Dual fluorescence lifetimes of pyrimidine: Intermediate radiationless decay

We have observed a dual fluorescence decay from the lowest n → π* excited singlet state of pyrimidine. The vibronic states 0-0, 6a¹, 12¹, 6a¹12¹, 12² and 6a¹12² have two exponential decays with lifetimes ranging from 2.7-0.7 nsec and from 410-234 ns at 0.02 torr. The ratio of pre-exponentials is pressure independent but the long decay is very sensitive to collisions. The four lower energy states have effective impact diameters of 16 A and the highest energy state is quenched by gas kinetic collision diameters (≈ 5.5 Å). The dual fluorescence decay and collisional fluorescence quenching by rotational relaxation is consistent with the available models of singlet-triplet mixed state decay. Using these models we have computed the rates for singlet-triplet crossing, the number of coupled triplet levels, and the decay rates for internal conversion. The model used our measured fluorescence decay parameters and our estimate of a triplet loss rate. The estimated triplet loss varies from 0.2 to 2.0 × 10⁶ s^?1 and the singlet internal conversion rate varies from ≈ 0.4 to 56 × 10⁷ s^?1. The singlet-triplet radiationless rate suggests that 50–100 times more triplet levels are effective in the state mixing than can be expected from the triplet vibronic density. Such an enhanced coupling of ro-vibronic triplet levels is 5–10 times larger than previously observed for the dicarbonyls. The observation of reduced collisional quenching of higher energy vibronic levels is quantitatively interpreted by a different model than used previously for the dicarbonyls.     

The phosphorescence of benzene obtained byab initio and semi-empirical calculations

Summary Radiative decay and phosphorescence of triplet stare benzene is doubly -orbital and spin- forbidden and is only activated through vibronic coupling among the manifold of triplet states. For this reason the determination of lifetime and transition moments for the decay of triplet benzene has posed a considerable challenge to both theory and experiment. In the present work we have addressed the triplet benzene problem at several levels of theory; by truncated perturbation theory and semiempirical, CNDO/S-CI, calculations; by complete sum-over-state calculations as implemented in recentab initio multiconfiguration quadratic response (MCQR) theory; and by direct MCQR calculations of vibronic phosphorescence. The vibronic coupling is in the two former cases treated by the Herzberg-Teller (H-T) perturbation theory, involving four main mechanisms for the phosphorescent decay of triplet benzene. The results and interpretations given by these approaches as well as their merits and limitations are presented and discussed in some detail. Our calculations indicate that the phosphorescent decay of the³ B _1u state takes place predominantly through vibronic coupling along thee _2g mode. We obtain a phosphorescence that is almost completely out-of-plane polarized, which is in line with more recent measurements by the microwave-induced delayed phosphorescence technique, and could reproduce quite well the intensity ratios for different vibronic bands obtained in that experiment. The final triplet state lifetime is the result of a delicate sum of contributions from several vibronic degenerate and non-degenerate modes. The direct vibronic phosphorescence calculations predict a long lifetime, about one minute — 68 seconds for the best wavefunction — and seem to focus on a doubling of the assumed, albeit not established, best experimental value for the radiative lifetime of triplet benzene; 30 seconds.Dedicated to Inga Fischer-Hjalmars on her 75th birthday     

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.     

CHLOROPHYLL PHOTOCHEMISTRY IN CONDENSED MEDIA—I. TRIPLET STATE QUENCHING AND ELECTRON TRANSFER TO QUINONE IN CELLULOSE ACETATE FILMS*

Chlorophyll-a was incorporated into cellulose acetate films and the triplet state decay kinetics and electron transfer from triplet to p-benzoquinone in aqueous solution was studied using laser flash photolysis and EPR. The triplet was found to decay by first order kinetics with a rate constant which was independent of Chl concentration. The triplet yield, however, was concentration dependent. These properties are due to quenching which occurs only at the singlet state level. In the presence of quinone, the triplet is quenched and, when the quinone is in an aqueous solution in contact with the film, Chl cation radical (C^±) as well as the semiquinone anion radical (Q^±) can be observed. The C decays by second order kinetics with a rate constant of 1.5 × 10⁶M^-1 s^-1. Although triplet conversion to radicals is slightly lower in the films as compared to fluid solutions (? 3 times), the lifetimes of the radicals are greatly increased (? 10³ times).     

