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–π-acceptor (D–π-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–π-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 (k^SB_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–π-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–π-A structural isomers.