Synthesis and fluorescence and electrochemical properties of D–π-A structural isomers of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type and benzofuro[2,3-c]oxazolo[5,4-a]carbazole-type fluorescent dyes

Abstract

Heteropolycyclic donor--acceptor (D--A) structural isomers of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type (2a-f) and benzofuro[2,3-c]oxazolo[5,4-a]carbazole-type fluorescent dyes (3a-f), which differ in the position of oxygen and nitrogen atoms of the oxazole ring, have been synthesized, and their photophysical and electrochemical properties have been investigated. The fluorescent dyes 2a-f exhibit much stronger intramolecular charge transfer (ICT) absorption and fluorescence bands than 3a-f. Both of the D-pi-A structural isomers showed bathochromic shifts of the fluorescence band and a decrease in the fluorescence quantum yields with increasing solvent polarity (i.e., positive fluorescence solvatochromism), and the bathochromic shifts for 3a-f were larger than those for 2a-f. The fluorescent dyes 2a-c and 3a-c, which have cyano and carboxyl groups, and carboxylic acid ester as electron-withdrawing substituents, exhibited significant fluorescence solvatochromic properties, compared to 2e, 2f, 3e and 3f without an electron-withdrawing substituent. The fluorescence solvatochromism of the fluorescent dyes were analyzed with the Lippert-Mataga correlation. Moreover, to elucidate the differences of photophysical properties among the fluorescent dyes, we have performed time-resolved fluorescence spectroscopic measurements, and a change of the electronic or molecular structures between the ground and excited states was evaluated by a comparison of the experimental radiative rate constant (k(r)) with the Strickler-Berg rate constant (kSB(r)). The electrochemical properties of 2a-f and 3a-f were determined by cyclic voltammetry, which demonstrated that these fluorescent dyes have similar HOMO energy levels, but different LUMO energy levels. Semi-empirical molecular orbital calculations (AM1 and INDO/S) have been carried out to elucidate the effects of the substituents and chromophore skeleton on photophysical properties of the two D-pi-A structural isomers. These studies indicated that the ICT characteristics in the excited state are different among the fluorescent dyes, which is a principal reason for the differences of the fluorescence solvatochromism and the substituent dependent fluorescence properties among the two D-pi-A structural isomers.