Comprehensive studies on dual excitation behavior of double proton versus charge transfer in 4-(N-substituted amino)-1H-pyrrolo[2,3-b]pyridines

Article abstract of DOI:10.1021/jp021243b

Comprehensive spectroscopic and dynamical studies on the dual excitation behavior of proton vs charge transfer for 4-(dimethylamino)-1H-pyrrolo[2,3-b]pyridine (DPP) and its related derivatives are reported. In cyclohexane, DPP dimer and/or dual hydrogen-bonded complex are formed with association constants K_a as high as ～4.2 × 10³ and 5.2 × 10⁴ M^-1 (e.g., the DPP/acetic acid complex) at 298 K, respectively, which upon electronic excitation undergo ultrafast rate (?6.7 × 10¹⁰ s^-1) of double-proton transfer, resulting in a unique tautomer emission. Dual fluorescence was observed in polar, aprotic solvents, in which the large Stokes shifted emission band originates from the charge-transfer species incorporating a dimethylamine and pyridine ring as electron donor and acceptor, respectively. Detailed solvent-polarity and temperature-dependent studies in combination with theoretical approaches have been performed to determine the excited-state charge-transfer properties such as dipole moment, orbital configuration, etc. Supplementary support for the dual charge/proton-transfer behavior was provided by the comparative spectroscopy and dynamics of various DPP-related derivatives. Further time-resolved measurements conclude that dual emissions share a common Franck-Condon excited state but undergo two independent relaxation channels. In protic solvents, such as ethanol, following fast solvent relaxation dynamics, the excited charge-transfer state further undergoes a solvent (i.e. alcohol) assisted proton-transfer reaction. The charge versus proton-transfer emission can be distinguished via the temporal spectral evolution. The results demonstrate DPP to be a unique model among 7-azaindole analogues in which the interplay between charge and proton-transfer reactions is operative in the excited state.

Full text of DOI:10.1021/jp021243b

J. Phys. Chem. A 2003, 107, 1459-1471
1459
Comprehensive Studies on Dual Excitation Behavior of Double Proton versus Charge
Transfer in 4-(N-Substituted amino)-1H-pyrrolo[2,3-b]pyridines
Chung-Chih Cheng,† Chen-Pin Chang,† Wei-Shan Yu,‡ Fa-Tsai Hung,§ Yun-I Liu,|
Guo-Ray Wu,| and Pi-Tai Chou*,‡
Department of Chemistry, Fu-Jen Catholic UniVersity, 242, Taipei, Taiwan, Republic of China, Department of
Chemistry, National Taiwan UniVersity, 106, Taipei, Taiwan, Republic of China, The National Hu-Wei Institute
of Technology, Yunlin, Taiwan, Republic of China, and Department of Chemistry and Biochemistry, National
Chung Cheng UniVersity, 621, Chia-Yi, Taiwan, Republic of China
ReceiVed: May 21, 2002; In Final Form: December 13, 2002
Comprehensive spectroscopic and dynamical studies on the dual excitation behavior of proton vs charge
transfer for 4-(dimethylamino)-1H-pyrrolo[2,3-b]pyridine (DPP) and its related derivatives are reported. In
cyclohexane, DPP dimer and/or dual hydrogen-bonded complex are formed with association constants Ka as
high as ∼4.2 × 103 and 5.2 × 104 M-1 (e.g., the DPP/acetic acid complex) at 298 K, respectively, which
upon electronic excitation undergo ultrafast rate (.6.7 × 1010 s-1) of double-proton transfer, resulting in a
unique tautomer emission. Dual fluorescence was observed in polar, aprotic solvents, in which the large
Stokes shifted emission band originates from the charge-transfer species incorporating a dimethylamine and
pyridine ring as electron donor and acceptor, respectively. Detailed solvent-polarity and temperature-dependent
studies in combination with theoretical approaches have been performed to determine the excited-state charge-
transfer properties such as dipole moment, orbital configuration, etc. Supplementary support for the dual
charge/proton-transfer behavior was provided by the comparative spectroscopy and dynamics of various DPP-
related derivatives. Further time-resolved measurements conclude that dual emissions share a common Franck-
Condon excited state but undergo two independent relaxation channels. In protic solvents, such as ethanol,
following fast solvent relaxation dynamics, the excited charge-transfer state further undergoes a solvent (i.e.
alcohol) assisted proton-transfer reaction. The charge versus proton-transfer emission can be distinguished
via the temporal spectral evolution. The results demonstrate DPP to be a unique model among 7-azaindole
analogues in which the interplay between charge and proton-transfer reactions is operative in the excited
state.
1. Introduction
for the catalytic type of reaction where the guest molecule (e.g.,
acetic acid) remains unchanged during the proton-transfer
In their seminal studies, Taylor et al.1 found that the first
excited singlet state of 7-azaindole (7AI) dual hydrogen-bonded
(HB) dimer undergoes an excited-state double proton transfer
(ESDPT), resulting in a large Stokes-shifted tautomer emission
(e.g., λmax ∼ 480 nm in cyclohexane). This dual hydrogen-
bonding system has ever since been recognized as a simplified
model for the base pair of DNA. At the molecular level, the
ESDPT process provides one possible mechanism for the
mutation due to a “misprint” induced by the proton-transfer
tautomerism of a specific DNA base pair during replication,
recording an error message.2-5 On this basis, much research
has focused on the spectroscopy and dynamics1-3,6-17 as well
as theoretical approaches18-20 of ESDPT in the 7AI dimer.
Conversely, via the formation of a 7AI(host)/guest HB complex,
the dynamics of ESDPT incorporating various types of guest
molecules has also received considerable attention through both
experimental21-26 and theoretical approaches.27-30 Studies of
7AI complexed to single molecules of carboxylic acids and
lactams have revealed rapid ESDPT reaction,26b-d especially
reaction. In bulk alcohols and water the dynamics of solvent
assisted ESDPT in 7AI and its analogue 7-azatryptophan have
been successfully applied to probe the solvation and/or protein
dynamics.21-25 During the past few years, focus on the chemical
modification of 7AI has received particular interest to study
the substituent effect on the proton transfer reaction. It has been
shown that the number as well as the position of the nitrogen
atom in either a five- or six-member ring system of 7AI alters
the relative ESDPT thermodynamics, especially in the noncata-
lytic type of proton-transfer reaction.31 The generalization of
amine-imine type of tautomerism in 7AI and its corresponding
analogues has been verified via the hydrogenation of the C2-
C3 double bond in 7AI, forming 7-azaindoline.32 The results
open up a study of ESDPT extended to a DNA base such as
adenine derivatives possessing similar type of tautomerism
proposed in the mutation process.33 On the other hand, based
on the design of 3-cyano-7-azaindole, a steady state tautomer
emission in aqueous solution has been unambiguously resolved
and successfully applied as a model to closely examine various
proposed mechanisms of excited-state proton-transfer reactions
for 7AI in pure water.34
* To whom correspondence should be addressed.
† Department of Chemistry, Fu-Jen Catholic University.
‡ Department of Chemistry, National Taiwan University.
§ The National Hu-Wei Institute of Technology.
| Department of Chemistry and Biochemistry, National Chung Cheng
University.
In another approach, in the previous communication35 we
reported the dual excitation behavior of excited-state biprotonic
10.1021/jp021243b CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/15/2003

