Intramolecular excited-state interactions in phenol-styrene bicromophoric systems: A photochemical and photophysical study

Article abstract of DOI:10.1016/S0040-4020(01)01132-2

The intramolecular excited state interaction between phenol and styrene in a series of trans-2-cinnamylphenols bearing electron-donating substituents at the phenolic ring has been directly observed as an exciplex emission in acetonitrile. The photochemica

Full text of DOI:10.1016/S0040-4020(01)01132-2

TETRAHEDRON
Pergamon
Tetrahedron 58 32002) 115±120
Intramolecular excited-state interactions in phenol±styrene
bicromophoric systems: a photochemical and photophysical study
p
M. Consuelo Jimenez, Miguel A. Miranda and Rosa Tormos
Â
   Â
Departamento de Quõmica, Instituto de Tecnologõa Quõmica UPV-CSIC, Universidad Politecnica de Valencia, Camino de Vera s/n,
Apartado 22012, E-46071 Valencia, Spain
Received 5 June 2001; revised 4 October 2001; accepted 13 November 2001
AbstractÐThe intramolecular excited state interaction between phenol and styrene in a series of trans-2-cinnamylphenols bearing electron-
donating substituents at the phenolic ring has been directly observed as an exciplex emission in acetonitrile. The photochemical reactivity
and the relatively low ¯uorescence quantum yields as compared with model compounds are additional evidences for the phenol/styrene
interaction. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
photophysical and photochemical properties. This is
remarkable; as such phenol/ole®n intramolecular exciplexes
were unknown. The mostclosely relaetd examples have
been found by Lewis etal. in bichromophoric amine/styrene
systems,11 where the conformation of chemically reactive
intramolecular exciplexes appears to control the regio-
selectivity of the observed photocyclisation reactions.
Bichromophoric compounds are interesting models for the
study of energy and electron transfer processes. They also
have application for the development of new materials 3i.e.
photoconducting polymers) and for mimicking some
biological processes such as photosynthesis.1,2
trans-2-Cinnamylphenols are bichromophoric compounds
containing phenol and styrene moieties connected
through a methylenic spacer. The introduction of suitable
substituents allows to reproduce at will a wide range of
intramolecular proton, electron or energy transfer
processes.3±10
2. Results and discussion
The structures of the substrates studied are given in Chart 1.
trans-2-Cinnamylphenols 1a±d were prepared as described
in the literature for the parent compound trans-2-cinnamyl-
phenol.12 The corresponding propanes 1aH±dH, required
for comparative photophysical studies, were obtained by
catalytic hydrogenation of 1a±d. Compounds 1a±d,13±16
1aH,bH15 and 1dH17 were known, and their structures
were unambiguously con®rmed by comparison of their
spectroscopic data with those reported in the literature.
In general, photocyclisation of trans-2-cinnamylphenols to
dihydrobenzofurans and dihydrobenzopyrans occurs mainly
via intramolecular excited state proton transfer, through
zwitterionic intermediates 3Fig. 1, path a). However, intro-
duction of electron-donating substituents at the phenolic
nucleus enhances the contribution of excited state intra-
molecular electron transfer, which becomes the predominat-
ing mechanism of photocyclisation 3Fig. 1, path b).
2.1. Photophysical studies
The emission properties of 1a±d are summarised in Table 1,
together with those measured under the same conditions for
their corresponding hydrogenated derivatives 1aH±dH,
where the styrenic chromophore has been suppressed and
the emission takes place exclusively from the phenolic
moiety. Data for b-methylstyrene have also been included
for comparison.18
We have recently communicated the ®rst direct observation
of an intramolecular excited state interaction between
phenol and styrene, detected as an exciplex emission.10
Although such interaction has been described for a single
compound 3trans-2-cinnamyl-4-methoxyphenol 1a), in the
present work we show that other analogues with electron
donating substituents at the phenolic ring exhibit similar
The ¯uorescence spectra of 1a 3Table 1, entry 1) in hexane
displayed a single band with lmaxˆ320 nm, clearly attribut-
able to emission from the lowest lying phenolic singlet
3compare trace B, Fig. 23a) with trace A, Fig. 23b)). The
same result was obtained by exciting either at 250 nm
Keywords: trans-2-cinnamylphenols; bichromophoric compounds;
exciplex; intramolecular interaction; photocyclisation.