A spectroscopic investigation of aggregations in chlorophyll solutions

Abstract— Absorption and emission spectra are reported for polar and non-polar solutions of chlorophyll a and chlorophyll b. These spectra can be interpreted in terms of the formation of chlorophyll dimers and more highly aggregated forms. The phosphorescence spectra of polar and non-polar solutions of chlorophyll a are identical and are associated with emission from a π-π-* triplet state.     

Nonradiative decay processes in benzene

We investigate the behavior of single vibronic level nonradiative decay rates in benzene and benzene-d₆. The effects of excitation in a promoting mode which undergoes frequency and geometry changes in the S₁ relaxation (to T₁ or S₀) are considered in detail. Calculated relative nonradiative decay rates are compared with experimental values and are used to assign triplet state vibrational frequencies to the ν_s, ν₁₀ and ν₁₆ vibrations. This comparison also indicates that none of these modes, nor the modes ν₁ and ν6, are likely to be the dominant promoting modes for the S₁ → T₁ decay. Some simple expressions are given which provide good estimates of the vibronic state dependence of the non-radiative decay rates. In conjuction with experimental decay rate data, these estimates can aid in guiding spectral assignments of vibronic bands. Simple but general theoretical criteria are derived which are useful in determining those vibrations which are poor (or good) accepting modes. Our previous theory is generalized to consider absolute nonradiative decay rates. The results are used to suggest a possible mechanism for the “channel three” decay process observed by Callomon . Although the numerical applications presented here are to benzene electronic relaxation processes, the theoretical developments also apply to and the calcultions illustrate general features of nonradiative decay in the statistical limit.     

Quenching of chlorophyll a singlets and triplets by carotenoids in light-harvesting complex of photosystem II: comparison of aggregates with trimers

Laser-induced changes in the absorption spectra of isolated light-harvesting chlorophyll a/b complex (LHC II) associated with photosystem II of higher plants have been recorded under anaerobic conditions and at ambient temperature by using multichannel detection with sub-microsecond time resolution. Difference spectra (ΔA) of LHC II aggregates have been found to differ from the corresponding spectra of trimers on two counts: (i) in the aggregates, the carotenoid (Car) triplet–triplet absorption band (ΔA>0) is red-shifted and broader; and (ii) the features attributable to the perturbation of the Q_y band of a chlorophyll a (Chla) by a nearby Car triplet are more pronounced, than in trimers. Aggregation, which is known to be accompanied by a reduction in the fluorescence yield of Chla, is shown to cause a parallel decline in the triplet formation yield of Chla; on the other hand, the efficiency (100%) of Chla-to-Car transfer of triplet energy and the lifetime (9.3 μs) of Car triplets are not affected by aggregation. These findings are rationalized by postulating that the antenna Cars transact, besides light-harvesting and photoprotection, a third process: energy dissipation within the antenna. The suggestion is advanced that luteins, which are buried inside the LHC II monomers, as well as the other, peripheral, xanthophylls (neoxanthin and violaxanthin) quench the excited singlet state of Chla by catalyzing internal conversion, a decay channel that competes with fluorescence and intersystem crossing; support for this explanation is presented by recalling reports of similar behaviour in bichromophoric model compounds in which one moiety is a Car and the other a porphyrin or a pyropheophorbide.     

Vibronic coupling of pyrene in the first ππ* excited state

The two-photon fluorescence excitation spectrum of pyrene in n-hexane and n-heptane matrices has been measured at 10 K in the region of the first electronic transition (26800–30200 cm^?1). The spectrum consists of a rich number of sharp bands, being in general better resolved in n-hexane than in n-heptane matrix. Shpol'skii multiplets have been observed for the most intense bands. A strong two-photon band dominates the spectrum = 1495 cm^?1 from the 0—0 line and was assigned to B_1u × b_1u = A_g symmetry. Other weaker vibronic origins occur in the spectrum which were correlated to vibrational modes of b_1u, b_2u, b_3u and a_u symmetry. Intense vibronic bands are observed close to the origin of the second electronic transition and were interpreted as combination bands of B_1u × b_1u × b_3g symmetry. A two-photon vibronic theory to account for their intensity is proposed where the electronic moment is linearly expanded in powers of the nuclear displacements.     

Vibronic relaxation in parabenzoquinone: Its effect on the decay rate of rotary spin echoes

We have observed that the decay rates of rotary spin echoes in the lowest triplet state T₀ of parabenzoquinone (PBQ) and toluquinone (TQ) increase exponentially with the temperature. The activation energy in TQ is equal to the trap depth but in PBQ it corresponds with the energy differences between T₀ and a higher lying vibronic level T_e. From the experimental data we obtain the rates for excitation to and decay from this excited vibronic level in PBQ.     

