STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS—VI. AB INITIO CONFIGURATION INTERACTION CALCULATIONS ON THE GROUND AND LOWER EXCITED SINGLET AND TRIPLET STATES OF ETHYL BACTERIOCHLOROPHYLLIDE-a AND ETHYL BACTERIOPHEOPHORBIDE-a‡

Abstract— Ab initio quantum mechanical calculations on ethyl bacteriochlorophyllide-a (Et-BChl-a) and ethyl bacteriopheophorbide-a (Et-BPheo-a) are presented, including self-consistent-field (SCF) molecular orbital studies on the ground states using the molecular fragment procedure, and configuration interaction (CI) calculations on the low-lying singlet and triplet states and absorption spectra. A characterization and comparison of many of the higher-lying molecular orbitals obtained from the SCF studies is presented. The estimated first ionization potentials are 5.66 and 5.97 eV for Et-BChl-a and Et-BPheo-a, respectively. Excited state calculations show that the visible spectrum of both molecules consists of an intense, y-polarized S₁← S₀ transition and a weakly-allowed, x-polarized S₂← S₀ transition. Both S₁ and S₂ states are ¹(π, π*) in character, and are described by a four-orbital model. Transitions to the remaining calculated states, S₃-S₁₂, appear in the Soret region of the spectrum of both molecules. However, only transitions to S₉(‘x’), S₁₀(‘x’) and S₁₁(‘y’) of Et-BChl-a, and S₇(‘x’) and S₁₀(‘y’) of Et-BPheo-a are of high intensity. The composition of the high intensity Soret states is ¹(π, π*) and strongly “four-orbital” in nature. The lowest triplet state, T₁, is predicted to lie 9752 cm^-1 and 7880 cm^-1 above S₀ for Et-BPheo-a and Et-BChl-a, respectively. In each molecule T₂ and S₁ are nearly degenerate, suggesting a favorable pathway for intersystem crossing. Calculated T_n← T₁ transitions indicate that the y-polarized T₁₂← T₁ transition in Et-BChl-a corresponds to the observed intense 24,400 cm^-1 absorption in the triplet-triplet spectrum of BChl-a. A similar type spectrum is also predicted for BPheo-a.     

Photoluminescence of Acenaphthenone in Organic Solvents and Aqueoug Media

The configuration of the lowest excited state of acenaphthenone, S₁(π, π^*) or T₁(π, π^*), depending on the solvent, dominates photoluminescence. The T₁(n, π^*) state in aprotic organic solvents is responsible for the phosphorescence of acenaphthenone. The wavelengths of the phosphorescence measured in benzene are 576 nm and 635 nm (vibronic) with 3.3 × 10^?4 quantum efficiency. However, the S₁(π, π^*) state in protic solution which dominates the fluorescence emission depending upon acidity is the most distinctive feature of acenaphthenone. The wavelengths of the emissions are 446 nm under water solvation with 0.185 quantum efficiency and 538 nm with 0.097 quantum efficiency under high acidity. The emission at 446 nm is assigned from a H-bonded keto-form excited state, whereas the emission at 538 nm is probably due to the excited state of protonated keto-form. The pK_a value in aqueous solution measured by diminution of fluorescence in basic solutions is 12.5 ± 0.4.     

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.     

