LOWEST EXCITED TRIPLET-STATE DISSOCIATION CONSTANTS OF PURINES IN AQUEOUS SOLUTION. SUBSTITUENT EFFECTS*

Abstract— Lowest triplet-state dissociation constants of purine and eight of its derivatives have been determined from the shifts of the 0–0 bands of phosphorescence occurring upon protonation and deprotonation (dissociation), in rigid aqueous solutions at 77°K. Triplet-state dissociation constants (p K _aτ) are found to be significantly different from the corresponding ground–state p K _a-values, differences ranging from 0.6 to 3.8 p K units. In the equilibria between neutral and anion species, purines are more acidic in the triplet state, due to derealization of the negative charge of the triplet-state anion. In the equilibria between cation and neutral species, the basicity of purine, amino, methyl, and benzyl-amino derivatives is increased in the triplet state compared to the ground state. The anomalously low basicity of the halogeno- and methylmercapto-derivatives in their triplet state is attributed to the increased interactions of dir orbitals of halogens and sulfur atoms with π orbitals in the triplet state. In general, substituent effects on the p K _a of purines appear to be parallel in the ground and excited triplet states.     

Ab initio molecular orbital studies of flavin radicals and the lowest triplet state of isoalloxazine

Ab initio calculations have been performed on the anion, neutral and cation radicals of isoalloxazine (isoalloxazine^?, 5H-isoalloxazine and 1,5-dihydroisoalloxazinium respectively) and on the lowest excited triplet state of isoalloxazine within the RHF and UHF SCF method. The calculated spin distribution is in reasonable agreement with experiments, provided the molecules are assumed to be planar. However, total energy considerations suggest that the radicals are non-planar in the gaseous phase.     

Position-dependent heavy atom effect in the subrates of the lowest triplet state of dichloronaphthalenes

By means of the microwave-induced delayed phosphorescence (MIDP) technique, the dynamics of the lowest triplet state T₀ of several dichloronaphthalenes in naphthalene and durene are investigated. It is shown that the position dependence is very selective in the nonradiative decay of the spin-sublevels: The out-of-plane spin state is nearly not affected, while the in-plane states are strongly affected. The radiative rates, on the other hand, are not selectively influenced by the position of substitution. Besides, the results show that the dynamics of the lowest triplet state is determined by the type of substitution, rather than by the symmetry of the molecule. The spin—orbit coupling of the in-plane states seems to profit from some distortion the guest molecule experiences in the naphthalene host lattice.     

Time-resolved infrared spectroscopy of the lowest triplet state of thymine and thymidine

Vibrational spectra of the lowest energy triplet states of thymine and its 2′-deoxyribonucleoside, thymidine, are reported for the first time. Time-resolved infrared (TRIR) difference spectra were recorded over seven decades of time from 300 fs to 3 μs using femtosecond and nanosecond pump-probe techniques. The carbonyl stretch bands in the triplet state are seen at 1603 and 1700 cm⁻¹ in room-temperature acetonitrile-d₃ solution. These bands and additional ones observed between 1300 and 1450 cm⁻¹ are quenched by dissolved oxygen on a nanosecond time scale. Density-functional calculations accurately predict the difference spectrum between triplet and singlet IR absorption cross sections, confirming the peak assignments and elucidating the nature of the vibrational modes. In the triplet state, the C4O carbonyl exhibits substantial single-bond character, explaining the large (70 cm⁻¹) red shift in this vibration, relative to the singlet ground state. Femtosecond TRIR measurements unambiguously demonstrate that the triplet state is fully formed within the first 10 ps after excitation, ruling out a relaxed ¹nπ^* state as the triplet precursor.     

The lowest triplet state of tetramethyl-1,3-cyclobutanedithione. III. Single-crystal Zeeman spectroscopy

Pulsed, high-field Zeeman spectra of the lowest singlet-to-triplet transition in single-crystal tetramethyl-1,3-cyclobutanedithione (TMCBDT) have been measured. The analysis of the spectra was performed using an extension of the theory of Castro and Hochstrasser which allows for an arbitrary alignment of the magnetic field with respect to the principal crystal axes. From this study we provide evidence that in TMCBDT (1) the lowest triplet is ³A_u (D_2h molecular symmetry). (2) only one spin-orbit coupling route is active. (3) a substantial zero-field splitting (2.5 ± 1.0 cm⁻¹) is present. (4) the magnetic and molecular axes are rotated by 90°. and (5) the spin-orbit contribution to the triplet-state g values is small compared to its effect on the zero-field splitting parameters.     

