Stilbene Photophysics

The main luminescence parameters traditionally measured are the frequency of maximal intensity Vmax, intensity I, the quantum yield , the hfetime of the exited state T, polarization, parameters of Raman spectroscopy, and excited-state energy migration. The usefulness of the fluorescence methods has been greatly enhanced with the development of new experimental techniques such as nano-, pico-, and femtosecond time-resolved spectroscopy, single-molecule detection, confocal microscopy, and two-photon correlation spectroscopy. 

Owning these properties, stilbenes exhibit a considerable potential for miscellaneous traditional and new applications, such as systems involving photoredox processes usable for solar energy conversion, information storage systems, chemical sensors and biosensors, photovoltaic devices, and so on. In this chapter, we focus on recent developments in stilbene photophysics. 

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Time-resolved femtosecond absorption, fluorescence, IR, and Raman spectroscopy elucidate the molecular structure evolution during ultrafast chemical reactions [7-11]. The technique provides in real time direct insight into the structural dynamics of various systems including photoisomerization. In this chapter, we briefly describe modem methods of studying photochemical and photophysical processes that have been employed or can be employed in the stilbene photophysics and photochemistry. 

All these recent developments demonstrate that the fast and ultrafast absorption, fluorescence, and vibrational spectroscopies continue to evolve synergetically and at a rather rapid pace. It is our hope that even more progress in the instrumentation and its application in stilbene photophysics and photochemistry would follow in the coming years. 

Lapouyade R, Czeschka K, Majenz W, Rettig W, Gilabert E, Rulliere C (1992) Photophysics of donor-acceptor substituted stilbenes. A time-resolved fluorescence study using selectively bridged dimethylamino cyano model compounds. J Phys Chem 96(24) 9643-9650... 

Letard JF, Lapouyade R, Rettig W (1993) Synthesis and photophysical study of 4-(N-monoaza-15-crown-5) stilbenes forming TICT states and their complexation with cations. Pure Appl Chem 65 1705-1712... 

J. F. Letard, R. Lapouyade, and W. Rettig, Structure-photophysics correlations in a series of 4-(dialkylamino) stilbenes Intramolecular charge transfer in the excited state as related to the twist around the single bonds, J. Am. Chem. Soc. 115, 2441-2447 (1993). 

Diphenylethene (Stilbene). This molecule has been the subject of many photophysical and photochemical investigations and the subject of several reviews. It is the prototypical alkene for studies of photoisomerization. Transient spectroscopic measurements in the picosecond time domain have been performed on electronically excited tran -stilbene in a wide range of environments. Selections from these studies are described here. 

The photochemistry and photophysics of trans-stilbene derivatives (24) have been utilized by Whitten and co-workers to understand the relaxation characteristics of media such as micelles, monolayers, and LB films [144,145]. For example, the d>,rans to Cis for stilbene derivatives show the following trend solvent system methylcyclohexane > SDS micelle multilayer assemblies (with arachidic acid). In fact, no isomerization is observed in multilayer assemblies. This is the trend expected on the basis of how readily the media can respond to stilbene shape changes during isomerization process. 

Similar studies in zeolites having different sizes of channels/cages also reveal the importance of the directionality of free volume within the reaction cavity [159]. Both the photochemical and the photophysical behavior of trans-stilbene and longer all trans-a,w-diphenyl polyenes critically depend on the zeolite in which they are included. In pentasil zeolites (ZSM-5, -8, and... 

The photo-induced electron transfer of l,4-bis(methylene)cyclohexane in acetonitrile-methanol solution with 1,4-dicyanobenzene (DCB) affords two products, both consistent with nucleophilic attack on the radical cation followed by reduction and protonation or by combination with DCB ).63 In the absence of a nucleophile, the product mixture is highly complex, as is the case under electro-oxidative conditions. Under UV irradiation, /nmv-stilbene undergoes dimerization and oxygenation (to benzaldehyde) by a single-electron mechanism in the presence of a sensitizer such as 2,4,6-triphenylpyrilium tetrafluoroborate (TPT).64 This reaction was found to yield a similar product mixture with the sulfur analogue of TPT and their relative merits as well as electrochemical and photophysical properties are discussed. 

Saeva mentions, in an early review [10] on LC materials and aspects of their photochemistry and photophysics, the irradiation of an unspecified stilbene containing polymer and changes in its physical properties. Creed et al. [25,61,62] have reported several observations of the photophysics and photochemistry of a rra/w-stilbene 4,4 -dicarboxylate containing MCLC polyester, 29, one of a series of such polymers with different spacers synthesized by Jackson, Morris, and coworkers [63]. This polymer is partly crystalline in the as cast state and has a N mesophase over a narrow temperature range (177-186°C). In solution, the structured UV-Vis absorption [62] and fluorescence [25,61] spectra are almost... 

The conjugated systems in Fig. 2 have inversion and e-h symmetry. The acenes are the prototypical systems of molecular exciton theory[23]. The photophysics of stilbene and polyparaphenylene vinylene (PPV) have been extensively studied [14], separately at first and together since the preparation [24] of light-emitting diodes based on PPV. Pyrene and perylene appear in many contexts, while the polydiacet-... 

Papper, V., Likhtenshtein G.I., Pines D. and Pines E. (1998) Tran.s-4-4 disabstituted stilbenes Linear free-energy relationship in photochemistry and photophysics. In Recent Research Development in Photochemistry and Photobiology. V. 1 Transwold Research Network. Trivandrum, pp. 205-250. 

At present, the accessible physico-chemical properties have been studied mainly in liquid solution and the overall results provide evidence of their intrinsic high dipolar character. Moreover, compounds of type 1 may be ideal substrates for the study of their photophysical and other physical properties, especially for unconventional extended rr-systems 15-20, which are push-pull aza analogues of (E)-stilbene. Their capacity for specific physical behavior merits further exploration. 

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