1460 J. Phys. Chem. A, Vol. 107, No. 10, 2003
Cheng et al.
7.10 (d, J ) 6.0 Hz, 1H), 7.48 (d, J ) 3.0 Hz, 1H), 7.67 (d, J
) 8.0 Hz, 1H), 8.18 (d, J ) 6.0 Hz, 1H), 13.50 (s, 1H, pyrrole
NH).
4-Chloro-1H-pyrrolo[2,3-b]pyridine (C7H5ClN2) (3). A 0.5
g sample of 2 was added in portions to the cooled phosphoric
trichloride solution (5 mL), and the mixture was refluxed for 5
h. Phosphoric trichloride was then distilled off under reduced
pressure followed by the dilution of water (5 mL). The solution
was basified with sodium carbonate to allow the precipitation
for ∼1 h. The precipitate was filtered off to obtain 3. 1H NMR
(200 MHz, DMSO-d6) δ 6.55 (d, J ) 3.4 Hz, 1H), 7.22 (d, J )
5.1 Hz, 1H), 7.63 (d, J ) 3.4 Hz, 1H), 8.23 (d, J ) 5.1 Hz,
1H), 10.25 (s, 1H, pyrrole NH).
4-(Dimethylamino)-1H-pyrrolo[2,3-b]pyridine (C9H11N3, DPP).
A mixture of 3 (2 mmol) and dimethylammonium chloride (10
mmol) was heated in an oil bath at 180°C for 5 h. After cooling,
the dark brown reaction mixture was worked up by flash column
chromatography (eluent: 10-20% methanol by weight in
dichloromethane) to obtain DPP. 1H NMR (200 MHz, CDCl3)
δ 3.27 (s, 6H), 6.18 (d, J ) 6.0 Hz, 1H), 6.64 (d, J ) 3.8 Hz,
1H), 7.12 (d, J ) 3.8 Hz, 1H), 7.96 (d, J ) 6.0 Hz, 1H), 11.8
(s, 1H, pyrrole NH).
Figure 1. Structures of DPP and its corresponding proton/charge-
transfer isomers, methylated derivatives, and analogues.
1-Methyl-4-(dimethylamino)-1-H-pyrrolo[2,3-b]pyridine
(C10H13N3, 1MDPP). 1MDPP was synthesized by adding sodium
hydride (100 mg) to the THF solution containing DPP (0.15
g), followed by the addition of methyl iodide (100 mg). The
resulting product, 1MDPP, was purified by column chroma-
tography (eluent 1:1 v/v hexanes: ethyl acetate). 1H NMR (200
MHz, CDCl3) δ 3.28 (s, 6H), 3.89 (s, 3H), 6.20 (d, J ) 6.0 Hz,
1H), 6.62 (d, J ) 3.6 Hz, 1H), 6.94 (d, J ) 3.6 Hz, 1H), 8.03
(d, J ) 6 Hz, 1H).
transfer vs charge transfer in 4-(dimethylamino)-1H-pyrrolo-
[2,3-b]pyridine (DPP, see Figure 1). The results make DPP a
unique model among hydrogen-bonded complexes to study the
interplay between two fundamental mechanisms fine-tuned by
solvent polarity as well as hydrogen-bonding perturbation. In
this paper we have further performed comprehensive spectro-
scopic and dynamic studies on DPP and its relative derivatives,
aimed at gaining a detailed insight into the electronic properties
and corresponding relaxation dynamics. Ground-state thermo-
dynamics as well as excited-state proton-transfer dynamics for
DPP dimer and DPP/guest complexes have been deduced in
nonpolar solvents. Detailed solvent-polarity-dependent studies
in combination with semiempirical calculations led us to derive
the excited-state charge-transfer properties such as dipole
moment, orbital configuration, etc. Finally, both temperature
and time dependent fluorescence spectroscopy and dynamics
were performed to resolve the excited-state charge-versus-
proton-transfer dynamics.
7-Methyl-4-(dimethylamino)-1H-pyrrolo[2,3-b]pyridine
(C10H13N3, 7MDPP). 7MDPP was synthesized by the reaction
of DPP (0.1 g) and methyl iodide (0.5 g) in THF under N2
atmosphere. NaOH (2.5N, 10 mL) was then added, and the
mixture was stirred for ∼20 min to obtain crude 7MDPP. The
product was further purified by column chromatography (elu-
1
ent: ethyl acetate and then methanol). H NMR(CDCl3, 400
MHz) δ 3.38 (s, 6H), 4.08 (s, 3H), 6.05 (d, J ) 7.2 Hz, 1H),
6.76 (d, J ) 2.8 Hz, 1H), 7.35 (d, J ) 7.2 Hz, 1H), 7.47 (d, J
) 2.8 Hz, 1H).
4-Amino-1H-pyrrolo[2,3-b]pyridine (C7H7N3, APP). APP was
prepared according to a previously reported procedure.39 Briefly,
a mixture of 0.2 g (1.11 mmol) of 4-nitro-1H-pyrrolo[2,3-b]-
pyridine-7-oxide40,41 and 0.8 g of Fe powder in 8 mL of glacial
acetic acid was heated with stirring at 100 °C for 2 h. The cooled
mixture was diluted with 20 mL of water, adjusted to pH ∼
10-11 with NaOH, and then continuously extracted with diethyl
ether (∼250 mL). The extract was dried over anhydrous Na2-
SO4 and then evaporated on a rotary evaporator to obtain APP.
1H NMR (200 MHz, DMSO-d6) δ 6.48 (d, J ) 7.0 Hz, 1H),
6.82 (d, J ) 3.0 Hz, 1H), 7.28 (d, ) J 3.0 Hz, 1H), 7.87 (d, J
) 7.0 Hz, 1H), 12.03(s, 1H, pyrrole NH).
Various solvents are of spectrograde quality (Merck, Inc.)
and used right after received. 2-Methyltetrahydrofuran (2MTHF)
and acetonitrile showed traces of fluorescence impurities and
were fractionally distilled prior to use.
2.2. Measurements. Steady-state absorption and emission
spectra were recorded by a Varian (Cary 3E) UV-vis spectro-
photometer and a Hitachi (F4500) fluorimeter, respectively. Both
wavelength-dependent excitation and emission response of the
fluorimeter have been calibrated according to a previously
reported method.31b Room-temperature fluorescence quantum
yields were measured using quinine sulfate/1.0 N H2SO4 as
2. Experimental Section
2.1. Materials. The synthesis of DPP was performed accord-
ing to procedures used in previous reports with slight
modification.36-38 Brief methodology and spectral characteriza-
tion in each step for synthesizing DPP and related derivatives
are described below. For clarity, structures of DPP and its
corresponding analogues are depicted in Figure 1.
7-Hydroxy-1H-pyrrolo[2,3-b]pyridinium m-Chlorobenzoate
(C14H11ClN2O3) (1). A 1.2 g (5.9 mmol) sample of 85%
m-chloroperoxybenzoic acid was added to 0.5 g (4.23 mmol)
of 1H-pyrrolo[2,3-b]pyridine dissolved in 15 mL of redistilled
1,2-dimethoxyethane The resulting solution was stirred at room
temperature for 1.5 h and during this period the product
gradually precipitated. The mixture was then cooled and the
light yellow product was isolated by filtration and washed with
diethyl ether to give 1 (0.45 g, ∼75%).
1H-Pyrrolo[2,3-b]pyridine-7-oxide (C7H6N2O) (2). A 1.0 g
(3.4 mmol) sample of 1 in 10 mL of water was basified to pH
∼ 9 with saturated K2CO3 solution, followed by the extraction
of chloroform. The solution was then concentrated to obtain 2.
1H NMR (200 MHz, DMSO-d6) δ 6.58 (d, J ) 3.0 Hz, 1H),

Excitation Behavior of DPPs
J. Phys. Chem. A, Vol. 107, No. 10, 2003 1461
reference, assuming a yield of 0.564 with 365-nm excitation.42
For the temperature-dependent emission measurement a 3rd
harmonic of the Nd:YAG laser (355 nm, 8 ns duration,
Continuum Surlite II) pumped optical parametric oscillator
(OPO) served as a light source. The output of the fundamental
laser frequencies was then frequency doubled by a BBO crystal
to obtain tunable excitation frequencies between 270 and 320
nm. The emission was collected at a right angle with respect to
the excitation source and detected by an intensified charge
coupled detector (ICCD, Princeton Instrument, Model 576G/
1). Typically, 100 laser shots were collected and averaged in
each data acquisition period. Low-temperature measurements
were performed in an Oxford cryostat (Model CCC1104) with
temperature regulator (Model ITC502).
Pico-nanosecond lifetime measurements were achieved by
using either a second (380-420 nm) or third (260-275 nm)
harmonic of the femtosecond Ti-Sapphire oscillator (82 MHz,
Spectra Physics) as an excitation source coupled with a pulse
picker (NEOS, model N17389). This combination gives an
output repetition rate of 8 MHz to avoid the accumulation of
residue from the previous excitation pulse due to the longer
fluorescence decay components. An Edinburgh OB 900-L time-
correlated single photon counter was used as a detecting system.
Since the fwhm of the excitation pulse is typically ∼100 fs,
the resolution is limited by the detector response of ∼30 ps.
The fluorescence decays were analyzed by the sum of expo-
nential functions with an iterative convolution method reported
previously.42 This procedure should allow partial removal of
the instrument time broadening and consequently renders a
temporal resolution of ∼15 ps.
Figure 2. (s) The concentration-dependent absorption spectra of DPP
in cyclohexane, in which DPP (C0) was prepared at (a) 1.1 × 10-4, (b)
2.1 × 10-4, (c) 4.3 × 10-4, (d) 8.6 × 10-4, (e) 1.7 × 10-3 M. The
absorption spectra are normalized at 305 nm. (- -) The absorption
spectrum of 1MDPP (1.0 × 10-5 M) in cyclohexane. Insert: Plot of
A0/(A - A0) versus 1/(A - A ) at 310 nm for b-e and a best linear
x
0
least-squares fitting curve using eq d. See section 3.1 for the definition
of A0 and A.
to the amine-like monomer species is unambiguous. Upon
increasing the concentration, changes in the spectral features
were observed in which the spectra reveal red shift with the
appearance of a new 0-0 onset peak maximum at 310 nm (see
Figure 2a-e). In comparison, the absorption features of 1MDPP
in cyclohexane reveal concentration independence. Since the
difference between DPP and 1MDPP lies in the dual hydrogen-
bonding sites in DPP, the results unambiguously conclude the
formation of DPP dimer and/or higher-order aggregates in
cyclohexane through the dual hydrogen-bonding effect. Similar
to that proposed in the 7AI self-association,3,26b-d we first
assume a dominant self-dimerization form possibly possessing
a cyclic type of hydrogen bonding configuration where the dual
hydrogen-bonding sites, i.e., the N(1)H hydrogen and the
pyridinic nitrogen in DPP (i.e., N(7)), act as a proton donor
and acceptor, respectively. Consequently, a competitive equi-
librium between DPP monomer and its corresponding self-
association is established, and the dimerization constant Ka can
be expressed as
Detailed methods of the theoretical approach (e.g. 6-31G(d,
p) and higher levels) for the thermodynamics of HB complexes
in the ground state have been described previously.43 For
calculating the physical parameters such as dipole moment,
cavity radius, etc., of DPP, various methods including AM1,
PM3, or MP2/6-31G(d, p) were applied. The complexity of
molecular structure makes an ab initio approach in the excited
state extremely time consuming and in some cases even not
feasible. Instead, the properties of various low-lying excited
singlet states were estimated by an INDO/S-CI calculation based
on an AM1 geometry optimized ground-state structure. All
singly excited configurations from the 15 highest occupied to
the 15 lowest unoccupied (225 configurations) molecular orbitals
were involved in the computation.
Cp
Ka )
(a)
(C0 - 2Cp)2
where C0 is the initial concentration of DPP, and Cp denotes
the concentration of the DPP dimeric form. In the case of 7AI
and its corresponding analogues, significant red shift of the
absorption spectrum near the onset of the monomer S0 f S1
(ππ*) absorption is commonly observed upon dimeri-
zation.1-3,31-34 For instance, the absorbance at wavelengths of
>310 nm can be exclusively attributed to the dimeric absorption
of 7AI.3 On the the basis of the Beer-Lambert law, eq a can
thus be rewritten as
The following sections are organized according to a sequence
of steps where we first performed detailed absorption and
fluorescence titration experiments to determine the thermody-
namics and ESDPT properties of DPP self-dimerization and
hydrogen-bonding complexes in nonpolar solvents. Subse-
quently, dual emission properties of DPP in polar, aprotic
solvents were investigated and discussed on the basis of the
excited-state charge-transfer property. Finally, the differentiation
between solvent diffusive reorganization and solvent relaxation
to affect the solvent assisted ESDPT dynamics in alcohols was
elaborated.
C0
A
1
1
1
2
ꢀD
)
+
(b)
K xA
ꢀ
x x
D
a
3. Results
where A and ꢀD denote the absorbance (in the 1 cm path-length
cell) and the molar extinction coefficient of the dimer, respec-
tively, at a selected wavelength. According to the plot of C0/A
3.1. Hydrogen-Bonding Association in Nonpolar Solvents.
When the concentration was prepared to be as low as 9.2 ×
10-6 M, DPP exhibits the lowest singlet πf π* absorption band
maximum at 303 nm (ꢀ303 ∼ 6750 M-1cm-1). The spectral
feature is similar to that of 1MDPP (see Figure 2) generally
treated as a nonproton-transfer model due to the lack of the
pyrrolic N-H proton. Thus, the assignment of the 303-nm band
x
as a function of 1/A, Ka can then be deduced by the ratio for
the intercept versus the square of the slope.
For the case of DPP, however, significant overlaps between
monomeric and dimeric species were observed throughout the
concentration-dependent UV-vis absorption measurement. As

