Arthur M. Halpern

Medium effects on fluorescence quantum yields and lifetimes for coumarin laser dyes

Abstract Fluorescence quantum yields and lifetimes of coumarin dyes are sharply reduced in polar solvents if amine substituent groups are free to rotate. The polar solvent effect is interpreted in terms of relaxation of excited dye from an initial planar conformation to a twisted zwitterionic state.

Solvent effects on photophysical parameters for coumarin laser dyes

Abstract The red shifts of absorption bands of coumarin dyes in polar solvents correlate with the solvent polarity-polarizability parameter π ∗ . Emission frequencies correlate well with the hydrogenbonding parameter, α and show a regular dependence on π ∗ if a solvent polarizability correction term is incorporated for chlorinated hydrocarbon and aromatic solvents. Quantum yield and lifetime measurements reveal that the reduced emission observed for dyes with non-rigid structures is associated with enhanced nonradiative decay in polar solvents. The solvent effect on fluorescence yield is interpreted in terms of the intervention of planar and non-planar excited species which appears to be general for polar dyes.

Photoisomerization of bis(9-anthryl)methane and other linked anthracenes. The role of excimers and biradicals in photodimerization

A series of linked anthracenes capable of storing photon energy through endoergic valence photo-isomerization have been studied. Photophysical and photochemical characteristics of the systems have been completely characterized by measurement of fluorescence quantum yields and lifetimes, and efficiencies for forward and reverse isomerization. The release of energy stored in photoisomers has been measured using kinetic and calorimetric techniques. From emission and lifetime data the respective roles of excimers and biradicals in anthracene photodimerization have been defined. (Author)

Electronic Structure of Cage Amines: Absorption Spectra of Triethylenediamine and Quinuclidine

The gas‐phase electronic absorption spectra of triethylenediamine and quinuclidine have been measured. Each compound shows two strong bands in the regions 1650–2500 A and 1650–2300 A, respectively, which have well‐resolved vibrational structure, as well as very weak, structured absorption at longer wavelengths (2560–2700 A for triethylenediamine and 2300–2500 A for quinuclidine). An essentially complete vibrational analysis of all but one of these bands has been accomplished. The results indicate a substantial change in the equilibrium positions of the nuclei in the excited states relative to the positions in the ground state. For quinuclidine, the most prominent excited state vibrational mode can be correlated with the ground‐state mode which corresponds to an approach to planarity of the nitrogen‐bearing end of the molecule. For triethylenediamine, ambiguities in the ground‐state assignments preclude conclusions about the upper state geometry. By comparisons with the spectra of other amines, the strong ...

IMPROVED ANALYSIS OF TIME‐RESOLVED SINGLE PHOTON COUNTING FLUORESCENCE DATA OF EXCIMER/EXCIPLEX SYSTEMS USING A DUAL ANALYSIS ITERATED RECONVOLUTION PROGRAM ON A DEDICATED LABORATORY MICROCOMPUTER

Abstract A novel iterative reconvolution program for the simultaneous analysis of two sets of time‐resolved single photon counting (TRSPC) fluorescence data is described. Each set of data may be fit to a function containing up to three exponential terms and any of the parameters may be constrained to both sets. This dual analysis approach offers great advantages for the study of a coupled photokinetic scheme such as an excimer or exciplex system, both in terms of the accuracy of the results and in the simplicity and speed of the analysis.

The temperature dependence of saturated amine fluorescence

Abstract The temperature dependence of fluorescence intensity of several saturated amines was studied. For “flexible” amines such as triethylamine (TEA), dimethylethylamine (DMEA), triethylemine, tri- n -octylamine (TOA), and N-methylpiperidine (NMP), it was found that I f increased as the temperature was increased. For example, in DMEA, I f increases by a factor of ca. 77 between −100 and 100°C. These results are interpreted in terms of an activated configurational change from the initially-formed (nonfluorescent) state to a relaxed (fluorescent) state in these compounds. Activation energies for these conversions range from about 1.8 kcal/mole for TOA to about 4.3 kcal/mole for TEA. Rigid, bicyclic amines having bridgehead N-atoms such as 1-azabicyclo [2.2.2] octane (ABCO) and its [3.3.3] analog were found to have temperature independent fluorescence intensities. These results were assumed to reflect the difference in energetics between the initially-formed and relaxed states relative to the “flexible” amines.

