G.S. Kastha

A Novel Spectroscopic Method for the Investigation of Structural Phase Transitions in Molecular Crystals

Abstract A simple experimental technique for the investigation of structural phase transitions in molecular crystals in the temperature range 77-300 K has been developed and applied to a wide variety of organic molecular crystals. The ‘Photokinematical Approach’, as the method has been termed, utilizes the responsiveness of photoluminescence of guest aromatic ketone molecules to the changes in crystalline structure of host molecular crystals as sensitive probe for the identification of structural phase transformations in host molecular crystals. In this method the prediction of the number of distinct polymorphs a given molecular crystal may exist, their corresponding optimum temperatures of existence and the transition temperatures is based on the observation of characteristic changes in the luminescence intensity of the guest ketone molecules that occur in response to host crystalline modifications. The reliability of the method has been established from the conformity and reproducibility of the results ...

On the origin of some anomalous luminescence behavior of isomeric cyanopyridines

The results of a comparative study of the absorption and emission properties and the triplet yields of the isomeric cyanopyridines (CNP) and benzonitrile under various environmental conditions are reported. All the three CNP isomers are found to be nonfluorescent but exhibit a fairly strong phosphorescence emission of π, π character in rigid polar media at 77 K. ΦP values of the CNP are found to be sensitively dependent on the rigidity (or viscosity) and crystalline structure of the solvent medium. Whereas the 3‐CNP molecule phosphoresces with moderately strong intensity in hydrocarbon solvents, the 2 and 4 isomers are completely nonluminescent in glassy or highly disordered crystalline phases of hydrocarbons, e.g., n‐alkanes, cyclohexane (CH), and methylcyclohexane (MCH). A dramatic reappearance of the phosphorescence emission from these two isomers (with anomalously large red‐shifted spectral positions) has, however, been observed in crystalline CH and MCH matrices. Unlike the 3 isomer, both the 2‐ and ...

Phosphorescence enhancement in diphenyl ether in rigid ethanol glass at 77 K

Abstract The phosphorescence yield of diphenyl ether increases markedly relative to phenol in ethanol glass at 77 K. Analysis of the results reveals that the phosphorescence enhancement is attributable to increased spin—orbit coupling arising from the non-planarity of the lone-pair orbital of the oxygen atom in diphenyl ether.

Luminescence properties of ultra-pure pyridine

Abstract Pyridine, when freed of impurities (viz.pyrazine), has been found to exhibit weak but readily observable dual fluorescence (S 2 (ππ ∗ )→ S 0 - major, S 1 (nπ ∗ )→ S o - minor) as well as dual phosphorescence (T 1 (ππ ∗ )→ S o - major, T 2 (nπ ∗ )→ S o - minor) emissions in its pure crystalline state and in a variety of rigid matrices at 77 K. The T 1 (ππ ∗ )→ S o phosphorescence is a broad (360 – 540 nm) and structureless emission with a λmax at ~430 nm and a lifetime ∼2s. The S 2 (ππ ∗ → S o fluorescence (λmax at ∼290 nm) and the total phosphorescence spectra, however, show a faint vibrational structure in crystalline matrices. The luminescence quantum yields and the phosphorescence lifetime of pyridine are observed to be sensitively dependent on the impurity content in the sample and on the rigidity and crystalline structure of the host medium.

Excited state charge-transfer effect on the phosphorescence emissions of 4-aminopyridine at 77 K

Absorption and phosphorescence spectral characteristics of 4-aminopyridine (AMP) have been studied in polar and non-polar solvents at 77 K. Absorption data provide evidence that the excited singlet π, π * states of the molecule possess substantial charge-transfer (CT) character. The 4-AMP molecule is nonfluorescent in all the solvents used in the investigation but phosphoresces strongly in ethanol glass at 77 K. The phosphorescence emissions have a lifetime and negative polarization characteristic of π,π * origin. From the observations that in acidic ethanol glassy medium the phosphorescence quantum yield somewhat decreases and the lifetime moderately increases as compared to that in neutral ethanol glass but the negative polarization persists, it has been concluded that all the coupling schemes π 1 , π * (CT) SO ¯ π 3 , π * , π 1 , π * SO ¯ n 3 , π * vib ¯ π 3 , π * and π 1 , π * vib ¯ n 1 , π * SO ¯ π 3 , π * are responsible for the phosphorescence characteristics of 4-AMP molecule. The substantial reductions in the values of the phosphorescence quantum yield and lifetime of 4-AMP in non-polar rigid matrix at 77 K have been attributed to enhanced nonradiative process arising from increased vibronic interactions between the closely spaced 3 n,π * and 3 π,π * states of the molecule in such environment.