1462 J. Phys. Chem. A, Vol. 107, No. 10, 2003
Cheng et al.
a result, upon considering the absorbance attributed to both
monomer and dimer at any selected wavelength, modification
of eq b is necessary. A revised equation of b with an arbitrary
cell path length of λ has been deduced and expressed as
C0l
)
A - ꢀMC0l
l
1
1
2
‚
+
(c)
ꢀ - 2ꢀ
K
A - ꢀ C l ꢀD - 2ꢀM
x
x x
D
M
a
M 0
No assumption is made in eq c except that the value of the
molar extinction coefficient of the monomer ꢀM,at the analyzed
wavelength needs to be obtained prior to the deduction of Ka
value. Multiplying ꢀM on both sides, eq c can be further
simplified to
Figure 3. The absorption spectra of DPP (8.0 × 10-6 M in
cyclohexane) as a function of free acetic acid concentration Cg of (a)
0, (b) 8.3 × 10-6, (c) 1.2 × 10-5, (d) 1.5 × 10-5, (e) 2.0 × 10-5, (f)
2.5 × 10-5 M. Insert: The plot of A0/(A - A0) at 310 nm as a function
of 1/Cg in curves b-f and a best least-squares fitting curve using eq e.
2
A0
lꢀM
2ꢀM
1
1
)
‚
+
(d)
acid absorption titration spectra reveal significant overlap
between uncomplexed DPP monomer and 1:1 DPP/acetic acid
hydrogen-bonded complex. In consideration of self-dimerization
for acetic acid the competitive equilibrium can be expressed as
A - A0
ꢀ - 2ꢀ
K
A - A
ꢀD - 2ꢀM
x
x x
D
M
a
0
where A0 in eq d simply denotes the absorbance of the monomer
at the selected wavelength, assuming that no dimer is formed
at the prepared concentration. A0 can be obtained by performing
the concentration dependent study at a sufficiently low con-
centration so that monomer exists predominantly (e.g., 9.2 ×
10-6 M in the case of DPP). By knowing the proportionality of
the dilution, A0 values can thus be obtained in each prepared
concentration. In this study, the wavelength at 310 nm was
selected due to the relatively higher absorptivity upon dimer-
ization (vide infra). Under a sufficiently low concentration where
only monomer exists, ꢀ3M10 was measured to be ca. 2835
M-1cm-1. A plot of A0/(A - A0) at 310 nm as a function of
In the above expression, the possibility of a single hydrogen
bonding formation between DPP(N(CH3)2) site and acetic acid
is neglected due to its rather small association constant. Since
DPP is complexed by stoichiometrically equivalent guest
molecules (e.g., acetic acid), while acetic acid is in excess, the
relationship between the measured absorbance as a function of
the free acetic acid concentration Cg can be expressed by
1/(A - A ) shown in insert of Figure 2 reveals sufficiently
x
0
linear behavior, supporting the assumption of dimeric formation
in concentrated DPP. Accordingly, by fixing the ꢀM and l()
1.0 cm) values, the best linear least-squares fit using eq d to
the insert of Figure 2 gives ꢀ3D10 and Ka values to be (1.4 ( 0.2)
× 104 M-1cm-1 and (4.2 ( 0.4) × 103 M-1, respectively.
Knowing that one dimer actually consists of two DPP chro-
mophores, the extinction coefficient for each DPP in the dimeric
form is thus deduced to be ꢀ3D10/2 ∼ 7100 M-1 cm-1. In
comparison to that of the 7AI dimer (Ka ∼ 2.2 × 103 M-1 in
cyclohexane26d), the higher dimerization constant in DPP may
be rationalized by increasing the basicity of the pyridinic
nitrogen resonantly enhanced by the electron donating property
of the para-dimethylamino functional group (vide infra). This
viewpoint can be further supported by adding acetic acid as
the guest molecule, forming a DPP/acetic acid dual hydrogen-
bonded complex.
A0
ꢀM
1
)
+ 1
(e)
(
)
A - A0 ꢀM - ꢀC KaCg
where ꢀM and ꢀC in eq e denote molar extinction coefficients of
DPP monomer and the hydrogen-bonded complex, respectively,
at a specific wavelength. On the basis of the negligible
consumption of acetic acid upon forming the DPP/acetic acid
complex, Cg can be deduced independently by the self-
association equation of the acetic acid; namely,
Figure 3 shows the absorption spectra of DPP as a function
of the acetic acid concentration in cyclohexane, in which the
initial concentration of DPP C0 was prepared to be as low as
1.2 × 10-5 M to avoid self-dimerization. The formation of DPP/
acetic acid hydrogen-bonded complexes is clearly shown by the
growth of an absorption band at ∼310 accompanied by the
appearance of an isosbestic point at ca. 295 nm throughout the
titration, indicating the existence of equilibrium with a common
intermediate. In comparison, although 1MDPP provides two
hydrogen-bonding sites, i.e., the pyridinic and dimethylamino
nitrogen, negligible spectral changes were observed upon adding
acetic acid up to 10-3 M. The result unambiguously supports
the formation of a 1:1 DPP/acetic acid complex incorporating
cyclic dual hydrogen bonds at N(1)H and N(7) positions. Similar
to that observed in the concentration-dependent study the acetic
(4K′Cg0 + 1) - 8K′C 0 + 1
x
a
a g
0
Cg ) Cg
-
[
]
4K′
a
0
where Cg is the initially prepared acetic acid concentration.
The self-association constant K′ of acetic acid has been
a
reported to be 3.7 × 104 M-1 44 in n-heptane, which was used
in the case of cyclohexane due to their similar solvent polarity.
A straight-line plot of A0/(A - A0) as a function of 1/Cg at a
selected wavelength of 310 nm (see insert of Figure 3) supports
the validity of the assumption of a 1:1 DPP/acetic acid complex
formation. Consequently, a best linear least-squares fit using
eq e deduces Ka to be (5.2 ( 0.5) × 104 M-1 which is
approximately twice as large as that of the 7AI/acetic acid

