Intramolecular Charge Transfer Dynamics of 4 Dimethylaminobenzonitrile Ultrafast
Intramolecular Charge Transfer Dynamics of 4 -(Dimethylamino)benzonitrile: Ultrafast Branching Followed by a Two-Fold Decay Mechanism PEDRO B. COTO, and LUIS SERRANO-ANDRÉS, Instituto de Ciencia Molecular (ICMOL), Universitat de Valéncia, Valencia, Spain THOMAS GUSTAVSSON, Laboratoire Francis Perrin, CEA/DSM/DRECAM/SPAM-CNRS URA 2453 CEA/Saclay, France TAKASHIGE FUJIWARA, and EDWARD C. LIM, Department of Chemistry and The Center for Laser and Optical Spectroscopy, The University of Akron, Akron OH.
Steady-state absorption and emission spectra of DMABN Abs LE Normal fluorescence from the lowest ππ* (1 Lb type) Locally excited (LE) state Abs LE ICT • Lippert and co-workers, Adv. Mol. Spectrosc. p. 443 (1962). A large red-shifted fluorescence from Intramolecular charge transfer (ICT) state • Grabowski and co-workers, Chem. Phys. Lett. 19, 315 (1973); Nouv. J. Chim. 3, 443 (1979).
1. Geometry of the Intramolecular Charge Transfer (ICT) State Twisted ICT (TICT) Amino group has a perpendicular conformation to benzonitrile plane Planar ICT (PICT) The pseudo-Jahn–Teller model with quinoidal conformation Rehybridized ICT (RICT) Conversion of the cyano triple bond into a bend double bond (sp → sp 2)
2. Electronic pathways in photoexcited DMABN in polar solvents Two-State Conversion Mechanism Temporal profiles of LE and ICT fluorescences Double-exponential decay Time-Correlated Single Photon Counting (TCSPC) Exc@267 nm, System Response > 30 ps
This talk focuses combined experimental and theoretical studies on ICT dynamics of DMABN: 1. Ultrafast spectroscopic techniques Femtosecond fluorescence upconversion, femtosecond-transient absorption, and picosecond time-resolved fluorescence (OKG) 2. Ab initio high-level quantum mechanical calculations Multireference perturbation level of theory (CASPT 2//CASSCF) Structures and energies for ICT and intermediate states, and their ICT reaction pathways 3. Photophysics in DMABN may be more complicated: Existence of two distinct ICT states of DMABN in polar solvents. A new mechanism of ultrafast branching followed by a two fold decay.
Femtosecond Fluorescence Upconversion Setup
Fluorescence upconversion of DMABN in acetonitrile 2 -D image plot + Contour plot Time-evoluted spectra LE ICT • T. Gustavsson, B. P. Coto, L. Serrano-Andrés, T. Fujiwara and E. C. Lim J. Chem. Phys. , 131, 031101 (2009).
Fluorescence Upconversions of DMABN in Acn
Global Fit Analyses in the Fluorescence Upconversions of DMABN in Acn at λexc = 267 nm (all decay constants in ps). Observed wavelength 325 nm 350 nm 450 nm τ1 a 0. 239 (rise) τ2 a 3. 067 τ3 a 550 nm 575 nm 600 nm 3. 067 (rise) 169. 8 b FWHMc No. of rund a Accuracies 500 nm 286 fs 333 fs 332 fs 301 fs 2 3 3 1 4 1 1 of the common decay constant of τ1, τ2, and τ3 are ± 0. 003, ± 0. 024, and ± 25. 6 ps, respectively. b It may appear a nominal decay constant since the LE and ICT bands overlap in this spectral regime, which also accounts for having a large inaccuracy in τ3. c Full width at half maximum of a Gaussian function used as the system response in the fit. d No. of traces included in the global fit. Precursor–successor relationship between the LE and the ICT states
Femtosecond Time-Resolved Transient Absorption Setup
DMABN/acn Exc@267 nm ICT 4. 3(1) ps 4. 0(1) ps 4. 33(4) ps
Potential Energy Profiles of DMABN Transitions πσ* ← πσ* λcal (nm) λobs (nm) fcalc 633 680 0. 55 ππ* ← ππ* 524 540 0. 07 ππ* ← ππ* 423 460 0. 08 ICT 400 410 0. 08 TDDFT/BP 86/cc-p. VDZ • J. -K. Lee, T. Fujiwara, W. G. Kofron, M. Z. Zgierski and E. C. Lim J. Chem. Phys. , 128, 164512 (2008).
