Computational Investigations about the Ground and Excited States’ Properties of Trans-4-N,N-Dimethylamino-4’-Nitro-Stilbene (DNS) and Trans-4-N,N-Dimethyl-Amino-4’-Cyanostilbene (DCS) Derivatives

During the past several years, the field of electro optical organic and nonlinear optical material has received extensive attention [1-3] because of their possible application as fluorescence probes, electro optical switches and chemical sensors. Due to the electron donating and accepting groups on opposite sides, such material, which is connected by a highly delocalized electron system, is important for the nonlinear optical properties. Stilbene and donor–acceptor (DA)substituted stilbenes are the molecules which have been used in photo physics and photochemistry [4-7]. More or less, the interest results from the complex mechanism relevant to bond twisting in these molecules. The torsional motion of the central C=C bond in transstilbene and its analogs are responsible for the trans-cis isomerization which does not proceed through conical intersection in the excited state potential energy surface [8]. It is known that twisting of the benzonitrilo (acceptor) and dimethylamino (donor) C-N bond in N,N-dimethylaminobenzonitrile (DMABN) could lead to a twisted intramolecular charge transfer (TICT) state, which is responsible for the dual fluorescence [9,10] or displays stokes-shifted fluorescence [11] in polar solvents. TICT-based dual fluorescent systems have been used as ratio metric fluorescent chemo sensors [12]. What is more, the TICT concept has inspired the design of donor-acceptor systems which have ultra large hyperpolarizability [13]. Twisting motion of the NO2 group in nitro aromatics could result in the ultrafast fluorescence quenching [14] and facilitate internal conversion of the lowest singlet excited state. 4-dimethylamino-4’-cyanostilbene (DCS), is a typical stilbene which involves the process of photo induced intramolecular charge transfer (CT) in polar solvents [15-18]. Trans-4-(N,N-dimethylamino)4’-nitrostilbene (DNS) is a nonlinear optical dye which contains potential torsional modes in the excited states [19]. Torsional motion about a specific bond in the excited state, such as the trans-cis photo isomerization of alkenes [20,21], is an important nonradiative decay channel for many π-conjugated systems.