Combined Theoretical and Experimental Study on Intramolecular Charge Transfer Processes in Star-Shaped Conjugated Molecules

Abstract

In this work, 1,3,5-tristyrylbenzene (1) and 1,3,5-tristyryl-s-triazine (2) derivatives decorated with carbazole moieties have been synthetized for the first time. Their photophysical properties and the nature of their low-lying electronic excited state were investigated combining steady-state and time-resolved fluorescence spectroscopy, along with TD-DFT calculations. We found that the fluorescence emission of these compounds is highly sensitive to the polarity of the solvent, particularly for the s-triazine derivative whose color varies from blue to green or yellow depending on it. This behavior was associated to pronounced intramolecular charge transfer (ICT) processes from the

Understanding the Driving Mechanisms of Enhanced Luminescence Emission of Oligo(styryl)benzenes and Tri(styryl)‐s‐triazine

“…Here, it should also be remembered that the Strickler–Berg relation establishes that the radiative rate constant ( k r ) is directly related to f . Therefore, the presence of nitrogen atoms in the core leads to a luminescence deactivation, as already reported in a previous study on the photophysical properties of carbazolyl derivatives of 1 and 3 . In this sense, a fluorescence quantum yield in dichloromethane of 15 % was reported for a star‐shaped molecule analogous to 1 and 3 , but with a pyrimidine central ring (compound 4 ) .…”

“…It must also be noted that the fluorescence maximum wavelength is only sensitive to the polarity of the solvent for compound 3 (the emission energy decreases as the polarity of the solvent increases; see Table ). This fact can be associated with intramolecular charge‐transfer processes from the styryl branches, where HOMO is mainly localized, to the s ‐triazine ring, which has a high contribution to the LUMO . The chosen method (TD‐M06‐2X/6‐31G*) showed a reasonable performance in the calculations of electronic vertical transition energies for the studied compounds, particularly for compounds without nitrogen atoms in the core.…”

“…Full geometry optimizations were performed by using the Gaussian 09 (Revision D.01) suite of programs at the M06‐2X/6‐31G* level of theory . We chose the metahybrid M06‐2X functional due to its satisfactory performance in the calculation of optoelectronic properties and noncovalent interactions of related compounds . The molecular geometries of the ground state, S 0 , and the first excited state, S 1 , were optimized and the vibrational modes were calculated to check the absence of imaginary frequencies.…”

“…[50,51] Therefore, the presence of nitrogen atomsi nt he core leads to al uminescence deactivation, as already reported in ap revious study on the photophysical properties of carbazolyl derivatives of 1 and 3. [48] In this sense, af luorescenceq uantum yield in dichloro-methaneof15% wasreported for astar-shaped molecule analogous to 1 and 3,b ut with ap yrimidine central ring (compound 4). [52] This value is higher than that of the quantum yield measuredf or the s-triazined erivative( compound 3; F F = 0.4 %), but significantly lower than that reported for the tribranched molecule withoutn itrogen atoms (compound 1; F F = 40.0 %) in the same solvent.…”

“…[76] We chose the metahybrid M06-2X functional due to its satisfactory performance in the calculation of optoelectronic properties and noncovalent interactions of related compounds. [43,47,48] The molecular geometries of the ground state, S 0 ,a nd the first excited state, S 1 ,w ere optimized and the vibrational modes were calculated to check the absence of imaginary frequencies. The solvent environment was described by the polarizable continuum model (PCM), as implemented in the Gaussian package.…”