Wavelength-dependent stereodifferentiation in the fluorescence quenching of asymmetric naphthalene-based dyads by amines

Article abstract of DOI:10.1021/jp047996a

In the present contribution, wavelength has been used as a tunable parameter to achieve selective control of the photophysics of two novel asymmetric bichromophoric dyads composed of naphthalene units, i.e., 6-methoxynaphthalene (NPX) and 1-methylnaphthalene (NAP) derivatives, with different electronic properties, connected by an amide spacer [(S,S) and (S,R)-NPX-NAP]. As model systems, relevant monochromophoric compounds (NPX-M and NAP-M) have also been investigated. While upon excitation at 325 nm the light energy remained in the NPX moiety, at 290 nm an efficient singlet - singlet energy transfer (ΦSSET of about 97%) from the NAP unit to the NPX chromophore dominated. A remarkable Stereodifferentiation was observed in the excited-state quenching by triethylamine via exciplex formation. The results demonstrate that it is possible to control configuration-dependent interactions in the excited state by wavelength tuning. This can be rationalized through intramolecular interactions of π systems leading to modulation of the redox properties.

Full text of DOI:10.1021/jp047996a

© Copyright 2005 by the American Chemical Society
VOLUME 109, NUMBER 12, MARCH 31, 2005
ARTICLES
Wavelength-Dependent Stereodifferentiation in the Fluorescence Quenching of Asymmetric
Naphthalene-Based Dyads by Amines
Sergio Abad,† Uwe Pischel,*,‡ and Miguel A. Miranda*,†
Instituto de Tecnolog´ıa Qu´ımica, UniVersidad Polite´cnica de Valencia, AV. de los Naranjos s/n,
E-46022 Valencia, Spain, and REQUIMTE/Departamento de Qu´ımica, Faculdade de Cieˆncias,
UniVersidade do Porto, Rua Campo Alegre, 4169-007 Porto, Portugal
ReceiVed: May 10, 2004; In Final Form: January 27, 2005
In the present contribution, wavelength has been used as a tunable parameter to achieve selective control of
the photophysics of two novel asymmetric bichromophoric dyads composed of naphthalene units, i.e.,
6-methoxynaphthalene (NPX) and 1-methylnaphthalene (NAP) derivatives, with different electronic properties,
connected by an amide spacer [(S,S) and (S,R)-NPX-NAP]. As model systems, relevant monochromophoric
compounds (NPX-M and NAP-M) have also been investigated. While upon excitation at 325 nm the light
energy remained in the NPX moiety, at 290 nm an efficient singlet-singlet energy transfer (ΦSSET of about
97%) from the NAP unit to the NPX chromophore dominated. A remarkable stereodifferentiation was observed
in the excited-state quenching by triethylamine via exciplex formation. The results demonstrate that it is
possible to control configuration-dependent interactions in the excited state by wavelength tuning. This can
be rationalized through intramolecular interactions of π systems leading to modulation of the redox properties.
Introduction
several parameters, such as temperature, pressure, or solvent;
in the solid phase the chiral information can also be provided
by a rigid environment.11,12 In this context, it would be highly
desirable to control configuration- or conformation-dependent
interactions in the excited state by adequate selection of the
excitation wavelength. To our knowledge, this possibility has
been explored only for few examples,6-8,10,13 despite its potential
interest.
Wavelength has been used as a tunable parameter to gain
selective control of photophysical and photochemical processes.1-5
Wavelength-dependent photophysics and/or photochemistry has
been clearly demonstrated in a number of examples.3,6-10 The
case of bi- (or poly-) chromophoric compounds is of particular
interest, in connection with the possibility of light absorption
at a specific chromophore, keeping it localized until finishing
the photochemical reaction. This is the origin of so-called
“orthogonal” photochemistry.4,5
Several factors are known to control the kinetics of excited-
state interactions via electron transfer, exciplex formation,
hydrogen transfer, or energy transfer.14-17 Extensive investiga-
tions have been devoted to steric18-21 or stereoelectronic
effects,21-23 spin multiplicity,21,24 and electronic effects.21,25-28
In our recent research we became interested in the question of
whether chirality might influence the quenching of excited states.
Our conceptual approach includes the design of diastereomeric
dyads, among them benzophenone/hydrogen donor and naph-
Recently, asymmetric photochemistry has attracted consider-
able attention.11,12 Its control can be achieved by variation of
* To whom correspondence should be addressed. M.A.M.: tel, +34 96
3877 807; fax, +34 96 3877 809, e-mail, mmiranda@qim.upv.es. U.P.:
tel, +351 22 608 2885; fax, +351 22 608 2959; e-mail, upischel@fc.up.pt.
† Instituto de Tecnolog´ıa Qu´ımica, Universidad Polite´cnica de Valencia.