Corresponding author. Tel.: 134-96-387-7343; fax: 134-96-387-7349;
e-mail: mmiranda@qim.upv.es
p
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020301)01132-2

Â
M. C. Jimenez et al. / Tetrahedron 58 12002) 115±120
116
Figure 1. Photocyclisation pathways of cinnamylphenol: 3a) via excited state proton transfer 3b) or excited state electron transfer.
Chart 1.
Table 1. Photophysical data of 1a±d and 1aH±dH
Entry
Compound
l
max 3em)a
tF 3ns)b
a
fF
1
2
3
4
5
6
7
8
9
1a
1aH
1b
1bH
1c
1cH
320 3420)
320 3320)
310 3360)
310 3310)
310 3310,360)
310 3310)
310 3310)
310 3310)
308 3307)
0.056 30.051)
0.197 30.342)
0.010 30.012)
0.100 30.327)
0.024 30.057)
0.127 30.364)
0.012 30.012)
0.063 30.327)
0.35
5.1
2.5
0.5
0.7 314%), 3.0 386%)
1.2 367%), 6.8 333%)
1.5 313%), 5.8 387%)
1.7 335%), 5.2 365%)
3.4
1d
1dH
b-methylstyrenec
8.1
The data have been obtained upon excitation at 290 nm.
a
The emission maxima and the ¯uorescence quantum yields have been measured in hexane and acetonitrile 3in parentheses).
l3exc)ˆ280 nm, solvent acetonitrile. The data in hexane are not included, because a number of them were near the time resolution of our instrumentation.
Data taken from Ref. 18.
b
c
3styrene) or at 290 nm 3phenol), due to ef®cient energy
be exergonic according to the Rehm±Weller Eq. 31)19
transfer between the excited singlets of both chromophores.
By contrast, a completely different behaviour was observed
in the more polar solvent acetonitrile 3Table 1, entry 1, data
in parentheses). The most remarkable result was the
presence of a much longer wavelength band centred at
420 nm and the concomitant disappearance of the phenolic
emission at320 nm 3see Fig. 23b)). For compound 1aH
3which does not possess styrenic subunit), the emission
wavelength 3lmaxˆ320 nm) does not depend on the polarity
of the solvent 3Table 1, entry 2).
p
DGꢀkcal mol21† ˆ 23:06ꢀED=D 2 EA=A † 2 E
ꢀ1†
1z
2z
where ED/D1z and EA/A2z are the redox potentials of the donor
and acceptor moieties and Ep is the singlet energy of the
phenolic chromophore. Using the excitation energy
determined from the intersection between normalised
excitation and emission spectra and the reported literature
values for the redox potentials,20 the resulting Gibbs free
energy associated with exciplex formation would be ca.
225 kcal mol21
.
The band observed for 1a in acetonitrile is clearly attribu-
table to a charge-transfer exciplex, whose formation would
The emission spectra were also obtained for solutions of 1a

Â
M. C. Jimenez et al. / Tetrahedron 58 12002) 115±120
117
Figure 2. Fluorescence spectra of: 3a) the isolated b-methylstyrene 3trace A) and 1aH 3trace B); 3b) 1a in hexane 3trace A), in acetonitrile 3trace B) and in a 9:1
dichloromethane±acetonitrile mixture 3trace C).
in solvents of intermediate polarity and in a variety of
solvent mixtures. For instance, using CH2Cl2/CH3CN 39:1,
v:v), both the monomer and the exciplex bands were
observed 3see trace C, Fig. 23b)). As expected for a charge
transfer exciplex, the position of the longer wavelength
band clearly depended on the solvent polarity 3data not
shown).21,22
As a summary of the photophysical studies, the inter-
chromophoric interaction in the excited states of the studied
trans-2-cinnamylphenols is evidenced by: 31) formation of
intramolecular exciplexes in polar solvents and/or 32) the
sharp decrease of the ¯uorescence quantum yield as
compared to model compounds.
