Domcke, W.

Sobolewski, A. L.

Staib, A.

Sudholt, W.

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John von Neumann Institute for ComputingAb Initio Calculations and Molecular DynamicsSimulations of Intramolecular Charge Transferin 4-(N,N-Dimethylamino)benzonitrileW. Sudholt, A.L. Sobolewski, A. Staib, W. Domckepublished inModern Methods and Algorithms of Quantum Chemistry,J. Grotendorst (Ed.), John von Neumann Institute for Computing,Ju¨lich, NIC Series, Vol. 2, ISBN 3-00-005746-3, p. 43, 2000.c© 2000 by John von Neumann Institute for ComputingPermission to make digital or hard copies of portions of this work forpersonal or classroom use is granted provided that the copies are notmade or distributed for profit or commercial advantage and that copiesbear this notice and the full citation on the first page. To copy otherwiserequires prior specific permission by the publisher mentioned above.http://www.fz-juelich.de/nic-series/Ab Initio Calculations and Molecular Dynamics Simulationsof Intramolecular Charge Transferin 4-(N,N-Dimethylamino)benzonitrileWIBKE SUDHOLTa, ANDRZEJ L. SOBOLEWSKIb, ARNULF STAIBa, WOLFGANG DOMCKEcaInstitute of Theoretical Chemistry, Heinrich-Heine-University, D-40225 Düsseldorf, GermanybInstitute of Physics, Polish Academy of Sciences, PL-02-668 Warsaw, PolandcInstitute of Physical and Theoretical Chemistry, Technical University of Munich, D-85747 Garching, GermanyC NNCH3CH3DMABN4-(N,N-Dimethylamino)benzonitrile (DMABN) is the prototype of a group of organic donor-acceptorcompounds that can undergo intramolecular charge transfer (ICT) in the excited singlet state. In polar solvents, itexhibits dual fluorescence: Apart from “normal” emission out of the locally-excited (LE) state, which is alsopresent in the gas phase and in nonpolar solvents, in polar solvents a second, “anomalous”, strongly red-shiftedband is observed in the spectrum, attributed to a charge-transfer (CT) state [1]. This phenomenon has beenmostly explained by the twisted ICT (TICT) mechanism [2], which claims that the electron transfer is induced bya rotation of the amino group around its bond to the benzene moiety. According to the wagged ICT (WICT)model [3], another possible source of charge separation is the pyramidalization of the amino nitrogen. Recently, anew concept for understanding the dual fluorescence, called rehybridized ICT (RICT) [4], which involvesbending and stretching of the nitrile group, was proposed.In the poster, we will present single-point calculations of CASPT2 potential-energy profiles and CASSCFdipole moments at CIS optimized geometries of isolated DMABN along the TICT, WICT and RICT reactionpaths in the lowest excited singlet states [5]. All results appear to be strongly dependent on the amount ofelectron correlation included. Already in the Franck-Condon region a moderately polar Lb and a strongly polar Lastate are found with the Lb state lying below the La state. Twisting is the sole reaction coordinate that leads to anintersection of the two states and appropriate dipole-moment changes. Wagging causes only minor changes of thepotential-energy profiles and dipole moments. Along the rehybridization reaction path, another CT state isstrongly stabilized, but does not become the global singlet excited-state minimum.Solvent effects along the TICT reaction path were examined by combining the CASPT2 potential energiesand CASSCF atomic charges with molecular dynamics (MD) simulations in the solvents cyclopentane andacetonitrile [6]. Adiabatic potential-energy profiles and vertical energy gaps were evaluated using equilibriumMD simulations. Solvent effects in cyclopentane turn out to be negligible. In acetonitrile, the La state is shiftedbelow the Lb state and develops a potential-energy minimum at the fully-twisted geometry. Fluorescence at thisgeometry is strongly red-shifted, mainly due to the increase of the ground-state energy upon twisting. Thecalculated vertical excitation energies are in good agreement with experiment, whereas the Stoke’s shift inacetonitrile is underestimated by about 0.4 eV. The solvation dynamics in acetonitrile directly after absorptionwas investigated by nonequilibrium MD simulations. The solvent response is very rapid and consists of two parts,the major of which is completed within about 0.2 ps.The present calculations support the TICT mechanism for the occurence of dual fluorescence in DMABN inpolar solvents.[1] E. LIPPERT, W. LÜDER, F. MOLL, W. NÄGELE, H. BOOS, H. PRIGGE, I. SEIBOLD-BLANKENSTEIN, Angew.    Chem. 73 (1961) 695.[2] K. ROTKIEWICZ, K. H. GRELLMANN, Z. R. GRABOWSKI, Chem. Phys. Lett. 19 (1973) 315.[3] W. SCHUDDEBOOM, S. A. JONKER, J. M. WARMAN, U. LEINHOS, W. KÜHNLE, K. A. ZACHARIASSE, J. Phys.    Chem. 96 (1992) 10809.[4] A. L. SOBOLEWSKI, W. DOMCKE, Chem. Phys. Lett. 250 (1996) 428; 259 (1996) 119.[5] A. L. SOBOLEWSKI, W. SUDHOLT, W. DOMCKE, J. Phys. Chem. A 102 (1998) 2716;    W. SUDHOLT, A. L. SOBOLEWSKI, W. DOMCKE, Chem. Phys. 240 (1999) 9.[6] W. SUDHOLT, A. STAIB, A. L. SOBOLEWSKI, W. DOMCKE, J. Phys. Chem. A, submitted for publication.

Ab Initio Calculations and Molecular Dynamics Simulations of Intramolecular Charge Transfer in 4-(N,N-Dimethylamino)benzonitrile

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Ab Initio Calculations and Molecular Dynamics Simulations of Intramolecular Charge Transfer in 4-(N,N-Dimethylamino)benzonitrile

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