‡ REQUIMTE/Departamento de Qu´ımica, Faculdade de Cieˆncias, Uni-
versidade do Porto.
10.1021/jp047996a CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/04/2005

2712 J. Phys. Chem. A, Vol. 109, No. 12, 2005
Abad et al.
CHART 1
the excitation wavelength was kept around 0.2. (S)-6-Methoxy-
R-methyl-2-naphthaleneacetic acid [(S)-naproxen] in acetonitrile
was used as the fluorescence standard (Φf ) 0.47 under
nitrogen).45
Synthesis of the Dyads and the Model Compounds. The
synthesis of the dyads was accomplished by reaction of 0.8
mmol (S)-6-methoxy-R-methyl-2-naphthaleneacetic acid [(S)-
naproxen] with 1 mmol (S)- or (R)-(1-naphthyl)ethylamine in
10 mL of dry dichloromethane. 1-(3-Dimethylaminopropyl)-N-
ethylcarbodiimide hydrochloride (EDC, 1.0 mmol) was used
for the activation of the acid. After standard workup, the
compounds were purified by flash chromatography (ethyl
acetate/n-hexane, 7:3).
Model compound NPX-M was synthesized by reaction of 0.8
mmol (S)-6-methoxy-R-methyl-2-naphthaleneacetic acid with
1.0 mmol diethylamine in the presence of 1.0 mmol EDC in 10
mL of dry dichloromethane. The preparation of NAP-M was
accomplished by reacting 0.9 mmol (R)-(1-naphthyl)ethylamine
with a slight excess of acetyl chloride (1.1 mmol) in the presence
of 1.2 mmol triethylamine in 10 mL of dry dichloromethane.
After standard workup, the products were purified by flash
chromatography (ethyl acetate/n-hexane, 7:3). All compounds
were subsequently recrystallized from mixtures of n-hexane and
dichloromethane and characterized by 1H and 13C NMR
spectroscopy and elemental analysis.
thalene/electron donor combinations.29-36 For the majority of
the investigated dyads, pronounced diastereodifferentiation has
been observed, albeit the size of the effect is dependent on the
conformational and thermodynamic requirements of each sys-
tem.
In the present contribution, we report on the photophysics of
two novel asymmetric and bichromophoric dyads (NPX-NAP)
composed of naphthalene units with different electronic proper-
ties, connected by an amide spacer (cf. Chart 1). As model
systems, the relevant monochromophoric compounds NPX-M
and NAP-M (cf. Chart 1) have been investigated. The signifi-
cantly different absorption spectra of both naphthalene units
allow for selective or at least preferential excitation of one or
the other chromophore in the dyads. A remarkable influence of
the chiral information on intermolecular exciplex-induced
quenching by triethylamine37-44 has been observed.
2(S)-(6-Methoxynaphth-2-yl)-N-[(1(S)-naphth-1-yl)ethyl)]pro-
1
pionamide [(S,S)-NPX-NAP]. H NMR (300 MHz, CDCl3) δ
(ppm): 1.55 (d, J ) 6.8 Hz, 3H, CH3), 1.57 (d, J ) 7.2 Hz,
3H, CH3), 3.68 (q, J ) 7.2 Hz, 1H, CH), 3.88 (s, 3H, CH3O),
3.68 (s, 1H, NH), 5.84 (q, J ) 6.8 Hz, 1H, CH), 7.05-7.13
(m, 2H, arom. CH), 7.17-7.43 (m, 5H, arom. CH), 7.51-7.63
(m, 3H, arom. CH), 7.68 (d, J ) 8.1 Hz, 1H, arom. CH), 7.77
(d, J ) 8.1 Hz, 1H, arom. CH), 7.91 (d, J ) 8.3 Hz, 1H, arom.
CH). 13C NMR (75 MHz, CDCl3) δ (ppm): 18.5 (CH3), 20.8
(CH3), 44.9 (CH), 47.1 (CH), 55.3 (CH3O), 105.6, 118.9, 122.3,
123.4, 125.0, 125.7, 126.0, 126.2, 126.3, 127.4, 128.1, 128.6
(arom. CH), 128.9 (arom. C), 129.2 (arom. CH), 131.0, 133.7,
133.8, 136.3, 138.3, 157.7 (arom. C), 173.1 (CO). Anal. Calcd
for C26H25NO2: C, 81.43; H, 6.57; N, 3.65. Found: C, 81.59;
H, 6.60; N, 3.68.
Experimental Section
Materials. All materials for the synthesis of the dyads and
model compounds were purchased from Aldrich and used
without further purification. n-Hexane as the solvent for
absorption and fluorescence measurements was of spectroscopic
quality from Merck. Silica gel (230-400 mesh) from Scharlau
was used for column chromatography. Ethyl acetate and
n-hexane from Scharlau were the eluting solvents for flash
chromatography. Triethylamine (99%) from Aldrich was used
as received.