fs-Transient Absorption Spectra of DMABN in Acetonitrile The πσ* state-mediated ICT TICT ππ* πσ* • M. Z. Zgierski and E. C. Lim J. Chem. Phys. , 122, 111103 (2005).
fs-TAS of DMABN in acetonitrile at longer time delay TICT -0. 2 – 3. 7 ns T T Time profiles for the TAS bands
Picosecond Time-resolved Fluorescence Setup w/OKG Kerr media : benzene Optical time-gate width: ~ 1 ps
ps-Time-resolved Fluorescence Spectra of DMABN in Acetonitrile 0 – 25 ps window LE ICT Nonradiative decay The decay time of ICT fluorescence should be essentially the same as that of deduced by TA.
ICT Emission ICT Absorption Fast Rise τ1 Slow Decay τ2 DMABN 3. 1 ps ~ 2. 9 ns in Acn 6. 0 ps ~ 2. 3 ns in Et. OH 4. 3 ps ~ 4. 8 ns in Acn 9. 9 ps ~ 3. 3 ns in Et. OH fs. Expt. methods upconversion ps-TRFS TCSPC ps-TRFS Existence of two charge-transfer state of DMABN in polar solvents: A dark ICT (from the πσ* state), probed by absorption A fluorescent ICT (formed from the LE state), detected by emission The two different ICT states may be non-communicating Accurate characterization of the potential-energy profiles: Ab initio multireference perturbation theory (CASPT 2) quantummechanical calculations
πσ* LE 121. 3˚ p. TICT 53. 4˚ 31. 4˚
Adiabatic energies, dipole moments, and charge distributions in the low-lying excited states in DMABN System a In Vacuo Solution ΔEa |μ|b Chargec ΔE |μ| Charge LE 3. 88 6. 532 -0. 337 3. 81 7. 941 -0. 397 πσ* 5. 41 13. 111 -0. 492 3. 87 22. 822 -0. 739 p. TICT 3. 92 14. 357 -0. 707 3. 60 16. 307 -0. 736 TICT 3. 80 13. 287 -0. 807 3. 28 15. 882 -0. 881 Adiabatic energy (in e. V) relative to the ground state. b Modulus of the dipole moment (in Debye). c Mulliken charges in the Ph. CN moiety.
Adiabatic energy diagram of DMABN CASPT 2//CASSCF model – in vacuo πσ* 1 L a 1 L p. TICT b TICT LE path 1 GS path 2
Adiabatic energy diagram of DMABN CASPT 2//CASSCF model – acetonitrile (PCM model) 1 L b 1 L a πσ* LE TICT path 1 GS path 2
f = 0. 231 f = 0. 001 Schematic energy diagram for 2 -fold decay mechanism of the photoexcited DMABN in polar solvents
Summary Ultrafast spectroscopic studies (fs-upconversion, ps-TRFS, and fs-TAS) + High-level ab initio multireference perturbation theory (CASPT 2) quantum-mechanical calculations performed for DMABN. A new mechanism with an initial branching to two different reaction paths with the three different ICT states (πσ*, TICT, and p. TICT) opening simultaneously in polar environments. The dark TICT formed from the πσ* state, which is only probed by TAS; the fluorescent ICT state formed from the LE state is assigned as the p. TICT state. The excited-state relaxation dynamics of DMABN – an ultrafast branching from initially photoexcited 1 La state, followed by a two-fold energy decay via distinct ICT states.
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