Synthesis, Optical and Electrochemical Properties of Benzofuro[2,3-<i>c</i>]carbazoloquinol Fluorescent Dyes

Abstract

Benzofuro [2,3-c]carbazoloquinol derivatives, a new type of fluorescent dyes, were derived from the corresponding quinone, and their optical and electrochemical properties were investigated by photoabsorption and fluorescence spectroscopy, cyclic voltammetry (CV) and density functional theory (DFT) calculation. The quinol derivatives in 1,4-dioxane showed the photoabsorption band at around 435 nm (molar extinction coefficient (εmax) = ca. 6000-8000 M -1 cm -1 ) and the fluorescence band at around 520 nm (fluorescence quantum yield (Φfl) = 0.24-0.28). The CV demonstrated that the quinol derivatives exhibit an irreversible oxidation wave at around 0.28 V versus Fc/Fc + . The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the quinol derivatives which were calculated using DFT at the B3LYP/6-31G(d,p) level are in good agreement with the experimental results. Key word: Benzofuro[2,3-c]carbazoloquinol; Photoabsorption; Fluorescence; Redox 2.1. General 1 H NMR spectra were recorded on a JNM-LA-400 (400 MHz) FT NMR spectrometer with tetramethylsilane (TMS) as an internal standard. IR spectra were recorded on a Perkin Elmer Spectrum One FT-IR spectrometer by ATR method. Cyclic voltammograms were recorded in acetonitrile/Et4NClO4 (0.1 M) solution with a three-electrode system consisting of Ag/Ag + as reference electrode, Pt plate as working electrode, and Pt wire as counter electrode, by using a Hokuto Denko HAB-151 potentiostat equipped with a functional generator. Column chromatography was performed on silica gel (KANTO CHEMICAL, 60N, spherical, neutral). Photoabsorption spectra were observed with a Shimadzu UV-3150 spectrophotometer, and fluorescence spectra were measured with a Hitachi F-4500 spectrophotometer. The fluorescence quantum yields (Φfl) were determined by a Hamamatsu C9920-01 equipped with CCD by using a calibrated integrating sphere system (λ ex = 325 nm). SynthesisGeneral synthetic procedure for YK-1 and YK-2: To a THF solution of BfCzQ-1 under an Ar atmosphere was added ethereal solution of organolithium (2.5 equivalent BuLi or PhLi) at -108 °C over 15 min. After stirring for 15 min at room temperature, the reaction was quenched with saturated NH4Cl solution. The solvent was evaporated, and the residue was extracted with CH2Cl2. The organic extract was washed with water. The CH2Cl2 extract was evaporated, and the residue was chromatographed on silica gel (CH2Cl2 : AcOEt = 3 : 1 as eluent) to give YK-1 or YK-2 with recovery