Theoretical DFT study of phosphorescence from porphyrins

Geometrical structure of free-base porphin (H₂P) and Mg- and Zn-porphyrins together with their vibrational frequencies and vibronic intensities in phosphorescence are investigated by density functions theory (DFT) with the standard B3LYP functional. These molecules have a closed-shell singlet ground state (S₀) and low-lying triplet (T₁) excited states of ππ* type. The S₀–T₁ transition probability and radiative lifetime of phosphorescence (τ_p) of these molecules are calculated by time-dependent DFT utilizing quadratic response functions for account of spin–orbit coupling (SOC) and electric-dipole transition moments including displacements along active vibrational modes. The infrared and Raman spectra in the ground singlet and first excited triplet states are also studied for proper assignment of vibronic patterns. The long radiative lifetime of free-base porphin phosphorescence (τ_p ∼ 360 s at low temperature limit, 4.2 K) gets considerably shorter for the metalloporphyrins. An order of magnitude reduction of τ_p is predicted for Mg-porphyrin but no change of phosphorescence polarization is found. A forty times enhancement of the radiative phosphorescence rate constant is obtained for Zn-porphyrin in comparison with the H₂P molecule which is accompanied by a strong change of polarization and spin-sublevel radiative activity. A strong vibronic activity of free-base porphin phosphorescence is found for the b_2g mode at 430 cm⁻¹, while the 679 and 715 cm⁻¹ vibronic bands of b_3g symmetry are less active. These and other out-of-plane vibrations produce considerable changes in the radiative constants of different spin sublevels of the triplet state; they also promote the S₁ → T₁ intersystem crossing. Among the in-plane vibrations the a_g mode at 1614 cm⁻¹ is found very active; it produces a long progression in the phosphorescence spectrum. The time-dependent DFT calculations explain the effects of the transition metal atom on phosphorescence of porphyrins and reproduce differences in their phosphorescence and EPR spectra.     

Photophysics of diplatinum polyynediyl oligomers: chain length dependence of the triplet state in sp carbon chains

The series of polyynes with the structure trans, trans-[Ar-Pt(P 2)-(C[triple bond]C) n -Pt(P 2)-Ar], where P = tri( p-tolyl)phosphine, Ar = p-tolyl, and n = 3, 4, 5, 6 (6, 8, 10, 12 sp carbon atoms), has been subjected to a comprehensive photophysical investigation. At low temperature ( T < 140 K) in a 2-methyltetrahydrofuran (MTHF) glass, the complexes exhibit moderately efficient phosphorescence appearing as a series of narrow (fwhm < 200 cm (-1)) vibronic bands separated by ca. 2100 cm (-1). The emission is assigned to a (3)pi,pi* triplet state that is concentrated on the sp carbon chain, and the vibronic progression arises from coupling of the excitation to the -C[triple bond]C- stretch. The 0-0 energy of the phosphorescence decreases with increasing sp carbon chain length, spanning a range of over 6000 cm (-1) across the series. Transient absorption spectroscopy carried out at ambient temperature confirms that the (3)pi,pi* triplet is produced efficiently, and it displays a strongly allowed triplet-triplet absorption. In the MTHF solvent glass ( T < 140 K), the emission lifetimes increase with emission energy. Analysis of the triplet nonradiative decay rates reveals a quantitative energy gap law correlation. The nonradiative decay rates can be calculated by using parameters recovered from a single-mode Franck-Condon fit of the emission spectra.     

Thermodynamic Control of Intramolecular Singlet Fission and Exciton Transport in Linear Tetracene Oligomers

We newly synthesized a series of homo- and hetero-tetracene (Tc) oligomers to propose a molecular design strategy for the efficient exciton transport in linear oligomers by promoting correlated triplet pair (TT) dissociation and controlling sequential exciton trapping process of individual doubled triplet excitons (T+T) by intramolecular singlet fission. First, entropic gain effects on the number of Tc units are examined by comparing Tc-homo-oligomers [(Tc)_n: n=2, 4, 6]. Then, a comparison of (Tc)_n and Tc-hetero-oligomer [TcF₃-(Tc)₄-TcF₃] reveals the vibronic coupling effect for entropic gain. Observed entropic effects on the T+T formation indicated that the exciton migration is rationalized by number of possible TT states increased both by increasing the number of Tc units and by the vibronic levels at the terminal TcF₃ units. Finally, we successfully observed high-yield exciton trapping process (trapped triplet yield: Φ_TrT=176 %).     