Excitation Behavior of DPPs
J. Phys. Chem. A, Vol. 107, No. 10, 2003 1463
Figure 4. A. The fluorescence spectra of DPP in cyclohexane where
DPP was prepared at (a) 4.2 × 10-5 and (b) 6.5 × 10-5 M. Spectra
was normalized at 330 nm. The excitation spectra (6.5 × 10-5 M)
monitored at (o o) 340 and (‚ ‚) 500 nm. (- -) Absorption and emission
spectra of 7MDPP in cyclohexane. B. The fluorescence spectra of DPP
(8.0 × 10-6 M in cyclohexane) as a function of the acetic acid
concentration of (a) 2.0 × 10-6, (b) 8.30 × 10-6, (c) 1.20 × 10-5, (d)
1.33 × 10-5, (e) 1.65 × 10-5 M. The excitation spectra of c monitored
at (o o) 340 and (‚ ‚) 500 nm.
Figure 5. The fluorescence spectra of DPP in various solvents: (a)
ethyl ether (34.5), (b) ethyl acetate (38.1), (c) dichloromethane (40.7),
(d) acetonitrile (45.6), (e) ethanol (51.9). ET(30) value in kcal/mol for
each solvent is specified in parentheses. The excitation wavelength is
290 nm.
complex (∼2.2 × 104 M-1),26d further supporting the proposed
stronger proton-accepting property on the pyridinic nitrogen.
The fluorescence spectra of DPP in cyclohexane reveal strong
concentration dependence (see Figure 4A). At sufficiently low
concentrations where the monomer mainly exists in the equi-
librium, a very weak, normal Stokes shifted emission maximum
at ∼327 nm was observed (the FN band, Φf ∼ 9.24 × 10-4).
Upon increasing the concentration, a large Stokes-shifted
emission maximum at 440 nm (the FT band) gradually appears.
Different excitation spectral features were obtained upon
monitoring at FN and FT bands where the excitation spectrum
of the FT band is red shifted by ∼10 nm with respect to that of
the FN band. The rise time of both emission bands cannot be
resolved by our current photon counting system with a response
limit of ∼15 ps. The results in combination with resolvable,
different relaxation dynamics for FN (τobs ∼ 70 ps) and FT (τobs
∼ 2.8 ns) bands unambiguously conclude that they originate
from different ground-state species and one cannot be the
precursor of the other. In comparison, 1MDPP, which is
considered a nonproton-transfer model compound, exhibits a
concentration independent, normal Stokes-shifted emission
maximum at 328 nm (τf ∼ 85 ps; Φf ∼ 1.36 × 10-3). On the
other hand, 7MDPP, the analogue of proton-transfer tautomer
for DPP, reveals a single fluorescence band maximum at 470
nm in cyclohexane (τf ∼ 4.1 ns; Φf ∼ 9.8 × 10-2). It is thus
conceivable to conclude that the FN band is ascribed exclusively
to the monomer emission, while the FT band with a Stokes shift
(peak-to-peak) of ∼7850 cm-1 can be unambiguously assigned
to the tautomer emission resulting from ESDPT of the DPP
dimer.
Figure 6. The excitation spectra of DPP (1.5 × 10-5 M) in ethyl acetate
monitored at (a) (- -) 500 nm, (b) (- ‚) 340 nm, and (c) (s) the
absorption spectrum.
noncatalytic (dimer) types26c of ultrafast ESDPT take place in
DPP dual hydrogen-bonded complexes.
3.2. In Polar, Aprotic Solvents. In contrast to the large self-
association for DPP in cyclohexane, both absorption and
emission spectra reveal concentration independence within the
studied concentrations of 10-6-10-3 M in all polar, aprotic
solvents. The results can be rationalized by the large (polar)
solvent stabilization energy for the solvated DPP monomer,
preventing self-association or even higher-order aggregation.
However, unlike the single, solvent-polarity-dependent Stokes-
shifted emission observed in 7AI,45 remarkable dual emission
was observed for DPP in polar, aprotic solvents (Figure 5).
These two emission bands, specified as FN and FC (the long-
wavelength band) originating from a common ground-state
species, are ascertained by the same fluorescence excitation
spectra of DPP in, e.g., ethyl acetate (see Figure 6). The
excitation spectra, within experimental error, are also identical
with the absorption spectrum, indicating that both emissions
result from a common Franck-Condon excited state. In contrast
to the nearly solvent-independent absorption maximum, peak
frequencies for both FN and FC bands reveal solvent-polarity
dependence, decreasing as the empirical parameter of solvent-
polarity ET(30) increases (see Figure 5 and Table 1). For
example, the peak frequency of the FN band is red shifted by ∼
Upon exciting the DPP/acetic acid complex, a similar ESDPT
reaction was observed, resulting in a proton-transfer tautomer
emission (λmax ∼ 440 nm) of which the excitation spectrum is
red shifted by ∼10 nm with respect to the uncomplexed
monomer (see Figure 4B). The emission monitored at > 430
nm exhibits a single-exponential decay rate (τobs ∼ 5.2 ns)
accompanied by system response limited rise dynamics (i.e.,
kpt > 6.7 × 1010 s-1). The results unambiguously demonstrate
that in nonpolar solvents both catalytic (acetic acid) and

1464 J. Phys. Chem. A, Vol. 107, No. 10, 2003
Cheng et al.
TABLE 1: Physical Parameters of Solvents as Well as Photophysical Properties of DPP, 1MDPP, and 7MDPP in Various
Solvents at Room Temperature
(× 104 cm-1)(× 10-3
)
(× 10-3
)
(× 10-2
)
ET(30)
kcal/mol
τF
τF
τF
N
C
T
solventa
∆f
v˜abs
v˜F
v˜F
v˜F
ΦF
ΦF
ΦF
(ns)
(ns)
(ns)
N
C
T
N
C
T
DPP
2.272
CHE
ether
EA
DCM
ACN
EtOH
30.9
34.5
38.1
40.7
45.6
51.9
0
3.300
3.286
3.278
3.250
3.248
3.234
3.054
3.042
3.037
2.998
2.985
2.990
0.92
2.19
1.31
2.68
2.37
2.08
10.31
0.070
0.102
0.080
0.120
0.110
0.050
0.45
0.31
0.35
0.51
2.30
2.80
0.1684
0.1997
0.2184
0.3062
0.2890
2.500
2.457
2.392
2.325
1.10
2.30
6.70
20.8
2.183
2.175
6.67
2.74
0.22b
3.60
0.17b
3.17
MeOH
55.4
0.3095
3.242
2.982
1.01
0.042
1MDPP
CHE
ether
EA
DCM
ACN
EtOH
MeOH
30.9
34.5
38.1
40.7
45.6
51.9
55.4
0
3.229
3.223
3.219
3.190
3.200
3.179
3.183
3.043
3.038
3.001
2.984
2.980
2.981
2.972
NA
1.36
1.81
2.60
3.11
3.07
2.50
1.70
0.085
0.095
0.128
0.131
0.162
0.055
0.048
0.1684
0.1997
0.2184
0.3062
0.2890
0.3095
2.514
2.471
2.405
2.331
2.262
2.212
0.70
1.00
1.50
6.50
6.80
4.90
0.15
0.18
0.22
1.18
1.33
1.05
7MDPP
CHE
EA
DCM
ACN
EtOH
30.9
38.1
40.7
45.6
51.9
0
2.759
2.813
2.829
2.837
2.845
2.119
2.146
2.148
2.150
2.174
98.0
97.0
137
108
87.0
4.065
4.258
4.618
4.121
3.405
0.1997
0.2184
0.3062
0.2890
a CHE: cyclohexane. EA: ethyl acetate. DCM: dichloromethane. ACN: acetonitrile. b The rise time.
550 cm-1 from ethyl ether to acetonitrile, while it is as large as
1745 cm-1 for the FC band. In a comparative study the proton-
transfer tautomer analogue 7MDPP exhibits a normal fluores-
cence band, of which the peak frequency reveals slight solvent-
polarity dependence, being 21,187 cm-1 (470 nm) in cyclohexane
and 21 500 cm-1 (465 nm) in acetonitrile (see Table 1). The
drastic difference in spectral properties between 7MDPP emis-
sion and the FC band of DPP in combination with the lack of
solvent proton donating site to assist the proton-transfer reaction
lead us to exclude the assignment of the FC band in polar, aprotic
solvents to a proton-transfer tautomer emission.
Perhaps the strongest support for this viewpoint is given by
1MDPP; a nonproton- transfer model for DPP. 1MDPP revealed
similar dual emission and solvent-polarity-dependent spectral-
shifted properties (see Figure 7 and Table 1) although the
relative intensity between FN and FC bands is different from
that observed in DPP (see Figures 5 and 7 for comparison). In
consideration of the low ionization energy for the dimethylamino
substituents, the results lead us to propose that the FC band
originates from an excited-state intramolecular charge transfer
(ESICT) reaction incorporating dimethylamine (electron donor)
and pyridine ring (electron acceptor) in DPP. Further verification
of the charge transfer emission can also be given by the synthesis
of APP (see Figure 1) in which only the FN band was observed
in various polar, aprotic solvents studied. For instance, APP
exhibited single, normal Stokes shifted emission band maxima
at ∼335 and 343 nm in CH2Cl2 and CH3CN, respectively (not
shown here). Theoretically, removing an additional methyl group
leads to an increase of ionization energy of ∼0.4 eV for simple
aliphatic amines.46 Assuming a similar trend holds in the
aromatic system, APP is expected to possess higher ionization
energy of ∼0.8 eV than DPP, rationalizing its lack of charge-
transfer property in the excited state.
Figure 7. The fluorescence spectra of 1MDPP in various solvents:
(a) ethyl ether, (b) ethyl acetate, (c) dichloromethane, (d) acetonitrile,
and (e) ethanol. The excitation wavelength is 290 nm. * denotes the
Rayleigh scattering peak.
on the difference in permanent dipole moments between ground
and excited states. The magnitude of the excited-state dipole
moments can thus be estimated by a method incorporating the
fluorescence solvatochromic shift.47-51 If the dipole moments
of the solute are approximated by a point dipole in the center
of a spherical cavity with a radius a0, on the basis of nearly
solvent independent absorption properties and negligence of the
solute polarizability one obtains
2(bµe - bµg)2
vac
The solvent-polarity dependent emission property can be
specified more quantitatively according to the theory derived
from dielectric polarization, specifying that the spectral shift
of the fluorescence upon increasing the solvent polarity depends
ν˜f ) ν˜f
-
∆f
(f)
3
hca0
vac
where v˜f and v˜f in eq f are the spectral position (in terms of