SPECTROSCOPIC STUDIES OF PURINES—I. FACTORS AFFECTING THE FIRST EXCITED LEVELS OF PURINE*

Abstract— Studies of purine absorption and emission in seven solvents differing greatly in dielectric constant and hydrogen bonding potential, reveal a variety of solvent effects. For example, the resolution of structure in the absorption spectrum, the position and/or intensity of the X₂ absorption band, the intensity of fluorescence, the magnitude of the long wave-lenth tail, and the position of the X₁ absorption band are differentially affected—in the order listed—by the solvents tested. Even though it is possible to correlate the extent of decrease in the n-π* tail with increasing solvent dielectric constant, probably alterations in all of these spectroscopic parameters depend most critically upon the ability of the various solvents to form hydrogen bonds with the hydrogen on N9 and/for with the non-bonding electrons on the purine nitrogens: it is tentatively concluded that the probability of hydrogen bonding is directly correlated with the electronegativity of the aza nitrogens (N7 > N3 > N1). In solvents like isopropanol not all of the non-bonding electrons must be solvated maximally in most purine molecules since there is appreciable fluorescence under conditions where a long wavelength tail is readily observed in the absorption spectrum (alternatively some noa-bonding electrons may not te relevant to fluorescence quenching.) Decreases in fluorescence yield are associated with red shifts in the fluorescence maximum, and in the solvents of highest polarity the fluorescence yield is again small indicating that glycerol and water can enhance radiationless tunneling—presumably by altering Franck-Condon configurations and/or improving electronic-vibrational coupling between solute and solvent. The quantum yield is uniform throughout the atsorption band for a given solvent, but studies in aqueous buffers varying from pH 1 to 11 show that the fluorescence yield is greater for charged than for neutral molecules. Further, the fluorescence excitation peak is red shifted in powders. Since phosphorescence is the predominant emission at 777deg;K and increases in fluorescence can be correlated with the presumed solvation of non-bonding electrons, the singlet excited state of lowest energy in ‘unperturbed’ purine must be n-π* in nature. The shape of the phosphorescence band and the decay lifetime of ? 1 sec at 77°K lead to the conclusion that the emitting triplet is a π-π* state. The eight vibrational structures in phosphorescence emission can be readily grouped into two progressions: there is an average separation of about 1300 cm^-1 between peaks within a given progression, and the two sets are mutually displaced by about 500 cm-^l. Individual vibrational peaks are favoured in different solvents and the whole band may be shifted up to 500 cm^-l. Even larger shifts are observed in charged purine molecules and in powders (up to 3000 cm^-l) and the presumed 0–0 band is not observed.     

STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS—VIII. AB INITIO QUANTUM MECHANICAL CHARACTERIZATION OF THE ELECTRONIC STRUCTURE AND SPECTRUM OF THE BACTERIOPHEOPHORBIDE a ANION RADICAL

Ab initio configuration interaction wavefunctions and energies are reported for 16 doublet states of the anion radical of ethyl bacteriopheophorbide a (Et-BPheo a^-˙), and are employed in an analysis of the electronic absorption spectrum. The lowest excited doublet state D₁ is predicted to lie 8601 cm^-1 above the ground state D₀; the D₁← D₀ transition is nearly forbidden, with a computed oscillator strength f= 0.002. The visible absorption spectrum is shown to consist of transitions to three ²(π, π*) states, D₂, D₃, and D₄. The D₄← D₀ transition (y-polarized, f= 0.91) appears to account for observed intense absorption at 15 800 cm^-1. The Soret band of Et-BPheo a^-˙ is shown to consist of transitions to several ²(π,π*) states, D₇-D₁₅. Transitions of particularly high intensity include D₇← D₀ (y-polarized, f= 0.72), D₁₀← D₀ (y-polarized, f= 1.1), D₁₂← D₀ (xy-polarized, f= 0.86) and D₁₅← D₀ (y-polarized, f= 0.83). Spin density data and plots are used to describe and compare the general features of the unpaired spin distributions in D₀ and D₁, which are in reasonable agreement with other reported calculated values and available experimental data for D₀.     

STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS — V. AB INITIO CONFIGURATION INTERACTION CALCULATIONS ON THE GROUND AND LOWER EXCITED SINGLET AND TRIPLET STATES OF ETHYL CHLOROPHYLLIDE a AND ETHYL PHEOPHORBIDE a

