Stereoselective fluorescence quenching by photoinduced electron transfer in naphthalene-amine dyads

Article abstract of DOI:10.1039/b301414b

Intramolecular chiral recognition in electron-transfer-induced fluorescence quenching has been observed for diastereomeric dyads composed of a naphthalene chromophore and an amine.

Full text of DOI:10.1039/b301414b

Stereoselective fluorescence quenching by photoinduced electron transfer
in naphthalene-amine dyads
Uwe Pischel,*ab Sergio Abada and Miguel A. Miranda*a
a Instituto de Tecnología Química, Universidad Politécnica de Valencia, UPV-CSIC, Av. de los Naranjos
s/n, E-46022 Valencia, Spain. E-mail: mmiranda@qim.upv.es; Fax: +34 96 387 78 09; Tel: +34 96 387
78 07
b CEQUP/Departamento de Química, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre
687, 4169-007 Porto, Portugal
Received (in Cambridge, UK) 13th February 2003, Accepted 12th March 2003
First published as an Advance Article on the web 2nd April 2003
Intramolecular chiral recognition in electron-transfer-in-
duced fluorescence quenching has been observed for diaster-
eomeric dyads composed of a naphthalene chromophore and
an amine.
oscillator strength nor spectral position. This indicates the
absence of significant ground-state interaction between the
naphthalene chromophore and the amine.
The fluorescence spectra of (S,S)-NPX-PYR, (R,S)-NPX-
PYR, and (S)-naproxen in acetonitrile are characterised by a
typical emission band centred at 350 nm (Figure 1).8 The
excitation spectra present the same bands as the absorption
spectra, which unambiguously confirms the origin of the
emission as coming from the naphthalene residue. Steady-state
fluorescence measurements upon excitation at 280 nm of
optically matched (A = 0.2) aerated acetonitrile solutions of
NPX-PYR diastereomers revealed a fluorescence quenching for
both compounds in comparison to parent naproxen (Figure 2).
The generally accepted quenching mechanism in such systems
is photoinduced electron transfer from the amine to the aromatic
residue. The thermodynamics for this process can be calculated
with the mentioned Rehm-Weller equation using: Eox = +0.96
V vs. SCE for triethylamine,9 Ered = 22.60 V vs. SCE for
2-methoxynaphthalene,10 E* = 3.69 eV for naproxen,8 and C =
20.06 eV for the coulombic term. An exergonic driving force of
The interaction of singlet-excited aromatics with electron-
donating amines provides a classical example of photoinduced
electron transfer reactions.1,2 Several aspects of this photoreac-
tion including the influence of factors like solvent polarity,3
electronic structure of the electron donor,4 and singlet-excited
state energy are connected with the driving force of electron
transfer by the well-known Rehm-Weller equation:
DGet = Eox(D) 2 Ered(A) 2 E* + C.
Less straightforwardly predictable is the influence of geometri-
cal parameters. Irie and co-workers investigated the inter-
molecular quenching of (R)-(2)-1,1A-binaphthyl by chiral
benzylamines.5 In nonpolar solvents a stereoselective photo-
physics was observed. However, in acetonitrile no stereodiffer-
entiation was noted. The authors argued that in polar media
electron transfer becomes dominant, while in nonpolar solvents
exciplex formation is the major quenching pathway. The latter
is more subjected to geometrical influences due to a closer
interaction in form of an orbital overlap between donor and
acceptor. Other related work included binaphthol and its
derivatives, whose fluorescence was quenched by chiral
amines.6 This led to their application as chiral chemosensors.7
In order to investigate chiral discrimination in electron-
transfer induced intramolecular quenching, we have synthe-
sized two dyads from the enantiomers of a naphthalene
derivative, i.e., (S)-(+)- and (R)-(2)-6-methoxy-2-naphthylpro-
pionic acid (naproxen, NPX), and N-methyl-(S)-pyrrolidineme-
thanol (PYR). The synthesis was realised by conversion of (R)-
or (S)-naproxen with thionyl chloride into the activated acyl
chloride and esterification of the latter with the hydroxyl-
functionalised pyrrolidine derivative in dichloromethane. Both
compounds were purified by preparative reverse-phase HPLC
with acetonitrile as eluent. Their identity was confirmed by 1H-
and 13C-NMR spectroscopy.†
Fig. 1 Normalised fluorescence emission (solid line) and excitation (dotted
line) spectra of (S,S)-NPX-PYR in aerated acetonitrile.
