Excitated state properties of 20-chloro-chlorophyll a

The excited-state and lasing properties of 20-chloro-chlorophyll a in ether solution were compared to those of chlorophyll a. Desactivation parameters and cross-sections were obtained from non-linear absorption spectroscopy in combination with a physico-mathematical methods package. The Cl substituent at C-20 (1) increases both intersystem crossing and internal conversion, (2) produces a blue-shift of the S1 absorption spectrum, and (3) leads to pronounced photochemistry

CONCERTED ELECTRON AND PROTON MOVEMENT IN QUENCHING OF TRIPLET C-60 AND TETRACENE FLUORESCENCE BY HYDROGEN-BONDED PHENOL-BASE PAIRS

The quenching of triplet Cm and tetracene fluorescence by phenols is strongly enhanced by added pyridines. Evidence that this is due to quenching by hydrogen-bonded phenol-base pairs is given by the close agreement between equilibrium constants for hydrogen-bond formation derived from kinetic measurements and from independent spectroscopic data. The effect is attributed to a trimolecular transition state in which electron transfer from the phenol to the excited molecule is concerted with proton movement from the incipient strongly acidic phenol cation radical to the hydrogen-bonded base

Oxidation of triplet C-60 by hydrogen-bonded chloranil: Efficient formation, spectrum and charge-shift reactions of C-60+center dot cation radical

The rate of oxidative quenching of C-3(60) by chloranil (CA) in CH2Cl2 is much enhanced by added trifluoroacetic acid (TFA) or hexafluoro-2-propanol (HFIPA). These additives have similar hydrogen-bonding powers but differ widely in their proton acidities. In both cases, quenching rate constants calculated for H-bonded CA increase sharply with additive concentration. H-bonded clusters around the quinone are postulated in which electron transfer is coupled to fast protonation of CA(-.) by TFA, and strong H-bonding or solvation of charged radicals by HFIPA. This is consistent with observed neutral semiquinone formation, higher radical yields, and much slower back reactions for TFA. The C-60(+.) spectrum (epsilon = 25 000 +/- 2000 M-1 s(-1) at 980 nm) shows low absorption throughout the visible region. Charge-shift reactions Of C-60(+.) and arenes follow Rehm-Weller-Marcus kinetics and afford efficient preparation of arenel(+.) cation radicals using visible light

Photodisaggregation of chlorophyll a and b dimers

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ANOMALOUS SOLUTION SPECTRA OF DIPYRRYL METHINE SALTS

$^{1}$ W. West in Neblette’s Handbook of Photography and Reprography, Seventh Edition (J. M. Sturge Edition, Van Nostrand Reinhold Company, 1977) Chapter 4, 87-9.Author Institution:We have found the hydrobromide and hydrochloride salts of substituted dipyrryl methines to behave non-ideally in some solvents. In acetonitrile, for example, their spectra sharpened as the concentration increased with a concurrent increase in the extinctin coefficient at the peak wavelength. Maximum absorption shifted towards longer wavelength as the concentration increased. The largest changes were observed for the tetramethyl hydrobromide derivitive, where an eight-fold increase in the extinction coefficient accompanied a twenty-fold increase in concentration. These results are similar to those reported for cyanine $dyes^{1}$. In this preliminary report we shall stress the observations and present the variation in extinction coefficients and peak wavelength in several solvents, ranging in polarity from benzene to dimethyl sulfoxide. Qualitative and semi-quantitative results will be given. We will discuss some of the implications of this work to the photochemical studies of these compounds, which can serve as models for the porphyrins. Some comments will also be made on the stability of these solutions with reference to their spectra

EXTERNAL HEAVY-ATOM INDUCED PHOSPHORESCENCE EMISSION OF FULLERENES - THE ENERGY OF TRIPLET C-60

The external heavy atom induced phosphorescence of C60 and C70 is obtained at 77 K in a glass containing ethyl iodide and gives 12 690 +/- 30 cm-1 as a precise lower limit for thc energy of triplet C60. Triplet lifetime measurements show that the heavy atom effect is relatively much greater on the S1 --> T1 than on the T1 --> S0 radiationless transitions, as expected from the smaller Franck-Condon constraints in the former process

Coupled electron-proton transfer in interactions of triplet C-60 with hydrogen-bonded phenols: Effects of solvation, deuteration, and redox potentials

The quenching of triplet C60 by phenols is greatly enhanced by addition of pyridines, which also lower the phenol voltammetric oxidation potentials. Flash photolysis shows that the products of this quenching reaction are the C60 ¥- anion radical, neutral phenoxy (or naphthoxy) radicals, and protonated pyridines. Analysis of the second-order kinetics gives quenching rate constants and values of formation constants of hydrogen-bonded phenolpyridine pairs. The latter agree with those derived from absorption spectra over a wide range of phenols, pyridines, and solvents. Significant deuterium kinetic isotope effects are observed, indicating the importance of both proton transfer and hydrogen bonding in enhancing the rate. Quenching rates and radical yields both increase with solvent polarity. It is concluded that this quenching process involves interaction between 3C60 and a hydrogen-bonded phenolpyridine complex, in which electron transfer from the phenol to 3C60 is concerted with proton transfer from phenol to hydrogen-bonded pyridine