SENSITIZED TRIPLET FORMATION OF CHLOROPHYLL-A AND ß-CAROTENE

The naphthalene-sensitized formation of triplet excited chlorophyll-a (Chl-a) and all-transß-carotene has been studied by pulse radiolysis. The rate constants for transfer of triplet energy from naphthalene to Chl-a and all-transß-carotene in benzene at 25°C are (3.6 ± 0.6)·10⁹M^-1 s^-1 and (10.7 ± 1.2)·10⁹M^-1 s^-1, respectively. The decays of the excited triplet states of naphthalene, Chl-a and all-transß-carotene all follow a mixed first-and second-order mechanism. The first-order rate constant for triplet decay is strongly dose dependent for naphthalene but only slightly dependent and independent of dose for Chl-a and all-transß-carotene, respectively. The rate constants for triplet-triplet annihilation are (1.4 ± 0.3)·10⁹M^-1 s^-1 for Chl-a and (3.6 ± 0.4)·10⁹M^-1 s^-1 for all-transß carotene. The nearly constant ratio k(ß-carotene)/k(Chl-a) for the bimolecular triplet energy transfer rate constants is discussed in terms of the molecular shapes of the two molecules. The energetics of the triplet-triplet annihilation of all-transß-carotene are discussed, and it is proposed that production of the excited ¹A_B state may be a major route in the annihilation process.     

Untersuchung reversibler Photoprozesse durch Blitzlichtphotolyse IV. Triplettzustände des 1, 3, 6′, 8′- tetramethyl-dehydrodianthrons

By means of the flash photolysis technique, transient absorption spectra attributed to tetramethyl-dehydrodianthrone (TMD) in both the photochromic and triplet states have been investigated in polymethylmethacrylate matrices and in the solvent triacetin. In polymethylmethacrylate matrices and in rigid glasses of triacetin the triplet state of TMD is heavily populated. Triplet-triplet absorption and phosphorescence measurements show that below 180°K the triplet decay follows first order kinetics with the decay constant k=11,3 ± 0,1 s^?1. In incompletely solidified triacetin glass it is possible to monitor the transient absorption of the photochromic and the triplet state simultaneously. It is shown that the photochromic state ¹A₁* is not generated via the triplet state. Therefore the authors suggest a kinetic scheme characterised by a direct singlet state - photochromic state transition.     

Comparative sensibility of S1 ←S0 and S2 ←S0 indole electronic transitions to environment perturbations. The positions of the 0—0 bands in polar media

The modifications under environmental perturbations of the well separated two first electronic systems of carbazole are used to demonstrate the possibility of including solute-ether complexes in polyethylene films. The application of this technique to indole allows to differentiate the overlapping ¹L_a and ¹L_b transitions in the spectrum of hydrogen bonded indole. The ground state interactions of indole, 5-methoxyindole and 3-methylindole in several well defined environment conditions (hydrogen bond in a polar cage, hydrogen bond in a nonpolar cage) are investigated through UV absorption spectroscopy between 293 and 88 K. A strong red shift of the ¹L_a bands under the hydrogen bonding occurs with all the indoles investigated. The comparative analysis of the spectra in three types of environment, allows to give the accurate location of the first bands for the two overlapping vibronic systems of these molecules in polar media. Particularly, it is unambiguously shown that the first ¹L_a band and the first ¹L_b band of indole in alcohols are superposed at 288 nm. These results offer reliable new bases for the study of the emission properties of indole and its derivatives.     

