Donor-acceptor stilbenes

The formation of a TICT state is often invoked even if no dual fluorescence is observed. For donor-acceptor stilbenes (PCT-2 and PCT-3), the proposed kinetic scheme contains three states the planar state E reached upon excitation can lead to state P (non-fluorescent) by double-bond twist, and to TICT state A by singlebond twist, the latter being responsible for most of the emission. 

In calcium chelators Indo-1 (17) and Fura-3 (18b) (Figure 2.9),(18) the fluoropho-res have donor-acceptor stilbene-like structures rigidified so as to avoid photoisomerization. Based on the same principle, Fura-2 (18a)a8) is one of the most popular calcium indicator for microscopy of individual cells because, in contrast to Quin-2 (see Section 2.2.5.), the excitation spectrum is blue shifted on cation binding, thus allowing intensity-ratio measurements. On the other hand, there is almost no shift of the emission spectrum, which can be interpreted along the same line as DCM-crown (see earlier in this section). 

We decided to explore the possibility of performing electric field poling of a NLO-active compound at room temperature and solidifying the material by in-situ photopolymerization. In essence, instead of cooling the sample below Tg, we proposed to raise the Tg of the material above the poling temperature. This paper reports on systems in which the active NLO species is present either as a dopant in a photocrosslinkable host matrix or as the matrix itself. We also describe the preparation and use of several new donor-acceptor stilbene compounds similar to 1. 

Unfortunately, 7 is a poor solvent for donor-acceptor compounds, and a maximum of 1 wt % concentration was achieved for 1. Limited solubility is a common problem with host-guest systems we found however, that the solubility of donor-acceptor stilbene and azobenzene compounds can be improved considerably by die attachment of bulky side-groups such as butyl or allyl to the chromophores. In this way, loading levels above 15 wt % were possible for certain compounds. On the other hand, simple cyclic side-groups such as pyrrolidino (12) led to poorly soluble materials (Figure 1). 

Fig. 11. a Example of a positively solvatokinetic ground state isomerization reaction (the activation barrier decreases with increasing solvent polarity) for an aza-dervative of a donor-acceptor stilbene [94], As outlined above, related donor-acceptor-stilbenes exhibit negative solvatokinetic behavior for the excited-state reaction, b For the merocyanine dye shown, the ground state trans-cis-isomerization across the central bond shows negative solvatokinetic behavior [95], the expectation for the excited slate reaction is positive solvatokinetic behavior... 

Donor-acceptor stilbene systems with A,A-dimethylamino groups as the donor and pyridine, thiophene, quinoline and other aryl groups as acceptors exhibit electrogenerated chemiluminscence by intramolecular charge transfer. The influence of A-substituents (methyl and phenyl) on the photophysical behaviour of fra/7 -4-aminostilbene has been evaluated.Calculations have been carried out on ra -4-dimethylamino-4 -cyanostilbene, and the torsion within the central ethenic moiety was calculated. The isomerization of such systems was discussed. ... 

Breitung, E.M., C.F. Shu, and R.J. McMahon. 2000. Thiazole and thiophene analogues of donor-acceptor stilbenes Molecular hyperpolarizabilities and structure-property relationships. J Amer Chem Soc 122 1154-1160. 

The two-photon absorption technique was applied for investigating cross-linked polyurethane containing modifled stilbene as potential frequency doubling or electro-optical controlling devices [38], properties of some donor-acceptor molecules with stilbene and azobenzene molecules as backbone [39], 4-fluorophenylethynyl-and 4-nitro-( )-stilbenylethynylruthenium complexes [40], series donor-acceptor stilbene analogues [41], and so on. 

Stimulated by extensive research activities on donor/acceptor substituted stilbenes, Mullen and Klarner have reported a donor/acceptor substituted poly(4,4 -biphenyl-diylvinylene) derivative (85) in which the NR2 donor and CN acceptor substituents are located on the vinylene unit [111]. The synthesis is based on a C-C-coupling reaction of in situ generated carbanion functions with a (pseudo)cation function, followed by a subsequent elimination of MeSH with formation of the olefinic double bond. 

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... 

Rettig W, Strehmel B, Majenz W (1993) The excited states of stilbene and stilbenoid donor-acceptor dye systems. A theoretical study. Chem Phys 173(3) 525-537... 

The scope of the Patemo-Buchi cycloaddition has been widely expanded for the oxetane synthesis from enone and quinone acceptors with a variety of olefins, stilbenes, acetylenes, etc. For example, an intense dark-red solution is obtained from an equimolar solution of tetrachlorobenzoquinone (CA) and stilbene owing to the spontaneous formation of 1 1 electron donor/acceptor complexes.55 A selective photoirradiation of either the charge-transfer absorption band of the [D, A] complex or the specific irradiation of the carbonyl acceptor (i.e., CA) leads to the formation of the same oxetane regioisomers in identical molar ratios56 (equation 27). 

Donor-Acceptor and Donor-Donor Substituted Stilbenes... 

The Patterno-Buchi coupling of various stilbenes (S) with chloroanil (Q) to yield fran -oxetanes is achieved by the specific charge-transfer photo-activation of the electron donor-acceptor complexes (SQ). Time-resolved spectroscopy revealed the (singlet) ion-radical pair[S+% Q" ] to be the primary reaction intermediate and established the electron-transfer pathway for this Patterno-Buchi transformation. Carbonyl quinone activation leads to the same oxetane products with identical isomer ratios. Thus, an analogous mechanism is applied which includes an initial transfer quenching of the photo-activated (triplet) quinone acceptor by the stilbene donors resulting in triplet ion-radical pairs. ... 

The photolysis of donor-acceptor systems provides unique synthetic opportunities. Direct irradiation of the donor-acceptor systems, such as systems containing arene and amine components, leads to intramolecular electron transfer, that is, to amine cation-radical and arene anion-radical moieties. After generation, these moieties undergo cyclization reactions providing efficient synthetic routes to fV-heterocycles with a variety of ring sizes. Thus, direct irradiation of secondary amino-ethyl and aminopropyl stilbenes leads to benzazepines in improved yields (Hintz et al. 1996). As known, benzazepines are used in medicine as antidepressants. Scheme 7.44 illustrates ion-radical cyclization with the formation of benzazepine derivative (65% yield). 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

The ground states of electron donor-acceptor complexes between fraws-stilbene and electron-deficient alkenes (fumaronitrile, dimethyl fumarate and maleic anhydride) are formed and the isomerization of /runs-stilbene (and of fumaronitrile to maleic nitrile) has... 

Many other organic and some organometallic materials have been examined by the powder SHG method and/or tested by EFISH techniques. They will not be discussed in detail here but are listed by structural classes for completeness. Several compendia of materials responses have been published. (38,54,55) Clearly, the largest single class of second order materials consists of donor-acceptor substituted aromatics. The class has been extended to stilbenes and diarylacetylenes and... 

The desire to increase P values above those of molecules used in the earliest materials has led to the exploration of organic compounds as the active components of second order NLO devices. Considerable effort has been expended in the synthesis and analysis of candidate molecules, which are largely donor-acceptor substituted conjugated n systems. (2) Examples of compound classes whose members display large nonresonant P include azo dyes, stilbenes, polyenes, merocyanines, stilbazolium salts, and quinoid... 

---