Internal conversion , substituent effect

Figure 12. Molecular structures of some monosubstituted benzenes studied via TRPES in order to determine the quantitative accuracy of the extracted internal conversion rates. Three different electronic substituents were used, C=0, C=C, and C=C, leading to different state interactions. The effects of vibrational dynamics were investigated via the use of methyl group (floppier), as in a-MeSTY and ACP, or a ring structure (more rigid), as in IND, side-group additions. Both BZA and ACP have favorable Type (I) ionization correlations, whereas STY, IND, a-MeSTY, and ACT have unfavorable Type (II) ionization correlations.

Abstract Substituent effects on ultrafast electronic relaxation (internal conversion) of nucleobases... 

However, flash photolysis and substituent-effect studies indicate that these complexes are orders of magnitude more photoactive when the lowest-energy ES of the complex is LF in character Complexes with low-energy MLCT states show little photolability. The photochemistry for these complexes can be explained by an ES model where initial MLCT excitation is followed by rapid internal conversion-intersystem crossing to the lowest energy ES, and the nature of the ES determines the overall photoactivity . ... 

---