Three series of 2-thienylpolyenic compounds I (R = Ac, CO2Et, cyano; n = 0-4) have been synthesized. Their electronic absorption spectra are compared with those of the corresponding isosteric phenylpolyenic series. When compared with the Ph compounds, the electronic absorption peaks of I are all red-shifted by about 25-30 nm, being about the increase in wavelength caused by lengthening a phenyl-polyenic chain by one unit. If the equivalent of the 2-thienyl group is taken as 3 instead of 2, the electronic absorption peaks of I conform to the rule of homologous linearity with correlation coefficients >0.99. In addition, the homologous line for the 2-thienyl series lies very close to that of the Ph series. The difference between the two lines, represented quant. by the ratio DlTh/DlE, is quite close to unity, where DlTh is the increment of the wavelength caused by the replacement of a Ph group by an isosteric 2-thienyl ring, and DlE is the increment by lengthening the phenyl-polyenic chain by one ethylenic unit.

Key words: ULTRAVIOLET SPECTROPHOTOMETRY, ETHANOL, STRUCTURE AND PROPERTY CORRELATION, CYCLOHEXANE, POLYENE P, THIOPHENE P, EQUIVALENT, POSITION EFFECT