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Excited state properties of linear polyenes: Fluorescence, fluorescence probe and photoisomerization studies of donor-acceptor diarylbutadienes

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Title Excited state properties of linear polyenes: Fluorescence, fluorescence probe and photoisomerization studies of donor-acceptor diarylbutadienes
 
Creator SINGH, AK
 
Subject intramolecular charge-transfer
pull diphenyl-polyenes
microheterogeneous media
electronic-structure
visual pigments
pi-star
photophysics
bacteriorhodopsin
photochemistry
spectroscopy
 
Description The excited state structure and dynamics of linear polyenes have been the subject of numerous investigations in recent sears. This is because linear polyenes like retinal (vitamin A aldehyde) play crucial roles in biological photo-sensory and photo-energy transductions. However, a complete understanding of the excited state properties of linear polyenes and related photobiological processes is not yet available. In the last few years, we have carried out photochemical and photophysical studies of donor-acceptor diphenylbutadienes with specific focus on the question whether or not dipolar twisted excited states are involved in the photoprocesses of linear polyenes. Our approach consists of examining the effects of substituents and media on the singlet excited states of donor-acceptor 1,4-diphenylbutadienes. In this context, we have chosen donor-acceptor 1,4-diphenylbutadienes [RC6H4(CH=CH)(n)C6H4R1, n = 2, R and R-t are various substituents including CN, NO2, COOMe, NMe2, OMe] as model systems and investigated their uv-vis absorption fluorescence and photoisomerization properties in organic solvents of varying polarity, 1,4-dioxane-water binary mixtures, and in micelles of SDS, CTAB and Triton-X-100. The uv-vis absorption spectra of the donor-acceptor dienes are not altered significantly by changing the solvent polarity. However, donor-acceptor groups present on the phenyl rings influence the absorption spectra as expected in terms of mesomeric effects. In contrast, pronounced solvatochromic fluorescence is observed in most of the donor-acceptor dienes studied. The dienes substituted with strong acceptor groups like NO2 and COOR have been found to exhibit much red-shifted fluorescence, which is highly dependent on solvent polarity; Low-temperature fluorescence studies in ethanol-methanol glass suggest conformational relaxation in the excited state of these dienes. The fluorescence quantum yields of the dienes are generally low, particularly in polar-protic solvents. The dienes with strong acceptor group like NO2 and COOR exhibit large dipole moment changes (similar to Delta mu, 20-28 D) upon excitation. Further, the solvatochromic fluorescence of dienes linearly correlates reasonably well with solvent polarity parameters such as E-T (30) and pi*. Time-resolved fluorescence of some of the dienes is characterized by single exponential fluorescence decay with generally increasing lifetimes in polar solvents. Irradiation of the donor-acceptor E,E-dienes yields the corresponding E,Z-isomer due to one-photon; one-bond photoisomerization of the chain C=C double bond lying close to the acceptor group. The photoisomerization quantum yield increases with increasing solvent polarity. The fluorescence properties of some of the donor-acceptor dienes have been employed to probing the microenvironment of micelles in terms of their micropolarity, probe solubilization site and critical micelle concentration. Both fluorescence lambda (max) as well as the fluorescence intensity can Ire used to probe the microenvironment of the micelles. It has been inferred that the photoprocesses of donor-acceptor diphenylbutadienes initially involve formation of delocalized pi pi* excited state. In polar solvent, tills state can have B-u(+) character, and being ionic in nature, it is stabilized by polar solvents and substituents leading to red-shifted fluorescence. In non-polar solvents, however, the excited state can be of A, nature resulting in weak emission due to forbidden nature of the 2A(g)(-) --> A(g) transtion. Further, the delocalized pi pi* locally excited state (LE) can get stabilized by non-radiative methods to an intramolecular charge transfer (ICT) state. Depending on the polarity of the medium and solvent reorganization the ICT state can decay to a conformationally relaxed twisted nob-planar state from which much red-shifted fluorescence can occur. The photoisomerization can occur from the polar configurationally perpendicular relaxed species (P*) that can exist in equilibrium with the ICT as well as with the conformationally relaxed twisted non-planer excited state depending on the electronic effects of the substituents and the polarity of the solvent. Further, the mechanism of formation, and involvement of such dipolar excited states in photobiological processes mediated by polyenes linear, particularly retinylidene polyenes, is also highlighted. These studies have brought out interesting features of the structure and potential. energy surface:of the singlet elicited state of linear polyenes and further provided new directions for the development of fluorescence probes as reporters of the microenvironment of organized assemblies.
 
Publisher INDIAN CHEMICAL SOC
 
Date 2011-08-02T11:11:45Z
2011-12-26T12:53:51Z
2011-12-27T05:40:56Z
2011-08-02T11:11:45Z
2011-12-26T12:53:51Z
2011-12-27T05:40:56Z
2001
 
Type Article
 
Identifier JOURNAL OF THE INDIAN CHEMICAL SOCIETY, 78(2), 53-72
0019-4522
http://dspace.library.iitb.ac.in/xmlui/handle/10054/8773
http://hdl.handle.net/10054/8773
 
Language en