Photodimers of a soluble tetracene derivative. Excimer fluorescence from the head-to-head isomer

Article abstract of DOI:10.1021/ol049695p

Equation presented. Irradiation of 5,12-didecyloxytetracene (1) leads to photodimers P₁ (planosymmetric) and P₂ (centrosymmetric). P₁ is characterized by naphthalene excimer fluorescence, whereas P₂ emits naphthalene monomer fluorescence.

Full text of DOI:10.1021/ol049695p

ORGANIC
LETTERS
2004
Vol. 6, No. 12
1899-1902
Photodimers of a Soluble Tetracene
Derivative. Excimer Fluorescence from
the Head-to-Head Isomer
,†
Jens Reichwagen,† Henning Hopf,* Andre´ Del Guerzo,‡
,‡
Jean-Pierre Desvergne,* and Henri Bouas-Laurent‡
Institut fu¨r Organische Chemie, TU-Braunschweig, Postfach 3329,
D-38023 Braunschweig, Germany, and Laboratoire de Chimie Organique et
Organome´tallique, CNRS UMR 5802, UniVersite´ Bordeaux 1,
F-33405 Talence, France
h.hopf@tu-bs.de; jp.desVergne@lcoo.u-bordeaux1.fr
Received February 20, 2004
ABSTRACT
Irradiation of 5,12-didecyloxytetracene (1) leads to photodimers P (planosymmetric) and P (centrosymmetric). P is characterized by naphthalene
1
2
1
excimer fluorescence, whereas P emits naphthalene monomer fluorescence.
2
Polycyclic aromatic hydrocarbons, especially tetracene and
pentacene, are of current interest for their potential applica-
tions in organic electronic devices, due to their optoelectronic
properties.1 They are also known for their photoreactivity.2
Tetracene was indeed the first photochromic organic com-
pound described in the literature.3 These acenes display a
very low solubility in organic solvents, and the majority of
studies were hence performed in the solid state.4 This low
solubility strongly limits their applications (inter alia intro-
duction into matrices, organization on surfaces) and hampers
photochemical studies in solution, which are hence less
documented than those of anthracene and its derivatives.5
Tetracene was found to photodimerize at a lower rate than
anthracene.2d By irradiation of a suspension in benzene (1
× 10-3 M), it was reported to form two photodimers: head-
to-head (hh), or planosymmetric, and head-to-tail (ht), or
centrosymmetric.2c To our knowledge, no photodimer of
† Institut fu¨r Organische Chemie.
‡ Universite´ Bordeaux 1.
(1) (a) Odon, S. A.; Parkin, S. R.; Anthony, J. E. Org. Lett. 2003, 5,
4245-4248. (b) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002,
4, 15-18. (c) Brinkmann, M.; Graff, S.; Straupe´, C.; Wittmann, J.-C.;
Chaumont, C.; Nuesch, F.; Aziz, A.; Schaer, M.; Zuppiroli, L. J. Phys.
Chem. B 2003, 107, 10531-10539. (d) Nichols, J. A.; Gundlach, D. J.;
Jackson, T. N. Appl. Phys. Lett. 2003, 83, 2366-2368. (e) Kitamura, M.;
Imada, T.; Arakawa, Y. Appl. Phys. Lett. 2003, 83, 3410-3412.
(2) (a) Scho¨nberg, A. PreparatiVe Organic Photochemistry; Springer-
Verlag: Berlin, 1968; p 99. (b) Wei, K. S.; Livingston, R. Photochem.
Photobiol. 1967, 6, 229-232. (c) Lapouyade, R.; Nourmamode, A.; Bouas-
Laurent, H. Tetrahedron 1980, 36, 2311-2316 and references therein. (d)
Castellan, A.; Lapouyade, R.; Bouas-Laurent, H. Bull. Soc. Chim. Fr. 1976,
201-209. (e) Bouas-Laurent, H.; Desvergne, J.-P. In Photochromism,
Molecules and Systems, revised ed.; Du¨rr, H., Bouas-Laurent, H., Eds.;
Elsevier: 2003; Chaper 14. (f) Bjarneron, D. W.; Petersen, N. O. J.
Photochem. Photobiol. A: Chem. 1992, 63, 327-335. (g) Dabestani, R.;
Nelson, M.; Sigman, M. E. Photochem. Photobiol. 1996, 64, 80-86. (h)
Yamamoto, S.; Grellman, K. H. Chem. Phys. Lett. 1982, 92, 533-540.
(3) Bouas-Laurent, H.; Du¨rr, H. Pure Appl. Chem. 2001, 73, 639-665.
(4) (a) Innone, M.; Scott, G. W. Chem. Phys. Lett. 1990, 171, 569-574.
(b) Katul, J. A.; Zahlan, A.-B. J. Chem. Phys. 1967, 47, 1012-1014.
(c) Mu¨ller, H.; Ba¨ssler, H.; Vaubel, G. Chem. Phys. Lett., 1974, 29, 102-
104.
(5) (a) Bouas-Laurent, H.; Castellan, A.; Desvergne, J.-P.; Lapouyade,
R. Chem. Soc. ReV. 2000, 29, 43-55. (b) Bouas-Laurent, H.; Castellan,
A.; Desvergne, J.-P.; Lapouyade, R. Chem. Soc. ReV. 2001, 30, 248-263.
10.1021/ol049695p CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/12/2004