Spectroscopic Measurements. UV/vis absorption measure-
ments were performed on a Shimadzu UV-2101PC spectrometer.
Fluorescence emission and excitation spectra were recorded on
a Photon Technology International (PTI) LPS-220B fluorimeter.
All spectra, except for relative measurements of fluorescence
quenching, are corrected for the instrument response. Lifetimes
were measured with a lifetime spectrometer (TimeMaster
fluorescence lifetime spectrometer TM-2/2003) from PTI by
means of the stroboscopic technique, which is a variation of
the boxcar technique. As the excitation source, a hydrogen/
nitrogen flashlamp (1.8 ns pulse width) was used. The kinetic
traces were fitted by monoexponential decay functions using a
reconvolution procedure to separate from the lamp pulse profile.
Measurements were done in N2-outgassed n-hexane at room
temperature (23 °C) in cuvettes of 1 cm path length. For
fluorescence quantum yield measurements, the absorbance at
2(S)-(6-Methoxynaphth-2-yl)-N-[(1(R)-naphth-1-yl)ethyl)]pro-
1
pionamide [(S,R)-NPX-NAP]. H NMR (300 MHz, CDCl3) δ
(ppm): 1.45 (d, J ) 6.4 Hz, 3H, CH3), 1.52 (d, J ) 7.0 Hz,
3H, CH3), 3.55 (q, J ) 7.0 Hz, 1H, CH), 3.87 (s, 3H, CH3O),
5.79 (s, 1H, NH), 5.86 (q, J ) 6.4 Hz, 1H, CH), 7.07-7.15
(m, 2H, arom. CH), 7.27-7.51 (m, 5H, arom. CH), 7.57 (s,
1H, arom. CH), 7.62 (d, J ) 8.9 Hz, 1H, arom. CH), 7.67 (d,
J ) 8.5 Hz, 1H, arom. CH), 7.72 (dd, J ) 7.6 Hz, J ) 1.5 Hz,
1H, arom. CH), 7.79-7.85 (m, 1H, arom. CH), 8.00-8.07 (m,
1H, arom. CH). 13C NMR (75 MHz, CDCl3) δ (ppm): 18.6
(CH3), 20.4 (CH3), 44.8 (CH), 46.9 (CH), 55.3 (CH3O), 105.7,
119.1, 122.5, 123.4, 125.1, 125.7, 126.0, 126.1, 126.4, 127.4,
128.2, 128.6 (arom. CH), 128.9 (arom. C), 129.2 (arom. CH),
131.0, 133.6, 133.8, 136.5, 138.3, 157.6 (arom. C), 173.0 (CO).
Anal. Calcd for C26H25NO2: C, 81.43; H, 6.57; N, 3.65.
Found: C, 80.92; H, 6.74; N, 3.73.
N,N-Diethyl-2(S)-(6-methoxynaphth-2-yl)propionamide (NPX-
M). 1H NMR (300 MHz, CDCl3) δ (ppm): 0.99 (t, J ) 7.2 Hz,
3H, N-CH2CH3), 1.09 (t, J ) 7.2 Hz, 3H, N-CH2CH3), 1.50
(d, J ) 6.8 Hz, 3H, CH3), 3.10 (dq, J ) 14.0 Hz, J ) 7.2 Hz,
1H, N-CH2), 3.27 (dq, J ) 14.0 Hz, J ) 7.2 Hz, 1H, N-CH2),
3.37 (dq, J ) 14.0 Hz, J ) 7.2 Hz, 1H, N-CH2), 3.54 (dq, J
) 14.0 Hz, J ) 7.2 Hz, 1H, N-CH2), 3.90 (s, 3H, CH3O),
3.94 (q, J ) 6.8 Hz, 1H, CH), 7.07-7.15 (m, 2H, arom. CH),

Fluorescence Quenching of Naphthalene-Based Dyads
J. Phys. Chem. A, Vol. 109, No. 12, 2005 2713
Figure 1. Absorption spectra of the model compounds NPX-M (full
line) and NAP-M (dashed line) in n-hexane. The arrows indicate the
two selected wavelengths for specific excitation of one or the other
chromophore. The inset shows the summed up spectra of the model
compounds (full line) and the spectrum of (S,S)-NPX-NAP (dashed
line), both normalized to 1 at 333 nm.
Figure 2. Normalized absorption (left) and fluorescence spectra (right)
of (a) NAP-M and (b) NPX-M in n-hexane. The fluorescence spectra
were obtained with λexc ) 290 nm. The inset in panel a shows the
normalized fluorescence spectrum (left) and the forbidden absorption
band at 313 nm (right). From the intersection, the 0-0 excitation energy
(E0-0) was extracted.
