Tuning excited-state charge/proton transfer coupled reaction via the dipolar functionality

Article abstract of DOI:10.1021/jp0476390

Based on design and synthesis of I, II, and III, we demonstrate an ingenious approach to fine-tuning the excited-state intramolecular charge transfer (ESICT) coupled excited-state intramolecular proton transfer (ESIPT) reaction via the dipolar functionality of the molecular framework. Both I and II exhibit remarkable dual emission due to the different solvent-polarity environment between ESICT and ESIPT states, while the interplay of two charge-transfer entities in III leads to ESIPT decoupling from the solvent-polarity effect, resulting in a unique proton-transfer tautomer emission. The results make further rational design of the ESICT/ESIPT coupled systems feasible simply by tuning the net dipolar effect. Accordingly, systematic investigation of the correlation in regards to the difference in dipolar vectors between ESICT and ESIPT versus solvent-polarity induced barriers becomes possible.

Full text of DOI:10.1021/jp0476390

6452
J. Phys. Chem. A 2004, 108, 6452-6454
Tuning Excited-State Charge/Proton Transfer Coupled Reaction via the Dipolar
Functionality
Pi-Tai Chou,* Chien-Huang Huang, Shih-Chieh Pu, Yi-Ming Cheng, Yi-Hong Liu,
Yu Wang, and Chao-Tsen Chen*
Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan, R. O. C.
ReceiVed: June 1, 2004; In Final Form: June 28, 2004
Based on design and synthesis of I, II, and III, we demonstrate an ingenious approach to fine-tuning the
excited-state intramolecular charge transfer (ESICT) coupled excited-state intramolecular proton transfer
(ESIPT) reaction via the dipolar functionality of the molecular framework. Both I and II exhibit remarkable
dual emission due to the different solvent-polarity environment between ESICT and ESIPT states, while the
interplay of two charge-transfer entities in III leads to ESIPT decoupling from the solvent-polarity effect,
resulting in a unique proton-transfer tautomer emission. The results make further rational design of the ESICT/
ESIPT coupled systems feasible simply by tuning the net dipolar effect. Accordingly, systematic investigation
of the correlation in regards to the difference in dipolar vectors between ESICT and ESIPT versus solvent-
polarity induced barriers becomes possible.
Due to its fundamental importance in chemical reactions, the
excited-state intramolecular proton transfer (ESIPT) reaction has
received much attention.1 One important issue of current interest
regarding ESIPT reactions should be ascribed to the ESIPT
coupled excited-state intramolecular charge transfer (ESICT).
Considering that the Franck-Condon excited state of an ESIPT
molecule undergoes a gigantic dipolar change due to the charge
separation, the normal and proton-transfer tautomer equilibrium
polarizations might be far separated. Since the large dipolar
change in solution is normally coupled to solvent polarization
effects, the relative energies between normal and tautomer in
the excited state are a function of the solvent polarization
coordinate. As a result, the long-range polarization interactions
may result in a solvent-induced barrier channeling into the
proton-transfer reaction.
Seminal studies on molecules exhibiting remarkable solvent
polarization dependent ESIPT reaction dynamics should be
ascribed to two prototypical systems, namely 4′-N,N-diethy-
lamino-3-hydroxyflavones (I) (see Scheme 1)2a-d and para-N,N-
dimethylamino-methylsalicylate.2e,f For the case of system I,
on one hand, ESICT takes place from the diethylamino nitrogen
(electron donor) to the carbonyl oxygen (electron acceptor). On
the other hand, similar to its parent molecule 3-hydroxyflavone
(3HF),3 I also undergoes ESIPT from the hydroxyl proton to
the carbonyl oxygen (see Scheme 1). The difference in solvent-
polarity environment between charge transfer (CT) and proton
transfer (PT) states leads to a remarkable ESICT/ESIPT coupled
reaction. In polar, aprotic solvents, in contrast to a unique PT
tautomer emission observed in 3HF,3 dual emissions are
resolved, consisting of CT and PT bands.2a-d Applying the
Marcus electron-transfer model, Swinney and Kelley2c were able
to describe the overall reaction dynamics as well as to deduce
the solvent-induced barrier for I by a combination of solvent
polarity and proton-transfer reaction coordinates. In contrast to
the ultrafast rate (<50 fs) of ESIPT in 3HF,4 the proton tunneling
rate for I in polar solvents is slower than the rate of solvent
relaxation. Thus, a nonadiabatic type of proton transfer takes
place essentially after solvent equilibration is established in the
excited state.2c Such an intriguing ESICT/ESIPT coupled
property has led to intensive studies on I and its analogues.
Most of the approaches focus on the wavelength tunability upon
replacing the 2-phenyl ring by other functionalized aromatics,5
the applications in metal ion recognition,6 and the use of CT/
PT emission intensity ratio as indicators to probe the empirical
solvent polarity.7
Based on the chemical design, we herein demonstrate an
ingenious approach to fine-tuning the ESICT/ESIPT coupled
reaction via the dipolar orientation of the molecular framework.
The synthetic strategy lies in that if the dialkylamine, which
acts as an electron donor at the C(4′) position in I, plays a key
role in ESICT, then switching the diethylamine from C(4′) to
C(7) position, forming II (Scheme 1), may result in a similar
ESICT/ESIPT coupled reaction. However, the resulting net
dipole moment, the orientation of which is affected by the CT
effect, is expected to alter with respect to that of I. Moreover,
if 2-phenyl pyrone (region A in Scheme 1) and flavone (region
B) chromophores are strongly coupled so that electrons are
delocalized throughout the entire molecular framework, it would
be intriguing to functionalize 3HF by adding dialkylamine at
both the C(4′) and C(7) positions, forming III, of which the
net dipole moment may be associated with the interplay between
two charge-transfer systems, namely I and II. Thus, switching
the dipolar property of the CT state may greatly tune the ESICT/
ESIPT coupling behavior among I-III.
I-III were synthesized by the Claisen condensation.2a While
I has been reported, detailed synthetic schemes for II and III
as well as their corresponding methoxyl derivatives, IIa and IIIa,
respectively, are elaborated in the Supporting Information.
Figure 1 shows the emission spectra of I-III in various polar
solvents, and detailed photophysical data are listed in Table 1
(see Supporting Information for the absorption spectra). Similar
to the steady-state emission behavior of I shown in Figure 1A,2
II exhibits dual emission, in which the peak wavelength of the
higher energy band is strongly dependent on the solvent polarity
* Corresponding author. E-mail: chop@ntu.edu.tw; chenct@ntu.edu.tw
10.1021/jp0476390 CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/10/2004

