Michael Kasha

Excited state proton-transfer spectroscopy of 3-hydroxyflavone and quercetin☆

Abstract The yellow-green luminescence of 3-hydroxyflavone and quercetin at room temperature in solution arises from a tautomer of the molecules produced by excited state proton-transfer across a barrier in the double-minimum hydrogen-bonding potential. At 77 K in 2-methylbutane rigid matrix, a normal UV-violet fluorescence is observed in correspondence with the UV absorption. Excitation spectra and deuteration effects confirm the mechanism.

Confirmation of the Anomalous Fluorescence of Azulene

The light emission properties of the azulene molecule and several alkyl‐substituted azulenes have been studied. In confirmation of recent results of Beer and Longuet‐Higgins, it is found that anomalous (S″→S) fluorescence occurs in azulene, with vmax at 26 802 cm—1 (rigid glass solution in EPA at 77°K). Red‐shifts for this fluorescence are found in several alkyl‐substituted azulenes, in qualitative agreement with substitution theory. The absence of normal (S′→S) fluorescence and the absence of any phosphorescence (T→S) emission to 11 000 A, the apparatus limit, are also confirmed. The emission observations are discussed, and alternative interpretations are given.

APPLICATION OF THE EXCITON MODEL TO MONO-MOLECULAR LAMELLAR SYSTEMS*

Summary The molecular exciton model is applied to specific mono‐molecular lamellar structures. For a variety of in‐plane and out‐of‐plane arrangements of transition dipoles exciton‐band energy level diagrams and specific formulas are given. These results permit a quantitative comparison of the absorption spectrum of a monomolecular lamellar aggregate with that of the single molecule. The experimental parameters are the dipole length of the absorption band studied in the single molecule, the intermolecular distance of molecular centers in the lamella, and geometrical parameters. The dipole‐dipole interaction sums are given in several cases with an accuracy of two percent. The Appendix summarizes the functional expressions in terms of lattice parameters in each case.

Lowest Triplet State of Anthracene

A study has been made of the electronic transitions between the lowest triplet state of anthracene and its ground state. Low‐temperature spectroscopic study of the phosphorescence revealed a sharp emission band with a 0,0‐frequency (T→S) at 14 927 cm—1 (EPA rigid glass solution at 77°K). A vibrational analysis in terms of known Raman frequencies indicates that the transition is of a u→g allowed type. The T←S absorption band was observed in anthracene with the 0,0‐frequency at 14 850 cm—1 (solution at 293°K). The study of the phosphorescence spectra of seven different chlorinated anthracenes confirmed the assignment of the anthracene lowest triplet. This study re‐establishes the earlier value obtained by Lewis and Kasha, and rejects the recent value deduced by Reid. A correlation of the above results for anthracene with calculations and observations on related polyacenes indicates that the lowest triplet of benzene is 3B1u (in D6h), and for naphthalene, anthracene, and naphthacene the lowest triplet is 3B2...

CORRELATION OF ORBITAL CLASSIFICATION OF MOLECULAR ELECTRONIC TRANSITIONS WITH TRANSITION MECHANISM: THE AROMATIC AMINES*

El par de electrones aislados del heteroátomo tiene dos orbitales; uno de no unión, orbital ‐n, el cual es del tipo s̀y es ortogonal con respecto a los orbitales de los electrones; y otro de union orbital l, que está relacionado con los electrones π, a un grado en que depende del ángulo de torsión entre el eje del orbital l y el orbital 2pπ del anillo aromático vecino. La excitación de electrones a partir de los orbitales n ó l a orbitales de anti‐unión del tipo π es tratada como una transición de transferencia de carga int amolecular. Se desarrolla un detallado análisis teórico del elemento matricial órbita‐spin para una mezcla singlete‐triplete de las promociones 1 a π en anilinas, usando dos centros integrados. El momento de la transición singlete triplete depende del ángulo de torsión entre el eje del orbital 1 y el piano del anillo aromático. Se provee evidencia experimental para predicir la dependencia angular usando una serie de anilinas restringidas estericamente con distintos ángulos de torsion. El recíproco de la vida media de la fosforecencia de estas anilinas tiene una correlación muy satisfactoria con la función teórica, lo cual apoya el uso del elemento matricial spin‐órbita.