7.40 (dd, J ) 8.5 Hz, J ) 1.9 Hz, 1H, arom. CH), 7.62 (s, 1H,
arom. CH), 7.68 (d, J ) 3.2 Hz, 1H, arom. CH), 7.70 (d, J )
3.0 Hz, 1H, arom. CH). 13C NMR (75 MHz, CDCl3) δ (ppm):
12.9 (N-CH2CH3), 14.3 (N-CH2CH3), 21.0 (CH3), 40.3 (N-
CH2), 41.7 (N-CH2), 43.1 (CH), 55.4 (CH3O), 105.7, 118.9,
125.5, 126.3, 127.4, 129.2 (arom. CH), 129.3, 133.5, 137.7,
157.6 (arom. C), 172.9 (CO). Anal. Calcd for C18H23NO2: C,
75.76; H, 8.12; N, 4.91. Found: C, 75.57; H, 8.10; N, 4.90.
N-[(1(R)-Naphth-1-yl)ethyl]acetamide (NAP-M). 1H NMR
(300 MHz, CDCl3) δ (ppm): 1.67 (d, J ) 6.6 Hz, 3H, CH3),
1.97 (s, 3H, COCH3), 5.66 (s, 1H, NH), 5.92 (q, J ) 6.6 Hz,
1H, CH), 7.42-7.59 (m, 4H, arom. CH), 7.80 (d, J ) 7.7 Hz,
1H, arom. CH), 7.87 (d, J ) 7.7 Hz, 1H, arom. CH), 8.10 (d,
J ) 8.3 Hz, 1H, arom. CH). 13C NMR (75 MHz, CDCl3) δ
(ppm): 20.7 (CH3), 23.4 (COCH3), 44.7 (CH), 122.6, 123.5,
125.2, 126.8, 127.7, 128.5, 128.8 (arom. CH), 131.2, 134.0,
138.2 (arom. C), 168.9 (CO). Anal. Calcd for C14H15NO: C,
78.84; H, 7.09; N, 6.57. Found: C, 78.35; H, 7.32; N, 6.67.
Figure 3. Normalized absorption (left) and fluorescence (λexc ) 290
nm, right) spectra of (S,R)-NPX-NAP in n-hexane.
Results and Discussion
preferentially each of the chromophores. These absorption
features have consequences for the wavelength-dependent
photophysics of NPX-NAP.
Fluorescence Measurements. The fluorescence spectra
obtained upon excitation of the model compounds in n-hexane
at 290 nm showed typical fine-structured bands, but with
different maxima (cf. Figure 2).45,46 For NAP-M the maximum
was found at 321 nm. On the other hand, the fluorescence
maximum of NPX-M was shifted to 347 nm.
Strikingly, upon excitation at 290 and 325 nm the fluores-
cence spectra of the bichromophoric dyads solely displayed an
emission band centered at 347 nm (cf. Figure 3). This
fluorescence can be unambiguously identified as originating
from NPX, based on comparison with the model compounds.
The latter observation indicates that when the major part of the
excitation energy at λexc ) 290 nm is absorbed by NAP, an
efficient singlet-singlet energy transfer from NAP to NPX must
happen in the dyads (see below).
From the intersection between normalized absorption and
emission spectra of the dyads NPX-NAP, an excited singlet state
energy (E0-0) of 3.70 eV can be extracted for both diastereo-
mers. This value compares favorably with the one determined
for the model compound NPX-M (3.70 eV) and the reported
value of 3.69 eV for (S)-naproxen.47 The E0-0 of the model
UV/Vis Absorption Properties. The absorption spectra of
both model compounds NPX-M and NAP-M in n-hexane are
shown in Figure 1. Both chromophores display fine-structured
absorption bands, typical for naphthalene and derivatives.45,46
The spectrum of the bichromophoric dyad (S,S)-NPX-NAP is
shown in the inset of Figure 1 and compared to the sum of the
spectra of the model compounds. It can be clearly seen that the
spectra of the bichromophoric dyads are a superposition of the
absorption spectra of the respective model compounds NAP-M
and NPX-M. This is indicative of the absence of any ground-
state interaction between both chromophores. Furthermore, no
significant differences in the spectra of both diastereomeric
dyads were noted; i.e., chiral information has significant impact
on neither the spectral distribution nor the oscillator strength
of the π,π*-transitions.
A comparison of the spectra of the naphthalene model
compounds revealed two favorable wavelengths for the excita-
tion of the NPX-NAP dyads: 290 and 325 nm (arrows in Figure
1). At 325 nm, exclusively NPX was excited, since NAP has
no absorption at this wavelength. On the other hand, at 290 nm
NAP absorbed the main fraction of photons (compare ꢀ290
)
4884 and 1058 M-1 cm-1 for NAP-M and NPX-M, respec-
tively). This enabled us to excite specifically or at least

2714 J. Phys. Chem. A, Vol. 109, No. 12, 2005
Abad et al.
