Excited state interactions in phenol/olefin bichromophoric compounds: Direct detection of an intramolecular exciplex

Article abstract of DOI:10.1039/b005026l

The emission from an intramolecular exciplex detected in trans-4-methoxy-2-(3-phenyl-2-propenyl)phenol (1) is the first direct evidence for the excited state interchromophoric interaction in phenol-styrene systems.

Full text of DOI:10.1039/b005026l

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Excited state interactions in phenol/olefin bichromophoric compounds: direct
detection of an intramolecular exciplex
Francisco Galindo, M. Consuelo Jiménez, Miguel A. Miranda* and Rosa Tormos
Departamento de Química-Instituto de Tecnología Química UPV-CSIC, Universidad Politécnica de Valencia,
Camino de Vera s/n, Apdo 22012, 46071 Valencia, Spain. E-mail: mmiranda@qim.upv.es
Received (in Liverpool, UK) 22nd June 2000, Accepted 2nd August 2000
First published as an Advance Article on the web 29th August 2000
The emission from an intramolecular exciplex detected in
trans-4-methoxy-2-(3-phenyl-2-propenyl)phenol (1) is the
first direct evidence for the excited state interchromophoric
interaction in phenol–styrene systems.
(Fig. 1c). By contrast, a completely different behaviour was
observed in the more polar solvent acetonitrile. The most
remarkable result was the presence of a much longer wave-
length band centered at 420 nm and the concomitant dis-
appearance of the phenolic emission at 320 nm (see Fig. 1b).
This new band is clearly attributable to a charge-transfer
Bichromophoric compounds have attracted considerable inter-
est as models for the understanding of energy or electron
transfer processes. They have also found application for
mimicking some biological processes (i.e. photosynthesis) and
for the development of new materials such as photoconducting
polymers.1
trans-2-Cinnamylphenols are bichromophoric compounds
containing phenol and styrene subunits connected by a
methylene spacer. Upon the introduction of suitable sub-
stituents, these simple systems allow the reproduction at will of
a wide range of intramolecular proton, electron or energy
transfer processes.2
Previously, the photochemical reactivity of trans-2-cinna-
mylphenols and their relatively low fluorescence quantum
yields, as compared with models containing the isolated
chromophores, has been attributed to an intramolecular inter-
action in the excited state between the two chromophores.3
However, no direct evidence has been provided in support of
such interaction. In the present work, direct detection of an
intramolecular exciplex has been achieved in the case of trans-
4-methoxy-2-(3-phenyl-2-propenyl)phenol (1), by measuring
the emission spectra in acetonitrile. To our knowledge, this is
the first case of an intramolecular exciplex involving a phenol
chromophore.
The emission properties of 1 are summarized in Table 1,
together with those measured under the same conditions for the
isolated 4-methoxyphenol chromophore. Thus, the fluorescence
spectra of 1 in hexane displayed a single band with lmax
=
320 nm, clearly attributable to emission from the lowest lying
phenolic singlet (compare trace B, Fig. 1a with trace A, Fig. 1b).
The same result was obtained by exciting either at 250 nm
(styrene) or at 290 nm (phenol), clearly due to efficient energy
transfer between the excited singlets of both chromophores
Table 1 Photophysical data of 1 and the reference compound 4-methoxy-
phenol in hexane and acetonitrile at room temperaturea
Compound
Solvent
lemission/nm fF
tF(ns)
1
Hexane
Acetonitrile
Hexane
320
420
320
320
0.056
1.0
5.1
1.9
2.1
0.051
0.143
0.141
Fig. 1 Fluorescence spectra of: (a) the isolated b-methylstyrene (trace A)
and 4-methoxyphenol (trace B); (b) 1 in hexane (trace A), in acetonitrile
(trace B) and in a 9+1 dichloromethane–acetonitrile mixture (trace C); (c) 1
in acetonitrile; excitation spectrum (trace A), emission upon excitation at
250 nm (trace B) and emission upon excitation at 290 nm (trace C).
4-Methoxyphenol
Acetonitrile
a lexcitation = 290 nm.
DOI: 10.1039/b005026l
Chem. Commun., 2000, 1747–1748
This journal is © The Royal Society of Chemistry 2000
1747