Excitation Behavior of DPPs
J. Phys. Chem. A, Vol. 107, No. 10, 2003 1465
the results are summarized in Table 1. Both FN and FC bands
show single-exponential decay dynamics with system response
limited rise time (i.e. < 15 ps). The results lead us to conclude
that the FN band cannot be a precursor for the FC band. This in
combination with the same excitation spectra for the dual
emission indicates that both FN and FC bands originate from a
common Franck-Condon excited state followed by two inde-
pendent relaxation pathways. More details regarding the quan-
titative analysis of the excited-state dipole moment as well as
relaxation dynamics are elaborated in the discussion section.
3.3. In Protic Solvents (Alcohols). Intriguing excited-state
properties were observed in alcohols. In the steady-state
measurement, for instance, DPP exhibited dual emission maxima
at 335 nm (FN) and 460 nm in ethanol (see Figure 5). While
ν˜max of the FN band lies on the linear plot, the correlation for
ν˜max of the long-wavelength band versus ∆f deviates signifi-
cantly from the linear behavior of the FC band in aprotic solvents
(see Figure 8A). The result on one hand may indicate ESICT
is operative, while the charge transfer species is subject to the
specific hydrogen-bonding interactions with protic solvents. On
the other hand, it could reflect different photophysical phenom-
ena other than the charge-transfer event that occurred in the
polar, aprotic solvents. Although the former proposed mecha-
nism may be partially supported by a similar dual emission for
1MDPP, where the deviation of the Lippert’s plot was also
observed for the long wavelength band in ethanol (or methanol,
see Figure 8B), some supplementary evidence leads us to
discount this proposed mechanism. First, the apparent quantum
yield of the long wavelength band (Φf ∼ 6.67 × 10-2) for DPP
in ethanol is higher than that of the FC band in the polar, aprotic
solvents by ∼ 1 order of magnitude. In comparison, for 1MDPP,
the yield in ethanol is about the same as those of the FC band
in aprotic solvents (see Table 1). Second, the peak maximum
of the long-wavelength emission band for DPP (460 nm) is red
shifted by ∼ 15 nm from that for 1MDPP in ethanol (445 nm).
In comparison, the difference in ν˜max for the FC band is
negligible between DPP and 1MDPP in polar, aprotic solvents
(see Table 1). It thus seems likely that the large Stokes shifted
emission band observed in ethanol for DPP and 1MDPP may
exhibit intrinsically different characteristics.
Figure 8. The fluorescence maxima of (A) DPP, (B) 1MDPP (in terms
of wavenumber) as a function of ∆f in various solvents of (1)
cyclohexane, (2) ethyl ether, (3) ethyl acetate, (4) dichloromethane,
(5) acetonitrile, (6) ethanol, and (7) methanol. (O) The high-frequency
band. (b) The low-frequency band.
wavenumber) of the solvation equilibrated fluorescence maxima
and the value extrapolated to the diluted gas-phase, respectively,
bµg and bµe are the dipole moment vectors of the ground and
excited states, and ∆f is the Lippert solvent polarity parameter47
which is generally expressed as
Perhaps the strongest support for the above viewpoint was
given by the time-resolved measurement. Drastically different
excited-state dynamics were observed between DPP and 1MDPP
in ethanol. By monitoring at > 410 nm, a single-exponential
decay component with a lifetime of 1.3 ns was resolved for
1MDPP in ethanol, while the rise time is beyond the system
response of 15 ps. In contrast, for DPP the time-dependent
fluorescence can be well fitted by two components expressed
as
ꢀ - 1
2ꢀ + 1
1 n2 - 1
∆f )
-
2
2
2n + 1
where ꢀ and n denote the static dielectric constant and the
refractive index of the solvent, respectively. The plot of spectral
maxima for both FN and FC bands as a function of ∆f is shown
in Figure 8A and B for DPP and 1MDPP, respectively. As
predicted by eq f, a linear relationship is found for both FN and
FC bands from ethyl ether to acetonitrile. For DPP the calculated
slope of -1.24 × 104 cm-1 for the FC band is much steeper
than that of -2.02 × 103 cm-1 for the FN band. Similar slopes
of -2.60 × 103 and -1.3 × 104 cm-1 were obtained for FN
and FC bands, respectively, in 1MDPP. Apparently, for both
DPP and 1MDPP the change of excited-state dipole moment
with respect to the ground state is expected to be larger in the
FC band, consistent with its assignment of the charge-transfer
emission. Furthermore, the proton-transfer tautomer emission,
i.e., the FT band, in cyclohexane can be clearly distinguished
from the FC band by its far deviation from the Lippert’s plot in
the aprotic, polar solvents (see Figure 8A), further supporting
the different spectral properties between proton-transfer (FT) and
charge transfer (FC) emission.
F(t) ) a1e-k t + a2e-k t
(g)
1
2
Independent of the monitored wavelengths at >410 nm, k1
and k2 were deduced to be 220 ( 20 ps-1 and 3.6 ( 0.2 ns-1
,
respectively. However, instead of the positive values for the
fitted a2throughout the monitored wavelengths the sign of a1 is
wavelength dependent. When monitoring at >450 nm a negative
a1 was obtained as indicated by a rise component of 224 ( 30
ps (see Figure 9a), which gradually shifts to a positive value,
i.e., the appearance of a faster decay component, upon monitor-
ing at shorter emission wavelength (e.g., 410 nm, see Figure
9b). Further analyses revealed that |a1/a2| was not equal to 1.0
and was found to be wavelength dependent. When the time-
dependent fluorescence was monitored at the steady-state
emission maximum of ∼460 nm this value is deduced to be
Time-resolved measurements have also been performed for
both DPP and 1MDPP in various polar, aprotic solvents, and

1466 J. Phys. Chem. A, Vol. 107, No. 10, 2003
Cheng et al.
interesting comparison can be made by plotting ν˜max versus ∆f
for the FT band of DPP in cyclohexane (i.e. the dimeric form),
ethanol and methanol, giving rise to a sufficiently linear
correlation (see Figure 8A, dashed line), supporting their similar
spectral properties.
As shown in Table 1, the lifetime of normal emission (i.e.
the FN band) was ∼50 ps in ethanol, while the rise time of the
proton-transfer tautomer emission was well resolved to be 220
ps. It is thus very unlikely that the precursor of ESDPT
originates from the normal form of DPP (i.e. the species
attributed to the FN emission), but more plausibly through its
charge transfer species. Consequently, a mechanism incorporat-
ing charge/proton-transfer coupled reaction was tentatively
proposed in Scheme 1c. Upon Franck Condon excitation, the
charge-transfer reaction takes place, accompanied by fast solvent
relaxation dynamics, to reach a stabilized charge-transfer state.
The equilibrated charge-transfer state further undergoes solvent
(ethanol) assisted, irreversible ESDPT, resulting in a proton-
transfer tautomer emission. The rise kinetics of the tautomer
emission depend on the types of short-carbon chain monoal-
cohols used, being 170, 220, and 250 ps-1 in methanol, ethanol,
and propanol, respectively. In addition, a deuterium isotope
effect was observed in the rise dynamics. For example, kptD was
Figure 9. The logarithm plot of the time-dependent fluorescence of
DPP in ethanol and its corresponding best-fitted curve. The emission
wavelength was monitored at (a) 465 nm and (b) 410 nm. λex ) 266
nm.
∼0.5, indicating that ∼50% of the fluorescence promptly exists,
while the remaining 50% has not yet been populated. The results
confirm the existence of two species at >410 nm. Due to its
negligible emission intensity at >410 nm, the possibility of the
observed decay dynamics originating, in part, from the residue
of the normal emission (i.e., the FN band, λmax ∼ 335 nm) can
be excluded. DPP possesses a similar parent structure to 7AI
where the pyridinic nitrogen and pyrrolic hydrogen provide
proton donating and accepting sites, respectively. In the previous
section (3.1), DPP has been shown to undergo ESDPT in its
dimer and complexed form with acetic acid in cyclohexane.
Knowing that 7AI also undergoes alcohol catalyzed proton
transfer in the excited state,21-25,26a it is thus reasonable to
propose the occurrence of ESDPT for DPP in ethanol, resulting
in a proton-transfer tautomer emission (denoted by the FT band).
Evidence to support this viewpoint is given by 7MDPP, a
proton-transfer analogue of DPP, which exhibits a normal Stokes
shifted fluorescence with spectral and dynamic features (λmax
∼ 460 nm, τf ∼ 3.4 ns, and Φf ∼ 8.7 × 10-2) similar to that of
the long-wavelength band of DPP in ethanol. Furthermore, an
measured to be ∼648 ps-1 in ethanol-d, giving kptH/kpt to be
D
2.9. It is thus believed that the mechanism of alcohol catalyzed
ESDPT in DPP should be similar to a two-step mechanism
proposed for 7AI and its corresponding analogues,29,34 incor-
porating equilibrium solvation followed by a rapid proton
transfer, which is possibly governed by a tunneling mechanism
shown in Scheme 1c. For this case kpt can be further expressed
by
k1
0
kpt )
kpt0 ) kpt e-∆G/RT
k-1
0
where kpt and ∆G, respectively, denote the proton tunneling
rate and difference in free energy between cyclic and neighbor-
SCHEME 1: The Proposed Photophysics of DPP in (a) Nonpolar Solvents (e.g., Cyclohexane); (b) Polar, Aprotic
Solvents (e.g., Acetonitrile); and (c) Protic Solvents (e.g., Ethanol).

