A Computational TDDFT Study on Intramolecular Charge Transfer in Di-tert-butylaminobenzonitriles and 2,4,6-Tricyanoanilines Takashige Fujiwara Department of Chemistry and Biochemistry The Ohio State University Marek Z. Zgierski National Research Council of Canada 69th ISMS FC10 Urbana-Champaign, IL E-mail: fujiwara@physics.osu.edu

Introduction and Motivation Unusual photophyical behaviors leading to intramolecular charge transfer (ICT) 3- or 4-(Di-tert-butylamino)benzonitrile (DTBABN) 2,4,6-Tricyano-N,N-dimethylaniline (TCDMA) 2,4,6-Tricyanoaniline (TCA) Efficient ICT formation in either polar/non-polar solvent. No indications for ICT formation in polar solvent. Computational investigations on the low-lying excited states (including the πσ*-state) leading to the ICT formations. Excited-state geometries and transition energies in the low-lying excited states: TDDFT on Gaussian 09 and Turbomole (VMN5) CC2-RI on Turbomole Solvation effect is not considered. Mechanism of photo-induced ICT formation, where πσ*-state plays a crucial role.

Steady-State Absorption and Fluorescence Spectra p-DTBABN m-DTBABN S.I. Druzhinin, S. Reddy Dubbaka, P. Knochel, S.A. Kovalenko, P. Mayer, T. Senyushkina, and K.A. Zachariasse, J. Phys. Chem. A,112, 2749(2008).

Optimized Structures of p-DTBABN ππ* πσ* TICT 124.6˚ 𝛥E = +0.00 eV 𝛥E = –0.37 eV 𝛥E = –0.78 eV 13.8 D 19.7 D 15.7 D TD/BP86/cc-pVDZ level of theory

TDDFT Calculations for DTBABN Isomers Adiabatic Energies and Dipole Moments in the Low-lying Excited States in DTBABN Isomers. State p-DTBABN m-DTBABN ΔEa |μ|b ππ* 3.53 13.1 3.24 12.8 πσ* 3.02 19.7 3.13 18.3 ICT 2.95 15.7 2.89 15.8 a Adiabatic energy (in eV) relative to the ground state. b Modulus of the dipole moment (in Debye). TD/BP86/cc-pVDZ level

Adiabatic Potential Energy Surfaces Single-Point TDDFT energy from the ππ* optimized structure Single point TDDFT energy from the πσ* structure p-DTBABN show that the lowest πσ* state is naturally switched to the ICT state: Straightening the Cph-C≡N angle, A slight torsion of the di-tert-butylamino group to its nearly perpendicular, Shortening of the C-N bond length. TD/BP86/cc-pVDZ adiabatic potentials

Adiabatic Potential Energy Surfaces Among dialkylaminobenzonitriles, the only meta-substituted compound that shows the ICT formation in a polar environment. TD/BP86/cc-pVDZ adiabatic potentials

The ICT Mechanisms in DTBABNs Two States Model Ultrafast ICT formation observed: The time scale of 60–70 fs of the ICT formation at room temp. S.I. Druzhinin, … K.A. Zachariasse, J. Phys. Chem. A, 112, 2749(2008). The πσ*-State Mediated ICT Model Render larger kinetic rate constants between the states involved. M.Z. Zgierski, E.C. Lim, and T. Fujiwara, Comp. Theor. Chem., 1036, 1 (2014).

Steady-State Absorption and Fluorescence Spectra TCA TCDMA ? Double exponential decays: 1.79 ns and ~ 2 ps decay/grow-in at higher/lower energy (423/541 nm) Single exponential decay: 4.2 ns K.A. Zachariasse, S.I. Druzhinin, V.A. Galievsky, S.Kovalenko, T.A. Senyushkina, P. Mayer, M. Noltemeyer, M. Boggio-Pasqua, and M.A. Robb, J. Phys. Chem. A,113, 2693 (2009).

TDDFT Optimized Structures of TCDMA ππ* πσ* TICT 28.7˚ 37.8˚ 124.0˚ TCDMA 𝛥E = +0.00 eV 𝛥E = +0.80 eV 𝛥E = –0.03 eV 𝛥E = +1.30 eV 𝛥E = +0.51 eV 𝛥E = +0.00 eV TCA TD/B3LYP/cc-pVDZ level of theory

Excited-State Absorption Spectra for TCA / TCDMA — Kinetically interconnected excited-states are present — No further spectral evolution over 10 ps K.A. Zachariasse, … and M.A. Robb, J. Phys. Chem. A,113, 2693 (2009).

CC2/cc-pVDZ Structures and Dynamics for TCDMA 29.3˚ 10.2 D 9.0 D 2.7 D ICT-AQ LE (ππ*) ICT-Q ~ 2 ps 𝛥E = +0.002 eV 𝛥E = –0.12 eV Rapidly interconnecting S1 conformers 1.79 ns Consistent with CASSCF computations by Zachariasse et al. ICT-AQ is not a stable conformer, having two imaginary vibrations. M.Z. Zgierski et al., Comp. Theor. Chem., 1036, 1 (2014).

Summary and Acknowledgment TDDFT (or CC2) computations were performed on the low-lying excited states of di-tert-butylaminobenzonitriles and tricyanoanilines that exhibit unusual photophysical behaviors leading to the ICT formation. Ultrafast ICT formation in p-DTBABN and m-DTBABN is due to the sequential mechanism of ππ*→ πσ*→ ICT, involving conical intersections among the closely-lying excited-states. For TCDMA, the presence of a TICT state that lies below the initially photoexcited ππ* state, responsible for the ultrafast dynamics observed in the excited-state absorption in acetonitrile. In both cases for TCDMA and TCA, the πσ* state locates significantly higher in energy than the ππ* state, thus precluding the πσ*→ ICT formation. Dr. Edward C. Lim, The University of Akron, U.S.A Collaborator Support Ohio Supercomputer Center