2.2. Photochemical studies
Likewise, the shape and the position of the emission band
for 1b 3Table 1, entry 3) changed with the solvent polarity,
being the wavelength shift associated with the exciplex
formation less pronounced 350 nm) than in the case of 1a
3100 nm).
After determining the photophysical properties of 1a±d,
their preparative photochemistry was investigated.
Although the absorption spectra of 1a±d showed their
mostinetnse maximum below 300 nm, irradiaiton using
quartz-®ltered light gave rise to a high degree of polymer-
isation. In order to prevent this problem, a pyrex ®lter was
used for the photolysis experiments. All the samples were
exhaustively deoxygenated before irradiation, and the
solvents used were hexane 3as non-polar solvent), aceto-
nitrile 3as polar solvent) and acetone 3triplet-photosensitised
irradiations).
The ¯uorescence spectrum of 1c 3Table 1, entry 5) in aceto-
nitrile displayed two maxima: the monomer emission at
shorter wavelength 3310 nm), identical to that obtained in
hexane and a second one appearing at360 nm, due ot
exciplex emission.
Although in principle the t-butyl derivative 1d could also
give rise to an exciplex, the spectra recorded for this
compound in polar and non polar solvent3Table 1, enrty
7) were superimposable 3lmax 310 nm). The lack of exciplex
emission is still unclear although it could be due to steric
reasons.
In general, irradiation of 1a±d led to the cis isomers 2a±d
together with the cyclic ethers 3a±d and 4a±d 3Fig. 3).
Compounds 4a±d were known.16,24,25
The results are summarised in Table 2. Irradiation of 1a in
hexane and acetonitrile 3Table 2, entries 1 and 2) resulted in
the almost complete consumption of the starting material,
accompanied by formation of the six-membered ring
compound 4a as the major photoproduct. Minor amounts
of the cis isomer 2a and the ®ve-membered ring product
3a were also obtained in hexane. Acetone-photosensitised
irradiation of 1a 3Table 2, entry 3) led exclusively to the cis
isomer 2a. Similar photolysis results were obtained for
1b±d.
In general, the ¯uorescence quantum yields of 1a±d were
markedly lower than those for the hydrogenated derivatives
1aH±dH and for b-methylstyrene. On the other hand, the
obtained values for 1aH±dH were higher in acetonitrile
than in hexane, as expected for simple phenols.23
The ¯uorescence lifetimes in acetonitrile are also given in
Table 1 3last column). Although the trends are far from
being clearcut, it appears that the exciplexes live longer
when the phenolic nuclei are more strongly activated 3for
instance, tF31a or 1c).tF31b)).
As the predominating formation of dihydrobenzopyrans 4 is
considered to be an indication for the involvement of
Figure 3. Products obtained upon irradiation of 1a±d.

Â
M. C. Jimenez et al. / Tetrahedron 58 12002) 115±120
118
Table 2. Photolysis of trans-2-cinnamylphenols 1a±d
Entry
Substrate
Conditionsa
Conversion
Productdisrtibuiton 3%)
2
3
4
1
2
3
4
5
6
7
8
1a
1a
1a
1b
1b
1b
1c
1c
1c
1d
1d
1d
A
B
C
A
B
C
A
B
C
A
B
C
94
96
63
70
58
42
67
73
4
Traces
100
37
20
100
30
Traces
100
Traces
22
10
Traces
±
18
±
±
5
Traces
±
9
8
±
86
99
±
45
80
±
65
99
±
91
70
±
9
100
100
51
10
11
12
59
100
a
A: hexane, B: acetonitrile, C: acetone.
intramolecular excited state electron transfer 3Fig. 1, path
b), detection of a charge transfer exciplex is compatible with
the preparative photochemistry of the trans-2-cinnamyl-
phenols. This is further supported by the fact that the
yield of 4 increases in the more polar solvent acetonitrile
in the cases where an exciplex is detected. The lower regio-
selectivity towards product 4 in hexane mightbe an
indication for the participation of the proton transfer
mechanism 3Fig. 1, path a) to some extent.
