CCL:G: UV-Vis spectra

Dear CCLers,
 I have a question regarding the simulation of electronic transitions :
 I calculated the UV-Vis spectra of two molecules using TD-DFT with a
 B3LYP/6-31G(d,p) + PCM
 The results are very good for one of the two moleules and very bad
 for the other.
 The two molecules are based on a 1-10-phenanthroline substituted in
 position 3 & 8 by two bithienyls. The spectrum of this molecule is very
 well reproduced. If I add 2 methyls in position 2 & 9 of the
 phenanthroline this induces a twist and breaks a bit the conjugaison.
 For this second molecule, the experimental spectrum shows a red shift
 of 70nm but only 20nm according to the calculation.
 Do you have an idea of the reason of this differance ? is it linked with
 the geo-optimisation (should I increase the basis set size ?) or with
 Another question : is it possible to calculate fluorescence spectra
 using Gaussian or another QM software ?
 Thank you for your help, best regards.
 Jérôme KIEFFER  :

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