Photophysics and Photochemistry of Naphthoxazinone Derivatives Santi Nonell,† Lourdes R. Ferreras,† Alvaro Can˜ete,‡ Else Lemp,‡ German Günther,‡ Nancy Pizarro,‡ and Antonio L. Zanocco*,‡ Grup d’Enginyeria Molecular, Institut Quı´mic de Sarria`, UniVersitat Ramon Llull, Via Augusta 390, E-08017, Barcelona, Espan˜a, and Facultad de Ciencias Quı´micas y Farmace´uticas, Departamento de Quı´mica Orga´nica y Fisicoquı´mica, UniVersidad de Chile, Casilla 233, Santiago - 1, Santiago, Chile
[email protected] ReceiVed January 17, 2008
The photophysics and photochemistry of a series of naphthoxazinones have been studied using a combination of methods ranging from steady-state and time-resolved spectroscopic techniques to product analysis. The photophysics of naphthoxazinone derivatives is very dependent on the structure: phenanthrene-like compounds exhibit higher fluorescence quantum yield than the less aromatic anthracenelike homologous. The latter, exhibit a substantial degree of charge transfer in the excited singlet state. These compounds are fairly photostable in the absence of additives, yielding a single photoproduct arising from the triplet state. The presence of electron donors such as amines increases the photoconsumption quantum yield and changes the product distribution, the primary photoproduct being a dihydronaphthoxazinone that photoreacts further yielding ultimately an oxazoline derivative.
Introduction Benzoxazinone derivatives are a class of compounds exhibiting spectral and photophysical properties of great interest such as broad first absorption band, emission in the red, intense fluorescence in both organic solutions and crystalline state, large dipole moment increase in the excited state, large Stokes shifts, and short fluorescence lifetimes.1–4 Regarding these properties, it has been suggested that this type of compounds can be employed as quantum counters, wavelength shifters, fluorescent solar concentrators, fluorescent probes for biological systems, and laser dyes.5–13 In spite of this interest, few studies addressing the photochemistry of aryloxazinones have been carried out. Light induced reactions of 1,4-benzoxazin-2-ones with electrondeficient olefins have been described by Nishio et al.14,15 Irradiation of a mixture of 3-methyl-1,4-benzoxazin-2-one and an excess of methacrylonitrile under nitrogen atmosphere gave two stereoisomeric azetidine derivatives. In the presence of an excess methyl methacrylate also two stereoisomeric photocy-
* To whom correspondence should be addressed. Phone: 56-2-6782877. Fax: 56-2-6782868. † Universitat Ramon Llull. ‡ Universidad de Chile.
10.1021/jo800039r CCC: $40.75 2008 American Chemical Society Published on Web 06/14/2008
cloadducts were formed (Scheme 1). Formation of the photocycloadducts was completely quenched by oxygen and transstilbene suggesting a reaction involving the triplet excited state. No cycloadduct formation was observed when the methyl substituent of the benzoxazinone was replaced with a phenyl (1) Le Bris, M. T. J. Heterocyclic Chem. 1984, 21, 551–555. (2) Le Bris, M. T. J. Heterocyclic Chem. 1985, 22, 1275–1280. (3) Le Bris, M. T.; Mugnier, J.; Boursons, J.; Valeur, B. Chem. Phys. Lett. 1985, 1, 124–127. (4) Le Bris, M. T. J. Heterocyclic Chem. 1989, 26, 429–433. (5) Dupuy, F.; Rullie`re, C.; Le Bris, M. T.; Valeur, B. Opt. Commun. 1984, 51, 36–40. (6) Mugnier, J.; Dordet, Y.; Pouget, J.; Le Bris, M. T.; Valeur, B. Sol. Energy Mater. 1987, 15, 65–75. (7) Monsigny, M.; Midoux, P.; Le Bris, M. T.; Roche, A. C.; Valeur, B. Biol. Cell 1989, 67, 193–200. (8) Monsigny, M.; Midoux, P.; Depierreux, C.; Bebear, C.; Le Bris, M. T.; Valeur, B. Biol. Cell 1990, 70, 101–105. (9) Depierreux, C.; Le Bris, M. T.; Michel, M. F.; Valeur, B.; Monsigny, M.; Delmotte, F. FEMS Microbiol. Lett. 1990, 67, 237–243. (10) Fery-Forgues, S.; Le Bris, M. T.; Guette, J. P.; Valeur, B. J. Phys. Chem. 1988, 92, 6233–6237. (11) Fery-Forgues, S.; Le Bris, M. T.; Guette, J. P.; Valeur, B. J. Chem. Soc., Chem. Commun. 1988, 5, 384–385. (12) Khochkina, O. I.; Sokolova, I. V.; Loboda, L. I. Russ. Phys. J. 1988, 31, 510–513. (13) Fery-Forgues, S.; Le Bris, M. T.; Mialocq, J.-C.; Pouget, J.; Rettig, W.; Valeur, B. J. Phys. Chem. 1992, 96, 701–710. (14) Nishio, T.; Omote, Y. J. Org. Chem. 1985, 50, 1370–1373. (15) Nishio, T. J. Chem. Soc., Perkin Trans. I 1990, 565–570.
J. Org. Chem. 2008, 73, 5371–5378 5371
Nonell et al. SCHEME 1
TABLE 1.