Intramolecular energy transfer between the saturated amine and carbonyl chromophores

Abstract The photophysical properties of the rigid, bicyclic amino-ketone 1-azabicyclo(2.2.2)octan-3-one (ABCONE) are compared with similarly structured amines and ketones. The fluorescence spectrum and the radiative rate constant, kR, of ABCONE was found to be similar to those of the ketones. It is suggested that ABCONEs absorption at ca. 235 nm correlates with the low-lying, weak transition observed in the free amines. Energy transfer from the second excited state to the (π* ← n) ketone state was determined to be about 50%.

Time-domain analysis in pressure perturbation spectroscopy: Optimized potential surfaces in argon-perturbed methyl iodide

Abstract A technique for the quantitative analysis of pressure-perturbed electronic absorption spectra is described using the dipole time correlator. It is based on the iterative optimization of reconvoluted spectra generated from the Fourier transform of the unperturbed spectrum and a model-dependent correlator. The method is applied to the B←X Rydberg transition of methyl iodide perturbed by 83 atm of argon; only two-body interactions are considered. The CH 3 I-Ar ground state is described by a 6–12 potential; the excited state by a three-parameter, modified Buckingham 6-exp potential. The results of the five-parameter, global optimization are discussed in terms of the ground state one-component 6–12 parameters of CH 3 I and Ar, and the implied changes in the interaction distance and well depth of the excited state.

Reversible intermolecular energy transfer between saturated amines and benzene in non-polar solution

Abstract Excitation of a mixture of dimethylethylamine (DEMA) and benzene in n -hexane at 222 nm primarily produces excited amine, while at 261 nm excited benzene predominantly results. The fluorescence spectra appreciably overlap. With 222 nm excitation, DEMA fluorescence is quenched by benzene at the diffusion-controlled rate; this quenching results with nearly unit efficiency in sensitized benzene fluorescence. With 261 nm excitation, some sensitized DEMA fluorescence is observed: the rate constant for tins process is ≈ 2.6 × 10 9 M −1 s −1 .

Saturated amine fluorescence quenching in non-polar solution by various organic molecules

Abstract The fluorescence quenching of a prototype saturated amine N,N-diethylmethylamine (DEMA) was examined in n-hexane solution at 25 °C by measuring the quencher concentration dependence of the DEMA fluorescence lifetime. The quenchers studied included N,N-dimethylformamide (DMF), acetic acid, ethyl acetate, acetonitrile, 2-butyne, dimethyl sulfoxide (DMSO), tetramethylethylene, perfluorocyclohexane, n-propyl bromide and oxygen. In all cases, within the respective quencher concentration ranges studied, no evidence of exciplex fluorescence was obtained; moreover, with the exception of oxygen, linear Stern-Volmer kinetics were observed. DEMA fluorescence quenching by these molecules is generally discussed in terms of possible energy transfer and electron (or charge) transfer mechanisms, although hydrogen bonding is considered for the acetic acid system. It was found that, even for those molecules for which energy transfer is highly improbable, efficient quenching is observed if the quencher molecule is polar (e.g. DMF, acetonitrile, DMSO and n-propyl bromide). In general, non-polar molecules having high-lying S1 states are inefficient quenchers, perfluorocyclohexane being an exception. Finally, quenching by molecular oxygen and by a secondary amine is mentioned. Complications arising from the ground state association between the amine and oxygen are discussed. The intermolecular quenching by a secondary amine is used to rationalize the very weak emissivity of N-methylpiperazine.