Excitation Behavior of DPPs
J. Phys. Chem. A, Vol. 107, No. 10, 2003 1467
TABLE 2: The Magnitude (Unit: Debye) and Angle (Unit:
Degree) of Dipole Moment for DPP Calculated by Various
Theoretical Approaches
methoda
magnitude
angle
Ground State
3.0147
4.0238
2.727
MP2/6-31G(d, p)
HF/6-31G(d, p)
AM1
164.65b
164.16
159.80
157.76
143.13
PM3
2.595
INDO/S-CI
1.688
Excited State (Φ ) 0°)
6.1d
28.0c
36.7c
Excited State (Φ ) 90°)
13.2d
Figure 10. The time-dependent spectral evolution of DPP in ethanol
acquired at (a) (O) 0-150 ps, (b) (b) > 1.0 ns. λex ) 266 nm.
a The angle is specified as the orientation of dipole moment vector
relative to the C8-C9 bond (see Figure 1) counted positive clockwise.
b All semiempirical calculations were performed by HyperChem 6.03
on Win2000 professional at Intel PIII 733 computer. c The angle is
specified as the orientation of dipole moment vector relative to the
ground state (i.e., structure N, see Figure 1) using INDO/S-CI method.
d Values were obtained by the best fit of Figure 8A in polar, aprotic
solvents using eq f where magnitude of dipole moment in the ground
state and angles between ground and excited states were calculated by
MP2/6-31(d, p) and INDO/S-CI methods, respectively (see text).
bonded structures.29 A distinct difference in mechanism between
DPP and 7AI is that the precursor of the ESDPT in DPP is a
charge-transfer species, while the normal (neutral) species is
operative in the case of 7AI.
On the basis of the prompt population (<15 ps) of the charge-
transfer species followed by ESDPT, the time-dependent signal
for FC(t)(charge-transfer emission) and FT(t) (proton-transfer
emission) in ethanol can be expressed as
band, ascribed to the charge-transfer precursor and its subsequent
proton-transfer tautomer emission.
FC(t) ) RkrC(ν˜)[FC]0e-(k +k )t
RkrT(ν˜)kpt[FC]0
C
pt
4. Discussion
4.1. Excited-State Charge-Transfer Properties. Much of
the evidence presented in previous sections has ambiguously
drawn the conclusion that the excited-state charge transfer
reaction is operative for DPP in polar solvents. In this section,
we have made an attempt to further explore the excited-state
charge transfer properties in DPP. One key issue is to see if the
ESICT mechanism incorporates the lock of a charge separation
through the rotation of the dimethyl amino group, namely the
twist intramolecular charge transfer (TICT).52-55 However,
excited-state proton transfer is prohibited in N,6N6-dimethyl-
adenosine since the pyrrolic nitrogen has been attached to a
sugar moiety. Experimentally, a direct verification of such a
proposed mechanism may require the design of a rigid alkyl
amino substituent so that the rotation is sterically hindered with
respect to the parent 7AI, prohibiting the TICT dynamics.
Unfortunately such a synthetic goal has not yet been achieved
in our laboratory at this stage. Alternatively, the possibility of
a TICT mechanism operated in DPP was herein examined by
theoretical approaches.
C
pt
T
FT(t) )
[e-(k +k )t - e-k t
]
kT - kC - kpt
where R denotes the instrument factor, including sensitivity,
alignment, etc., of the detecting system, [FC]0 is the initial
population of the charge-transfer species in the excited state,
and krC(ν˜) and krT(ν˜) are radiative decay rates of charge-transfer
and proton-transfer tautomer emission, respectively. kC is the
decay rate of the FC band excluding the rate of proton-transfer
reaction, and kT denotes the observed decay rate of the tautomer
emission. It is reasonable to assume kpt to be .kC + kT in protic
solvents. As a result, the sum of FC(t) and FT(t) at a monitored
specific wavelength (e.g. > 400 nm) can be expressed as
FC(t) + FT(t) )
R[FC]0[(krC(ν˜) - krT(ν˜))e-k t + krT(ν˜)e-k t] )
pt
T
a1e-k t + a2e-k t
pt
T
According to the ab initio approach (MP2/6-31G(d, p)) the
geometry optimized structure of DPP in the ground state
revealed two carbon atoms in the dimethylamino group lying
on the same symmetry plane with respect to the parent 7AI
moiety (Φ ∼ 0°, structure N in Figure 1). Conversely, a DPP
structure with the dimethyl group being perpendicular to the
plane of 7AI moiety (Φ ∼ 90°, structure C) is calculated to be
higher in energy than that of the in-plane one by 4.5 kcal/mol.
The difference is more or less the same upon incorporating
semiempirical PM3-SMx solvation models57 to obtain the
solvation free energy in various aprotic solvents, which is then
added to the calculated “gas phase” energies. Thus, structure N
was concluded to be the most stable conformer in the ground
state, of which the magnitude of dipole moment is deduced to
be 3.01 D with its orientation 165° in plane relative to the C8-
C9 bond counted positive counterclockwise. The vector of dipole
moment was also calculated by various computation methods
including AM1, PM3, and Hartree-Fock methods incorporating
different basis sets. The results listed in Table 2 indicate that
Since krC(ν˜) and krT(ν˜) incorporate the Franck-Condon factor,
krC(ν˜) - krT(ν˜) is expected to be wavelength dependent,
rationalizing the variation of preexponential factor a1 in both
value and sign.
As predicted from the FC band of 1MDPP (see Figure 7),
the charge-transfer emission is expected to exhibit a peak
maximum at ∼445 nm in ethanol. In the steady-state measure-
ment, although the fluorescence yield of the charge-transfer
emission is obscured in ethanol due to the dominant rate of
ESDPT in DPP, an attempt has been made to resolve the
individual FC and FT bands by the time-dependent spectral-
evolution technique. Figure 10a shows the emission spectrum
obtained by integrating the photon-counting signal in a time
delay interval of 0-150 ps. It is apparent that the resulting
spectrum which has a maximum at ∼440 nm is blue shifted
versus that of ∼460 nm obtained at a time delay of >1.0 ns
(see Figure 10b). The time-resolved spectral evolution substanti-
ates our proposal of two emitting species in the long wavelength

1468 J. Phys. Chem. A, Vol. 107, No. 10, 2003
Cheng et al.
Figure 11. The electron-density distribution of the two lowest unoccupied and three highest occupied frontier molecular orbitals calculated (INDO/
S-CI) for DPP in the planar (Φ ) 0°) and perpendicular (Φ ) 90°) conformations.
TABLE 3: The Lowest Singlet Electronic Transitions
Calculated for DPP in the Planar (Φ ) 0°) and Twisted (Φ
) 90°) Conformations Using the INDO/S-CI Method (See
Experimental Section for the Detailed Description)
depending on the method applied, the magnitudes of the
calculated dipole moment diversify, whereas the orientation of
dipole moment is less affected.
Based on the results of INDO/S-CI calculation (see Experi-
mental Section), Figure 11 depicts the two lowest unoccupied
and three highest occupied frontier molecular orbitals calculated
for DPP in the planar (Φ ) 0°) and twisted (Φ ) 90°)
conformations, where Φ specifies the rotational angle of
N(CH3)2 relative to the parent 7AI plane. Except for the second
highest occupied orbital (HOMO-1) all rest frontier molecular
orbitals possess the same configuration between N and C
conformers. The π-symmetry configuration was found for
LUMO+1, LUMO and HOMO orbitals, while HOMO-2 is
mainly attributed to the lone pair orbital of the pyridinic
nitrogen. Instead of the π-symmetry for the HOMO-1 orbital
in N conformer, the HOMO-1 orbital in structure C possesses
a σ-symmetry where the charge density is largely localized on
the dimethylamino nitrogen. The lowest Franck-Condon singlet
excitation in the planar form was dominated by HOMO f
LUMO+1 and HOMO-1 f LUMO (32 206 cm-1) transition
ν˜
transition
(cm-1
)
f a
dominant configurationsb
Φ ) 0°
0.038
0
S0 f S1 (ππ*)
S0 f S2 (nπ*)
S0 f S3 (ππ*)
32206
33772
34916
H f L+1, H-1 f L
H-2 f L
H f L
0.3177
Φ ) 90°
0.001
0.086
0
S0 f S1 (σπ*)
S0 f S2 (ππ*)
S0 f S3 (nπ*)
27030
32258
32680
H-1 f L
H f L, H f L+1
H-2 f L
a Oscillator strength. b H: HOMO. L: LUMO.
1
possessing a ππ* configuration (see Table 3). In contrast, it
was altered to HOMO-1 (σ-symmetry) f LUMO (π-symmetry)
in the case of the C conformer (∼27 030 cm-1), indicating that
the lowest excited singlet state for structure C, to a certain
extent, possesses a charge transfer character where the electron

