state assignments in each of the Pt-PDI structures.     

Ab initio calculation of resonance Raman cross sections based on excited state geometry optimization

A method for the calculation of resonance Raman cross sections is presented on the basis of calculation of structural differences between optimized ground and excited state geometries using density functional theory. A vibrational frequency calculation of the molecule is employed to obtain normal coordinate displacements for the modes of vibration. The excited state displacement relative to the ground state can be calculated in the normal coordinate basis by means of a linear transformation from a Cartesian basis to a normal coordinate one. The displacements in normal coordinates are then scaled by root-mean-square displacement of zero point motion to calculate dimensionless displacements for use in the two-time-correlator formalism for the calculation of resonance Raman spectra at an arbitrary temperature. The method is valid for Franck-Condon active modes within the harmonic approximation. The method was validated by calculation of resonance Raman cross sections and absorption spectra for chlorine dioxide, nitrate ion, trans-stilbene, 1,3,5-cycloheptatriene, and the aromatic amino acids. This method permits significant gains in the efficiency of calculating resonance Raman cross sections from first principles and, consequently, permits extension to large systems (>50 atoms).     

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.     

Emission from an excited triplet state during ECL

The application of Marcus theory of electron transfer reactions for the case of radical ion chemiluminescence of 9,10-diphenylanthracene (DPA) gives a high rate constant value (10⁹–10¹⁰ M^?1 s^?1) for the formation of the second triplet state (T₂). It is suggested that the near infrared emission observed during electrochemiluminescence of DPA is due to T₂ → T₁ fluorescence based on the high yield of T₂ (≈0.7) in the electron transfer reaction.     

Resonance Raman de-enhancement caused by excited state mixed valence

Resonance Raman and absorption spectra of 9,10-bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl)-anthracene (2) are measured and analyzed. The contribution of the individual vibrational normal modes to the reorganization energy is investigated. Excited-state mixed valence in this system is analyzed using density functional theory electronic structure calculations. The resonance Raman excitation profiles exhibit a resonance de-enhancement effect around 20 725 cm-1, but a corresponding feature is not observed in the absorption spectrum. This unusual observation is attributed to the presence of a dipole-forbidden, vibronically allowed component of the split mixed valence excited state. The de-enhancement dip is calculated quantitatively and explained in terms of the real and imaginary components of the polarizabilities of the two overlapping excited states.     

Optically detected magnetic resonance study of the lowest excited triplet state of aromatic thioketones: Xanthione

The aromatic thioketone xanthione has been investigated by means of the optically detected magnetic resonance (ODMR) technique in a n-hexane matrix at ≈ 1.1 K. It was established that the short-lived red emission, which is characteristic for many thione molecules, is phosphorescence. At high temperatures (77 K) this phosphorescence originates mainly (>80%) from the T₁^z (n, π*) sublevel (k_z^(r) >k_x^(r), k_y^(r). At low temperature (1.1 K) the intersystem crossing following S₂ (π, π*) ← S₀ excitation is a highly spin-sublevel selective process which populates predominantly the T₁^x and T₁^y, levels. Hence, the slow spin—lattice relaxation phosphorescence at low temperature originates from these sublevels. A value of 0.0611 cm^?1 was obtained for the zero-field parameter |E|/hc. A lower limit of 0.66 cm^?1 has been found for the zero-field parameter |D|/hc. This value is considerably larger than those observed for ketones, and it is shown that spin—orbit coupling contributes strongly to the zero-field splitting.     

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.     

Interpretation of unusual absorption bandwidths and resonance Raman intensities in excited state mixed valence

Excited state mixed valence (ESMV) occurs in molecules in which the ground state has a symmetrical charge distribution but the excited state possesses two or more interchangeably equivalent sites that have different formal oxidation states. Although mixed valence excited states are relatively common in both organic and inorganic molecules, their properties have only recently been explored, primarily because their spectroscopic features are usually overlapped or obscured by other transitions in the molecule. The mixed valence excited state absorption bands of 2,3-di-p-anisyl-2,3-diazabicyclo[2.2.2]octane radical cation are well-separated from others in the absorption spectrum and are particularly well-suited for detailed analysis using the ESMV model. Excited state coupling splits the absorption band into two components. The lower energy component is broader and more intense than the higher energy component. The absorption bandwidths are caused by progressions in totally symmetric modes, and the difference in bandwidths is caused by the coordinate dependence of the excited state coupling. The Raman intensities obtained in resonance with the high and low energy components differ significantly from those expected based on the oscillator strengths of the bands. This unexpected observation is a result of the excited state coupling and is explained by both the averaging of the transition dipole moment orientation over all angles for the two types of spectroscopies and the coordinate-dependent coupling. The absorption spectrum is fit using a coupled two-state model in which both symmetric and asymmetric coordinates are included. The physical meaning of the observed resonance Raman intensity trends is discussed along with the origin of the coordinate-dependent coupling. The well-separated mixed valence excited state spectroscopic components enable detailed electronic and resonance Raman data to be obtained from which the model can be more fully developed and tested.