Frans-stilbene, fluorescence

Olson [1] was the first to postulate that electronic excitation of compounds carrying ethylenic double bonds leads to free internal rotation in excited states. Several years later Lewis [95] observed that frans-stilbene fluoresces whereas the cis form does not. He concluded that excited cis-stilbene is not converted to the fluorescing state of trans-stilbene, thus excluding free internal rotation from cis to trans in the excited singlet state. More detailed fluorescence studies by Saltiel et al. [81, 240, 241] have recently uncovered both a weak c fluorescence, [Pg.79]

For an experimental demonstration of the capabilities of this system, Taylor et al. [69] studied the dual fluorescence decay of frans-stilbene as a function of temperature between —10 and 30° C. The fluorescence comprised two components, a short one varying between 125 and 64 ps and a longer one varying from 690 to 1450 ps over the range of temperatures studied. Typical fluorescence decay curves are shown in Fig. 21. The fluorescence decay curves were recorded over a total integration time of 2 s which represented a summation of 3 x 108 fluorescence decay profiles. The fluorescence profile of a single-shot would not be observable above the noise level. 

Activated Processes. The temperature dependence of 4>f of frans-stilbene was analyzed by assuming a temperature dependent and a temperature independent deactivation step competing with fluorescence [9, 241, 306],... 

In the presence of an electron donor or acceptor, stilbenes can undergo a number of other reactions. Charge and electron transfer processes of frans-stilbene have been intensively studied by Lewis [492,493] and Hub et al. [494], Exciplex fluorescence was observed on addition of either secondary and tertiary amines or fumaronitrile to frans-stilbene in solvents of low polarity [492, 495-499]. Quenching of 9,10-dicyanoanthracene by stilbene leads to singlet exciplexes in nonpolar solvents [500] and to radical ions in acetonitrile [501,502],... 

Pathway for Cis -> Trans Photoisomerization. The cis - trans pathway is more difficult to analyze because of the problems in selective observation of fluorescence from m-stilbene at ambient temperatures [5, 81, 240, 241]. Therefore, more sophisticated techniques have to be applied. Sumitani et al. [315] determined the time interval for appearance of the fluorescence from t after pulsed excitation of c/s-stilbene This delay is only a few picoseconds. Since the cis - trans photoisomerization does not occur via excited states of DHP to a considerable extent (as postulated for bromostilbenes [105]) and since the rate constant for this pathway is larger than 1011 s an intersystem crossing step (involving 3c or 3p ) is not likely. Furthermore, no triplet intermediate has been observed in the nanosecond timescale by direct flash excitation of the cis form in solution [315] or in rigid glasses at — 196°C [96,114] in contrast to the results with frans-stilbene (Table 16b). This suggests that cis -> trans photoisomerization occurs via Eq. (15) ... 

Figure 5.18 The fluorescence decay rate of frans-stilbene as a function of the excitation energy. The rate increases abruptly above the barrier for isomerization at 1200 cm , an increase that is attributed to the isomerization reaction. The rate is extracted under the assumption that the fluorescence rate remains constant at (1/2.7 nsec )- Taken with permission from Syage et al. (1982).

Temperature dependence of the fluorescence quantum yields and fluorescence lifetimes of frans-4,4 -di-fert-butylstilbene in n-hexane and n-tetradecane allowed to define the index of refraction dependence of the radiative rate constants, kf= (3.9 — 1.8) X 10 s, and fluorescence lifetime [78]. This relationship was used to calculate torsional relaxation rate constants ktp> for traws-4,4 -dimethyl- and frans-4,4 -di-ferf-butylstilbene in the n-alkane solvent series. It was found that activation parameters for ktp, based on Eyring s transition state theory, adhered to the medium-enhanced thermodynamic barrier model relationship, AHtp = AHt + aEr, and to the isokinetic relationship. The isokinetic relationship between the activation parameters for the parent frans-stilbene led to an isokinetic temperature of P = 600K and brings it into agreement with the isokinetic temperature for activation parameters based on estimated microviscosities, qp, experienced by stilbene in its torsional motion. The authors concluded that only microviscosities raflier than shear viscosities, q, can be employed in the expression ktp = ktSq — b, when a = b. These data clearly indicated the important role of the media dynamics in the stilbene cis-trans photoisomerization. 

The effect of temperature on the quantum yields of isomerization and fluorescence was determined for frons-stilbene and for certain halogen-substituted frans-stilbenes (Dyck and McClure, 1962 Malkin and Fischer, 1962). It was found that as the temperature is decreased (below about 200°K) the trans cis quantum yield decreases but that the fluorescence quantum yield increases. These results suggest that an activated process (i.e., internal conversion or intersystem crossing) competes with fluorescence from S - and that this process is an essential step in the mechanism for isomerization. The results for intersystem crossing into T. ... 

Fluorescence Lifetimes and Picosecond Dynamics. The fluorescence lifetime (ts) of fran.s -stilbenes at room temperature is rather short. A value of approximately lOOps may be estimated from radiative rate constant ( r) using ts = f//rr [295]. Direct lifetime measurements with picosecond laser pulses confirm this finding (Table 14). Important progress in understanding of the excited state dynamics was achieved in several laboratories [314-375], especially by Yoshihara [314 319], Hochstrasser [262, 320 335], Fleming [343-348], Troe [352 355], Zewail [361-364] and their co-workers. Saltiel and Sun [28] extensively discussed the literature con-... 

The one- and two-photon excited fluorescence property and crystal structure of a substituted stilbene-type compound frans-4-diethylamino-4 -bromostilbene (DEARS) has been reported [17]. Results indicate that this compound has a strong two-photon-exdted blue fluorescence at 440 nm when the 700 nm laser is used as the pump source. The one- and two-photon absorption and fluorescence properties of a free radical photopolymerization initiator, ( , )-4- 2-[p -(]yf,N-di-w-butylamino)stil-ben-p-yl]vinyl pyridine (Figure 3.8), in various solvents have been investigated [18]. The dye has a moderate two-photon absorption cross section of = 0.91 x 10 cm s/photon at 532 nm. This compound showed a strong two-photon-induced blue fluorescence of432 nm when pumped with 800 nm laser irradiation. Quantum chemical calculation indicated that the new initiator possesses a large delocalized... 

Figure 3.14 Fluorescence emission of frans-4-dimethylamino-4 -amino-stilbene in a free state in 8 x 10 M toluene solution (closed squares) and an immobilized state cross-linked with cyanuric chloride to the coating BSA protein in toluene (open squares) and in glycerin (rhombus) [55], (Reproduced with permission from Elsevier.)...

Frequency upconversion of 800 nm ultrashort 175 fs optical pulses by two-photon absorption in a stilbenoid compound-doped polymer (PMMA) optical fiber was reported [28]. By the intensity-dependent transmission method, the two-photon absorption cross section was deduced. The combination of a well-designed organic chromophore incorporated into a fiber geometry is appealing for the development of an upconversion blue polymer laser. Upconversion fluorescence and optical power limiting effects based on the two- and three-photon absorption process of a frans-4,4 bis(pyrrolidinyl)stilbene were investigated [29]. The molecular TPA cross section three-photon absorption (3PA) cross section g3 at 720-1000 nm were measured. The 3PA-induced optical power-limiting properties were also illustrated at 980 nm. 

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