Excitation Behavior of DPPs
J. Phys. Chem. A, Vol. 107, No. 10, 2003 1469
density flows from dimethylamino substituent to the ring,
particularly the pyridinic nitrogen. In summary, theoretical
approaches for both ground and lowest-lying excited states of
DPP are qualitatively in accordance with a TICT model
originally proposed by Grabowski and co-workers.52 The
calculated charge-transfer emission of 27 030 cm-1 in a vacuum
is consistent with the extrapolation of ν˜max for the FC band to
∆f ) 0 (Figure 8A), which gives rise to an intercept of 26 950
cm-1 for DPP in the gas phase. Further solvent relaxation must
take place to stabilize the point dipole of the charge-transfer
species, resulting in a larger Stokes shifted emission upon
increasing the solvent polarity. It should also be noted that
analyses have not been performed in consideration of various
twisting angles. Thus, the possibility of TICT taking place at a
certain small but critical angle rather than the Φ ) 90°
conformer cannot be ruled out at this stage. Further studies on
the molecular design of an intrinsically twisted DPP analogue
are crucial to resolve this issue.
Figure 12. The plot of apparent quantum yield (Φapp) for the FN band
of DPP as a function of the temperature (T). Insert: The plot ln{(1/
Φapp - 1 - knr/kr)kr} versus 1/T from 298 to 80 K and a best linear
least-squares fitting curve using eq i.
version. Consequently, these two states decay independently,
and the FN state cannot be the precursor of the FC state.
A strong support for this model was given by the temperature-
dependent fluorescence spectra of DPP in 2MTHF shown in
Figure 12. The FN emission revealed strong temperature
dependence. Its apparent quantum yield Φapp increased from
1.8 × 10-3 at 298 K to a plateau (Φapp ∼ 0.47) at ∼90 K and
remained constant at lower temperatures. There is a sharp
increase of the FN intensity in the range between 150 and 100
K where a turning point at ∼115 K was observed. In contrast,
the intensity of the FC band was nearly temperature independent,
and only increased by ∼ 1.5-fold from 298 down to 80 K (not
shown here). The steady-state results were further compared
with the temperature-dependent relaxation dynamics. The
lifetime monitored at the FN band (e.g., 340 nm) follows the
steady-state pattern where it increased significantly from 70 ps
at 298 K to 3.2 ns at 80 K. Conversely, the FC band increased
only slightly from 1.5 to 2.1 ns. Under a system response limit
of 15 ps, rise dynamics cannot be extracted for either the FN or
FC band throughout the temperature-dependent study, further
indicating that at the studied temperatures, the FN species cannot
be the precursor of the FC band. Assuming a temperature-
independent radiative decay rate kr for the FN band, the observed
temperature-dependent decay rate kobs can be expressed as kobs
) kr + knr + knr(T). Accordingly, the apparent quantum yield
is deduced to
The excited-state dipole moments for the solvated DPP
structures attributed to either FN or FC bands could be estimated
from the slopes depicted in Figure 8. Because the ground state
dipole moment for DPP has not yet been determined experi-
mentally, we adopted the value of 3.01 D deduced from the
MP2/6-31G(d, p) method incorporating the electron correlation.
As mentioned in the previous section, the orientation of the
dipole moment is much more reliable than its magnitude,
especially in the excited state, among various computational
methods. On the basis of the INDO/S-CI method, the angles of
dipole moment between ground (structure N) and FN (the first
1ππ* state of structure N) or FC (the first 1σπ* state of structure
C) state were estimated to be 28.0° and 36.7°, respectively (see
Table 2). If charge transfer from the dimethylamino group to
the pyridinic nitrogen makes a key contribution, we would
expect a short-axis (C8-C9)-oriented excited-state dipole mo-
ment of the C state. This viewpoint is supported by the nearly
parallel orientation (∼ 180°) of bµC* relative to the C8-C9 bond
(see INDO/S-CI method in Table 2). Employing these calculated
angles as fixed parameters, magnitudes of excited-state dipole
moment for both FN and FC states can thus be estimated. In eq
f, the radius of spherical cavity ao was calculated to be 4.57 Å
based on a geometry optimized structure of DPP (HF/6-31G(d,
p)). Accordingly, a best fit of Figure 8 using eq f gives the
magnitude of dipole moment to be 6.1 and 13.2 D for N* and
C*, respectively (see Table 2). The combination of experimental
and theoretical approaches predicts a relative trend of |µC*| >
|µN*| > |µg| for DPP. A larger solvatochromic shift is then
expected for the charge-transfer state (i.e., the C* state) in
comparison to the normal state (N*), rationalizing the difference
in solvent-polarity dependence of FN and FC emissions (i.e.,
the ν˜max value in Figure 8).
4.2. Relaxation Dynamics in Aprotic Solvents. The time-
resolved fluorescence studies reveal that the rise time of the FC
emission band is irresolvable (i.e. < 15 ps), while a finite
lifetime of 30-160 ps was resolved for the FN band in various
solvents studied. Careful analyses also conclude that preexpo-
nential factors of the fitted decay dynamics for both FN and FC
bands are positive throughout the spectral region of 300-600
nm. No negative preexponential values can be resolved in our
current time correlated photon-counting measurement. This in
combination with identical excitation spectra suggests that FN
and FC states share a common Franck-Condon state. Upon
excitation, this initially prepared electronic state rapidly relaxes
to FN and FC states which become decoupled without intercon-
kr
Φapp
)
(h)
kr + knr + knr(T)
where knr denotes the temperature independent radiationless
decay rate constant, possibly involving internal conversion,
intersystem crossing, etc. The temperature-dependent radiation-
less decay rate constant knr(T) can be further expressed as an
Arrhenius type of thermally deactivated pathway of knr(T) )
Ae-E /RT, which is then plugged into eq h to obtain
a
knr
kr
Ea
1
Φapp
ln
- 1 -
kr ) ln A -
(i)
{(
) }
RT
As indicated by steady-state and time-resolved measurements,
the lifetime and intensity of the FN band at <90 K are nearly
temperature independent. Thus, knr(T) is assumed to be negli-
gible, and Φapp expressed in eq h can be simplified to kr/(knr
+
kr), which was measured to be ∼0.47 at 80 K. On the other
hand, kobs was resolved to be 3.1 × 108 s-1 at 80 K. Accordingly
kr and knr were deduced to 1.5 × 108 and 1.6 × 108 s-1
,

1470 J. Phys. Chem. A, Vol. 107, No. 10, 2003
Cheng et al.
respectively. With all parameters provided, the plot of ln{(1/
Φapp - 1 - knr/kr)kr} as a function of reciprocal of the
temperature reveals a straight line and Ea was deduced to be
2.1 kcal/mol with a frequency factor A of 3.6 × 1012 s-1 (see
insert of Figure 12).
methyladenosines, Albinsson and co-workers56 have proposed
a similar nonprecursor-sater type of TICT mechanism. In
contrast, Andreas et al.61 recently claimed a precursor-sater
relation for the dual emission of N,6N6-dimethyladenine deriva-
tives. For the case of DPP, the independent relaxation dynamics
between FN and FC bands leads us to tentatively adopt the
nonprecursor-sater TICT mechanism at this stage. Focus on
design and synthesis of DPP derivatives so that the electron-
donating site can be geometrically locked to test the proposed
TICT/ESDPT coupled mechanism is currently in progress. It is
believed that this approach coupled with further femtosecond
time-resolved measurements will gain more insight on this issue.
Finally, several derivatives of DPP and APP have been
proposed as potential antitumor agents.62,63 It is thus of
considerable interest to investigate whether the ESDPT reaction
catalyzed by, e.g., the carboxylic group will induce the antitumor
activity. Many amino acid side chains have abnormal pKa values
in proteins.64 Therefore, unionized carboxylic acids might be
involved in the hydrogen-bond interaction under biological
conditions. Formation of cyclic dual hydrogen bonds allows an
amino acid side chain incorporating carboxylic acid to discrimi-
nate between different nucleic acid bases, which serves an
important role in the selective recognition of nucleic acid bases
by proteins.65 Therefore, it is of great importance to probe the
dual hydrogen bonding protein-nucleic acid interaction65 on the
basis of the excited-state proton-transfer tautomerism of DPP
and APP.
The fast depopulation of the FN state may be tentatively
rationalized by the proximity of the nπ* state which has been
calculated to be in the second excited state and was estimated
to be only ∼4.4 kcal/mol above the lowest singlet ππ* state
(see Table 3). Note this value is on the same magnitude as the
activation energy (i.e., 2.1 kcal/mol) of the temperature-
dependent radiationless pathway for DPP in 2MTHF. As a result,
the Franck-Condon excited state may undergo a fast thermally
activated 1ππ* f 1nπ* internal conversion. The vibronic mixing
between nπ* and ππ* states may explain the extremely low
fluorescence yield of the FN state. In an extreme case, the
corresponding pseudo Jahn-Teller distortion may be incorpo-
rated, enhancing a nonradiative decay channel. Such a mech-
anism requires the molecule to be distorted along a nontotally
symmetric (i.e., out-of-plane) coordinate.58-60 Twisting the
exocyclic dimethylamino group possibly induces a nontotally
symmetric distortion. Accordingly, the relationship between
movement into the charge-transfer state and nonradiative decay
of the FN state might be considered an efficient deactivation
pathway. This proposed mechanism, however, is discounted via
studying the spectral properties of APP. Although ESICT is
prohibited in APP due to the high ionization energy of the amino
substituent, the florescence (i.e., the FN band) yield is still low
(∼10-3) in all polar solvents studied, indicating that radiationless
pathways other than the charge-transfer reaction dominate the
decay dynamics of the FN band.
Acknowledgment. This work was supported by the National
Science Council, Taiwan, R.O.C. (NSC89-2113-M-194-009).
References and Notes
Conclusions
(1) Taylor, C. A.; El-Bayoumi, M. A.; Kasha, M. Proc. Natl. Acad.
Sci. U.S.A. 1969, 63, 253.
In summary, we have reported dual photophysical properties,
i.e., the excited-state proton transfer versus charge-transfer
reactions, on DPP in different environments. To summarize the
viewpoints made in previous sections, Scheme 1a-c depicts
the photophysical pathways of DPP categorized by nonpolar,
polar, aprotic, and protic solvents. Several remarks can be
pointed out on the basis of Scheme 1: (1) The electron donating
property of the dimethylamino group on DPP enhances the dual
hydrogen bonding strength in both DPP dimer and DPP/acetic
acid complex, which upon excitation undergo ultrafast double
proton transfer (kpt > 6.7 × 1010 s-1), resulting in an imine-
like tautomer emission in nonpolar solvents. (2) The excited-
state intramolecular charge transfer takes place in polar, aprotic
solvents, of which the relaxation dynamics are independent of
the normal emission (i.e., the FN band). As supported by the
theoretical approach, the mechanism incorporating TICT may
be operative, resulting in a large change of the dipole moment.
(3) In protic solvents such as in ethanol, following the fast
excited-state intramolecular charge transfer and solvent relax-
ation, the solvent catalyzed proton-transfer reaction takes place
(e.g., ∼220 ps-1 in ethanol), resulting in an imine-like tautomer
emission. Thus, DPP provides an ideal model to study the dual
excitation behavior of proton transfer vs charge transfer, which
is unique among the 7AI analogues.
(2) Ingham, K. C.; Abu-Elgheit, M.; El-Bayoumi, M. A. J. Am. Chem.
Soc. 1971, 93, 5023.
(3) Ingham, K. C.; El-Bayoumi, M. A. J. Am. Chem. Soc. 1974, 96,
1674.
(4) Watson, J. D.; Crick, F. H. C. Nature (London) 1953, 171, 964.
(5) Watson, D. G.; Sweet, R. M.; Marsh, R. E. Acta Crystallogr. 1965,
19, 573.
(6) Hetherington, W. M., III; Micheels. R. H.; Eisenthal, K. B. Chem.
Phys. Lett. 1979, 66, 230.
(7) Fuke, K.; Yoshiuchi, H.; Kaya, K. J. Phys. Chem. 1984, 88, 5840.
(8) Waluk, J.; Herbich, J.; Oelkrug, D.; Uhl, S. J. Phys. Chem. 1986,
90, 3866.
(9) Fuke, K.; Kaya, K. J. Phys. Chem. 1989, 93, 614.
(10) Tokumura, K.; Watanabe, Y.; Udagawa, M.; Itoh, M. J. Am. Chem.
Soc. 1987, 109, 1346.
(11) Catala´n, J.; Kasha, M. J. Phys. Chem. A 2000, 104, 10812.
(12) Douhal, A.; Kim, S. K.; Zewail, A. H. Nature 1995, 378, 260.
(13) Folmer, D. E.; Poth, L.; Wisniewski, E. S.; Castleman, A. W., Jr.
Chem. Phys. Lett. 1998, 287, 1.
(14) Chachisvilis, M.; Fiebig, T.; Douhal, A.; Zewail, A. H. J. Phys.
Chem. A 1998, 102, 669.
(15) Takeuchi, S.; Tahara, T. J. Phys. Chem. A 1998, 102, 7740.
(16) Fiebig, T.; Chachisvilis, M.; Manger, M.; Zewail, A. H.; Douhal,
A.; Garcia-Ochoa, I.; de La Hoz Ayuso, A. J. Phys. Chem. A 1999, 103,
7419.
(17) Chou, P. T.; Liao, J. H.; Wei, C. Y.; Yang, C. Y.; Yu, W. S.; Chou,
Y. H. J. Am. Chem. Soc. 2000, 122, 986.
(18) Douhal, A.; Guallar, V.; Moreno, M.; Lluch, J.-M. Chem. Phys.
Lett. 1996, 256, 370.
(19) Guallar, V.; Batista, V. S.; Miller, W. H. J. Chem. Phys. 1999,
110, 9922.
(20) Catala´n, J.; Del Valle, J. C.; Kasha, M. Proc. Natl. Acad. Sci. U.S.A.,
1999, 96, 8338.
(21) McMorrow, D.; Aartsma, T. J. Chem. Phys. Lett. 1986, 125, 581.
(22) Moog, R. S.; Bovino, S. C.; Simon, J. D. J. Phys. Chem. 1988, 92,
6545.
(23) Konijnenberg, J.; Huizer, A. H.; Varma, C. A. G. O.J. Chem. Soc.,
Faraday Trans. 2 1988, 84 (8), 1163.
(24) (a) Moog, R. S.; Maroncelli, M. J. Phys. Chem. 1991, 95, 10359.
(b) Chapman, C. F.; Maroncelli, M. J. Phys. Chem. 1992, 96, 8430.
Although the spectroscopy and dynamics have been studied
comprehensively, several perplexities remain unsolved. The
main question lies in relation to the branching ratio as well as
the corresponding dynamics for the population of the FN and
FC species. While TICT seems to be operative in the case of
DPP, definitive proof and correlation with the twisting angles
remain to be explored. It should be noted that to explain the
spectroscopy and dynamics of dual emission for N,6N6-di-

