**1. Introduction**

The molecule, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)- 4*H*-pyran (DCM) belongs to the merocyanine dye category and is well-known in the literature as red fluorescent dye. The DCM dye consists of N,N-dimethylaniline group, electron donor and dicyanomethylene, and electron acceptor which are covalently attached by a π-conjugated moiety, 4*H*-pyran-4-ylidiene, in the form of electron donor-acceptor (D-π-A) architecture. The DCM was first reported by Eastman Kodak Company and initially used as dopant in developing red laser

materials [1]. However, in subsequent years it was found that DCM exhibit high fluorescence quantum efficiency, large Stokes shift, and solvatochromic behavior. Further, the absorption spectrum of DCM dye has minimum overlap with its fluorescence spectrum which was utilized in lasing action, for developing red lasers, and organic light-emitting diode (OLED) materials [2, 3]. Because of their interesting photophysical and optoelectronic properties, several research groups actively involved in developing DCM analogues not only for OLED application but also for logic gates, lasers, bioimaging, sensors, photovoltaics, and NLO applications. Way back in 2004, Chen reviewed how red emitting DCM derivatives have evolved as dopants for OLED device applications [4]. Later in 2012, Tian has published one review article which describes not only OLED applications of DCM-type materials but also fluorescent sensors, logic gates, photovoltaic sensitizers, nonlinear optical materials, bioimaging dyes, etc. [5]. Considering simple synthetic procedures of DCM derivatives [6], many optical sensors were reported based on DCM derivatives for recognizing various guest analytes, and the number of publications is rapidly increasing day by day. However, on the other hand, a comprehensive summery of DCM photophysical behavior has not been reported till date. Moreover, to the best of our knowledge, there is no single report that describes optical sensing behavior of DCM and its derivatives. The book chapter describes both the fundamental photophysics of DCM and recent progress on DCM derivatives as optical sensors.