Solvent Dependence of the Absorption and Fluorescence Transitions Abs (ε)/λEm a λmax max
hexane benzene acetonitrile methanol a
FIGURE 1. Chemical structures of the studied naphthoxazinones.
group and when the photolysis was conducted in the presence of amines such as triethylamine. In the last case the main photoreaction products are reductive dimers in moderate yields (Scheme 1). A reaction mechanism involving the formation of an exciplex between the oxazinone triplet and the olefin was proposed, whereby the exciplex evolves to a 1,4-birradical intermediate which ultimately leads to the observed products.14 In the presence of amines, electron transfer from the amine to the benzoxazinone triplet and subsequent hydrogen abstraction leading to the products has been suggested as the reaction mechanism. Building on the fact that the benzoxazinone excited states have substantial charge-transfer character, we hypothesized that substitution of the benzo- with a naphtho-group could substantially affect the photophysical and photochemical properties of the aryloxazinones. In this work, we report on the synthesis and photochemistry of three naphthoxazinone derivatives (Figure 1). These compounds all contain a phenyl group at the carbonyl’s R position, which should prevent the formation of cyclic photoproducts, and differ in the position where the oxazinone ring is fused to the naphtho group and in their relative orientation, resulting in significant changes in their photophysical properties. The properties evaluated for these compounds suggest that they are valuable candidates for technological applications, such as dyes, quantum counters, or fluorescent probes.
1
2
3
392 (19140)/453 398 (19020)/467 392 (18820)/475 394(18650)/476
350 (23290)/500, 483 356 (23150)/514 348 (24140)/538 346 (23630)/548
398 (12110)/459 396 (12370)/478 396 (11270)/490 396 (11045)/511
Abs , λEm in nm; ε in M-1 cm-1 λmax max
the Franck-Condon transitions) overestimate λmax by about of 20 nm, however analysis of molecular orbital indicates a π-π* transition in all cases. The results are comparable to those reported for benzoxazinone derivatives2 and structurally related compounds such as coumarins.16 Values of λAbs max and molar absorption coefficient are collected in Table 1. The polarity of the solvent has a large effect on the fluorescence spectrum of compounds 1 - 3. The red shift is quite large in polar solvents: for compound 2, the position of the emission maximum (λEm max) shifts from 500 nm in n-hexane to 548 nm in methanol, while for compound 3 λEm max shifts from 459 nm in n-heptane to 506 nm in ethylene glycol. Representative values of λEm max in selected solvents are collected in Table 1. These shifts can be analyzed in terms of various solvatochromic scales. We have chosen the LSER equation (eq 1) introduced by Kamlet et al.:17,18
P ) P0 + aπ * + bR + cβ + dFH2
(1)
where P is an energy-dependent property, π* accounts for dipolarities and polarizabilities of solvent, R is related to the hydrogen bond donor solvent ability, β indicates the solvent capacity as hydrogen bond acceptor, and FH2 is the square of Hildebrand parameter, which accounts for the solvent cohesive energy density and models the cavity effects.17,19,20 Compared to other empirical scales such as ET(30),21 equation 1 was established studying the behavior of many different chromophores and therefore offers the advantage of its wider applicability. Also, it enables the separation of specific hydrogenbonding effects. The studied naphthoxazinone derivatives are not hydrogenbond donors but only hydrogen-bond acceptors mainly on the carbonyl and the imino groups; hence, the dependence of λEm max on solvent is expected to involve only two parameters, the dipolarity/polarizability π* and the hydrogen-bonding ability R.17 Indeed, the results of multilinear regression analysis by employing StatView 5.0 statistical software confirm that the values of λEm max depend only on the microscopic solvent parameters π* and R, showing red-shifts in solvents able to stabilize charges and dipoles as well as in solvents with high R values (Table 2). Correlation equations result from purely statistical criteria. The overall correlation equation quality is indicated by the sample size, N, the product correlation coefficient, R, and the
Results and Discussion Absorption and Fluorescence Spectra. The absorption spectra of compounds 1-3 are nearly independent of solvent polarity. Compound 2 exhibits the maximum of the lowestenergy absorption band (λAbs max) centered around 350 nm whereas for compounds 1 and 3 the maxima appear between 392 and 398 nm. Spectra calculations employing DFT formalism (B3LYP6311 g+ for structure optimization and ZINDO-S to calculate 5372 J. Org. Chem. Vol. 73, No. 14, 2008
(16) Kovac, B.; Novak, I. Spectrochim. Acta 2002, 58, 1483–1488. (17) Taft, R.W.;.; Kamlet, M. J. J. Am. Chem. Soc. 1976, 98, 2886–2894. (18) Kamlet, M. J.; Hall, T. N.; Boykin, J.; Taft, R. W. J. Org. Chem. 1979, 44, 2599–2604. (19) Kamlet, M. J.; Abboud, J. L. M.; Abraham, M. H.; Taft, R. W. J. Am. Chem. Soc. 1977, 99, 6027–6038. (20) Kamlet, M. J.; Abboud, J. L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48, 2877–2887. (21) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry, 3rd ed.; WILEY-VCH: Weinheim, 2003.
Photophysics and Photochemistry of Naphthoxazinone LSER Correlation Equations (υ j)υ j + aπ* + br + cβ 2 ) for the Dependence of the λEm (Expressed in cm-1) on the + dGH max Solvent Parameters
TABLE 2.
νj0
a
b
-1079.9 69.9 -15.5