TABLE 1: Photophysical Parameters of the Dyads and the
Relevant Model Compounds
with
and
290
NAP-M
ꢀ
Φf
Φf
τf/ns
(λexc
290 nm)b 325 nm)b
τf/ns
) 0.82
) 0.18
(λexc
)
(λexc
)
)
(λexc
)
290
290
ꢀ
ꢀ
+ ꢀ
NPX-M
290 nm)a
325 nm)a
NAP-M
(S,S)-NPX-NAP 0.34 ( 0.02 0.59 ( 0.03
(S,R)-NPX-NAP 0.34 ( 0.02 0.61 ( 0.02
13.2
13.2
14.1
52.7
13.4
13.4
14.4
NPX-Mc
NAP-Md
0.35 ( 0.02 0.62 ( 0.02
0.18 ( 0.01
290
NPX-M
ꢀ
290
NAP-M
290
NPX-M
a Fluorescence quantum yield in deaerated acetonitrile, measured with
(S)-naproxen as the standard; see Experimental Section. b Fluorescence
lifetime measured at λobs ) 347 nm in deaerated acetonitrile; measured
with single-photon-counting; 5% error. c (S)-enantiomer. d (R)-enanti-
omer.
+ ꢀ
τNAP ) τNAP-M(1 - ΦSSET
)
(2)
(3)
1
1
kSSET
)
-
τNAP τNAP-M
NAP-M, 3.93 eV, is in excellent agreement with the value for
1-methylnaphthalene (3.90 eV).48 From these data it becomes
clear that singlet-singlet energy transfer from NAP to NPX is
thermodynamically favored (∆GSSET ) -0.23 eV).
With the known molar absorption coefficients of the model
compounds, the lifetime of NAP-M, and the fluorescence
quantum yield of NPX-M (λexc ) 290 nm), the following
lifetime τNAP and rate constant kSSET are calculated for the two
diastereomeric dyads NPX-NAP: τNAP ) 1.8 ns; kSSET ) (5.3
( 0.3) × 108 s-1. These data are the same for both dyads.
According to eq 1, the quantum yield of energy transfer amounts
to about 97% ((3%) indicating a high efficiency of the process.
Due to the almost quantitative energy transfer, no residual
fluorescence of NAP can be seen in the fluorescence spectra of
the dyads.
To account for the quantification of radiative processes in
the dyads, fluorescence quantum yields and lifetimes were
measured for both model compounds and bichromophoric dyads.
The data are compiled in Table 1.
The photophysical data for NPX-M and NAP-M are in good
agreement with those reported for their respective parent
chromophores, e.g., naproxen (Φf ) 0.47 and τf ) 10.8 ns; in
acetonitrile)45 and 1-methylnaphthalene (Φf ) 0.26 and τf )
67.0 ns; in cyclohexane).49 By comparing the fluorescence
quantum yields in Table 1, two conclusions can be drawn: (1)
the fluorescence quantum yield upon excitation of the dyads at
290 nm is ca. 2 times larger than the quantum yield of the
NAP-M model compound and is practically identical with that
of NPX-M, indicating an efficient energy transfer, and (2) the
fluorescence quantum yields of the dyads and NPX-M are larger
for excitation at 325 nm than for 290 nm, pointing to an
increased contribution of an additional decay channel at shorter
wavelengths.
The calculation of the spectral overlap integrals for singlet-
singlet energy transfer in our dyads yielded the following values
for the application of the Fo¨rster (Jdipole-dipole) or Dexter (Jexchange
)
mechanism: Jdipole-dipole ) 1.23 × 10-12 cm6 mol-1, Jexchange
) 2.07 × 10-4 cm.17 The Fo¨rster theory defines a critical
distance (R0), where the energy transfer probability is 50% (eq
4), which was estimated to R0 ) 16.9 Å for the investigated
dyads NPX-NAP. Molecular modeling (AM1) of the dyads
resulted in a center-to-center distance of ca. 6.5 Å between both
naphthalene moieties. For such distance, a rate constant of kSSET
) 5.9 × 109 s-1 would be expected, based on dipole-dipole
interaction (eq 4). On the other hand, the observed rate constant
for the dyads is 1 order of magnitude lower. Thus, based on
our observations it cannot be decided clearly whether the Fo¨rster
or Dexter mechanism prevails. Due to the rather high flexibility
of the spacer in the dyads, also extended conformations with
larger donor-acceptor distances are expected as local minima.
For these situations (R g 10 Å), clearly Fo¨rster energy transfer
should be dominant, due to the missing orbital overlap between
donor and acceptor, as it is required for Dexter energy transfer.
However, for the further discussion (see below) related to the
quenching of both naphthalene chromophores by electron-
donating amines, the actual energy transfer mechanism has no
implications.