EFFECT OF THE BROMINE ATOM UPON THE DISSOCIATION CONSTANTS OF BROMOPYRIDINES,BROMOQUINOLINES AND 4-BROMOISOQUINOLINE IN THEIR LOWEST EXCITED SINGLET AND TRIPLET STATES*

Abstract— The lowest excited singlet-state dissociation constants (pK^S_a) of bromosubstituted pyridines, quinolines, and isoquinolines were determined from the pH-dependent shifts in their electronic absorption spectra. The lowest excited triplet-state dissociation constants (pK^T_a) of bromosubstituted quinolines and 4-bromoisoquinoline were obtained from the shifts of the 0–0 phosphorescence bands measured in rigid aqueous solution at 77 K. The pK^S_a values indicate that the basicity of these brominated nitrogen heterocycles is increased in the lowest excited singlet state by 2 to 10 orders of magnitude as compared with the ground state. The pK^T_a values are found to be significantly different from the corresponding ground-state pK_a values, indicating that the basicity of bromoquinolines is increased in the lowest excited triplet state by 1.7 to 3.0 pK units. The enhancement of the excited singlet-and triplet-state basicity of brominated nitrogen heterocycle derivatives as compared with the unsuhstituted parent compounds is attributed to the increased electron-donor conjugative interactions of the bromine atom pπ orbitals with π orbitals in the lowest excited singlet and triplet state.     

A diffusion quantum Monte Carlo study on the lowest singlet and triplet electronic states of BN molecule

Ab initio calculation of both the lowest singlet and triplet electronic states of BN has been performed by the fixed-node Ornstein-Uhlenbeck diffusion quantum Monte Carlo method with the floating spherical Gaussian orbitals and spherical Gaussian geminals. The Monte Carlo calculation gives equilibrium bond lengths and equilibrium harmonic frequencies of 1.3317(7) A and 1529(7) cm(-1), respectively, for the lowest triplet state and 1.2751(7) A and 1709(8) cm(-1), respectively, for the lowest singlet state. Also, the Monte Carlo calculation reports an energy separation of 178(83) cm(-1) between the two electronic states and recommends the ground state is the lowest triplet state.     

On the intrinsic population of the lowest triplet state of thymine

The population of the lowest triplet state of thymine after near-UV irradiation has been established, on the basis of CASPT2//CASSCF quantum chemical calculations, to take place via three distinct intersystem crossing mechanisms from the initially populated singlet bright 1pipi* state. Two singlet-triplet crossings have been found along the minimum-energy path for ultrafast decay of the singlet state at 4.8 and 4.0 eV, involving the lowest 3npi* and 3pipi* states, respectively. Large spin-orbit coupling elements predict efficient intersystem crossing processes in both cases. Another mechanism involving energy transfer from the lowest 1npi* state with much larger spin-orbit coupling terms can also be proposed. The wavelength dependence measured for the triplet quantum yield of pyrimidine nucleobases is explained by the location and accessibility of the singlet-triplet crossing regions.     

Assignment of the lowest triplet state of 9,10-anthraquinone single crystals

The lowest triplet state of 9,10-anthraquinone single crystals is assigned as ³B_1g located at 22153 cm^?1. The pure electronic transition to this state is forbidden in the centrosymmetric crystal, but is induced in an electric field. Combined Stark—Zeeman measurements provide an unambiguous symmetric assignment of this transition and clear up the difficulties of previous assignments.     

Studies of the triplet state of the proton-transfer tautomer in salicylaldehydes

The spectroscopy and dynamics of the low-lying triplet state of the proton-transfer tautomer in salicylaldehydes have been studied via internal heavy-atom effects coupled with a sensitive near-IR detecting system. For 3,5-diiodosalicylaldehyde a weak proton-transfer keto-tautomer phosphorescence was resolved with a maximum at 710 nm (τ_p1.8 μs, Φ_obs5.23×10⁻⁴) in a 77 K methylcyclohexane glass. The results, in combination with the time-resolved thermal lensing experiment, further deduced the triplet-state population yield and radiative decay rate to be 0.20 (298 K) and 3.12×10² s⁻¹, respectively. Consequently, the energetics and dynamics of the triplet states during a proton transfer cycle are discussed in detail.     

Ab initio MO and approximate density functional theory studies on the lowest singlet and triplet states of s and as-indacene

The lowest singlet and triplet states of s- and as-indacene have been studied by means of high-level ab initio MO and approximate density functional methods. Among the geometrical and energetical details discussed are the equilibrium structure of s-indacene (C_2h or D_2h), the structures and energies of the low-lying s-indacene triplet states, and the stability and geometry of the singlet and triplet states of as-indacene. It is shown that single-determinant-based methods, such as Hartree-Fock or MP 2, are not suited to properly describe these molecules. Instead, methods are required which explicitly take into account nondynamical and dynamical electron correlation. The results obtained by density functional theory-based methods compare very well with the most elaborate ab initio MO data and seem to provide an economical alternative even for molecules with a complicated electronic structure such as s- and as-indacene. © 1995 John Wiley & Sons, Inc.     