Substitution effect on the triplet state energy and decay of some disubstituted naphthalenes

Singlet- and triplet-state energy positions and triplet-state decay lifetimes of some 2,6- and 2,7-distributed naphthalene molecules, in which the substituents are OCH3, -CN; -OCH3, -CO2CH3; and -OCH3, -COOH, have been determined in ethanol glass at 77 K. It has been found that the first excited singlet-state energy, the triplet-state energy and the triplet-state decay lifetime of the two members in each of the systems show similar dependence on the relative positions of the substituents. All the three are lower in the 2,7-compound than in the 2,6-compound.

Fluorescence and phosphorescence properties of 2-cyano-6-methoxy and 2-cyano-7-methoxy naphthalene

Abstract Fluorescence and phosphorescence properties of 2-cyano-6-methoxy and 2-cyano-7-methoxy naphthalenes have been studied. It is observed that the experimentally measured fluorescence lifetimes of the two compounds are much smaller than, while their phosphorescence lifetimes are almost the same as, those of the parent naphthalene molecule. From the measured values of fluorescence quantum yield and fluorescence lifetime the intersystem crossing rate constants for the two molecules have been calculated. They are found to be ten times larger than that of naphthalene. On the other hand, the non-radiative triplet state decay rate constant in each of the two molecules is of the same magnitude as in the parent molecule.

Effect of ring aza substitution on the luminescence of aromatic ketones. Luminescence of isomeric acetylpyridines

The absorption and luminescence characteristics of 2-,3-and 4-acetyl-pyridines (ACP) and acetophenone have been examined in polar and nonpolar media at room temperature and low temperature (77K). All the ACP exhibit intense and structured phosphorescence emission ofnπ* character. Though the triplet yields of these ketones are very high, the phosphorescence quantum yield and lifetime of ACP decrease significantly in the order 4-ACP>3-ACP>2-ACP> acetophenone. The distinctive differences in the spectral properties and the luminescence characteristics of ACP from their phenylanalogue have been interpreted in terms of the perturbing influences of the inductive effect of the pyridinic nitrogen, the vibronic interaction between thenπ* andππ* states and the photochemical reaction occurring from the lowest triplet state.

Correlative studies on the absorption and emission characteristics of isomeric chlorobenzonitriles

Absorption, fluorescence and phosphorescence characteristics of isomeric chlorobenzonitriles have been studied and similarities and differences in these have been noted and compared with those of analogous compounds reported in the literature. The variations in absorption spectral characteristics have been explained and the correlation between the dipole moments of the molecules and the energy of the energy of the para isomers has been elucidated. Analyses of the phosphorescence data have led to the conclusion that internal heavy atom spin-orbit coupling effect is important in the chlorobenzonitriles and that the radiative and nonradiative rates from their triplet states may be reasonably rationalized on this basis.

Absorption and emission spectra of isomeric tolunitriles

Absorption and emission characteristics ofo-, m- andp-tolunitriles in polar and non-polar solvents under different conditions have been investigated in detail. Solvatochromic shifts of band origin of these molecules in non-polar solvents show that their dipolemoments in the first excited singlet state are almost the same while its value in the second excited singlet is larger in the metathan in the para-isomer. Vibronic analyses of the low temperatures absorption, fluorescence and phosphorescence spectra of all the three molecules have provided evidence that these molecules are slightly distorted in the first excited singlet state while such distortion in the phosphorescence emitting triplet state is larger. The data on fluorescence and phosphorescence quantum yield and phosphorescence lifetime of the tolunitriles are reasonably interpreted as showing that in these molecules, particularly m-andp- tolunitriles, the internal conversion rate from the first excited singlet to the ground state is probably small and that the charge transfer character of the triplet state in thep-isomer is larger than that in the meta.