ORIGINAL PAPER NIR-Emitting Boradiazaindacene Fluorophores -TD-DFT Studies on Electronic Structure and Photophysical Properties Kishor G. Thorat & Hanusha Bhakhoa & Ponnadurai Ramasami & Nagaiyan Sekar Received: 25 September 2014 /Accepted: 24 November 2014 /Published online: 11 December 2014 # Springer Science+Business Media New York 2014 Abstract Density Functional Theory [B3LYP/6-31G(d)] and Time Dependent Density Functional Theory [TD-B3LYP/6- 31G(d)] computations have been used to have more under- standing of the structural, molecular, electronic and photophysical parameters of recently synthesized near IR- emitting acid switchable di-styryl BODIPY dyes. The struc- tures have been optimized using function B3LYP and basis set used was 6-31G(d) for all the atoms and their geometries which are correlated with corresponding rotational isomers including rotational isomers of diprotonated forms in chloro- form solvent. The observed energies of the optimized mole- cules suggest that there may be rotation about C-C single bond as the observed energy barrier is very low. The results of TD- DFT suggest that there is very good match between the observed and calculated absorptions diprotonated forms of one molecule. There is also good match between experimental and theoretical emission of neutral forms. More deviations are observed in the case emission of the diprotonated forms. Keywords NIR-BODIPYs . Boradiazaindacenes . DFT . TD-DFT . Dipole moment . Fluorescent di-styryl BODIPYs Introduction Difluoroboraindacene (BODIPY) dyes discovered about 45 years ago [1] have become interesting class of highly fluorescent dyes and are used in several fields, as fluorescent labels [2], chemosensors [3–10], dye-sensitized solar cells [11, 12 ], and for photodynamic therapy [ 13]. Very good photophysical properties such as high fluorescence quantum yields (typically ranging from 0.6 to 1.0), high molar extinc- tion coefficients, good photostability and sharp absorption- emission bands in the visible region of spectrum are the reasons for their use in bioimaging, lasers applications, bio- labels and cation sensing [13–15]. But typical BODIPY mol- ecules absorb near 510–520 nm and emit at around 540 nm, considerably shorter wavelength region than what is referred to as the “optical window” (650–900 nm) that is required for a fluorophore to be useful for biological applications. The fluorophores showing photophysics in the region, 650– 900 nm, are very useful as there would be significant reduc- tion of background signal due to auto-absorption and auto- fluorescence of biomolecules in this region of electromagnetic spectrum. In addition, there will be deep penetration of near IR light, little cell rupture and low light scattering with the possibility to use low cost excitation light source [16]. The photophysical properties of a typical BODIPY dye can easily be tuned by modifications at various positions of boradiazaindacene core, like introduction of electron donating moieties at 3 and 5 positions [ 17, 18], replacement of meso C atom by N atom [19, 20], making rotatable moieties rigid [21–23], extension of conjugation by fusion of aryl moieties [24–28], and extension of π conjugation by the introduction styryl subunits at 3- and 5-positions of the BODIPY core [29, 30]. Electronic supplementary material The online version of this article (doi:10.1007/s10895-014-1481-1) contains supplementary material, which is available to authorized users. K. G. Thorat Tinctorial Chemistry Group, Department of Dyestuff Technology, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai 400 019, Maharashtra, India H. Bhakhoa : P. Ramasami (*) : N. Sekar (*) Computational Chemistry Group, Faculty of Science, Department of Chemistry, University of Mauritius, Réduit, Mauritius e-mail: ramchemi@intnet.mu e-mail: n.sekar@ictmumbai.edu.in N. Sekar e-mail: nethi.sekar@gmail.com P. Ramasami Department of Pharmaceutical Chemistry College of Pharmacy, King Saud University, P.O. Box. 2457, Riyadh 11451, Kingdom of Saudi Arabia J Fluoresc (2015) 25:69–78 DOI 10.1007/s10895-014-1481-1