substituted tetracenes has been described. In analogy with
anthracenes,6 it was anticipated that substitution with long-
chain alkoxy groups would substantially increase the solubil-
ity in common organic solvents and make the photochemical
studies easier.
The preparative irradiation was carried out on 500 mg of
1 (5 × 10-2 M) in argon-purged cyclohexane with a 1000
W halogen lamp in a glass (Duran: transmittance 5% at 285
nm, 10% at 290 nm) vessel for 5 days at ambient temper-
ature. Two photoproducts, P1 and P2 (Scheme 2, ratio 1:1,
Here, we describe the synthesis and the photodimerization
of 5,12-di-n-decyloxytetracene (1) as well as the determi-
nation of the structure of photodimers P1 and P2.
The synthesis of compound 1 is outlined in Scheme 1.
Although 5,12-tetracenequinone 4 is a commercial product,
Scheme 2
Scheme 1a
104 mg each, 0.092 mmol, 20%), were isolated as white
powders by silica gel column chromatography. P1, eluting
first, was also found to be more soluble in cyclohexane than
1
P2. The H NMR spectra (CDCl3) revealed the presence of
signals characteristic of four bridgehead protons (6,11- and
5,6c
6′,11′-positions) at δ 5.35 (P1) and 5.32 (P2).
Attempts to obtain single crystals suitable for X-ray
structure determination did not meet with success. Com-
pelling evidence of the hh structure of P1 (and hence ht
structure of P2) came from fluorescence spectroscopy. The
absorption (1, P1, P2) and fluorescence (P1, P2) spectra are
represented in Figures 1 and 2, respectively. P1 and P2 exhibit
a Conditions: (a) Zn, HOAc, ultrasound; (b) Na2CO3, CF3CH2OH;
(c) Zn, Me3SiCl, THF, ultrasound; (d) K2CO3, DMF, CnH2n+1Br;
compound 5 was not isolated. Overall yield 38%.
the preparation described here is straightforward and allows
scaling up to multigram quantities. The two-step formation
of 5,12-dialkoxytetracenes follows a route that has been
employed successfully for anthracene derivatives.7 The use
of Zn powder as a reductant and the ultrasound technique
were found to be more convenient than the published
procedures.7 The yields are limited by the back-reaction of
5 to 4 during the alkylation step.
(6) (a) Fages, F.; Desvergne, J.-P.; Bouas-Laurent, H. Bull. Soc. Chim.
Fr. 1985, 959-964. (b) Fages, F.; Desvergne, J.-P.; Frisch, I.; Bouas-
Laurent, H. J. Chem. Soc., Chem. Commun. 1988, 1413-1415.(c) Brotin,
T. The`se, Universite´ Bordeaux 1, Talence, France, 1990. The 1H NMR
chemical shift of the bridgehead protons of 1,4-didecyloxyanthracene
photodimer was found to be δ (CDCl3) 5.0 ppm.
(7) (a) Bouas-Laurent, H.; Lapouyade, R.; Brigand, C.; Desvergne, J.-
P. Comptes Rendus Acad. Sci., 1970, 270 C, 2167-2170. (b) Marquis, D.;
Desvergne, J.-P.; Bouas-Laurent, H., J. Org. Chem. 1995, 60, 7984-7996.
(c) Lu, L.; Chen, Q.; Zhu, X.; Chen, C. Synthesis 2003, 2464-2466.
Figure 1. UV-vis absorption coefficients (ε) of (s) 1, ([) P1,
and (]) P2 in CH2Cl2. Inset: 1 from 360 to 540 nm. Concentrations
of 1, 1.6 × 10-5 M; P1, 1.9 × 10-5 M; P2, 1.9 × 10-5 M at ambient
temperature.
1900
Org. Lett., Vol. 6, No. 12, 2004

