A comprehensive analysis of the spectral behaviour of the first UV band for t-stilbene (trans-1,2,diphenylethylene) and its rigid derivative 5,10-dihydro-indeno(2,1-a)indene in such a scarcely perturbative solvent as 2-methylbutane on lowering the temperature revealed unfulfillment of the Mulliken-Rieke rule and the fact that, unlike its rigid derivative, t-stilbene exhibits a sort of conformational tremor that results in spectrally significant structural distortion. (c) 2005 Elsevier B.V. All rights reserved.