Excitation Behavior of DPPs
J. Phys. Chem. A, Vol. 107, No. 10, 2003 1471
(25) (a) Negrerie, M.; Bellefeuille, S. M.; Whitham, S.; Petrich, J. W.;
Thornburg, R. W. J. Am. Chem. Soc. 1990, 112, 7419. (b) Negrerie, M.;
Gai, F.; Bellefeuille, S. M.; Petrich, J. W. J. Phys. Chem. 1991, 95, 8663.
(c) Negrerie, M.; Gai, F.; Lambry, J.-C.; Martin, J.-L.; Petrich, J. W. J.
Phys. Chem. 1993, 97, 5046. (d) Chen, Y.; Rich, R. L.; Gai, F.; Petrich, J.
W. J. Phys. Chem. 1993, 97, 1770. (e) Chen, Y.; Gai, F.; Petrich, J. W. J.
Am. Chem. Soc. 1993, 115, 10158. (f) Rich, R. L.; Chen, Y.; Neven, D.;
Negrerie, M.; Gai, F.; Petrich, J. W. J. Phys. Chem. 1993, 97, 1781. (g)
Gai, F.; Rich, R. L.; Petrich, J. W. J. Am. Chem. Soc. 1994, 116, 735. (h)
Smirnov, A. V.; English, D. S.; Rich, R. L.; Lane, J. Teyton, L.;
Schwabacher, A. W.; Luo, S.; Thornburg, R. W.; Petrich, J. W. J. Phys.
Chem. B 1997, 101, 2758 and references therein.
(26) (a) Chou, P. T.; Martinez, M. L.; Cooper, W. C.; McMorrow, D.;
Collin, S. T.; Kasha; M. J. Phys. Chem. 1992, 96, 5203. (b) Chang, C. P.;
Hwang W. C.; Kuo M. S.; Chou, P. T.; Clement, J. H. J. Phys. Chem.
1994, 98, 8801. (c) Chou, P. T.; Wei, C. Y.; Chang, C. P.; Chiu, C. H. J.
Am. Chem. Soc. 1995, 117, 7259. (d) Chou, P. T.; Wei, C. Y.; Chang, C.
P.; Kuo, M. S. J. Phys. Chem. 1995, 99, 11994.
(27) (a) Gordon, M. S. J. Phys. Chem. 1996, 100, 3974. (b) Chaban, G.
M.; Gordan, M. S. J. Phys. Chem. A 1999, 103, 185.
(28) (a) Ferna´ndez-Ramos, A.; Smedarchina, Z.; Siebrand, W.; Zgierski,
M. Z.; Rios, M. A. J. Am. Chem. Soc. 1999, 121, 6280. (b) Smedarchina,
Z.; Siebrand, W.; Ferna´ndez-Ramos, A.; Gorb, L.; Leszczynski, J. J. Chem.
Phys. 2000, 112, 566.
(29) Mente, S.; Maroncelli, M. J. Phys. Chem. A 1998, 102, 3860.
(30) Kyrychenko, A.; Stepanenko, Y.; Waluk, J. J. Phys. Chem. A 2000,
104, 9542.
(31) (a) Chou, P. T.; Wei, C. Y.; Wu, G. R.; Chen, W. S. J. Am. Chem.
Soc. 1999, 121, 12186. (b) Chou, P. T.; Wu, G. R.; Wei, C. Y.; Cheng, C.
C.; Chang, C. P.; Hung, F. T. J. Phys. Chem. B 1999, 103, 10042.
(32) Chou, P. T.; Wu, G. R.; Wei, C. Y.; Cheng, C. C.; Chang, C. P.;
Hung, F. T. J. Phys. Chem. B 2000, 104, 7818.
(39) Schneller, S. W.; Luo, J. K. J. Org. Chem. 1980, 45, 4045.
(40) Robison, M. M.; Robison, B. L. J. Am. Chem. Soc. 1955, 77, 457.
(41) Robison, M. M.; Robison, B. L. J. Am. Chem. Soc. 1956, 78, 1247.
(42) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(43) Chou, P. T.; Wei, C. Y.; Hung, F. T. J. Phys. Chem. B 1997, 101,
9119.
(44) Pimentel, G. C.; McClellan, A. L., Eds. The Hydrogen Bond, W.
H. Freeman and Co.: 1960, p 368.
(45) Collins, S. T. J. Phys. Chem. 1983, 87, 3202.
(46) Staley, R. H.; Taagepera, M.; Henderson, W. G.; Koppel, I.;
Beauchamp, J. L.; Taft, R. W. J. Am. Chem. Soc. 1977, 99, 326.
(47) Lippert, E. Z. Naturforsch. 1955, 10a, 541.
(48) Mataga, N.; Kubota, T. Molecular Interactions and Electronic
Spectra; Marcel Dekker: New York, 1970; p 385.
(49) Bilot, L.; Kawski, A. Z. Naturforsch. 1962, 17a, 621.
(50) Liptay, W. Z. Naturforsch. 1965, 20a, 1441.
(51) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum
Press: New York, 1983.
(52) Rotkiewicz, K.; Grellmann, K. H.; Grabowski, Z. R. Chem. Phys.
Lett. 1973, 19, 315.
(53) Rettig, W.; Bonacic-Kouteckr, V. Chem. Phys. Lett. 1979, 62, 115.
(54) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971 and
references therein.
(55) Bhattacharyya, K.; Chowdhury, M. Chem. ReV. 1993, 93, 507 and
references therein.
(56) Andre´asson, J.; Holme´n, A.; Albinsson, B. J. Phys. Chem. B 1999,
103, 9782.
(57) (a) Cramer, C. J.; Truhlar, D. G.J. Am. Chem. Soc. 1993, 115, 8810.
(b) Cramer, C. J.; Truhlar, D. G. J. Comput. Chem. 1992, 13, 1089.
(58) Lai, T.; Lim, E. C. Chem. Phys. Lett. 1979, 62, 507.
(59) Lim, E. C. J. Phys. Chem. 1986, 90, 6770.
(60) Madej, S. L.; Okajima, S.; Lim, E. C. J. Chem. Phys. 1976, 65,
1219.
(33) Chou, P. T.; Chen, Y. C.; Wei, C. Y.; Chen, W. S. J. Am. Chem.
Soc. 2000, 122, 9322.
(34) Chou, P. T.; Yu, W. S.; Wei, C. Y.; Chen, Y. M.; Yang, C. Y. J.
Am. Chem. Soc. 2001, 123, 3599.
(35) Chou, P. T.; Liu, Y. I.; Liu, H. W.; Yu, W. S. J. Am. Chem. Soc.
2001, 123, 12119.
(36) Schneller, S. W.; Luo, J. K. J. Org. Chem. 1980, 45, 4045.
(37) Clark, B. A. J.; Parrick, J. J. Chem. Soc., Perkin Trans. 1, 1974,
2270.
(61) Parusel, A. B. J.; Rettig, W.; Rotkiewicz, K. J. Phys. Chem. A 2002,
106, 2293.
(62) Suhadolnik, R. J. Nucleosides as Biological Probe; Wiley-Inter-
science: New York, 1979; pp 298-353.
(63) Antonini, I.; Claudi, F.; Cristalli, G.; Franchetti, P.; Grifantini, M.;
Martelli, S. J. Med. Chem. 1982, 25, 1258.
(64) Blow, D. M.; Steiz, T. A. Annu. ReV. Biochem. 1970, 39, 63. (b)
Lancelot, G. J. Am. Chem. Soc. 1977, 99, 7037.
(65) Stormo, G. D.; Fields, D. S. Trends Biochem. Sci. 1998, 23, 109..
(38) Girgis, N. S.; Larson, S. B.; Robins, R. K.; Cottam, H. B. J.
Heterocycl. Chem. 1989, 26, 317.