Frans-2-Styrylpyridine

The fact that the intraligand transition is lowest in energy in ClRe(CO)3-(fnms-4-styrylpyridine)2 also allows a direct assessment of the effect of the coordination on the decay properties of the intraligand excited state. Data for both the photoisomerization by direct irradiation, Table 19, and the triplet sensitized isomerization, Table 20, of ClRe(CO)3(fra s-4-styrylpyridine)2 and frans-styrylpyridine... 

Table 19. Direct irradiation of trcns-styrylpyridines and XRe(CO)3(frans-styrylpyridine)2 )...

Ultraviolet irradiation of the methiodide or hydrochloride of 2-styryl-pyridine (XCIII) in the solid state results in transformation of the trans-isomers to the corresponding dimers (XCVa, b) on the other hand irradiation in benzene solution gives both isomerization and dimerization.309 Dimer (XCVa) was produced in low yield on irradiation of XCIII in the powdered form in the presence of air.308 This is in contrast to the reported stability of XCIII toward photodimerization.109 Similar dimerizations have been reported in the case of 2,4-dichloro-3-cyano-6-styrylpyridine,164 2-styrylquinaldine (XCV) and frans-4,4 -diguanyl-stilbene bis(2-hydroxyethane sulfonate) (stilbamidine) (XCIV).82,83... 

Table 20. Triplet sensitized isomerization of f/vwis-styrylpyridines and XRe(CO)3 (frans-sty rylpyridine)21)...

The conversion of trans- to cis-stilbene by irradiation of either W(CO)6 or Mo(CO)6 is consistent with production of an excited metal carbonyl-stilbene complex which undergoes isomerization promptly.136, 137 The generation of the cis isomer represents movement away from the thermodynamic ratio of cis- and frans-stilbene which is 100% trans. Thus, an electronically excited state must be important in the reaction sequence. Two proposals have been made 137 (1) excitation of an M(CO) (stilbene)6 complex leads to a sigma bonded diradical intermediate as shown in reaction (49) or (2) electronic excitation may yield a perturbed IL excited state as for the styrylpyridine complexes discussed above. The sigma bonded olefin diradical... 

On the whole, fra/w-cto-isomerizations store less energy than valence isomerizations, but may be easier to sensitize to visible light 436 nm light, for example, absorbed by XRe(CO)8(/raw,s-3- or 4-styrylpyridine)2, where X = Cl, Br, or I, photoassists the frans-cfa-isomerization of excess styrylpyridine, which does not itself absorb at that wavelength. The mechanism is as shown in Scheme 1,... 

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