1. 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

2. 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.

3. 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).

4. Optimized Structures of p-DTBABN
ππ*
πσ*
TICT
124.6˚
𝛥E = 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

5. 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

6. 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

7. 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

8. 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).

9. 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).

10. TDDFT Optimized Structures of TCDMA
ππ*
πσ*
TICT
28.7˚
37.8˚
124.0˚
TCDMA
𝛥E = eV
𝛥E = eV
𝛥E = –0.03 eV
𝛥E = eV
𝛥E = eV
𝛥E = eV
TCA
TD/B3LYP/cc-pVDZ level of theory

11. 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).

12. 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 = 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).

13. 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