Letters
J. Phys. Chem. A, Vol. 108, No. 31, 2004 6453
SCHEME 1. Structures of Various Systems Studied and the Calculated (see text) Dipole Moments (arrows show
components along the z-x plane) for I, II and III in Singlet normal (S) and Proton Transfer (S′) States (angle specified
clockwise to the z-axis)
TABLE 1: Photophysical Properties of I-III in Various
Solvents at Room Temperature
Em (nm) (τ (ns) [preexponential factor])
Abs (nm)
Nd
Te
QY(%)f
I
401a
411b
405c
374
380
382
362
365
367
401
406
409
376
384
388
475 (0.47) 570 (0.38)
495 (0.72) 572 (0.78)
516 (0.42) 574 (0.41)
445 (0.12) 566 (0.11[-1.00], 1.30[1.02]) 13.8
460 (0.15) 564 (0.17[-1.00], 3.25[0.93]) 45.5
490 (0.39) 566 (0.39[-1.00], 1.80[1.15]) 19.7
5
17
9.6
II
IIa
III
IIIa
452 (0.40)
465 (0.79)
513 (4.05)
3.3
11.2
29.3
26.3
46.3
71.1
16.2
45.2
46.6
589 (3.41)
589 (3.65)
594 (2.81)
453 (0.42)
458 (0.95)
460 (1.07)
a In ethyl acetate. b In CH2Cl2. c In CH3CN. d Normal band. e PT
tautomer. f For I and II, QY(%)% is the sum of dual emission intensity.
Note that negative preexponential values indicate rise components.
Figure 1. Emission spectra of (A) I, (B) II, and (C) III (solid line)
and IIIa (dash line) in ethyl acetate (0), CH2Cl2 (4), and CH3CN (O).
The excitation wavelength is 350 nm.
independent, normal Stokes shifted emission (see Table 1 and
Figure 1C). The results lead us to propose that, upon excitation,
the interplay between two charge-transfer dipole vectors in III
yields a net dipole moment nearly the same as that of the PT
tautomer. ESIPT is thus decoupled from the solvent polarity
effect, resulting in a dominant PT tautomer emission.
A further dynamic approach allows us to establish a precur-
sor-successor relationship between CT and PT bands for II.
For example, in CH3CN the decay time of 390 ps in II
monitored at the CT band is identical to the rise time of the PT
band (see Table 1 and Figure 2). The results are in contrast to
I, in which the excited-state equilibrium between CT and PT is
established, as supported by their identical population lifetimes
(see Table 1 and also ref 2c). For the case of III, the rate of
ESIPT is too fast to be resolved by our time-correlated single
photon counting system (∼15 ps response time), supporting the
solvent-polarity decoupled ESIPT mechanism for III.
and is reasonably ascribed to a CT emission incorporating a
charge transfer from C(7)-diethylamine to carbonyl oxygen.
Conversely, the lower energy band with an anomalously large
Stokes shift of >7000 cm-1 reveals negligibly small solvent-
polarity dependence and is clearly assigned to a PT tautomer
emission. The charge-transfer property in II could be further
supported by its methoxyl derivative IIa, in which ESIPT was
prohibited and the resulting emission showed similar solvent-
polarity dependence with that of the CT band in II (see Table
1). Figure 1C depicts the emission spectra of III in various
solvents. In contrast to both I and II, III exhibits predominantly
a proton-transfer emission, of which the peak wavelength reveals
negligible solvent-polarity dependence. The results indicate that
the dipolar change in the excited state for III must be small
between normal and PT tautomer species so that ultrafast ESIPT
takes place. This viewpoint can be firmly supported by the non-
ESIPT analogue IIIa, which exhibits a nearly solvent-polarity
Theoretical approaches at an INDO/S-CI level based on AM1
geometry optimized ground state structures have been performed
in an attempt to rationalize the experimental results. All singly

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
at http://pubs.acs.org.
References and Notes
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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