Abstract— Ab initio configuration interaction wavefunctions and energies are reported for the ground state and many low-lying excited singlet and triplet states of ethyl pheophorbide a (Et-Pheo a) and ethyl chlorophyllide a (Et-Chl a), and are employed in an analysis of the electronic absorption spectra of these systems. In both molecules the visible spectrum is found to consist of transitions to the two lowest-lying ¹(π, π*) states, S₁ and S₂. The configurational compositions of S₁ and S₂ in both molecules are similar, and are described qualitatively in terms of a four-orbital model. The S₁← S₀ transition in each case is predicted to be intense, and is largely in-plane y-polarized, while the S₂← S₀ transition is predicted to be extremely weak and in-plane polarized. The orientation of the S₂← S₀ transition dipole is not conclusively established in the present calculations. The Soret band in both molecules is composed of transitions to no less than ten states (S_3-S₁₂ in Et-Chl a and S₃-S₇S₉-S₁₂. and S₁₄ in Et-Pheo a), which exhibit primarily (π, π*) character. The configurational compositions of these states are generally a complex mixture of excitations from both occupied macrocyclic π molecular orbitals and occupied orbitals with electron density in the cyclopen-tanone ring and the carbomethoxy chain. No clear correspondences are evident between respective Soret states of the two systems. Transitions to these states are generally intense and display a variety of in-plane polarizations. Two additional Soret states of Et-Pheo a, S₈ and S₁₃, exhibit primarily (n. π*) character. S₈ is characterized by excitations from u and non-bonding regions of the carbomethoxy chain, while S₁₃ is described by n →π* excitations involving the nitrogen atom of ring II. No corresponding (n, π*) states were found for Et-Chl a. In both molecules the lowest two triplet states, T₁ and T₂, are found to lie lower in energy than S₁. while T, and S₁ are approximately degenerate. The configurational compositions of T₁-T₄ of both molecules are nearly identical, and may be described by a four-orbital model. However, the compositions of T₁-T₄ differ sharply from those of S₁ and S₂. A number of higher-lying ³(π, π*) states of both molecules (T₅-T₁₃ in Et-Chi a and T₈-T₉, T₁₁-T₁₃ in Et-Pheo a) are found to have energies similar to the singlet Soret states, relative to S₀. They are characterized by a complex mixture of configurations which do not include significant contributions involving the four-orbital model. In addition, two ³(n, π*) states of Et-Pheo a, T₁₀ and T₁₄, are found, which are somewhat analogous to S₈ and S₁₃. Additional data presented include the charge distributions and molecular dipole moments of the S₀. S₁, and T₁ states of both molecules, as well as energies and oscillator strengths of computed S_n←S₁ and T_n←₁ transitions.     

The T3 (π, π) State of acridine

The 00 band maximum of the transition T₃(π, π^*) ← T₁ (π, π^*) of acridine occurs at ≈ 10200 ± 20 cm^?1 in inert (n-hexane, benzene, CCl₄), at 10220 ± 20 cm^?1 in polar (acetonitrile) and at 10170 ± 50 cm^?1 in hydrogen-bonding (methanol, 2-propanol and alkaline water) solvents. Based on the solvent-independent energy of T₁ (π, π^*), the T₃(π, π^*) state of acridine is estimated at 26050 ± 50 cm^?1 in all the solvents.     

Unraveling the Efficiency of Thioxanthone Based Triplet Sensitizers: A Detailed Theoretical Study

Photochemical activation by triplet photosensitizers is highly expedient for a green focus society. In this work, we have theoretically probed excited state characteristics of thioxanthone and its derivatives for their triplet harvesting efficiency using density functional theory (DFT) and time-dependent density functional theory (TDDFT). Absorption and triplet energies corroborate well with the available experimental data. Our results predict that both the S₁ and T₁ states are π-π^* in nature, which renders a high oscillator strength for S₀ to S₁ transition. Major triplet exciton conversion occurs through intersystem crossing (ISC) channel between the S₁ (¹π-π^*) and high energy ³n- π^* state. Apart from that, there is both radiative and non-radiative channel from S₁ to S₀, which competes with the ISC channel and reduces the triplet harvesting efficiency. For thioxanthones with −OMe (Me=Methyl) or −F substitution at 2 or 2’ positions, the ISC channel is not energetically feasible, causing sluggish intersystem crossing quantum yield (Φ_ISC). For unsubstituted thioxanthone and for isopropyl substitution at 2’ position, the S₁-T₁ gap is slightly positive ( ), rendering a lower triplet harvesting efficiency. For systems with −OMe or −F substitution at 3 or 3’ position of thioxanthone, because of buried π state and high energy π^* state, the S₁-³nπ^* gap becomes negative. This leads to a high Φ_ISC (>0.9), which is key to being an effective photocatalyst.     

