6454 J. Phys. Chem. A, Vol. 108, No. 31, 2004
Letters
dipolar vectors between ESICT and ESIPT as well as the
solvent-polarity induced barrier becomes possible, which may
be crucial to gain fundamental insights into research fields of
current interest regarding, e.g., proton-coupled electron transfer
in living systems.8 Unfortunately, for I-III possessing five-
membered ring hydrogen bonds, the intramolecular hydrogen
bonding strength is relatively weak. Thus, in protic solvents
such as ethanol and water, our preliminary studies have shown
that certain percentage of CdO- - -H-O- intramolecular
hydrogen bond in compound I-III is ruptured by forming
intermolecular hydrogen bond with protic solvents. As a result,
excited-state deprotonation instead of ESIPT takes place in these
protic solvents. This makes the discussion of the ESICT/ESIPT
reaction solely based on the solvent polarity very complicated.
Studies on the ESICT/ESIPT coupled reaction, in combination
with the excited-state deprotonation, for compounds I-III in
protic systems are currently in progress.
Figure 2. Time-resolved spectra of compound II (CT: monitored at
460 nm; PT: monitored at 640 nm; S: system response function).
excited configurations from the 10 highest occupied to the 10
lowest unoccupied molecular orbitals were involved in the
computation (see Supporting Information). Dipole moments
were calculated for I, II, and III in both ground and excited
states, and the dipole vectors along the z-x plane are depicted
in Scheme 1. For III, the orientation of the dipole moment is
similar between the normal ground (S0, 160°) and excited (S1,
155°) states, while significant differences were obtained for both
I and II (see Scheme 1). More importantly, while the dipole
vectors of excited CT (S1) and PT (S1′) states are substantially
different for both I and II (see Scheme 1), they are similar in
III (S1, (3.6 D, 155°), S1′(3.9 D, 140°)). Although this approach,
to a certain extent, is qualitative on the magnitude of the dipole
moment, according to different dipole orientations, ESICT/
ESIPT coupled reactions are expected for I and II, and solvent
polarities should play a key role in describing the reaction
dynamics and/or thermodynamics. Conversely, negligible changes
in dipolar orientations for III lead to a fast, solvent decoupled
ESIPT, consistent with experimental results.
Supporting Information Available: Detailed experimental
procedures, absorption, emission, time-resolved and X-ray
studies. This material is available free of charge via the Internet
References and Notes
(1) For recent reviews, see: (a) Scheiner, S. J. Phys. Chem. A 2000,
104, 5898. (b) Waluk, J. Conformational Aspects of Intra- and Intermo-
lecular Excited-State Proton Transfer and Conformational Analysis of
Molecules in Excited States; Waluk, J., Ed.; Wiley-VCH: New York, 2000.
(2) (a) Chou, P. T.; Martinez, M. L.; Clements, J. H. J. Phys. Chem.
1993, 97, 2618. (b) Chou, P. T.; Martinez, M. L.; Clements, J. H. Chem.
Phys. Lett. 1993, 204, 395. (c) Swinney, T. C.; Kelley, D. F. J. Chem.
Phys. 1993, 99, 211. (d) Parsapour, F.; Kelley, D. F. J. Phys. Chem. 1996,
100, 2791. (e) Gormin, D.; Kasha, M. Chem. Phys. Lett. 1988, 153, 574.
(f) Heldt, J.; Gormin, D.; Kasha, M. Chem. Phys. 1989, 136, 321.
(3) (a) Sengupta, P.; Kasha, M. Chem. Phys. Lett. 1979, 68, 382. (b)
McMorrow, D.; Kasha, M. J. Phys. Chem. 1984, 88, 2235.
(4) Ameer-Beg S.; Ormson, S. M.; Brown, R. G.; Matousek, P.; Towrie,
M.; Nibbering, E. T. J.; Foggi, P.; Neuwahl, F. V. R. J. Phys. Chem. A
2001, 105, 3709.
(5) (a) Falkovskaia, E.; Pivovarenko, V. G.; Valle, J. G. Chem. Phys.
Lett. 2002, 352, 415. (b) Klymchenko, A. S.; Pivovarenko, V. G.;
Demchenko, A. P. J. Phys. Chem. A 2003, 107, 4211.
(6) (a) Bublitz, G. U.; Boxer, S. G. Annu. ReV. Phys. Chem. 1997, 48,
213. (b) Klymchenko, A. S.; Demchenko, A. P. J. Am. Chem. Soc. 2002,
124, 12372.
(7) Klymchenko, A. S.; Demchenko, A. P. Phys. Chem. Chem. Phys.
2003, 5, 461.
(8) For example, see: Milligan, J. R.; Aguilera, J. A. Hoang, O.; Ly,
A.; Tran, N. Q.; Ward, J. F. J. Am. Chem. Soc. 2004, 126, 1682.
We have thus demonstrated for the first time a prominent
dipole orientation tuning ESICT/ESIPT coupled reaction. The
preservative net dipolar effect in III leads us to conclude that
2-phenyl pryone and flavone moieties are strongly coupled so
CT in III is essentially delocalized in the entire molecule. This
makes feasible further rational design of ESICT/ESIPT systems
by altering the net dipolar vector. Accordingly, systematic
investigation of the correlation in regards to the difference in