Benzo[g]coumarin-Based Fluorescent Probes for Bioimaging Applications

Received Mar 9, 2018; Accepted Apr 22, 2018

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1. Introduction

Coumarin (2H-chromen-2-one) is a chemical compound in the benzopyrone chemical class that can be found in many natural species [1, 2]. Coumarins possess variety of biological activities and unique photophysical properties. Among them, the fluorescent property received much attention recently due to its high quantum yield, high stability, and biological compatibility [3, 4]. The coumarin-based fluorescent dyes and molecular probes have been applied not only for basic science such as physics, chemistry, medical science, and clinical science but also for industry and engineering [5, 6]. In progress, various kinds of the expanded or combined structure of coumarin derivatives have been discovered. Among them, linearly extended derivatives, benzo[$g$]coumarin, show superior photophysical properties in the bioimaging applications over the other derivatives [7]. Moreover, a large two-photon absorbing ability with longer excitation and emission wavelengths in optical window, high photostability, and high chemical stability are the key advantageous features of benzo[$g$]coumarin derivatives. In this review, we describe a brief explanation of benzocoumarin analogues with photophysical properties, their synthetic methods, and the recently developed benzo[$g$]coumarin-based one/two-photon excitable fluorescent dyes/probes that can sense biologically important species.

2. Benzo[$g$]coumarins

Benzocoumarin derivatives can be categorized into four types depending on the position of the fused aromatic ring in the parent coumarin backbone (Figure 1): (1) benzo[c]coumarin (3,4-benzocoumarin) fused on 3,4-position; (2) benzo[$g$]coumarin (6,7-benzocoumarin) fused on 6,7-position; (3) benzo[f]coumarin (5,6-benzocoumarin) fused on 5,6-position; and (4) benzo[h]coumarin (7,8-benzocoumarin) fused on 7,8-position.

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2.1. Photophysical Properties of Benzo[$g$]coumarin Analogues

The photophysical property such as absorption and fluorescence emission of coumarin derivatives has been identified by many scientists in various fields. Among the derivatives, the functional group substitution on the 3- and 7-position gives large property changes from the original backbone. Typically, coumarin itself shows maximum absorbance and emission at a short wavelength (λ_max,abs = 330 nm; λ_max,emi = 380 nm) with poor fluorescence quantum yield (Φ_F) (Figure 2) [7]. However,...