The triplet state of 4-nitro-N,N-dimethynaphthylamine

Nanosecond laser photolytic studies of 4-nitro-N,N-dimethylnaphthylamine (4-NDMNA) in nonpolar and polar solvents at room temperature show a transient species with an absorption maximum in the 500-510-nm range. This species is assigned to the lowest triplet excited state of 4-NDMNA. The absorption maximum of this state is independent of solvent polarity, and its lifetime is a function of the hydrogen donor efficiency of the solvent. In n-hexane the lifetime 1/k of the triplet state is 9.1 × 10^?6 sec, while in acetonitrile 1/k is 2.0 × 10^?7 sec. The hydrogen abstraction rate constant k_H of the triplet state with tributyl tin hydride (Bu₃SnH) in n-hexane is 1.7 × 10⁷M^?1·sec^?1, while in the case of isopropyl alcohol as hydrogen donor, k_H is 4.0 × 10⁷M^?1·sec^?1. The activation energy for the hydrogen abstraction by the triplet state from Bu₃SnH in deaerated n-hexane is 0.6 kcal/mol. The lack of spectral shift with increasing solvent polarity, and the appreciable hydrogen abstraction reactivity of the triplet state, also independent of solvent polarity, seem to indicate that this excited state is an n-π* state which retains its n-π* character even in polar media.     

Factor group splitting in the lowest triplet state of p-benzoquinone-d4 crystals

Polarized Stark-modulated Zeeman absorption experiments on p-benzoquinone-d₄ single crystals at 2 K show the factor group splitting in the origin of the lowest B_1g (nπ^*) triplet state at 18649 cm^?1 to be 0.62±0.06 cm^?1. The ordering of the crystal states is such that the orbital plus state lies at higher energy. The absence of a measurable factor group solitting in the ³A_u (nπ^*) state at 12.1 cm^?1 from the origin is taken as a further confirmation of the vibronic nature of this state. The ZFS parameter D of this level is found to be ?10±3 GHz.     

Correlation of the zero-field splittings with the n,π* and π,π* triplet levels of benzaldehydes

The n,π^* and π,π^* triplet state energies of p-chlorobenzaldehyde and p-mathoxybenzaldehyde were determined in several hosts with the aid of phosphorescence and phosphorescence excitation spectra. A linear relationship expected from the theory considering spin-orbit interaction between the closely located n,π^* and π,π^* triplet states was found to be satisfied. The spin—orbit interaction parameter, <vb>G₂>vb>² was found to be 83 cm^?2 for benzaldehydes.     

Correlation between the n,π* triplet energy of some acetophenones and the corresponding electroreduction potentials

The half-wave reduction potentials, E_{$\text{1}\text{2}$}'s, of ten acetophenone derivatives have been determined in acetonitrile. A linear free-energy relationship was obtained between E_{$\text{1}\text{2}$} and the n,π^* triplet energy. This correlation enabled an estimate to be made of the n,π^* triplet energy of those acetophenones with π,π^* lowest triplet.     

Triplet excited states of nitronaphthalenes. III. 1,4-dinitronaphthalene

Nanosecond flash photolysis of 1,4-dinitronaphthalene (1,4-DNO₂N) in aerated and deaerated solvents shows a transient species with absorption maximum at 545 nm. The maximum of the transient absorption is independent of solvent polarity and its lifetime seems to be a function of the hydrogen donor efficiency of the solvent. The transient absorption is attributed to the lowest excited triplet state of 1,4-DNO₂N. The reactivity of this state for hydrogen abstraction from tributyl tin hydride (Bu₃SnH), K_q = 3.8 × 10⁸M^?1 sec, is almost equal to that of nitrobezene triplet state which has been characterized as an n → π* state. Based on spectroscopic and kinetic evidence obtained in the present work, the triplet state of 1,4-DNO₂N behaves as an n → π* state in nonpolar solvents, while in polar solvents the state is predominantly n → π* with a small amount of intramolecular charge transfer character.     

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.     

ENERGY TRANSFER-ENHANCED CHEMILUMINESCENCE OF ADAMANTANONE (n,π*) AND ESTER (π,π*) SINGLET AND TRIPLET EXCITED STATES IN THE THERMOLYSIS OF SILYLOXYARYL-SUBSTITUTED SPIROADAMANTYL DIOXETANES†

