PROTOLYTIC EQUILIBRIA IN THE LOWEST EXCITED SINGLET STATES OF 2-HYDROXYQUINOLINE AND 4-HYDROXYQUINOLINE

Abstract— Fluorescence spectra of 2- and 4-hydroxyquinoline in a variety of solvents indicate that the lactam-lactim tautomerism favoring the lactam structure in the ground state also favor the lactam structure in the lowest excited singlet state. Fluorimetric titrations show that the cations of both isomers become slightly more acidic in the lowest excited singlet state and that the anion of the 2 isomer becomes slightly more basic in the excited state. The excited anion of the 4 isomer is either identical in basicity to the ground state anion or does not equilibrate with the quinolone during the lifetime of the excited state. The Forster cycle is found to give a pK _α * values in generally poor agreement with those obtained from fluorimetric titrations. The reasons for this are discussed.     

ACID-BASE EQUILIBRIA OF ELECTRONICALLY EXCITED PHENAZINE

Abstract. Using the CNDO/S method, we have studied the lowest singlet ← singlet and triplet ← singlet transition energies of phenazine and its two conjugate acids. The spectra of the conjugate acids are not well reproduced by the calculations unless a water molecule is introduced to hydrogen bond to each < N⁺H group. The pattern of the pK's for the acid-base equilibria in the excited states is shown to conform with previously derived general patterns.     

THE TRIPLET EXCITED STATES OF BILIVERDIN AND BILIVERDIN DIMETHYL ESTER

Abstract— The triplet states of biliverdin and biliverdin dimethyl ester have been generated using pulse radiolysis excitation. Biliverdin triplet was formed by energy transfer from biphenyl triplet in acetone, absorbed throughout the wavelength range studied (380–1000 nm) and had a half-life of 11.7μs under the cpnditions chosen. Biliverdin dimethyl ester triplet was formed by energy transfer from biphenyl triplet in benzene, likewise absorbed throughout the wavelength range studied (360–1000 nm) and had a half-life of 6.7μs under the conditions used. Both biliverdin and biliverdin dimethyl ester efficiently quench anthracene, naphthacene, but not μ-carotene, triplet states. On the other hand. neither μ-carotene nor oxygen were found to quench the triplet states of biliverdin or biliverdin dimethyl ester. Estimates or limits for the rate constants of all these quenching reactions were obtained. These reactivities suggest that the triplet levels of both biliverdin and biliverdin dimethyl ester lie around 90 kJ mol^-1. The triplet energy transfer rate from bilirubin to biliverdin dimethyl ester in benzene was measured to be 1.9 × 10⁹ M^-1 s^-1. The singlet-triplet intersystem crossing efficiencies of both molecules were very low, limits of 0.004 and 0.001 being found for biliverdin and biliverdin dimethyl ester, respectively, using 347 nm laser excitation.     

TRIPLET EXCITED STATES OF PYRIMIDINE NUCLEOSIDES AND NUCLEOTIDES

Abstract— The triplet absorption spectra, lifetimes, extinction coefficients, e_TT, and intersystem crossing quantum yields to the lowest triplet T₁, ø_T1, of thymidine, thymidine monophosphate, uridine and uridine monophosphate, have been determined in various solvents at 300 K.
The effect of H-bonding on ø_T1, of these nucleosides and nucleotides and also of uracil has been determined and discussed. This effect allows, an ordering of ^l,3 n, π* and ^1,3 π, π* states in protic and aprotic solvents.     

EXCITED SINGLET AND TRIPLET PRODUCTS GENERATED FROM OXIDATION OF THE -CO-CH- CONTAINING MOLECULES

Abstract— In a previous study. we found indirect chemiluminescence from air oxidation of various simple ketones in alkaline aprotic solvents containing 9,10-diphenylanthracene and 9,10-dibromoanthracene. By summarizing the experimental results. it was concluded that excited products. mainly in the triplet state. were generated from air oxidation of those simple compounds having a -CO-CH-group via dioxetane intermediates.
Recently. we have succeeded in observing an intense direct chemiluminescence due to the generation of the excited singlet product by the air oxidation of such anthracene derivatives having a -CO-CH-group as 9,10-diisobutyrylanthracene and related compounds. The finding lends strong support to the hypothesis that dioxetane or dioxetanone may be the critical intermediate to give fluorescent singlet products.     

EVIDENCE FOR THE PRODUCTION OF SINGLET OXYGEN BY PHOTOEXCITED ACRIDINE ORANGE

Abstract In contrast to previous results obtained with the nitroxide radical detection technique, generation of the specific ¹O₂ oxidation product of cholesterol shows that photoexcited acridine orange produces singlet oxygen.     