Phosphorescence properties and kinetics of the lowest triplet state T1 of a series of pyrene derivatives as obtained by ODMR techniques

The experimental results of an ODMR investigation of the radiative and nonradiative properties of the lowest electronically excited triplet state T₁ of a series of pyrene derivatives are presented. The paper gives the highly resolved phosphorescence spectra of 1,2-benzopyrene, naphtho-(2′, 3′ ; 1,2)-pyrene, anthraceno-(2′, 3′; 1, 2)-pyrene, 1,2:6,7-dibenzo-pyrene, 1,2-benzonaphtho-(2′, 3′; 6, 7)-pyrene and 1,2; 4,5-dibenzo-pyrene along with vibrational analyses and — in part — symmetry assignments. The correlation know from the literature between triplet energies and the ZFS-parameter D is discussed in terms of character orders. Three of the molecules exhibit an unusual deactivation behaviour similar to that of 3,4-benzo-pyrene where the out-of-plane level is not the slowest level. Several possibilities to explain this unexpected behaviour are discussed.     

Pseudo-jahn-teller distortion of pyridine in its lowest triplet state

We have performed an electron spin echo detected electron paramagnetic resonance study of the nitrogen hyperfine interaction in the lowest triplet state of pyridine. It is concluded that the molecule is non-planar in this state owing to pseudo-Jahn-Teller coupling between the ³B₁(nπ) and the close-lying ³A₁(ππ) states.     

Electronic structure of the lowest excited triplet state of 5,12-naphthacenequinone

Continuous-wave time-resolved EPR (cw-TREPR) and pulsed electron nuclear double resonance (ENDOR) studies have been carried out to clarify the electronic structure of the lowest excited triplet (Tl) state of 5,12-naphthacenequinone (5,12-NpQ) as well as 1,4-anthraquinone (1,4-AQ) and 6,13-pentacenequinone (6,13-PeQ). The Tl energy level and the D value of the zero-field splitting (ZFS) parameters only slightly decreased with the increasing pi-conjugated system. The Tl states of these linear para-acenequinones were assigned to the pi pi* character. In triplet 5,12-naphthacenequinone, more than 80% of the unpaired electron spins are localized on the naphthalene aromatic sub-system.     

Spin delocalization in the lowest triplet state of benzophenone

Ring atom hyperfine structure has been observed in the optically-detected magnetic resonance (ODMR) spectra of the lowest triplet states of 4,4′-difluoro- and 4,4′-dibromobenzophenone in a single crystal host at liquid helium temperatures. An analysis of this structure and its angular dependence yields a lower limit of 0.10 for the normalized π-orbital spin density in each of the para ring positions. These results, together with the demonstrated sensitivity of the zero-field splitting parameters to halogen substituents in the 4 and 4′ positions, suggest that a significant amount of spin density resides on the phenyl rings in the ³(n,π^*) state of benzophenone.     

Electron spin resonance studies of the excited triplet states of DL-5-hydroxytryptophan, 5-hydroxyindole, 6-hydroxynicotinic acid, indole and hippuric acid

Abstract— The electron spin resonances of the metastable triplet states of DL-5-hydroxy-tryptophan, 6-hydroxynicotinic acid, and other related compounds have been observed in various glasses at 77°K. These compounds are biologically interesting in connection with photochemical processes. The Δm= 2 transitions were observed and the spin-spin interaction parameter (D²+ 3E²1/2 was determined. However, for indole, it was also able to observe the Δm= 1 transitions from which D and E were determined separately. From the values of (D²+ 3E²)1/2, D and E whenever possible and from the decay times of the triplet states, it was possible to characterize the lowest triplets as π—π* states in all cases. Studies of the pH dependence of DL-5-hydroxytryptophan, 5-hydroxyindole, and 6-hydroxynicotinic acid indicate that the unionized molecule exhibits a resonance different from that of the related ion with a proton removed from the phenolic hydroxyl group. The zero-field splitting parameters D* and the lifetime of the triplet states have also been measured in various glasses and in Lucite. Both temperature and the specific nature of the host media affected the values of these parameters.     

Anisotropic spin—lattice relaxation in the triplet state of naphtralene-h8 in n-pentane

The temperature dependence of the individual spin—lattice relaxation rate constants (W_ij) between the lowest triplet sublevels of naphthalene-h₈ in a polycrystalline Shpol'skii matrix of n-pentane have been measured between 1.2 and 2.4 K in zero field. The total sublevel decay constants are evaluated and found to be independent of temperature. The W_ij are found to be highly anisotropic, but the anisotropy pattern differs from that observed previously in a durene matrix.     

Vibrational study of 8-quinolinol and 7-(4-ethyl-1-methyloctyl)-8-quinolinol (Kelex 100), two representative members of an important chelating agent family

Rather complete i.r. and Raman data are presented for 8-quinolinol and 8-quinolinol-OD. Moreover, special attention is paid to the vibrational modes of 7-(4-ethyl-1-methyloctyl)-8-quinolinol. The most significant results concern the assignment of the CO stretching and the OH in-plane bending vibrations.     

Transfer of a hydrogen atom to acridine in the triplet state

Conclusions The quenching of triplet acridine by substituted phenols, thiophenols, and anilines is accomplished with high rate constants by hydrogen atom transfer with the quantitative formation of the corresponding neutral radicals.The solvation of the reagents by ethanol molecules or a decrease in the electron donor capacity of the quencher is accompanied by a decrease in the reaction rate constants.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 178–181, January, 1989.     

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.