Hewlett±Packard 5988 A spectrometer; the ratio m/z and
the relative intensities 3%) are indicated for the signi®cant
peaks. Isolation and puri®cation were done by conventional
column chromatography on silica gel Merck 60 PF254 using
dichloromethane as eluent and by means of an isocratic
HPLC Waters equipment provided with a semipreparative
Ê
mporasile column 310 mm, 125 A), using hexane 399±
95%)/ethyl acetate 31±5%) as eluent 31.5 ml min21).
Fluorescence spectra were recorded with an Edinburgh
instrument.
Finally, formation of 2a±d upon irradiation in acetone is
consistent with the previously established mechanism,
where trans/cis isomerisation of cinnamylphenols is
accepted to occur from the styrenic triplet.3
4.2. General procedure for the ¯uorescence
measurements
Emission spectra were recorded in hexane or acetonitrile at
258C. The samples 3absorbance between 0.1 and 0.3) were
thoroughly purged with argon prior to ¯uorescence
measurements. Phenol was used as standard for determining
the ¯uorescence quantum yields.23
As a summary of the photochemical studies, the charge
transfer interchromophoric interaction in the excited state
of 1a±d is evidenced by the photocyclisation to six-
membered ring products.
4.3. General irradiation procedure
3. Conclusions
Solutions of 1a±d 35 mM) in 20 ml of the appropriated
solvents were placed into pyrex tubes surrounding a
centrally positioned quartz cooling jacket containing a
125 W medium-pressure Hg lamp and irradiated under Ar
for 1 h. The course of the reaction was followed by GC and
1H NMR; the degrees of conversion, the product distri-
butions and the mass balances were determined using
adequate standards. The reaction mixtures were submitted
to chromatographic separation, in order to obtain the pure
photoproducts.
The intramolecular excited state interaction between phenol
and styrene in trans-2-cinnamylphenols bearing electron-
donating substituents at the phenolic ring has been directly
observed as an exciplex emission. The photochemical
reactivity and the relatively low ¯uorescence quantum
yields as compared with model compounds are additional
evidences for the phenol/styrene interaction.
4. Experimental
4.1. General
4.4. Synthesis of the substrates
Compounds 1a±d were prepared following a previously
described literature method for the parent compound
trans-2-cinnamylphenol.12 The sodium saltobatined from
40.0 mmol of 4-methoxy-, 4-methyl-2,4-dimethyl- or 2-4-
di-t-butylphenol was added to trans-cinnamyl chloride
36.10 g, 40.0 mmol) in 100 ml of benzene. After re¯uxing
for 5 h, the solvent was distilled and the residue was treated
with 100 ml of Claisen's alkali 335.00 g of potassium
hydroxide in 25 ml of water and methanol up to 100 ml).
The alkaline solution was washed with hexane, acidi®ed
UV spectra were recorded in hexane in a Shimadzu
UV-160A; lmax 3nm) and log 1 values 3in parentheses) are
given for each absorption band. IR spectra were obtained
with a FTIR Hewlett±Packard 5965 instrument; nmax
1
3cm21) is given for all the absorption bands. H RMN and
13C NMR spectra were measured in CDCl3 with a 300-MHz
Varian Gemini-300 instrument; chemical shifts are reported
in d 3ppm) values, using TMS as internal standard. Mass
spectra were obtained under electron impact using a

Â
M. C. Jimenez et al. / Tetrahedron 58 12002) 115±120
119
with HCl and extracted with dichloromethane. Evaporation
of the solvent gave a residue, which was submitted to
column chromatography.
3100), 161 312), 145 310), 117 322), 115 328), 91 377), 57
360). Anal. calcd for C23H30O: C, 85.67; H, 9.34; Found: C,
85.60; H, 9.27.
4.5.5. 2-Benzyl-5-methyl-2,3-dihydrobenzofuran .3b).