The absorption spectra of both diastereomers show two fine-
structured bands with maxima at lmax = 273 nm and 332 nm,
akin to the parent naproxen.8 These absorptions are ascribed to
Fig. 2 Fluorescence emission spectra (lexc = 280 nm) of optically matched
solutions of (a) (S)-naproxen, (b) (S,S)-NPX-PYR, and (c) (R,S)-NPX-PYR
in aerated acetonitrile.
*
*
p,p -type transitions. Noteworthy, the p,p -transitions of the
diastereomers compared to naproxen are not altered, neither in
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DGet = 20.19 eV results. Notably, at the chosen concentration
of ca. 5 3 1025 M the contribution of intermolecular quenching
The reason for sensitivity of the electron transfer rate to the
chiral information in the present system must be sought in the
steric hindrance associated with the necessary approach of
donor and acceptor moiety.13 As the reaction is only moderately
exergonic, such steric effect could have a strong impact on the
actual height of the activation barrier. Finally, in an inter-
molecular control experiment (S)- and (R)-naproxen fluores-
cence was quenched by the acetyl ester of N-methyl-(S)-
pyrrolidinemethanol in acetonitrile. The rate constants are with
is less than 1% [with kq = 5.1 3 109 M21 21
for the
s
naphthalene/triethylamine system in acetonitrile11 and t0
7.44 ns for (S)-naproxen in aerated acetonitrile].
=
Most strikingly, the amount of quenching, i.e., the quantum
yield of electron transfer Fet, for the two diastereomers is
significantly different, which must be related to a ster-
eoselective electron transfer. Here the (S,S)-diastereomer shows
a lower efficiency than the (R,S) combination, i.e, Fet = 0.52
versus 0.66. The same trend is observed for the fluorescence
lifetime of the naphthalene chromophore, which was deter-
mined by time-correlated single-photon-counting measure-
ments (Figure 3). (S)-Naproxen itself has a lifetime of 7.44 ns in
aerated acetonitrile solution, while the lifetimes tf of (S,S)-
NPX-PYR and (R,S)-NPX-PYR are 3.02 ns and 2.35 ns,
respectively. Quantum yields for electron transfer can also be
derived from these data, i.e., 0.59 and 0.68 for (S,S)- and (R,S)-
NPX-PYR, respectively. Hence, the unimolecular rate constant
for electron transfer can be calculated with ket = Fet/tf. The
following values result: ket (S,S) = 1.8 3 108 s21 and ket (R,S)
= 2.8 3 108 s21 indicating a factor of ca. 1.6 for the
stereodifferentiation between both diastereomers.
1.2 and 1.0 3 1010 M21 21
s , respectively, close to diffusion
control. Therefore the enantioselectivity factor is strongly
reduced (cf. reactivity–selectivity principle). However, in this
case the (S)/(S) combination showed a slightly higher reactivity,
opposite to the intramolecular experiment.
The presented results suggest that exciplex formation is not
always a precondition for the observation of stereodifferentia-
tion in charge transfer processes.14
The financial support by MCYT (Grant No. BQU2001-2725)
and the Deutsche Forschungsgemeinschaft (Research Fellow-
ship for U.P.) is gratefully acknowledged.
