Mendeleev Communications Electronic Version, Issue 2. 1997 (pp. 47–86) On the photosensitized formation of 1O2 (1 2) by flavonoids Eugeny A. Venedictov* and Olga G. Tokareva Institute of Chemistry of Non-Aqueous Solutions, Russian Academy of Sciences, 153045 Ivanovo, Russian Federation. Fax: +7 0932 378 509; e-mail: eav@ihnr.polytech.ivanovo.su The quantum yields of the photosensitized formation of singlet molecular oxygen from luminescence experiments in benzene solutions of 3,5,7,3',4'-pentamethoxyflavone, 3-hydroxy-5,7,3',4'-tetramethoxyflavone and 5-hydroxy-3,7,3',4'-tetramethoxyflavone were found to be 0.49±0.05 and < 10–2, respectively.Flavonoids play an important role in biochemical processes.1 Therefore, the photochemical properties of these molecules are of interest in the study of light-induced biochemical processes which are not well understood.Here we focus on the photoactivation of molecular oxygen by flavonols. Singlet molecular oxygen (1O2) is a key intermediate in many photobiochemical reactions. From experiments on the photoformation of 1O2 by quercetin, it has been deduced that the quantum yield of 1O2 is less than 10–3.2 This fact was interpreted in terms of intramolecular hydrogen bonding.With the ultimate aim of further understanding the role of the C-5 and C-3 hydroxy groups in this process, we present the quantum yield of 1O2 observed in the solutions of quercetin derivatives. The substrate molecules employed were 3,5,7,3',4'-pentamethoxyflavone 1, 3-hydroxy-5,7,3',4'-tetramethoxyflavone 2 and 5-hydroxy-3,7,3',4'-tetramethoxyflavone 3.Compounds 1–3 were preparated by methylation of quercetin and rutin by standard methods and the authenticity of the samples were confirmed by UV spectroscopy.3 Benzene and [2H6]benzene were used as solvents. Near-infrared time-resolved luminescence was used as a diagnostic means for the analysis of the 1O2 formation.4 The excitation of flavonoids yields dramatically different results.Photosensitized luminescence from the 1D2-state of O2 in solutions of 1 was observed. In contrast to 1, the light-induced formation of 1O2 by 2 and 3 was not observed. The luminescence intensity in solutions of 1 were measured as a function of the delay time. The decay rate constant k is determined from the following equation (1), where It is the observed intensity at time t after excitation, I0 is the intensity at zero delay time, t is the delay time, k is the decay rate constant.One can see that value of k depends on the concentration of 1 in [2H6]benzene solutions: concentration/mol dm–3 (k/s–1) 9.94×10–5 (1.86×103), 6.1×10–5 (1.76×103), 4.35×10–5 (1.56×103), 2.52×10–5 (1.6×103). The differences in decay rate observed for different concentrations of 1 might be simply due to the quenching of 1O2.A linear least-squares extrapolation of the cancentration dependence of k yielded an intercept of 1.46×103 s–1, from which (k = 1/t) a limiting lifetime of (684±50)×10–6 s was calculated (for benzene the t value of 29.5×10–6 s was obtained). This plot has a slope of (4.0±1.0)×106 dm3 mol–1 s–1, corresponding to the total quenching rate constant of 1O2 by 1 according to the Stern-Volmer equation Here kQ is the quenching rate constants, [Qu] is the concentration of 1.The rate constants of the 1O2 quenching by 2 and 3, estimated from experiments on the quenching of the photosensitized luminescence of 1O2 by anthracene in [2H6]benzene solutions, are 2.0×106 and 2.5×106 dm3 mol–1 s–1 respectively.For the quantum yield of 1O2 we have5 where kr is the rate constant of the radiative channel, t is the lifetime of 1O2 in solution, and j is the luminescence quantum yield expressed as I0 is the initial luminescence intensity obtained by extrapolation of It to t = 0, Iexc is the excitation light intensity, and A is the absorbance density at the excitation wave-length.Quantum yields of 1O2 in solutions of 1–3 were determined by relative methods using a benzene solution of Pd-mesoporphyrin-IX dimethyl ester (gstd = 1.0)6 as standard. A 10 mm square cell was used for luminescence measurements with solution optical density A = 0.70 at 337nm. Assuming that Iexc (1 – 10–A) parameter is constant, the ratio of g in the solutions of both sensitizers is given by The result for 1 is g = 0.49±0.05 in benzene.The g values for 2 and 3 are substantially less than 10–2. One can see that the rate constant for the quenching of 1O2 by 1 is relatively low. This compound is similar in this respect to 2 and 3. However, the quantum yield of 1O2 in a solution of 1 is higher than the observed value g for 2 and 3. We associate these differences with the photophysical properties of flavones. It is well-known that the energy of 1O2 is 22.5 kcalmol–1 (1 cal = 4.184J).2,6 Spectroscopic studies7 suggest that the triplet state energy of singlet–triplet separation for 1 are 56 and 19 kcalmol–1, respectively.Similar data are predicted for the other flavonols.2 This is evidence that 1O2 can be sensitized by energy transfer from the triplet state of flavones to 1O2 (3Sg – ).Experimentally, g for 2 was found to be less than that for 1. This is due to the specific intramolecular interactions between the proton of the hydroxide group at the C-3 position with the D It I0e kt – = (1) 0.2 0.1 0 200 400 600 Luminescence intensity/V t /ms Figure 1 Time-dependence luminescence intensity of 1O2 (1Dg) in [2H6]benzene solution of 3,5,7,3',4'-pentamethoxyflavone (c =4×10–5 M). k k0 kQ Qu[ ] + = (2) g = j/krt, (3) I0 Iexc 1 10 A – –( ) ---------------------------------- = j (4) s td I0 I0 std ------- = g g (5)Mendeleev Communications Electronic Version, Issue 2. 1997 (pp. 47–86) carbonyl oxygen in the excited state of 2.8 It is important to note that this process leads to a decrease of the triplet state population of 2.2 The experimental results show that the hydroxide group at the C-5 position leads to a similar effect.Consequently the OH group at the C-5 position also plays a key role in the excited state deactivation of 3. For 3 H-transfer also quenches the excited states. References 1 B. Havsteen, Biochem. Pharmacol., 1983, 32, 1141. 2 A.P. Darmanjan, A. G. Kasatkina and N. P. Chrameeva, Khim. Fiz., 1987, 6, 1083 (in Russian). 3 The Chemistry of Flavonoid Compounds, ed. T. A. Geissman, Pergamon Press, Oxford–London–New York–Paris, 1962. 4 E. A. Venedictov, Opt. i Spektr., 1994, 77, 405 [Opt. Spectrosc. (Engl. Transl.), 1994, 77, 359]. 5 R. D. Scurlock and P. R. Ogilby, J. Phys. Chem., 1987, 91, 4599. 6 B. M. Dzhagarov, E. I. Sagun, W. A. Ganza and G. P. Gurinovich, Khim. Fiz., 1987, 6, 919 (in Russian). 7 A. C. Waiss Jr, R. E. Llundin, A. Lee and J. Corse, J. Am. Chem. Soc., 1967, 87, 6213. 8 P. F. Barbara, P. K. Walsh and L. E. Brus, J. Phys. Chem., 1989, 93, 29. 2000 1000 0 5×10–5 10–4 k /s–1 Concentration/M Figure 2 Dependence of the rate constant for the luminescence decay of 1O2 (1Dg) on the concentration of 3,5,7,3',4'-pentamethoxyflavone. Received: Moscow, 14th May 1996 Cambridge, 13th December 1996; Com. 6/03517E