Energy Transfer in the Dyads. A detailed analysis of the
parameters involved in the energy transfer results in the
following relations (eqs 1-3), which allow for the extraction
of the rate constants of intramolecular singlet-singlet energy
transfer (SSET) in the dyads. The fluorescence quantum yield
of NPX in the dyads at λexc ) 290 nm breaks up into two
contributions: the fluorescence which is sensitized by energy
transfer from NAP (first term in eq 1) and the fluorescence
which results from direct excitation of NPX (second term in eq
1). The contributions of both pathways are weighted by the
relative light absorption of both chromophores at 290 nm. For
low optical densities (as applies to our experiments), the molar
absorption coefficients can be used to define the respective
fractions of absorbed photons. With the help of eq 1, the
quantum yield for SSET in both dyads can be calculated.
Equation 2 allows for the determination of the NAP lifetime
τNAP in the dyads, and eq 3 defines the rate constant of SSET
(kSSET).
6
R0
1
τD
kdipole-dipole
)
( )
R
9000 ln 10κ2ΦD
128π5n4NA
6
with R0
)
Jdipole-dipole (4)
Quenching of the Model Compounds by Triethylamine.
Thermodynamics of Electron Transfer versus Exciplex
Formation. Typical photoinduced reactions of naphthalene and
its derivatives are electron transfer (et) or exciplex formation
(ex) with appropriate electron donors such as amines.37-44 First,
the fluorescence of model compounds NPX-M and NAP-M was
quenched by triethylamine (TEA). Since nonpolar n-hexane was
290
NAP-M
ꢀ
290
f,NPX
290
Φ
)
ΦSSET
Φ
+
f,NPX-M
290
290
(
)
ꢀNAP-M + ꢀNPX-M
290
NPX-M
ꢀ
290
f,NPX-M
Φ
(1)
290
290
(
)
ꢀNAP-M + ꢀNPX-M

Fluorescence Quenching of Naphthalene-Based Dyads
J. Phys. Chem. A, Vol. 109, No. 12, 2005 2715
Figure 5. Steady-state fluorescence quenching of (a) (S,S)-NPX-NAP
and (b) (S,R)-NPX-NAP by TEA in n-hexane. The fluorescence spectra
were obtained at λexc ) 325 nm. The insets show the development of
the exciplex emission bands.
Figure 4. Steady-state fluorescence quenching of NAP-M by triethyl-
amine (TEA) in n-hexane. The fluorescence spectra were obtained at
λexc ) 290. The inset shows the corresponding Stern-Volmer plot.
used as the solvent, exciplex formation should dominate over
photoinduced electron transfer. For the calculation of the free
energy for the formation of the radical ion pair (∆Get) in
n-hexane, eq 5 was used.50
2.6 eV
∆Get ) Eox - Ered - E0-0
+
- 0.13 eV (5)
ꢀ
With this equation, the free energy associated with radical
ion pair formation upon photoinduced electron transfer from
triethylamine can be calculated as ∆Get ) 1.02 and 0.77 eV
for NPX-M and NAP-M, respectively.51 Thus, this quenching
pathway is characterized by strongly endergonic thermo-
dynamics and is highly unlikely. For the free energy of exciplex
formation ∆Gex, eq 6 applies.50
Figure 6. Plots of the inverse lifetime of NPX versus TEA concentra-
tion. The data were obtained from time-resolved fluorescence quenching
of (S,S)-NPX-NAP (full circles) and (S,R)-NPX-NAP (open circles)
by TEA at λexc ) 325 nm in n-hexane.
µ2 ꢀ - 1
∆Gex ) Eox - Ered - E0-0
-
- 0.19 + 0.38 eV
(
)
F3
2ꢀ + 1
(6)
rate constants was observed in both cases [(8.1 ( 0.4) × 109
M-1 s-1 for NAP-M and (4.7 ( 0.2) × 109 M-1 s-1 for NPX-
M].
Using the same electrochemical potentials and excited singlet
state energies as in eq 551 and an average value of 0.75 eV for
µ2/F3,50 ∆Gex is equal to 0.16 and -0.09 eV for NPX-M and
NAP-M, respectively. Strikingly, these estimations are excel-
lently verified by the experimental observations. Indeed, for
NAP-M the quenching is accompanied by a broad and red-
shifted emission band at ca. 435 nm (cf. Figure 4), typical for
a naphthalene-amine exciplex as reported for several related
systems.37,39,40,52 On the other hand, no quenching was observed
for NPX-M as predicted by the endergonic thermodynamics for
exciplex formation.
