Loose bolt effect

The long lifetime has important consequences on the decay rates. First, we consider what affects the nonradiative rates (knr) which change the yields of fluorescence and phosphorescence. The nonradiative decay rate is often enhanced in molecules which have flexible constituents (the so-called loose-bolt effect). Therefore, both fluorescence and phosphorescence yields are generally larger for rigid molecules than flexible molecules. For the same reason, a rigid environment will increase the emission yields hence both fluorescence and phosphorescence yields often increase with increasing viscosity. 

Polymer 18 was synthesized in order to verify whether the molecular weight is influenced by side chain groups (it is not), but a notable difference in A, (da-da ) and was observed (17 and 18) despite the fact that the FWHM (2,3(X)-2,5(X)cm- ) were relatively constant all along this series. And similarly, the FWHM for the emission band varied only from 5,300 to 5,600 cm" going from 17 to 18. The loose bolt effect does not apply here because the photophysical parameters increase for... 

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Aromatic molecules that contain lengthy aliphatic side-chains generally tend to fluoresce less intensely than those without the side-chains because of greater opportunity for vibrational deactivation (the loose-bolt effect). In unsubstituted aromatic molecules, the rigidity of the aromatic ring results in lower probabilities of vibrational deactivation and hence, higher quantum yields. 

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