SYNTHESIS AND PHOTOPHYSICAL PROPERTIES OF FLUORESCENT 2-ARYL-1,3-DIALKYLBENZIMIDAZOLIUM IONS AND A 1-ALKLY-2-ARYLBENZIMIDAZOLE WITH EXCITED-STATE INTRAMOLECULAR PROTON-TRANSFER

To improve the solubility, photostability and fluorescence quantum yie ld (PHI) of 2-arylbenzimidazolium ions the N-1 hydrogen was replaced b y an alkyl group before N-3 was quaternized; this substitution did not reduce PHI, proving the absence of steric inhibition to attainment of coplanarity in the excited state. A related symmetrical 2,2'-aryleneb is(1,3-dimethyldibenzimidazolium ion) bad PHI = 0.86. The related 2,5- bis(1-methyl-2-benzimidazolyl)phenol had PHI = 0.38 at 22-degrees as w ell as a very large Stokes' shift due to proton-transfer. These findin gs supported a new insight into the electron distribution of the first excited singlet state of oligophenylenes. The compounds are of intere st as laser dyes and scintillation fluors.