D-π-A-A-π-D Prototype 2,2′-Bipyridine Dyads Exhibiting Large Structure and Environment-Sensitive Fluorescence: Synthesis, Photophysics, and Computation Monima Sarma, † Tanmay Chatterjee, † Susanta Ghanta, and Samar K. Das* School of Chemistry, University of Hyderabad, Central University P.O., Hyderabad 500 046, Andhra Pradesh, India * S Supporting Information ABSTRACT: A series of 4,4′-π-conjugated-2,2′-bipyridine chromophores (MS 1-8) were synthesized, and their photo- physical and thermal properties were investigated. The title “push-pull’ chromophores”, except MS 1, were integrated with both alkoxy and alkylamino donor functionalities that differ in their donation capabilities. The oligophenylenevinyl- ene (OPV) chromophores MS 4-8 are associated with a π-extended backbone in which the position and the number of alkoxy donors were systematically varied. All of the studied systems possess a D-π-A-A-π-D dyad archetype in which the A-A is the central 2,2′-bipyridine acceptor core that is electronically attached with the donor termini through π-linkers. The fluorescence quantum yields of the synthesized chromophores are found to be sensitive to the molecular archetype and the solvent medium. Out of the eight fluorescent compounds reported in this article, the compound MS 5 exhibits fluorescence in the solid state also. The modulating effect of the nature, position, and number of donor functionalities on the optical properties of these classes of compounds has further been comprehended on the basis of DFT and TD-DFT computation in a solvent reaction field. ■ INTRODUCTION The phenomenon of fluorescence has turned out to be an indispensable analytical technique in the various branches of science, most prominently, in the fields of analytical, biological, medical sciences, etc. 1 Among the diverse classes of organic π-systems, the materials that absorb electromagnetic radiation by virtue of an intramolecular charge transfer (ICT) and emit from the corresponding photoexcited state are most fascinating because of their notable applications in the field of molecular electronics, integrated photonic devices, non-linear optics (NLO), 2 etc. The elegant fabrication of an electron donor- acceptor (DA) or “push-pull” architecture can be carried out via the electronic union between the donor and acceptor mesomeric units in a chromophore, which is in turn associated with spontaneous charge redistribution between the function- alities (ICT). Consequently, much research has been con- ducted in designing and synthesizing diverse classes of DA-type fluorescent probes, which are associated with brilliant photo- physical behaviors, some typical examples of such fluorescent probes being acridine, 3 fluorescein, 4 cyanine, 5 rhodamine, 6 coumarin, 7 BODIPY, 8 squarines, 9 oligophenylenevinylenes (OPVs), 10 etc. The disconnection approach is a beautiful tool for organic chemists to devise a wide variety of chromophores wherein the nitrogen-containing heterocycles act as very promising building blocks to synthesize diverse classes of strongly emissive materials. 11 2,2′-Bipyridine derivatives are endowed with an extensive coordination/supramolecular chemistry. 12 However, in photoscience, it is advantageous to employ this N-hetero- biaryl compound because of the fact that easy derivatization of the pyridine rings offers introduction of an assorted class of donor end-capping functionalities to tune the optical properties of the relevant 2,2′-bipyridine based dyads. Over the past two decades, many research groups including Le Bozec, Beer, Abbotto, and others have accounted for the diverse end-capped 2,2′-bipyridine chromophores with moderate to strong emis- sion responses. 13-15 Recently, Ajayaghosh and co-workers have exemplified the chemo-sensing properties of the 2,2′-bipyridine based luminophores. 16 Transition metal complexes of the 2, 2′-bipyridine based DA systems are of topical interest due to their potential applicability in octupolar nonlinearity. 17 Hetero- leptic bis-thiocyanato ruthenium complexes, bearing a TiO 2 anchoring 2,2′-bipyridine ligand along with another auxiliary 2,2′-bipyridine ligand (known as antenna), have proved to be very promising photosensitizers for building high-performance dye-sensitized solar cell modules. 18 The exciting photophysical responses of the 2,2′-bipyridine based DA systems and their transition-metal complexes have drawn our recent attention. 19 The bipyridine based chromophores reported so far are either symmetrically (point group = C i ) or dissymmetrically (point group = C 1 ) substituted with alike donor functionalities (for example, alkylamino, etc.), whereas the photophysical properties of the associated heterodonor systems are less explored. In this article, we wish to report a series of styryl- and bistyryl-2,2′-bipyridine luminophores (MS 2-8, Chart 1) functionalized with both the alkoxy and amino functionalities in C i symmetrical fashion and their photophysical Received: October 7, 2011 Published: November 7, 2011 Article pubs.acs.org/joc © 2011 American Chemical Society 432 dx.doi.org/10.1021/jo202015m | J. Org. Chem. 2012, 77, 432-444