Spectroscopic studies of two dynamically different 1,4-dihydropyridine structures

Results of spectroscopic studies of solutions of two photoactive dihydropyridine derivatives: 1-methyl-2,4,4,6-tetraphenyl-1,4-dihdyropyridine and 4,4-(biphenyl-2,2'-diyl)2,6-diphenyl-1-methyl-1,4-dihydropyridine (hereafter referred to as DHP and BDH, respectively) are presented. The analysis of UV-Vis and NMR spectra points to a different behaviour of the two molecules under study, in spite of a close similarity of formal molecular structures. These results indicate that two isomers of BDH exist in solid matrices, as well as in liquid solutions, in the temperature range studied. No such feature has been detected in DIP. We attribute the difference to an enhanced stiffness of the BDH molecule resulting from a bridge between side phenyl groups.