Wavelengths cross-conjugated

The long wavelength absorption band, due to the cross-conjugation, is very sensitive to the solvent polarity. For example, l,8-bis(dimethylamino)-4-nitronaphthalene (108) reveals a positive solvatochromism with Xmax values of 411, 444, 463 and 484 nm in n-hexane, benzene, methanol and DMSO, respectively80. On the other hand, 4-[2- 4, 5 -bis(dimethylamino)naphth-l -yl -vinyl]-l-methylpyridinium perchlorate (128) displays a negative solvatochromism, reminiscent in this respect of the known solvatochromic betaine 129 (Table 12). 

Why do cross-conjugated alkenes absorb at shorter wavelengths than linearly conjugated alkenes Compare hexa-1,3,5-triene and 1,1-divinylethene. [PMO]... 

The Ti-jt transition of conjugated double bonds is above 200 nm with typical intensities of the order of log e 4. Its position can be estimated with the Woodward-Fieser rule. For cross-conjugated systems, the value for the chromophore absorbing at the longest wavelength must be calculated. 

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