Abstract— The thermal generation of singlet and triplet excited states from silyloxyaryl-substituted spiroadamantyl dioxetanes lab and the adamantylidineadamantane dioxetane (1c) was investigated by direct and enhanced chemiluminescence (CL). 9,10-Diphenylanthracene (DPA) and 9-fluorenone were used as energy acceptors in the singlet-singlet (S-S), naphthalene and europium chelate Eu(TTA)₃Phen (TTA = thenoyltrifluoroacetone, Phen = 1,10-phenanthroline) in the triplet-triplet (T-T) and 9,10-di-bromoanthracene (DBA) in triplet-singlet (T-S) energy transfer experiments. The direct chemiluminescence observed in the thermolysis of dioxetanes lab consisted of fluorescence derived from the singlet-excited adamantanones 2a,b. In the presence of naphthalene, selective T-S energy transfer with DBA (napthalene as quencher) displayed the adamantanone triplets 2a,b and with Eu(TTA)₃Phen (naphthalene as mediator) also the silyloxyaryl ester 3 triplets. From the Stern-Volmer constants (k_TNT_T⁰) the triplet lifetimes t⁰_t of these triplet state products were assessed. By using the Hastings-Weber standard, the total triplet excitation yield (φ^t) was estimated to be ca 20%. The energies of the first excited singlet and triplet states of the adamantanones 2a,b and the silyloxyaryl ester 3, the products of the thermally induced decomposition of dioxetanes la-c , were determined by semiempirical calculations (AMI-based configuration interaction), which included explicitly solvent effects on the excitation energies in terms of a self-consistent reaction field approach. The calculations revealed that the first excited singlet and triplet states of the adamantanones 2a,b are expectedly n,π*-type excitations while the silyloxyaryl ester 3 possesses π,π* character. The semiempirical computations suggest that excitation of the adamantanones 2a,b as well as the silyloxyaryl ester 3 is feasible in the thermolysis of the spiroadamantyl dioxetanes lab , which has been confirmed by the experimental energy transfer studies.     

Ab initio calculations on urea

Multiconfiguration wave functions constructed from contracted Gaussian-lobe functions have been found for the ground and valence-excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the ¹A₁(π → π*) state. The ¹A₁(π → π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three-configuration SCF calculation. The lowest excited states are predicted to be the n π → π* and π → π* triplet states. The lowest singlet state is predicted to be the n π → π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π → π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π → π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing.     

THE PHOTOEXCITED TRIPLET STATE OF TETRAPHENYL CHLORIN, MAGNESIUM TETRAPHENYL PORPHYRIN AND WHOLE CELLS OF CHLAMYDOMONAS REINHARDI. A LIGHT MODULATION-EPR STUDY

Abstract— The photoexcited triplet states of frozen solutions of tetraphenyl chlorin (TPC), magnesium tetraphenyl porphyrin (MgTPP) and whole cells of Chlamydomonas reinhardi have been studied by light modulation-EPR spectroscopy. The porphyrins were chosen to be studied as model compounds for chlorophyll molecules, From EPR spectra the zero field splitting parameters (ZFS) were calculated. For TPC, |D| = 0.0364 ± 0.0002 cm^-1, |E| = 0.0063 ± 0.0002 cm^-1. For MgTPP, |D| = 0.0310 ± 0.0002 cm^-1. For chloroplasts, |D| = 0.0280 ± 0.0004 cm^-1, |E| = 0.0032 ± 0.0004 cm^-1. In all compounds studied, except MgTPP, electron spin polarization (ESP) was observed. From the analysis of the kinetic curves at each canonical orientation we evaluated the spin lattice relaxation rate W, the depopulation rate constants k_p, and the ratio between the population rate constants, A_p, at zero magnetic field. For TPC in ethanol-toluene (5:1) k_x= (0.70 ± 0.10) × 10³ s^-1, k_y= (0.40 ± 0.07) × 10³ s^-1, k_x= (0.24 ± 0.05) × 10³ s^-1; A_x:A_y:A_z? 1.0:0.6:0.4; W= (2.60 ± 0.40) × 10³ s^-1. For MgTPP, only the total decay rate constant, k_T, was calculated: (1.5 ± 0.2) × 10 s^-1 in n-octane and (4.8 ± 0.8) × 10 s^-1 in ethanol. The results for TPC and MgTPP are compared to those reported previously for chlorophyll. It is concluded that the dynamics of the photoexcited triplet state in chlorophylls are mainly governed by the chlorin macrocycle. From the EPR spectrum and ZFS parameters of chloroplasts, we propose that both chlorophyll a and chlorophyll b are the main constituents of the EPR spectrum. From the analysis of the kinetic curves we obtain separately the kinetic parameters for chlorophylls a and b, k^a_x= (1.30 ± 0.20) × 10³ s^-1, k^a_y;= (0.85 ± 0.15) × 10³ s^-1k^a_x= (0.32 ± 0.05) × 10³ s^-1; A^a_x:A^a_y:A^a_z? 1.0:0.7:0.2; W^a= (1.20 ± 0.20) × 10³ s^-1; k^b_x= (0.56 ± 0.09) × 10³ s^-1, k^b_y= (0.30 ± 0.04) × 10³ s^-1, k^b_z= (0.06 ± 0.01) × 10³ s^-1; A^b_x:A^b_y:A^b_x? 1.0:0.6:0.1; W^b= (5.00 ± 0.80) × 10³ s^-1. These results are very close to those found separately for chlorophyll a and chlorophyll b oligomers in vitro.     