Depolarization dispersion curves of resonance Raman fundamentals of metalloporphyrins and metallophthalocyanines subject to asymmetric perturbations

A model is presented that allows the investigation of depolarization dispersion curves of a_1g,a_2g,b_1g and b_2g resonance Raman fundamentals in the region of the Q state of metalloporphyrins and metallophthalocyanines. This dispersion results from electronic and/or vibronic perturbations of A_2g,B_1g and B_2g symmetry due to asymmetric substituents and/or metal ion-ring interaction acting on the porphyrin (phthalocyanine) ring. The electronic perturbations affect the electronic configuration interaction pattern between the four orbital components of the Q and B states, yielding thereby similar depolarization dispersion curves for all modes of a given symmetry, whereas the vibronic perturbations affect selectively the vibronic coupling matrix of a particular mode. Depolarization dispersion curves resulting from A_2g and B_1g perturbations are treated separately, and many helpful perturbational formulas are given for use in analyzing experimental data. Examples of depolarization dispersion curves and excitation profiles of fundamentals of a_1g, a_2g, b_1g and b_2g symmetry are presented. It is shown that strong depolarization dispersion observed in copper chelate of mesoporphyrin IX dimethyl ester for a_1g and a_2g fundamentals can be explained in terms of an A_2g electronic perturbation and a vibronic a_2g perturbation suffered by the a_1g(1131 cm^?1) fundamental. Similarly, the depolarization dispersion curves observed for fundamentals in cytochrome c and Pt-phthalocyanine are explained in terms of an electronic B_1g perturbation, together with selective b_1g vibronic perturbations acting on the 1310 cm^?1 a_2g fundamental in cytochrome c and the 482 cm^?1 b_2g fundamental in Pt-phthalocyanine. The agreement between the depolarization dispersion curves predicted by our model and experimental data is shown to be satisfactory.     

Interpretation of the vibronic fine structure in the fluorescence spectrum of matrix isolated UF6

The emission spectra of UF₆ in an argon matrix are ascribed to a T_1g state subject to a strong Jahn-Teller effect and a weak crystal field and Herzberg-Teller effects. These effects account quantitatively for the observed triplet structure of the vibronic bands without involving different crystal sites.     

Ground and lowest excited triplet state dissociation constants of the 5-, 6- and 7-mononitro-8-quinolinols. Spectroscopic evidence for intramolecular hydrogen bonding in the lowest triplet state of 7-nitro-8-quinolinol

Abstract— The dissociation constants of 5-nitro-8-quinoIinol, 6-nitro-8-quinolinol and 7-nitro-8-quinolinol have been determined for the ground states of the molecules by potentio-metric titration, and for the lowest triplet states from the shifts of the O—O bands of phosphorescence occurring upon dissociation. The ground-state dissociation constants of the phenolic groups are found to be typical of ortho, meta, and para nitro phenols. The phosphorescences of all prototropic species derived from the 6-isomer appear to be anomalous and their origin is unknown. The anomalously low acidity of the neutral species of the 7-isomer in its triplet state, relative to that of the neutral species of the 5-isomer in its triplet state, is attributed to intramolecular hydrogen bonding in the lowest triplet sfate of the 7-isorner, between the phenolic proton and the 7-nitro group. The magnitudes of the two triplet-state disSociation constants of both the 5 and 7 isomers suggest that the predominant uncharged species in fluid solutions containing the triplet-state molecules are the excited zwitterions.     

Two-photon ionization spectrum of the 1Lb ← S0 transition in toluene

The multiphoton ionization (MPI) spectrum of toluene arising from the ¹B₂ (¹L_b) valence state has been investigated. The state participates as a two-photon resonance. A total of nine excited state fundamentals have been characterized, including three non-totally symmetric vibrations. The toluene MPI spectrum shows a strong resemblance to the two-photon fluorescence excitation spectrum with the strongest transitions taking place to the origin and excited state modes ν₁(a₁), ν₁₂(a₁) and ν₁₄(b)₂). The intensities of the observed fundamentals are rationalized in terms of Franck-Condon and vibronic coupling effects. A major conclusion is, that the primary mechanism for the activity of ν₁₂ is vibronic coupling.     

Excited state kinetics and stern-Volmer quenching plots in the photolysis of azoisopropane at 366 nm

The photochemistry of azoisopropane is reinvestigated at 366 nm over an extended pressure range by using n-butane as an added bath gas, and over a range of temperature from 277° to 180°C. The Stern-Volmer type plot of the N₂ product quantum yield is interpreted in terms of the decomposition of the vibrationally excited upper singlet and triplet states, with the onset of the dissociation of the vibrationally equilibrated triplet state as the temperature is increased. The energy barrier for the dissociation of the vibrationally equilibrated first triplet state is found to be 8.8 kcal/mole. Triplet sensitization experiments with biacetyl correlate with our observations, and it is suggested that the proposed mechanism is generally applicable to the photodissociation of acyclic azoalkanes at 366 nm, based on a comparison of our data with those of Wu and Rice on hexafluoroazomethane.