Oil. FTIR 3071, 3030, 2933, 1856, 1607, 1491, 1365,
For the preparation of 1aH±dH, the cinnamylphenols 1a±d
310 mmol) in ethyl acetate 325 ml) were hydrogenated in the
presence of palladium/charcoal 311%) until consumption of
the adequate volume of hydrogen. The solution was ®ltered
and on evaporation afforded pure 1aH±dH in quantitative
yield.
1
1337, 1296, 1241, 1115, 983, 921, 851, 807, 745, 700; H
NMR 7.34±7.23 3m, 5H, ArH), 6.95 3s, 1H, 4-ArH), 6.90,
6.67 3d1d, Jˆ8 Hz, 2H, 6,7-ArH), 4.98 3m, 1H, CH), 3.15
3m, 2H, CH2), 2.83 3m, 2H, CH2), 2.26 3s, 3H, CH3): 13C
NMR: 157.3 37a-CO), 137.5 310-CCH2), 129.6±125.6
33a-C, 4,6-CH, 5-CCH3, 2030405060-CH), 109.0 37-CH),
83.5 32-CHCH2), 42.0, 35.0 32-CHCH2, 3-CH2) 20.7
3CH3); MS 224 3M1,75), 133 3100), 132 332), 115 38),
105 361), 91 343), 92 314), 77 316). Exactmass calcd for
C16H16O 224.1201, found 224.1201.
4.5. Spectral data of the new compounds
4.5.1. 4,6-Dimethyl-2-.3-phenylpropyl)phenol .1c-H).
Oil. FTIR 3624, 3029, 2924, 2859, 1478, 1450, 1196, 800,
1
695; H NMR 7.32±7.15 3m, 5H, ArH), 6,79 3s, 2H, 3,5-
ArH), 4.39 3s, 1H, OH), 2.62 3m, 4H, CH2CH2CH2), 2.20,
2.22 3s1s, 6H, 4,6-ArCH3), 1.94 3m, 2H, CH2CH2CH2); 13
C
4.5.6. 2-Benzyl-5,7-dimethyl-2,3-dihydrobenzofuran .3c).
Oil. FTIR 3070, 3030, 2932, 1606, 1480, 1365, 1320, 1280,
NMR 149.5 31-COH), 142.3 310-C), 129.3±122.8 33,5-CH,
20,30,40,50,60-CH),35.7 3CH21-C0), 31.3, 29.6 32-CCH2CH3),
20.5, 15.9 34,6-CCH3); MS 240 3M1, 91), 149 316), 148
312), 136 3100), 135 355), 121 322), 105 316), 91 333);
Exactmass calcd for C 17H20O 240.1514, found 240.1516.
1
1210, 1137, 1036, 987, 850, 743, 700; H NMR 7.34±7.21
3m, 5H, ArH), 6.79, 6.73 3s1s, 2H, 4,6-ArH), 4.95 3m, 1H,
CH), 3.17 3m, 2H, CH2), 2.89 3m, 2H, CH2), 2.21 3s, 3H,
CH3), 2.19 3s, 3H, CH3); 13C NMR: 155.6 37a-CO), 137.6
310-CCH2), 130.2±125.8 33a-C, 4,6-CH, 5-CCH3, 2030405060-
CH), 122.9 37-CCH3), 83.1 32-CH), 42.0, 35.0 33-CH2,
2-CHCH2), 20.7 35-CCH3), 15.2 37-CCH3); MS 238
3M1,26), 147 3100), 131 37), 119 336), 117 38), 115 310),
91 337), 77 310). Anal. calcd for C17H18O: C, 85.67; H, 7.61;
Found: C, 85.64; H, 7.76.
