Stilbene excimer

This is the only reported example of a chemically productive reaction between a stilbene excimer or exciplex with a third molecule. ... 

A triplex has been proposed as an intermediate in some photoreactions. Yang and co-workers [54] found that addition of 1,3-dienes to anthracene excimers leads to different products than does its adition to monomeric excited arenes. Lewis and co-workers [55, 56] found that stilbene excimers can be intercepted by dimethyl fumarate to give an oxetane through a presumed triplex. More recently, Schuster and co-workers [57,58] studied the Triplex Diels-Alder reaction of 1,3-dienes with enol, alkene, and acetylenic dienophiles. Take 9,10-dicyanonaphthalene (DCN)/indene (IN)/l,3-cyclohexadiene (CHD) system as an example ... 

The only head-to-head polymer which has been examined for excimer fluorescence is polystyrene 25). Unfortunately, the synthetic route to this polymer leaves a number of stilbene-based structures in the sample, which have a lower-energy singlet state than either PS monomer (285 nm) or excimer (330 nm). Thus, fluorescence from these intrinsic stilbene traps was seen in the spectra of head-to-head PS in pure films and, to a lesser extent, in fluid solution. In the latter, the fluorescence of PS monomer was predominant, and the small amount of stilbene fluorescence was increased when a nonsolvent (methanol or cyclohexane) was added to the 2-methyl-tetrahydrofuran solution. In films of the polymer, stilbene fluorescence was the major spectral band, although some PS excimer fluorescence was also present in the spectrum. No monomer fluorescence at 285 nm was detected from films. Given the impure nature of the head-to-head PS sample, no conclusions on excimer formation in these systems could be drawn. 

The mechanism of stilbene photodimerization was first investigated by Stegemeyer (39) who observed that dimerization occurs upon irradiation of t-1, but not c-1. Thus, 11 and 12 are the syn and anti dimers, respectively, of t-1. Increasing the t-1 concentration was observed to decrease the fluorescence intensity of It, leading to the proposal of a singlet excimer mechanism for photodimerization as outlined in eqs. 2-6 ... 

According to the model for [2+2] cycloaddition shown in Fig. 2, it should be possible to reach the pericyclic intermediate upon irradiation of the cycloadduct. If a common intermediate is attained from the cycloaddition and cycloreversion processes, then the sum of the quantum yields for the two processes should equal unity. This has, in fact, been observed to be the case for several exciplex and anthracene excimer systems (49b,52). Stereospecific cycloreversion of stilbene dimers 11 and 12 to t-1 has been observed to occur upon 254 nm... 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

The stilbene carbon unit has also been peripherally bound to POPAM cores. Although it does not strictly belong to the hydrocarbon dendrimers, the formula of a G2 dendrimer of this type (Fig. 4.21) is depicted here as an example. It was obtained by alkylation of the corresponding eightfold mono-sulphonamide with 4-(bromomethyl)stilbene. Its fluorescence, E/Z isomerisation, photoisomerisation (see Section 5.2.2), and excimer formation were compared with those of non-dendritic stilbenes. The quantum yields of photoisomerisation (0.30) and fluorescence of the E isomer (0.014) of the dendrimer proved to be substantially lower [38]. 

The discovery of two excimer emitting polymorphs of the stilbene derivative 6-XXXVIII (Cohen et al. 1975) provided an opportunity to determine directly the relationship between excimer structure and its emission properties. The structure of Form A was determined crystallographically and indicated a stacking arrangement, while the structure of Form B, inferred from the cell constants and the photochemistry consistent with the topochemical principles (Section 6.4), was a pairwise arrangement... 

In a later study Theocharis et al. (1984), were able to make a direct determination of the relationship between intermolecular registry and excimer behaviour through the study of two conformational polymorphs of bis(/ -methoxy)-fran,y-stilbene 6-XXXIX. Irradiation of the orthorhombic form with a planar conformation is likely to form excimers which requires the approach of two non-parallel neighbouring molecules. Apparently, relaxation to the original molecular positions following excimer emission may not be possible, leading to a topotactic transformation to a monoclinic form, in which the molecules are not planar, but are parallel, since they are related by translation. 

Cohen, R., Ludmer, Z. and Yakhot, V. (1975). Structural influence on the excimer emission from a dimorphic crystalline stilbene. Chem. Phys. Lett., 34, 271. [232, 233f]... 

Another example of intramolecular CT complex formation is provided by trans-4-dimethvlamino-4 -(1-oxobutvl)stilbene Solvent effects on the spectrum give a value of 22D for the excited state dipole moment. The effect of electric field on the fluorescence of 4-(9-anthry1)-N.N.-2.3,5,G-hexamethy1-aniline shows this compound forms an excited state whose dipole moment does not change with solvent . Chiral discrimination in exciplex formation between 1-dipyrenylamine and chiral amines is very weak . In the probe molecule PRODAN (6-propionyl)-2-(dimethylamino)—naphthalene the initially formed excited state converts to a lower CT state as directly evidenced by time-resolved spectra in n-butanol. Rate constants for intramolecular electron transfer have been measured in both singlet and triplet states of covalently porphyrin-amide-quinone molecules . Intramolecular excimer formation occurs during the lifetime of the excited state of bis-(naphthalene)hydrazides which are used as photochemical deactivators of metals in polyethylene . ... 

Beyond cyano-OPVs, several other dyes that form excimers in their aggregated state have also been used for mechanosensing purposes. Pucci and coworkers have reported that poly(propylene) (PP) films containing bis(benzoxazolyl)stilbene (BBS) (Fig. 9a) change photoluminescence color upon tensile deformation [50]. In the as-prepared state, the emission spectrum (Fig. 9b) shows a broad excimer emission band around 500 nm, along with sharper monomer emission bands at 410, 430, and 455 nm. Mechanical deformation lowers the intensity of the excimer band, resulting in a photoluminescence color change (Fig. 9c). 

Water soluble p-sulfonato calbc[w]arenes (w = 8, la-lb and n=6, 2a-2b) was employed as host to control the outcome of photodimerization and photoisomerization of 4-stilbazoles [17]. Novel macrocydic and medium-size stilbenophanes tethered by silyl chains were synthesized, and their photochemical and photophysical properties were examined (Figure 5.5) [18]. Direct irradiation of macrocydic stilbenophanes stereoselectively gave intramolecular photocydoadducts, and the efficiency increased with decreasing distance between the two stilbene units. The triplet-sensitized photoreaction of stilbenophanes caused cis-trans photoisomerization. Photoreactions of as-fixed stilbenophanes under an oxygen atmosphere selectively led to phenanthrenophanes. Fluorescence quantum yields increased with the introduction of silyl substituents, and hence those of silyl-tethered stilbenophanes were larger than that of unsubstituted fraws-stilbene. Intramolecular excimer emis-... 

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