EXCITED STATES OF SIX-MEMBERED N-HETEROCYCLES. FLUORESCENCE, PHOSPHORESCENCE AND ACID-BASE EQUILIBRIA OF FIVE MONO- AND DIAZA-PHENANTHRENES IN THE LOWEST EXCITED (τ,τ) STATES

Abstract— Acid-base equilibria of five azaphenanthrenes (1-and 4-aza; 1,10-, 1,7-, and 4,7-diaza) in the two lowest excited (τ,τ) states have been determined by Förster's cycle. Pure electronic levels of the free bases and their conjugate acids have been located from the absorption, fluorescence and phosphorescence spectra. In diazaphenanthrenes the increase of the first pK_a and the decrease of the second pK_a under excitation has been observed, and an explanation is offered. In many cases the reactivity of the molecule excited to the lowest triplet state differs considerably from that in the ground state. The Pariser-Pam-Pople (PPP) calculations of the free bases have been performed. From the theoretical results, the changes of the net charge on nitrogen atoms under excitation and the magnitude of the S-T splitting in the L_a and L_b states have been discussed and compared with experiment.     

QUENCHING OF OXONINE EXCITED STATES BY EDTA VIA ELECTRON TRANSFER AND DEACTIVATION PROCESSES. A COMPARATIVE STUDY OF EXCITED SINGLET AND TRIPLET STATE REACTIVITY

The quenching of excited singlet oxonine by EDTA in aqueous solution leads mainly to deactivation of the dye to the ground state and, to a lesser extent, to electron abstraction. The rate constants for these processes have been measured and compared to those for the same reactions involving the oxonine triplet state. The rate constant of electron abstraction is about ten times greater via the singlet state than via the triplet state. However, the rate constant of deactivation to the ground state is 10³-10⁴ times greater for the excited singlet state than for the triplet state, so that the efficiency of electron transfer is much smaller for the singlet state.     

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 FLASH PHOTOLYSIS STUDY OF THE TRIPLET STATE OF ACRIDINE ORANGE IN BASIC SOLVENTS

Abstract— Photochemical reactions of acridine orange (AO) in basic aqueous and ethanolic solutions were studied using the flash photolysis technique. The absorption spectrum of AO was determined in detail (230–900 nm) and extinction coefficients were obtained. The decay of the triplet state is the result of a first order process, a triplet-triplet annihilation process and a quenching by the dye in the ground state. The main part of the triplet decays to the ground state; however the observation of semi-reduced AO shows that the decay is partly due to chemical reactions.
An efficient reversible reaction is observed on flashing aqueous solutions of AO containing tetramethyl- p -phenylenediamine: semi-reduced AO is formed in high yield by reaction between the triplet dye and the diamine. In addition, irreversible reactions of AO occur; these are shown to be due to the triplet state by the method of triplet energy transfer.     

INDICATION OF FAST TRIPLET FORMATION IN BIMOLECULAR QUENCHING OF EXCITED SINGLET STATES STUDIED BY CONVENTIONAL FLASH PHOTOLYSIS

The bimolecular quenching of the first excited singlet state of oxonine by allylthiourea leads to the formation of the triplet state of the dye. This has been proved by comparison with the triplet-triplet absorption spectrum of oxonine obtained by triplet-triplet energy transfer. The conventional flash experiments suggest that the dye triplet state is produced directly rather than by radical recombination.     

THE LOWEST EXCITED TRIPLET STATES OF CODEINE AND MORPHINE AND OF THEIR MOLECULAR SUBUNITS VERATROLE AND GUAIACOL

Abstract— The lowest excited triplet T₁ states of the two alkaloids, morphine and codeine, have been investigated by ODMR and emission spectroscopy. The electronic character of their T₁ states could be determined by comparing the T₁ properties with those of the constituent parts guaiacol and veratrole. The T₁ properties, e.g. phosphorescence spectra, zero-field splitting parameters and triplet sublevel selective decay rates are very similar for the constituent parts and the alkaloids. Thus, in analogy to veratrole and guaiacol, the T₁ state of codeine and morphine is assigned to a ππ* state mainly localized on the benzene ring and the attached alkoxy and hydroxy groups. There is no evidence for a contribution from the other subunits, trialkylamine and unsaturated alcohol, to the T₁ state of codeine and morphine.     

REACTIVITY OF ACRIDINE DYE TRIPLET STATES IN ELECTRON TRANSFER REACTIONS

Abstract—Rate constants, k _q , for the reaction of cationic and neutral acridine orange and 10-methylacridine orange triplet states (³AOH ⁺, ³AO, ³MAO⁺) with a series of electron donors have been measured. Two different protolytic forms of the semireduced dye radical are produced by acridine orange triplet quenching at various pH^M values in methanolic solution.
It is found that k ₄ decreases with increasing oxidation potential of the reducing agent for all triplet states in a manner which is expected for electron transfer reactions. The different reactivities of the cationic and neutral triplet forms can, therefore, be attributed to the difference in reduction potentials of these species. The difference in reduction potentials is related to the p K ^M values of triplet state, p K _T^M , and semireduced dye radical, p K ^M_S , by thermodynamic consideration. p K _T^M (³AOH⁺/³AO) is determined to be 11.2. From thisp K _S^M (AOH./AO;) is estimated to be 17–18. This is in striking contrast to the protolytic equilibrium of the semireduced dye radicals found to be pK^F= 4.1. We conclude that the last value represents the second protonation equilibrium (AOH⁺₂./AOH). This conclusion is confirmed by spectroscopic data.     