to compete at ca. 10-5M; (ii) the excimer/monomer ratio was
found to be the same at 10-6 and 10-5 M, respectively; (iii)
intermolecular excimer emission should have been observed
for P2 as well under the same conditions.
Although excimer emission is evidence of intermolecular
interactions of aromatic hydrocarbons in polymers or ag-
gregates,9,10 the present example is an interesting case of an
intramolecular emission used as a tool to distinguish between
two regioisomers.
The reaction quantum yield, measured at 439 nm in freeze
and thaw degassed cyclohexane, using the Parker actinom-
eter, was found to be equal to 6.5 × 10-3 (average of two
measurements) at a concentration of 8.3 × 10-3 M.
The regioselectivity of photodimerization differs from that
observed for the crossed photodimerization between 9,10-
dialkoxyanthracenes and anthracene; the only photoproducts
obtained (Scheme 3) were the 9,10:9′,10′-photodimers (7),
Figure 2. Fluorescence spectra of (s) P1 and (‚‚‚) P2 in hexane.
Concentration: 1.5 × 10-5 M at ambient temperature; λexc, 310
nm (the same spectra are obtained with λexc, 240 nm).
Scheme 3a
the normal monomer fluorescence of a substituted naphtha-
lene derivative (emitting between 340 and 425 nm); ad-
ditionally, and in contrast to P2, P1 presents a nonstructured
and red-shifted band (λmax ) 424 nm, νmax ) 23 585 cm-1).
The shift from the onset emission (29 585 cm-1) is ∆ν ≈
6000 cm-1, in keeping with known data in the naphthalene
series.8
That this emission is typical of an excimer is borne out
by the kinetic analysis using the single-photon counting
technique. The fluorescence decay of P1 (at ambient tem-
perature) was fitted with the following equations:
at λ ) 352 nm,
i(t) 0.50 exp(-t/1.3 ns) + 0.06 exp(-t/6.3 ns)
at λ ) 424 nm,
i(t) -0.18 exp(-t/1.6 ns) + 0.45 exp(-t/6.6 ns)
a Crossed photodimerization between 9,10-dialkoxyanthracene
(6) and anthracene (A). ΦR (6b + A) at 10-3 M ) 20 × 10-3 in
methylcyclohexane;6a some dianthracene (A2) is also formed (not
represented).
The similarity of the short decay time of the high energy
emission (352 nm) and of the rise time of the lower energy
emission (424 nm) is a typical signature for excimer states
populated by S1 excited states.8 The excimer nature of the
nonstructured red-shifted emission is corroborated by the
excitation spectra, which were found to match the absorption
spectrum regardless of the emission wavelength of observa-
tion (340 or 424 nm; see inset of Figure 2). The fluorescence
decay of P2 was found to be single-exponential at 352 nm
with τ ) 1.8 ns. The unusually short lifetimes for naphthalene
derivatives fluorescence are presumably related to the
dissociation of the photodimers in their excited states.2h
Intermolecular excimer formation must be discarded for the
following reasons: (i) the naphthalene monomer singlet-state
lifetime is too short (<2 ns) to allow a bimolecular reaction
which are thermally stable, and some dianthracene (A2).6a,11
The 9,10:1′,4′-cycloadduct (8), presumably also thermally
stable in comparison with other mixed compounds of the
same type,6b was not observed. The main reason is probably
the loss of stabilization energy12 that is higher to form 8 than
(9) (a) Valeur, B. Molecular Fluorescence, Principles and Applications;
Wiley-VCH: Weinheim, 2002. (b) Lahmani, F.; Zehnacker, A.; Desvergne,
J.-P.; Bouas-Laurent, H.; Colomes, M.; Kru¨ger, A. J. Photochem. Photobiol.
A: Chem. 1998, 113, 203-212.
(10) Jover, A.; Meijide, F.; Rodriguez Nunez, E.; Vasquez Tato, J.;
Mosquera, M.; Rodriguez Prieto, F. Langmuir 1996, 12, 1789-1793.
(11) Bouas-Laurent, H.; Lapouyade, R. Comptes Rendus Acad. Sci. 1967,
264 C, 1061-1064.
(12) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry Part
A; Plenum Press: New York, 1984; Chapter 9.
(8) (a) Birks, J. B. Photophysics of Aromatic Compounds; Wiley
Interscience: 1970.(b) Todesco R.; Gelan, J.; Martens, H.; Put, J.; De
Schryver, F. C. J. Am. Chem. Soc. 1981, 103, 7304-7312.
Org. Lett., Vol. 6, No. 12, 2004
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to obtain 7. This factor is absent in the case of the tetracene
derivative in which the reactivity of the 6,11-positions is
apparently higher than that of the 5,12-positions.
Similarly to Livingston,2b we observed that P1 (mp 70 °C)
and P2 (mp 125 °C) dissociate back to 1 in solution
(dissociation after 15 min of P1 ) 10% at 308 K and 13%
at 342 K; P2 ) 19% at 308 K and 36% at 342 K), but the
back-reaction has not been studied in depth so far. It is
surprising that the thermal stability of P2 is lower than
that of P1; this point deserves further investigation. In the
solid state, the two photodimers are stable for months, and
they dissociate to a small extent at 150 °C in a Schlenck
tube.
The results obtained in this work for tetracene will be
extended to pentacene. This would pave the way to the
introduction of tetracene and pentacene derivatives as
components in light-modulated systems.
Acknowledgment. The CNRS, the French Ministry of
Research, Re´gion Aquitaine, and the Fonds der Chemischen
Industrie are thanked for financial support.
Supporting Information Available: 1H and 13C NMR
data of 1, P1, and P2 and HRMS of 1. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL049695P
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Org. Lett., Vol. 6, No. 12, 2004