Photophysical induction of dual fluorescence of quercetin and related hydroxyflavones upon intermolecular H-bonding to solvent matrix

Abstract Quercetin (3,3′,4′,5,7-pentahydroxyflavone) in contrast to many hydroxyflavonols exhibits very little or no fluorescence in 298 K liquid solution or in the rigid glass protic solvents at 77 K. However, in the UV beam of the spectrofluorimeter a strong dual fluorescence builds up in a protic solvent on a time scale of 5–30 min. This is demonstrated to be a photophysical induction of fluorescence in the thermally annealed solvent matrix at 77 K. The 5-OH group in the vibrationally excited molecule can execute a motion converting its H-bond from an intramolecular to an intermolecular one, activating the 3-OH intramolecular proton transfer fluorescence.

Triplet‐Triplet Absorption Studies on Aromatic and Heterocyclic Molecules at 77 °K

Triplet‐triplet absorption spectroscopy has been extended both to numerous new molecules of current spectroscopic interest and to lower energy spectral regions (to 10 000 cm−1) of previously investigated molecules. Measurements have been carried out by the method of steady cross illumination using a modified Cary 14 spectrophotometer.Eight new T‐T transitions have been found in the present research. New lower energy bands have been studied in the previously studied molecules triphenylene, phenanthrene, and phenanthrene‐D10; in the latter two molecules, an evaluation was made of the absorption coefficient (e = 675), which has established the forbidden character of the observed bands. A tentative assignment as 3Lb←3La of the new triplet‐triplet transition at 12 075 cm−1 in phenanthrene has been made from theoretical considerations. Some absorptions found previously in the literature for phenanthrene and fluorene have been questioned. Triplet‐triplet absorption has been verified in diphenylamine by lifetime ...

The observation and assignment of the lowest multiplicity-forbidden transition in pyrazine

Abstract The T → S luminescence spectra at 77°K in rigid glass solution and the T ← S absorption spectra at 293°K in isopentane solution are reported for pyrazine and 2,5-dimethylpyrazine. The observed blue-shift of approx. 325 cm −1 upon conjugative substitution allows the characterization of the transition as 3 B 2u (n, π ∗ ) − 1 A g . The 0,0-band is observed in the low temperature emission spectrum at 25991 ± 15 cm −1 . The observed singlet-triplet split for the ( n, π ∗ ) state is approx. 5000 cm −1 . The f -number for the transition is calculated from the absorption to be 8–10 × 10 −8 . The observed vibrational progression in the spectrum involves the 609 cm −1 angular distortion frequency of pyrazine, whose occurrence is interpreted to indicate rehybridization of the σ-orbitals of the nitrogen upon n → π ∗ excitation. A discussion of the lowest triplet state of pyridine as a ( π, π ∗ ) state is included.

Phenomenology of solvent matrix spectroscopic effects

Abstract Rigid-matrix perturbations are applied to various molecular potentials in accordance with the Born-Oppenhelmer approach developed in an earlier paper. The potentials considered are the (a) Morse vibrational, (b) repulsive, (c) double-bond torsional, (d) double-minimum, and (e) pseudo-Jahn-Teller. Predictions are made concerning the novel phenomenology expected for each matrix-perturbed potential.

A comparative picosecond spectroscopic study of the competitive triple fluorescence of aminosalicylates and benzanilides

Abstract Picosecond time-resolved absorption spectroscopy and fluorescence excitation and decay dynamics studies have unravelled the complexities of triple competitive fluorescence in the aminosalicylates and benzanilides. The fluorescences observed are F1, the normal locally excited state emission S1 (LE) → S0; F′2, the proton-transfer tautomer emission S′1 (PT) → S′0 (PT); and F″2, the twisted intramolecular charge-transfer emission S″1(TICT) → S0. The triple fluorescence was observed for p-dimethylaminosalicylic acid methyl ester and for benzanilide, in comparison with other aminosalicylates and benzanilides. Structure selection and solvent perturbation permitted the “tuning in” of F1, F′2, and F″2 fluorescences individually and in combination.