4.5.2. cis-2-Cinnamyl-4-methylphenol .2b). Oil. UV 216
33.97), 245 34.08), 280 33.26); FTIR 3651 3OH), 3598 3OH),
3066, 3024, 2931, 1597, 1501, 1438, 1317, 1257, 1180,
1
1101, 913, 806, 759, 700; H NMR 7.76±7.24 3m, 6H,
ArH), 6.95 3s, 1H, 3-ArH), 6.90 3d, Jˆ8 Hz 1H, 5-ArH),
6.67 3d, Jˆ8 Hz, 1H, 6-ArH), 6.63 3d, Jˆ12 Hz 1H,
CH2CHvCH), 5.82 3dt, J1ˆ12 Hz, J2ˆ7 Hz, 1H,
CH2CHvCH), 4.67 3s, 1H, OH), 3.63 3d, Jˆ7 Hz, 2H,
CH2), 2.25 3s, 3H, CH3); 13C NMR: 151.5 31-COH), 137.0
4.5.7. 2-Benzyl-5,7-ditertbutyl-2,3-dihydrobenzofuran
.3d). Oil. FTIR 2963, 2850, 1700, 1474, 1365, 1230; H
1
NMR 7.55±7.30 3m, 5H, ArH), 7.10, 7.05 3s1s, 2H, 4,6-
ArH), 4.98 3m, 1H, CH), 3.17 3m, 2H, CH2), 2.92 3m,
2H, CH2), 1.35 3s, 9H, C3CH3)3), 1.29 3s, 9H, C3CH3)3);
13C NMR: 155.7 37a-CO), 141.2, 139.0 310-CCH2,
5-CC3CH3)3), 130.9 37-CC3CH3)3), 129.5±125.9 33a-C,
2030405060-CH), 121.6 34-CH), 119.4 36-CH), 82.9 32-CH),
42.1, 35.0 32-CHCH2, 3-CH2) 31.9, 31.8, 29.8, 29.4
35-CC3CH3)3, 7-CC3CH3)3; MS 322 3M1,35), 307 3100),
203 354), 161 311), 117 313), 115 315), 91 863). Exact
mass calcd for C23H30O 322.2296, found 322.2296.
310-CCHvCH),
127.0±130.0
33,5-CH,
CHvCH,
20,30,40,50,60-CH), 130.1 34-CCH3), 126.0 32-CCH2), 115.3
36-CH), 29.4 3CH2), 20.5 3CH3); MS 224 3M1, 60), 209 333),
133 385), 120 337), 116 321), 115 345), 105 346), 104 351), 91
3100), 77 330). Exactmass calcd for C 16H16O 224.1201,
found 224.1203.
4.5.3. cis-2-Cinnamyl-4,6-dimethylphenol .2c). Oil. UV
216 34.13), 242 34.13), 280 33.51); FTIR 3651 3OH), 3597
3OH), 3067, 3021, 2930, 1598, 1483, 1283, 1194, 1022, 853,
1
761, 700; H NMR 7.37±7.25 3m, 5H, ArH), 6.82 3s, 2H,
Acknowledgements
3,5-ArH), 6.64 3d, Jˆ12 Hz, 1H, CH2CHvCH), 5.82 3dt,
J1ˆ12 Hz, J2ˆ7 Hz, 1H, CH2CHvCH), 4.56 3s, 1H, OH),
3.62 3d, Jˆ7 Hz, 2H, CH2), 2.23 3s, 1H, CH3), 2.19 3s, 1H,
Financial supportby hte DGICYT 3GrantPB 97-0339) is
gratefully acknowledged.
13
CH3); C NMR: 150.0 31-COH), 136.6 310CCHvCH),
123.6±130.6 32-CCH2, 3,5-CH, 4C±CH3, CHvCH,
20,30,40,50,60-CH), 29.83CH2), 20.4 34-CCH3), 15.8
36-CCH3); MS 2383M1, 75), 223314), 147355), 134380),
119313), 115340), 913100). Anal. calcd for C17H18O: C,
85.67; H, 7.61; Found: C, 85.60; H, 7.56.
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216 34.13), 243 34.16); FTIR 3669 3OH), 3566, 3OH), 3069,
3029, 2966, 1597,1476, 1400, 1368, 1288, 1240, 1206,
1
1118, 967, 876, 736; H NMR 7.38±7.23 3m, 5H, ArH),
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3d, Jˆ12 Hz, 1H, CH2CHvCH), 5.82 3dt, J1ˆ12 Hz,
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Jˆ7 Hz, 2H, CH2), 1.35 3s, 9H, C3CH3)3), 1.27 3s, 9H,
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