KINETIC AND SPECTROSCOPIC FEATURES OF SOME CAROTENOID TRIPLET STATES: SENSITIZATION BY SINGLET OXYGEN

Abstract— Triplet-triplet absorption spectra of a series of carotenoid pigments in benzene solution have been determined by pulse radiolysis experiments. The natural lifetimes in deaerated solution have also been measured. They fall in the range 2–10 µ s as found for other carotenoids under similar conditions. Pulsed laser (337 nm) excitation of benzene solutions containing oxygen, carotenoid and a photosensitized molecule (anthracene) showed the generation of absorption spectra of the triplet states. These absorptions decayed by first order kinetics in such a way as to indicate that they were formed in reactions with singlet oxygen, itself generated by interaction with the anthracene triplet state. Bimolecular rate constants for energy transfer from O*₂ (¹△_g), to carotenoid have been evaluated.     

EXCITED STATES OF SIX-MEMBERED N-HETEROCYCLES. FLUORESCENCE, PHOSPHORESCENCE AND ACID—BASE EQUILIBRIA OF ELECTRONICALLY EXCITED PHENAZINE

Abstract— An account of a systematic study of the acid-base equilibria of phenazine in the two lowest excited (π,π) states is presented. Pure electronic levels of the free base and of both its protonated forms have been located by spectroscopic methods. Fluorescence, phosphorescence and corresponding absorption spectra have been measured. The O-O energies of the free base, of the singly-protonated species and of the doubly protonated form in the lowest triplet state (³L_α(π, π)) are: 15, 475 cm^-1, 14, 175 cm^-1 and about 9300cm^-1, respectively. This last value has been estimated from the experimentally determined S-T splitting in the other two forms. Corresponding energies of the lowest singlet state (^IL_α(π,π)) are: 23,500 cm^-1, 21,250cm^-1 and 17,300 cm^-1. The fluorescence of the free base has been found in polar as well as in non-polar solvents and has been checked by the fluorescence excitation spectrum. Fluorescence quantum yields for the free base have been measured: 8.6 times 10^-4 and 3.0 × 10^-5 in ethanol and hexane solutions, respectively. Emission in ethanol has been ascribed to (π,π), that in hexane —to (π, π). fluorescence. The changes of pK_α's under excitation, calculated from the Forster's cycle, are equal: δpK_a¹=+2.8±0.3; δpK_a¹¹?+10±1.5 in the lowest (π, π) triplet state and δpK_a¹=+4.8±0.5; δpK_a¹¹=+8.4 ± 0.5 in the lowest (π,π) singlet state. The δpK_a¹¹ in the triplet state is at least as high as that in the ¹L_a(π, π) state. P P P calculations of the electronic levels and of the molecular diagrams have been performed. The energies obtained exceed experimental values by not more than 0.5 eV. An increase of the net charge on nitrogen δp under excitation has been found to be +50, +70 and +19 per cent in the ¹L_a, ¹L_b and ³L_a states, respectively. A good correlation has been found between δpK_a¹ and δp in both excited states, which have been studied experimentally.     

PHOTOTAUTOMERISM OF HARMALINE AND HARMALOL IN THE EXCITED SINGLET STATE

Unusual behaviour of harmaline and harmalol by excited state proton transfer has been studied and the acidity constants for the different prototropic equilibria in the ground and excited singlet states have been reported.     

THE INFLUENCE OF THE NITRO GROUP ON THE DISSOCIATIONS OF SOME AROMATIC ACIDS AND BASES IN THEIR LOWEST EXCITED TRIPLET STATES*

Abstract— –Estimation of lowest excited triplet and singlet state dissociation constants of some nitro-aromatic acids and bases, from shifts in their phosphorescence and absorption spectra, respectively, indicate that intramolecular charge transfer to the nitro group is much more important in the lowest excited singlet state than in the ground or lowest excited triplet states. As a result, the effect of a nitro group on the acidity of the lowest excited singlet state of an acid or base is more exaggerated than that on the ground or lowest excited triplet state of the same compound. Furthermore, the basicity of the nitro group is greatly enhanced in the lowest excited singlet state. On this basis the increased rate of photoreduction of nitrobenzene in acidic solutions is found to be thermodynamically unfeasible in the lowest excited triplet state. Although the reaction is thermodynamically feasible in the lowest excited singlet state, the short lifetime of that state may make the reaction kinetically unfeasible. Rate-Hammett acidity profiles are therefore inadequate to alone establish the mechanism of photoreduction of nitrobenzene.     

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.