In order to support the proposed electron transfer mechanism
the experiment was performed in a less polar solvent, i.e., n-
hexane. However, no significant fluorescence quenching was
observed in the case of the dyads and therefore no stereodiffer-
entiation. Obviously, the thermodynamics disfavours electron
transfer to a high extent as a consequence of the destabilisation
of the intramolecularly formed radical ion pair. This serves as
corroboration of photoinduced electron transfer in the present
dyads. However, a closer inspection of the fluorescence spectra
in acetonitrile revealed a very small but significant red-shifted
broad emission band with a maximum at ca. 540–550 nm. It can
be ascribed to an exciplex emission in analogy to the
observations made for 2-naphthylmethylamine.12 The excita-
tion spectrum according to this emission matches the absorption
spectrum of the dyad, which confirms the origin of this
fluorescence. A comparison of the emission intensities of the
monomer (at 350 nm), i.e., the naphthalene chromophore, and
the exciplex (at 550 nm) reveals ratios of Imonomer/Iexciplex = 90
and 40 for (S,S)-NPX-PYR and (R,S)-NPX-PYR, respectively.
This can be identified as minor compared to 2-naphthylmethyla-
mine, where the exciplex emission in acetonitrile is ca. 4 times
more intense than the naphthalene fluorescence.12 Therefore,
formation of exciplexes is a minor pathway compared to full
electron transfer for the investigated systems.
Notes and references
†
(S,S)-NPX-PYR: 1H-NMR (300 MHz, CDCl3) d (ppm) 1.40–1.52 (1H,
m, CH2 PYR), 1.58 (3H, d, J = 7.2 Hz, CH3), 1.60–1.85 (3H, m, CH2 PYR),
2.12–2.22 (1H, m, CH2 PYR), 2.29 (3H, s, CH3N) 2.33–2.43 (1H, m, CH2
PYR), 2.96–3.03 (1H, m, CH PYR), 3.88 (1H, q, J = 7.2 Hz, CH), 3.91 (3H,
s, CH3O), 3.99–4.11 (2H, m, CH2), 7.09–7.16 (2H, m, arom. CH), 7.40 (1H,
dd, J = 8.5 and 1.9 Hz, arom. CH), 7.66–7.71 (3H, m, arom. CH). 13C-NMR
(75 MHz, CDCl3) d (ppm) 18.4 (CH3), 22.8 (CH2 PYR), 28.3 (CH2 PYR),
41.3 (CH3N), 45.4 (CH), 55.3 (CH3O), 57.6 (CH2 PYR), 63.7 (CH PYR),
67.1 (CH2), 105.6, 118.9, 126.0, 126.3, 127.0 (arom. CH), 128.9 (arom. C),
129.3 (arom. CH), 133.7, 135.7, 157.6 (arom. C), 174.7 (CO).
(R,S)-NPX-PYR: 1H-NMR (300 MHz, CDCl3) d (ppm) 1.40–1.53 (1H,
m, CH2 PYR), 1.58 (3H, d, J = 7.2 Hz, CH3), 1.62–1.90 (3H, m, CH2 PYR),
2.07–2.12 (1H, m, CH2 PYR), 2.28 (3H, s, CH3N) 2.33–2.43 (1H, m, CH2
PYR), 2.94–3.02 (1H, m, CH PYR), 3.87 (1H, q, J = 7.2 Hz, CH), 3.91 (3H,
s, CH3O), 4.04–4.08 (2H, m, CH2), 7.09–7.16 (2H, m, arom. CH), 7.40 (1H,
dd, J = 8.5 and 1.9 Hz, arom. CH), 7.65–7.71 (3H, m, arom. CH). 13C-NMR
(75 MHz, CDCl3) d (ppm) 18.5 (CH3), 22.8 (CH2 PYR), 28.5 (CH2 PYR),
41.5 (CH3N), 45.5 (CH), 55.3 (CH3O), 57.6 (CH2 PYR), 63.8 (CH PYR),
67.5 (CH2), 105.6, 118.9, 126.0, 126.4, 127.1(arom. CH), 128.9 (arom. C),
129.3 (arom. CH), 133.7, 135.7, 157.6 (arom. C), 174.6 (CO).
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Fig. 3 Fluorescence decay traces (lexc = 280 nm, lobs = 350 nm) of (a) (S)-
naproxen, (b) (S,S)-NPX-PYR, and (c) (R,S)-NPX-PYR in aerated acet-
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