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exciplex, whose formation would be exergonic according to the
Rehm–Weller eqn (1)4
DGf (kcal mol21) = 23.06 (ED/D 2 EA/A ) 2 E* (1)
+·
2·
+·
-·
where ED/D and EA/A are the redox potentials of the donor and
acceptor moieties, and E* is the singlet energy of the phenolic
chromophore. Using the excitation energy determined from the
intersection between normalized excitation and emission spec-
tra, together with the reported literature values for the redox
potentials,5 the resulting Gibbs free energy associated with
exciplex formation would be ca. 225 kcal mol21. The emission
spectra were also obtained for solutions of 1 in solvents of
intermediate polarity and in a variety of solvent mixtures. For
instance, using CH2Cl2–CH3CN (9+1, v+v), both the monomer
and the exciplex bands were observed (see trace C, Fig. 1b). As
expected for a charge-transfer exciplex, the position of the
longer wavelength band showed a clear dependence on the
solvent polarity (see Fig. 2 for the Lippert–Mataga plot with the
data obtained in pure solvents).6
Scheme 1
oxyphenol and b-methylstyrene in acetonitrile (5 3 1025
M
each) were studied. The emission spectra consisted of the bands
assignable to the isolated chromophores (either phenolic
maximum at 320 nm, upon excitation at 290 nm, or styrenic
maximum at 310 nm when exciting at 250 nm). However, using
much higher concentrations of the partners (0.1 M), the exciplex
emission at ca. 440 nm was clearly observable.
After determining the photophysical properties of 1, its
preparative photochemistry was also investigated. Irradiation of
1 for 1 h in acetonitrile and benzene, with the Pyrex-filtered
light of a medium pressure Hg lamp resulted in the almost
complete ( > 95%) consumption of the starting material,
accompanied by formation of the six-membered ring compound
2 as the major photoproduct. Minor amounts of the cis-isomer 4
and the five-membered ring product 3 were also obtained in
benzene. These results are summarized in Scheme 1.
As the formation of six-membered ring product 2 is
considered to be an indication for the involvement of intra-
molecular excited state electron transfer,2 detection of a charge-
transfer exciplex is compatible with the preparative photo-
chemistry of 1.
In summary, the intramolecular excited state interaction
between phenol and styrene has been directly observed for the
first time as an exciplex emission. This strongly supports the
previous mechanistic proposals to explain the photochemistry
of bichromophoric cinnamylphenols. Although such interaction
is reported here for a single compound (1), preliminary data in
hand show that other analogues with electron donating
substituents at the phenolic ring [such as 4-methyl, 4,6-di-
methyl- and 4,6-di-tert-butyl-2-(3-phenyl-2-propenyl)phenol]
exhibit similar photophysical and photochemical properties.
Financial support by the DGICYT (PB 97-0339) is gratefully
acknowledged. M. C. J. thanks the European Commision for a
grant (MCFI-1999-00101). F. G. thanks Ministerio de Educa-
ción y Cultura for a grant.
Fig. 2 Solvent polarity dependence of the exciplex emission maxima for 1.
Solvent parameter Df has been calculated as follows: Df = (e 2 1)/(2e + 1)
2 (n2 2 1)/(4n2 + 2) where e is the dielectric constant and n is the refraction
index of the solvent.
The quantum yield of emission at 420 nm was 0.05, and the
lifetime was relatively long (5.1 ns), as compared with the
monomer under the same conditions (1.0 ns). The exciplex
was quenched by oxygen (kq = 4.2 3 1010 M21 s21) and
by tetrabutylammonium hydrogensulfate (kq
=
3.3 3
109 M21 s21). The Stern–Volmer plot for quenching by the
ammonium salt is shown in Fig. 3. Similar salt effects have been
observed for other exciplexes; they have been attributed to
dissociation into radical ions, with the concomitant decrease of
fluorescence.7
Notes and references
1 S. Speiser, Chem. Rev., 1996, 96, 1953; V. Balzani and F. Scandola,
Supramolecular Photochemistry, Ellis Horwood Limited, Chichester,
1991; D. Gravel, S. Gauthier, F. Brisse, S. Raymond, M. D’Amboise, P.
Messier, B. Zelent and G. Durocher, Can. J. Chem., 1990, 68, 908; E.
Cotsaris, J. W. Verhoeven and N. S. Hush, J. Am. Chem. Soc., 1987, 109,
3258.
2 M. C. Jiménez, F. Márquez, M. A. Miranda and R. Tormos, J. Org.
Chem., 1994, 59, 197; M. C. Jiménez, P. Leal, M. A. Miranda and R.
Tormos, J. Org. Chem., 1995, 60, 3243; M. C. Jiménez, M. A. Miranda
and R. Tormos, Tetrahedron, 1997, 53, 14 729; M. C. Jiménez, M. A.
Miranda and R. Tormos, J. Org. Chem., 1998, 63, 1323; M. C. Jiménez,
P. Leal, M. A. Miranda, J. C. Scaiano and R. Tormos, Tetrahedron, 1998,
54, 4337.
3 M. T. Bosch-Montalvá, L. R. Domingo, M. C. Jiménez, M. A. Miranda
and R. Tormos, J. Chem. Soc., Perkin Trans. 2, 1998, 2175.
4 A. Weller, Z. Phys. Chem., 1982, 133, 93.
5 Eox for 4-methoxyphenol is +0.406 V vs. SCE and Ered of b-
methylstyrene is 22.54 V vs SCE as reported in Technique of
Electroorganic Synthesis, ed. N. L. Weinberg, part 2, John Wiley & Sons,
New York, 1975.
Fig. 3 Quenching of the exciplex fluorescence for 1 by [Bu4NHSO4] at
different molar concentrations: curve A) 0.0 M; B) 0.02 M, C) 0.03 M, D)
0.04 M, E) 0.05 M, F) 0.06 M, G) 0.07 M.
6 N. Mataga, Adv. Chem. Phys., 1999, 107, 431; The Exciplex, ed. M.
Gordon and W. R. Ware, Academic Press, New York, 1975.
7 B.-W. Zhang, Y. Cao, J.-W. Bai and J.-R. Chen, J. Photochem.
Photobiol., A: Chem., 1997, 106, 169.
All the above data were obtained using 5 3 1025 M solutions
of 1. To check whether the corresponding intermolecular
exciplex is also observable, equimolar mixtures of 4-meth-
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Chem. Commun., 2000, 1747–1748