Photochemistry and Photophysics
Our scientific background lies in the areas of photochemistry, physical-organic chemistry, and radical chemistry. One aspect of our work involves synthesis and characterization of organic chromophores and in particular fluorescent probes. Our spectroscopic experience ranges from fast time-resolved techniques (laser-flash photolysis, time-resolved fluorescence, time-correlated single-photon counting) to conventional fluorescence spectroscopy, UV-Vis spectrophotometry and NMR spectroscopy. In addition, we have a keen interest in the development of kinetic models, which includes the analysis of complex kinetic data by advanced computer-based approaches like the development of global fitting and numerical integration routines.
Intramolecular O−H···O Hydrogen-Bond-Mediated Reversal in the Partitioning of Conformationally Restricted Triplet 1,4-Biradicals and Amplification of Diastereodifferentiation in Their Lifetimes
J. N. Moorthy, S. Samanta, A. L. Koner, S. Saha, and Werner M. Nau, J. Am. Chem. Soc. 2008, 130, 13608–13617.
Mechanistic Rationalization of the Formation of Photoproducts of Diastereomeric Ketones
The photoreactivity and nanosecond transient phenomena have been investigated for a rationally designed set of ketones in order to gain comprehensive insights concerning the influence of intramolecularhydrogen bonding on (i) the lifetimes of triplet 1,4-biradicals and (ii) the partitioning of the latter between cyclization and elimination.
Kinetic Solvent Effects on Hydrogen Abstraction Reactions
A. L. Koner, U. Pischel, W. M. Nau, Org. Lett. 2007, 9, 2899-2902.
Kinetic solvent effects on hydrogen abstractions, namely, acceleration in nonpolar solvents, have been presumed to be restricted to O-H hydrogen donors. We demonstrate that also abstractions from C-H and even Sn-H bonds by cumyloxyl radicals and n, pi*-excited 2,3-diazabicyclo[2.2.2]oct-2-ene are fastest in the gas phase and nonpolar solvents but slowest in acetonitrile. Accordingly, solvent effects on hydrogen abstractions are more general, presumably due to stabilization of the reactive oxygen or nitrogen species in polar solvents.
New Insights into the Mechanism of Triplet Radical-pair Combinations. The Persistent Radical Effect Masks the Distinction between In-Cage and Out-of-Cage Processes
C. A. Chesta, J. Mohanty, W. M. Nau, U. Bhattacharjee, R. G. Weiss, J. Am. Chem. Soc. 2007, 129, 5012-5022.
Analyses of the chemical fates and dynamic courses of geminate singlet radical pairs, generated during photo-Fries reactions of aromatic esters and photo-Claisen reactions of aromatic ethers, have allowed intimate details of in-cage (i.e., cages of origin) and out-of-cage motions to be probed in a variety of media. Obtaining detailed kinetic information about the in-cage combination of triplet radical pairs however is not so simple due to its intersystem crossing (ISC) character and involves additional mechanistic considerations. Dibenzyl ketone (ACOB0) and its derivatives have been frequently employed as precursors of triplet radical pairs since they employ Norrish type I process, which results in arylacetyl/benzylic radical pairs that form different combinations of product distributions of type AA, AB, and BB (Scheme above).Steady-state and laser-pulsed irradiations of dibenzyl ketone (ACOB0) and derivatives with a p-methyl or a p-hexadecyl chain (ACOB1 and ACOB16, respectively) have been conducted in polyethylene films with 0, 46, and 68% crystallinities. It has been shown that the product distributions using the customary equation for calculating Fc based on product yields does not always provide the true cage effect. Careful analyses of the transient absorption traces, based upon the new model developed here, allow the correct cage effects to be determined even from ACOB0. The model also permits the rate constants for radical-pair combinations and escape from their cage of origin to be calculated using either an iterative fitting procedure or a very simple one which requires only k–CO and the intensities of the transient absorption immediately after the flash and after the in-cage portion of reaction by the benzylic radicals is completed.
Diastereomeric Discrimination in the Lifetimes of Norrish Type II Triplet 1,4-Biradicals and Stereo-Controlled Partitioning of their Reactivity (Yang Cyclization vs Type II Elimination)
N. J. Moorthy, A. L. Koner, S. Samanta, N. Singhal, W. M. Nau, R. G. Weiss Chem. Eur. J. 2006, 12, 8744-8749.
A representative mechanistic picture for Norrish-Yang Type II reactions
The stereochemistry at C2 and C3 carbons controls the partitioning of triplet 1,4-biradicals of ketones of type alpha, beta -dimethyl-gamma-phenylbutyrophenones among various pathways. Differences in the major reaction pathways, for example, cyclization (syn) and fragmentation (anti), adopted by the diastereomeric 1,4-radicals of these ketones have permitted unprecedented diastereomeric discrimination in their lifetimes to be observed by nanosecond laser flash photolysis. From quantum yield measurements and transient lifetime data, the absolute rate constants for cyclization and fragmentation of a pair of diastereomeric triplet 1,4-biradicals have been determined for the first time.