Optically detected magnetic resonance study of the phosphorescent states of thiouracils

The triplet states of 1-methyl-2-thiouracil (1-Me-s²U), 1-methyl-4-thiouracil (1-Me-s²s⁴U) and 1-methyl-2,4-dithiouracil (1-Me-s² s⁴) have been investigated by optically detected magnetic resonance in zero magnetic field. The zero field splittings (ZFS) and the individual sublevel kinetic parameters are reported. The ZFS (|D|, |E|) values (in cm^?1) are found to increase in the order: 1-Me-s² U (0.2895, 0.0728) < 1-Me-s⁴U, (0.605, 0.0500) < 1-Me-s²s⁴ U (0.870, 0.0458). The triplet state lifetimes decrease in the same order, and both effects are attributed to an internal heavy atom effect of sulfur substitution. The vibronic structure of the phosphorescence emission indicates that the thiouracil phosphorescent states are ³(π, π^*). The low phosphorescence quantum yields of 1-Me-s⁴ U and of 1-Me-s²s⁴U result from radiationless decay of the triplet state rather than from inefficient intersystem crossing from the excited singlet state. The efficient radiationless decay of the triplet state appears to be a feature of the S-substitution at the 4-position of uracil. Phosphorescence polarization measurements of the individuals triplet sublevel emissions at ca. 1.2 K are consistent with 1-Me-s²U and 1-Me-s⁴U being non-planar in the phosphorescent state; the thiouracil phosphorescence from each triplet sublevel is polarized in the average plane of the distorted molecule. In the absence of σπ separability, spin—orbit mixing of ¹(π, π^*) and ³(π, π^*) states is enhanced and the radiative properties of the triplet state may be dominated by this mechanism rather than by the mixing of ¹(n, π^*), ¹(σ, π^*), or ¹(π, σ^*) with ³(π, π^* states which generally is the dominant mechanism for planar aromatic molecules.     

Transient Species in the Photochemistry of Tiaprofenic Acid and Its Decarboxylated Photoproduct*

Photoexcitation of tiaprofenic acid (TPA) in aqueous medium leads with almost unitary efficiency to the lowest π,π* triplet, which is detected by transient absorption. The deactivation of this state occurs in the microsecond time domain and is dominated by a thermally activated spin-allowed process with –10 kcal/mol energy barrier. The occurrence of decarboxylation from an upper state, likely the second triplet of n, π* character, is confirmed by the study of the transients toward the final keto photoproduct, i.e. the benzoylthiophene ethyl derivative (DTPA). At neutral pH, upon adiabatic release of the CO₂ fragment, long-lived triplet biradicals and ground-state intermediates with a protonated carbonyl oxygen are formed. Laser flash photolysis of DTPA leads almost quantitatively to the lowest π,π* triplet, with similar T-T absorption features as those of TPA. However the DTPA triplet appears essentially unreactive in aqueous medium. In isopropanol H-abstraction from the solvent is demonstrated by the formation of the ketyl radical.     

Die Elektronenzustände von Perimeter-π-Systemen. II. Das Elektronenspektrum des trans-15, 16-Dimethyl-dihydropyrens

The electronic spectrum and the polarisation of the transitions have been determined in the region from 15000 to 50000 cm^?1 for the 2-acetamino derivative of trans-15, 16-dimethyl-dihydropyrene, whose spectrum resembles closely that of the parent compound and its di-ethyl analogue. It is shown that the sequence of states is ¹L_b, ¹L_a, ¹B_b, ¹B_a, in agreement with theoretical predictions which were deduced from a configuration interaction model for the D_2h-π-perimeter of these systems. The influence of inductive and hyperconjugative effects on the band positions and the band intensities has been discussed.