Excitation Emission Matrix (EEM) Spectroscopy and Computational Evaluation of Excited States of Carbazole – Bromobenzothiadiazole (CBB)




Joulaei Zonouz, Sara

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We examine the source of solvatochromism in a fluorescent organic dye using fluorescence Excitation Emission Matrix (EEM) spectroscopy, supported by ab initio calculations. The dye, carbazole–bromobenzothiadiazole (CBB), has a donor-, an acceptor-, and a bridging group connected by sigma bonds. The study of its fluorescence in liquid solutions shows a strong emission wavelength dependence on the solvent polarity but only a negligible dependence on solvent viscosity. In previous work, the polarity-induced solvatochromism was attributed to a twisted intermolecular charge transfer (TICT) state, where the excited state twists into a conformation in which a large dipole is generated between the carbazole (donor) group and the benzothiadiazole (acceptor) group. (1) Density Functional Theory (DFT) calculations are performed to map the excited state potential energy surface and the associated dipole moment in different solvent environments using the corrected linear response (cLR) solvent model (2). The calculations agree well with the observed energy differences and the solvatochromic shifts. To determine the configuration of the molecule in the excited state before emission, the dihedral angles between the three main groups of the molecule are investigated. Ab initio calculation indicates that the three dihedral angles between the donor-, bridging-, and acceptor-groups in the excited state change only by comparably small amounts between the Franck-Condon region and the potential minimum of the excited state, from which fluorescence is expected to occur. The molecular configuration in the excited state’s potential minimum is therefore close to the molecular structure in the Franck-Condon region, and not a “twisted” structure indicative of a TICT structure. Interestingly, we calculate a second local minimum on the excited state in the global excited state form when the third dihedral angle, θ_3 =90°. However, the energy in this twisted form is much higher than that of the first minimum configuration (θ_3= 0°). Since the second excited state minimum is not accessible upon excitation of the molecule emission is calculated to occur from the lowest excited state, which was, indeed, observed. Consequently, in this research, the solvatochromic shift is due to the charge separation around the acceptor domain, which is partially induced by the polarity of the environment. This work raises questions about the presence of TICT state in other molecules. We argue that the observation of solvatochromatic shifts, and the presence of donor and acceptor moieties alone are not sufficient to positively identify a TICT state. We found that TD-DFT calculation require a sophisticated solvent model to quantitatively model the solvatochromic shift. Also, we observed the natural transition orbitals are far more useful in identifying donor and acceptor moieties compared to the commonly used canonical orbitals. Excitation Emission Matrix (EEM) spectroscopy serves as a useful experimental tool to identify excitation and emission pathways.



TICT, Excitation Emission Matrix Spectroscopy, TD-DFT, Carbazole-Bromobenzothiadiazole, corrected linear response