Comparison Study of the Site-Effect on Regioisomeric Pyridyl–Pyrene Conjugates: Synthesis, Structures, and Photophysical Properties

To investigate the
“site effect” of pyridyl substituents
on a pyrene core, four regioisomeric monopyridyl-pyrene (1–4) and five regioisomeric dipyridyl-pyrene (5–9) conjugates were synthesized and characterized and their structures
confirmed by single-crystal X-ray diffraction. The photophysical properties
and related frontier orbital features of these compounds have been
studied both experimentally and theoretically and demonstrate the
dependence of the properties of the compounds on the position of substitution
of the pyridyl moieties connecting to the pyrene core. It was found
that the absorption spectra of 2- and 4-substituted pyrene derivatives
display similar and weak influence on the S2 ← S0 excitations, whereas they are quite different from those
of 1-substituted isomers. The emission spectra of 1- and 4-substituted
pyrenes are quite similar, whereas those of 2-substituted isomers
display the largest bathochromic shift. The 1,6-disubstituted compound 5 exhibits a near-unity emission quantum yield in solution,
which is nearly three times higher than those of other regioisomeric
dipyridyl-pyrenes. In addition, the tetrasubstituted 1,6-dipyridyl-3,8-di-n-butyl-pyrene (10) exhibits the highest solid-state
quantum yield of 0.24 among all of the 10 pyridyl-pyrenes prepared
in this study.