Stilbene triplet, lifetime

On the other hand, reactions in which the return to So occurs from a "non-spectroscopic minimum (Fig. 3, path g) are probably the most common kind. The return is virtually always non-radiativef). This may be the very first minimum in Si (Ti) reached, e.g., the twisted triplet ethylene, or the molecule may have already landed in and again escaped out of a series of minima (Fig. 3, sequence c, e). For instance, triplet excitation of trans-stilbene 70,81-83) gives a relatively long-lived trans-stilbene triplet corresponding to a first spectroscopic minimum in Ti. This is followed by escape to the non-spectroscopic , short-lived phantom twisted stilbene triplet, corresponding to a second and last minimum in Ti. This escape is responsible for the still relatively short lifetime of the planar nn triplet compared to nn triplet of, say, naphthalene. A jump to nearby So and return to So minima at cis- and trans-stilbene geometries complete the photochemical process ). 

In non-viscous organic solvents the fluorescence efficiency of most SNA derivatives is comparable to that of TS as are the yields of trans to ois photoisomerization (33,3A). The double chain derivatives, MSNA and MSN, show substantially higher fluorescence yields (0.2 0.02) in non-viscous solvents with a corresponding decrease in the trans to ois isomerization efficiency (3A). In the sensitized isomerization, the MSN derivatives investigated thus far show only photoisomerization but there is an increase in the triplet lifetime by a factor of 2 however there is no change in the decay ratio of the isomerization precursor and the sum of the benzophenone sensitized isomerization efficiencies of ASA is approximately unity. When water insoluble SNA or MSNA molecules are spread as a film, either pure or in mixtures with insoluble fatty acids, at the air-water interface, it is found that the films show comparable behavior on compression to those of pure fatty acid with an indicated area per molecule of the surfactant stilbenes very close to that of a linear fatty acid (oa. 202 /molecule) (35). This suggests strongly that the TS chromophore in these molecules offers suitable molecular dimensions to pack into a crystalline-like array of linear parafin chains. 

Third, the triplet lifetime of the 2-styrylanthracene is in the order of 1(X) p,s, whereas the triplet lifetime of stilbene is 60 ns. 

Table 4 lists the quantum yields of cis-trans isomerization determined on triplet sensitization as well as the T-T absorption spectra and the triplet lifetimes for stilbene and related compounds [33,77-92]. 

At room temperature the triplet lifetime of trans- and cis-stilbene is 62 ns [412, 443, 444, 448]. Values around 100ns were observed for trans-azastil-benes [428] and some phenylethylenes [299], whereas those for 2-vinyl-biphenyls [449] or l-(aryl)-2-(l-naphthyl)ethylenes [441] are longer. Shorter lifetimes were reported for styrene (26ns in benzene) [441,450]. Introduction of three methyl groups prolongs the lifetime of triplet styrene from 22 to 91 ns [448], The effects of structural features on the lifetimes of... 

The absorption and emission spectra and transient triplet-triplet (T-T) absorption spectra of five fluorostilbenes were studied over a wide temperature range, down to 90 K, and compared with stilbene [12]. The phosphorescence spectra of 5,10-dihydro-5,10-dimethylindolo[3,2-b]indole, its 2,7-dicarbethoxy derivatives, and various S and Se analogues of the latter were measured in liquids, glasses, and low-temperature inert gas or N2 matrices [13]. Properties of the triplet state (T-T) absorption spectra (Figure 3.7) and triplet lifetimes of trans-l-(R)-2-(2,4-dinitrophenyl)ethylenes (R 1-naphthyl Ila, 9-anthryl Ilh, styryl lie and 4-(2, 4 -dinitro)- rans-stilbenyl Ild)... 

This prediction was substantiated experimentally for several donors with triplet excitation energies in the 45-68 kcal per mole region. Besides establishing the existence of a trans triplet in solution, the results with azulene allowed the evaluation of the ratio kn/(kis - - kig) = 50 liter mole. Since, by analogy to the quenchii of anthracene (Ware, 1962) and benzophenone (Hammond and Leermakers, 1962a) by azulene, the value of k would be expected to be at least close to that for a diffusion-controlled process, it is possible to estimate the effective lifetime of stilbene triplets, (kjg -j- fc ) to be of the order of 10 sec. 

Due to the relatively long lifetime of the sensitiser triplet state and the possibility of integrating data on the stilbene photoisomerisation, the apparent characteristic time of the method can reach hundreds of seconds. This unique property of the cascade system and, therefor triplet-photochrome technique, allows the investigation of slow diffusion processes, including encounters of proteins in membranes using very low concentrations of both the triplet and photochrome probes. 

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 . ... 

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

TABLE 16a Absorption Maximum and Lifetime of the Triplet of Stilbene Analogues in Glassy Media"... 

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