The bimolecular quenching rate constant of NAP-M by
triethylamine was obtained in two ways: (i) from the Stern-
Volmer plot shown in the inset of Figure 4 and (ii) by measuring
the NAP-M fluorescence lifetimes in the presence of increasing
TEA concentrations. The two values were practically identical
[(6.5 ( 0.3) × 109 M-1 s-1]. This quenching rate constant is 1
order of magnitude below diffusion control and accounts very
well for the estimated exergonicity of exciplex formation.
To verify our observations, a control experiment was
performed, using 1,4-diazabicyclo[2.2.2]octane (DABCO) as a
stronger electron-donating amine [Eox ) 0.68 V versus 0.88 V
for triethylamine (saturated calomel electrode (SCE), aceto-
nitrile)].53 Now for both model compounds NAP-M and NPX-M
an exergonic thermodynamics for exciplex formation is ex-
pected.54 And indeed, dynamic quenching accompanied by high
Fluorescence Quenching of NPX-NAP Dyads by Trieth-
ylamine at λexc ) 325 nm. Excitation of the dyads at 325 nm
leads exclusively to excited states of NPX. Contrary to the
observations made for the model compound NPX-M (see
above), the fluorescence of the NPX-chromophore in both dyads
is quenched by triethylamine in a dynamic process. The
bimolecular quenching rate constants can be simply determined
by application of the Stern-Volmer equation: I0/I ) τ0/τ ) 1
+ kqτ0[TEA]. Bimolecular quenching rate constants obtained
from steady-state fluorescence measurements were (1.1 ( 0.1)
× 108 M-1 s-1 and (7.4 ( 0.4) × 107 M-1 s-1 for (S,S)- and
(S,R)-NPX-NAP, respectively (cf. Figure 5). The values derived
from time-resolved measurements were somewhat smaller [(8.5
( 0.4) × 107 M-1 s-1 and (4.9 ( 0.2) × 107 M-1 s-1 for (S,S)-
and (S,R)-NPX-NAP, respectively, cf. Figure 6], presumably
due to a small contribution of static quenching in the steady-
state experiments.
Evidently, exciplexes are involved in the quenching mech-
anism of NPX in the dyads, as can be seen by the typical broad
and red-shifted emission band (cf. insets in Figure 5). Interest-
ingly, the diastereomeric dyads show significant stereodiffer-
entiation in the reactivity toward electron-donating triethylamine.
The rate constants differ by a factor of 1.7.

2716 J. Phys. Chem. A, Vol. 109, No. 12, 2005
Abad et al.
CHART 2
Figure 7. Steady-state fluorescence quenching of (a) (S,S)-NPX-NAP
and (b) (S,R)-NPX-NAP by TEA in n-hexane. The fluorescence spectra
were obtained at λexc ) 290 nm. The insets show the development of
the exciplex emission bands.
The different behavior of the NPX-chromophore in the model
compound NPX-M and in the dyads (NPX-NAP) implies that
the reduction potential of NPX is increased in the NPX-NAP.
On the other hand, a change of the excited state energy of NPX
cannot be deemed responsible, as clearly can be seen from the
matching absorption spectra of model compounds and dyads
(cf. inset in Figure 1). The increased NPX reduction potential
in the dyads can be explained by a π-π interaction of both
chromophores, which shifts electron density from NPX to NAP.
Therefore, NPX acts as a better electron acceptor in the dyads
(cf. Chart 2).
To achieve a favorable interaction between the π-systems of
both chromophores, the spacer has to attain a certain conforma-
tion, whose stability might be strongly dependent on the
configuration of both chiral centers. Obviously, this is facilitated
when both asymmetric carbons are in their S configuration, as
indicated by the higher quenching rate constant of NPX in the
(S,S) dyad.
Fluorescence Quenching of NPX-NAP Dyads by Trieth-
ylamine at λexc ) 290 nm. Quenching experiments of dyad
Figure 8. Plots of the inverse lifetime of NAP versus TEA concentra-
tion according to eq 8. The data were obtained from fluorescence
quenching of (S,S)-NPX-NAP (full circles) and (S,R)-NPX-NAP (open
circles) by TEA at λexc ) 290 nm in n-hexane.
fluorescence by triethylamine were also performed at λexc
)
290 nm, where light is preferentially absorbed by NAP. The
high quenching rate constant of the model compound NAP-M
by triethylamine [kq ) (6.5 ( 0.3) × 109 M-1 s-1] suggests
that exciplex formation with excited NAP in the dyads can
compete with singlet-singlet energy transfer to NPX (see above)
already at amine concentrations of ca. 0.1 M. As for the model
compound NAP-M, exciplex emission at ca. 435 nm was
observed with increasing fluorescence quenching of the dyads
at λexc ) 290 nm (cf. Figure 7).
