Substituent effects in electronic spectroscopy: Correlations with Platt spectroscopic moments

Abstract

Two correlation equations for the intensity of the 1Lb band in benzene derivatives were tested: the classic theory of Platt, Eq. (1), and the linear modification suggested by Ballester, Eq. (2). A systematic and statistically correct comparison of mono- and disubstituted derivatives revealed that Eq. (1) holds for substituents which are not conjugated or only slightly conjugated with the benzene ring, both in the ground and in the excited state (alkyls, Cl, Br, I, CH2X, CHal3). Some symmetrical properties of the substituents might be also of importance but their spectral properties (the spectroscopic moment itself, or the spectral shift) are not decisive. The conjugated substituents deviate from Eq. (1) in such a sense that the cumulative substituent effect is less than predicted; a part of the deviation may be approximately described by Eq. (2) but an exact range of validity cannot be determined. The spectroscopic moments depend mainly on the mesomeric effect of the substituent and quite little on its inductive effect; they cannot, however, be expressed only by a combination of these two effects as determined from the ground state properties. This was demonstrated on the series of compounds C6H5CH2X since the correlation with σI constants revealed only a general trend.