290
f,NPX-M
was calculated with k
) Φ2f,9N0PX-M/τN29P0X-M. Furthermore,
the lifetime of NPX in the dyads at each triethylamine
concentration (τNPX,TEA) is known by time-resolved fluorescence
quenching of NPX-NAP at λexc ) 325 nm. Based on eq 7 the
lifetime of the NAP chromophore (τNAP,TEA) at each triethy-
lamine concentration was calculated and then plotted according
to eq 8 (cf. Figure 8). The slope of a plot according to eq 8
yields the bimolecular rate constant for quenching by exciplex
formation between triethylamine and NAP for each dyad: kq,NAP
) (5.3 ( 0.3) × 108 M-1 s-1 and (1.1 ( 0.1) × 109 M-1 s-1
for (S,S)-NPX-NAP and (S,R)-NPX-NAP, respectively. From
the intercept, the lifetime of NAP without added quencher can
be calculated: τNAP ) 2.0 ns. This lifetime is in excellent
agreement with the one derived from the treatment of the SSET
process in the dyads (see above).
The determination of the bimolecular rate constant of NAP
quenching in the dyads by triethylamine is based on eqs 7 and
8.
290
f,NPX,TEA
Φ
)
290
NAP-M
ꢀ
290
τNAP,TEAkSSET
τ
k
+
NPX,TEA f,NPX-M
290
290
(
)
ꢀNAP-M + ꢀNPX-M
Similar to the quenching of NPX at λexc ) 325 nm, a
significant stereodifferentiation in the reactivity of NAP was
observed for both diastereomeric dyads. But now, the (S,R)-
diastereomer reacts faster by a factor of 2.1, while for NPX
quenching (S,S)-NPX-NAP is more reactive toward triethyl-
amine. Conclusively, there is an inversion of diastereodiffer-
entiation upon going from NPX to NAP.
290
NPX-M
ꢀ
290
τ
k
(7)
(8)
NPX,TEA f,NPX-M
290
290
(
)
ꢀNAP-M + ꢀNPX-M
1
1
)
+ kq,NAP[TEA]
τNAP,TEA τNAP
Equation 7 describes, similar to eq 1, the two contributions
The observed inverted behavior of NAP compared with NPX
in the dyads is not unexpected, following our explanation of a
changed reduction potential of the chromophores in the dyads,
mediated by configuration-dependent π-π interactions. If, as
to the fluorescence of NPX in the dyads at each triethylamine
concentration at λexc ) 290 nm, i.e., sensitization by SSET and
direct excitation. The rate constant for radiative decay of NPX

Fluorescence Quenching of Naphthalene-Based Dyads
J. Phys. Chem. A, Vol. 109, No. 12, 2005 2717
(18) Jacques, P.; Allonas, X.; Suppan, P.; von Raumer, M. J. Photochem.
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shown above (cf. Chart 2), NPX is a better electron acceptor in
the (S,S) dyad, NAP must be a worse acceptor in this
diastereomer compared with the (S,R) combination. Therefore,
the diastereodifferentiation for NAP is opposite to that for NPX.
Generally, NAP is a worse electron acceptor in the dyads
compared with the model NAP-M, as documented by the smaller
quenching rate constants in the dyads (0.5-1.1 × 109 M-1 s-1
versus 6.5 × 109 M-1 s-1).
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Conclusions
In essence, we have demonstrated wavelength-dependent
stereodifferentiation in the photophysical processes of novel
bichromophoric and diastereomeric naphthalene dyads. While
upon excitation at 325 nm the light energy remains in the
naproxen-derived moiety (NPX), at 290 nm efficient singlet-
singlet energy transfer (ΦSSET of about 97%) from the naph-
thalene-derived chromophore NAP to NPX dominates. Inter-
molecular quenching experiments of dyad fluorescence by
triethylamine as the electron donor have been performed. The
role of exciplexes in the quenching mechanism has been
established. For this process a pronounced diastereodifferen-
tiation has been observed. Based on thermodynamic consider-
ations and the presence of typical exciplex emission, we were
able to conclude on the alteration of the electron acceptor
properties of the naphthalene derivatives in the dyads. This effect
is dependent on the electronic interaction and, therefore, on the
configuration-dependent interaction between both chromophores.
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Acknowledgment. The authors thank the Generalitat Va-
lenciana (Grupos 03/082) and the MCYT (Grant BQU 2001-
2725 and doctoral fellowship to S.A.) for financial support. The
Deutsche Forschungsgemeinschaft (DFG) and the Universidad
Polite´cnica de Valencia are gratefully acknowledged for research
fellowships for U.P.
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Supporting Information Available: A table with fluores-
cence quantum yields and lifetimes of dyads and model
compounds in aerated n-hexane, a table with quenching data
of dyads and model compounds by triethylamine in aerated
n-hexane, 1H and 13C NMR spectra of dyads and model
compounds, and AM1 optimized folded conformers of the
dyads. This material is available free of charge via the Internet
at http://pubs.acs.org.
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