3762
J. Phys. Chem. 1994,98, 3762-3769
Excited Triplet State Interactions with Molecular Oxygen: Influence of Charge Transfer on the Bimolecular Quenchfng Rate Constants and the Yields of Singlet Oxygen (O;,'Ag) for Substituted Naphthalenes in Various Solvents F. Wilkinson,' D. J. McGarvey,+and A. F. Olea* Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire LE1 1 3TU, U.K. Received: October 27, 1993; In Final Form: January 3, 1994'
The bimolecular rate constants k& for oxygen (02(3Z~)) quenching and the efficienciesE with which singlet oxygen (Of('AJ) is thereby produced are reported for a range of substituted naphthalene triplet states in acetonitrile, benzene, and cyclohexane. The magnitudes of k& and f' are inversely correlated, and both d) of the naphthalene derivative and parameters exhibit pronounced sensitivity to the oxidation potential some dependence on the solvent. Since, within the range of naphthalenes studied, the triplet state energy (ET) remains largely constant and the molecules are structurally similar, the dominant variable is the free energy change (AGCT) for charge transfer to molecular oxygen. It is demonstrated that the large variations observed depend on the energy of the substituted naphthalene/molecular oxygen charge-transfer (CT) in k& and states, 1.3(Me+...02*-). In acetonitrile, for example, the respective magnitudes of k& a n d f i are 7.2 X lo9 dm3 mol-' s-1 and 0.33 for 1-methoxynaphthalene compared with 1.4 X lo9 dm3 mol-' s-* and 0.74 for 1-cyanonaphthalene. In the nonpolar solvent cyclohexane, the C T state energy levels are raised (by -14 kJ mol-') relative to the energy levels in acetonitrile and benzene and this is reflected in decreased oxygen quenching rate constants ((1-3) X lo9 dm3 mol-' s-') and increased efficiencies of singlet oxygen production (0.56-1.0), particularly for those naphthalenes which contain electron-donating substituents. In all three solvents the k& and f;f values for naphthalenes containing strong electron-withdrawing substituents (e& -CN,-NOz) remain largely constant. In order to account for the observed data, it is necessary to invoke a potential barrier (AG*) to charge-transfer formation or the formation of exciplexes with significant C T character in the quenching step.
(e
discuss in this paper, if 02.- remains associated with Me+as a radical pair in the CT states lJM(*+.-02*-) such that only charge The nature of the interactions which occur between molecular recombination occurs, then photophysical CT-mediated quenching oxygen and the electronically excited singlet and triplet states of results. organic molecules (1M*,3M*) is a subject which has intrigued Numerous interesting studies have been carried out in the researchersin photochemistry for several decades'" and continues previous 20 years in attempts to elucidatethe mechanism of excited to provide a wealth of unresolved questions, which maintains a state quenching by o ~ y g e n . ~ + - ' In ~ Jparticular, ~~~ the classic highcurrent level of activity in this field.6'8 Theenormous interest work of Gijzeman, Kaufman, and Porter published in 19732.3has in this area derives from the fact that molecular oxygen is been a reference point for many subsequent investigations ubiquitous in nature and that it possesses several unusual properties including our 0wn.~7~JOIt is well-known that the bimolecular which account for its ability to efficiently quench organic excited quenching rate constants vary by large amounts (108-10'0 dm3 states, virtually without exception. These special properties mol-' s - I ) ~ ~ and the quantum yield of O;('A ) production varies include (i) the triplet spin multiplicity of 02(32J which can from O-100%.18 The magnitudes of k& a n d t depend on various enhance intersystem crossing processes in organic molecules, (ii) parameters such as (i) the excited state en erg^,^^^ (ii) the the presence of two low-lying excited singlet states O;('AJ and multiplicity of M*,5J2J7(iii) the electron configuration of M* O;('Z:) with energies of 94 and 157 kJ mol-', respectively, (n?r*, AT*, CT),4J4J5J9923(iv) the redox properties of M*,9#21(v) which may be populated by exchange energy-transfer quenching structure^,^ and (vi) the solvent environment. of most excited triplet states and some excited singlet ~ t a t e s , ~ * ~ J ~molecular J~ We have recently shown that for a series of substituted and (iii) the relative ease of reduction of 0 2 to superoxide ( 0 2 ' - ) naphthalenes in benzene the efficiency of singlet oxygen prowhich, as we report here, can have a strong influence on the duction during oxygen quenching of triplet states u',)increases oxygen quenching rate and the measured yield of singlet oxygen, with the oxidation potential of the naphthalene derivao;('AJ, production. tive.9 In addition, we demonstrated that k& exhibits an inverse Thus the quenching of excited states by oxygen has several correlation with the oxidation potential of the naphthalene photophysical consequences of which the most important are (i) derivativebeing quenched, which is evidence for the participation fluorescence and triplet state quenching, (ii) enhanced intersystem of CT interactions within excited s t a t e 4 2 complexes formed crossing (1M* 3M* and 3M* M), and (iii) production of during the quenching process. We also measured recently12the Oi('Ag). The production of 0 2 ' - is, strictly speaking, a photoefficiencies of singlet oxygen production during quenching of chemical consequence of oxygen quenching. However, as we both the first excited singlet and triplet states u", and respectively) of a range of substituted anthracenes in cyclohexane. Present address: Department of Chemistry, University of Keele, Newcastle, Staffordshire ST5 5BG, U.K. In contrast to naphthalene derivatives, anthracene derivatives t On leave from the Departamento de Quimica, Facultad de Ciencias, have efficiencies of singlet oxygen production from the triplet Universidad de Chile, Santiago, Chile. state,fz, all equal to unity, while the efficiency of singlet oxygen 0 Abstract published in Advance ACS Abstracrs, March 1, 1994.
Introduction
(ex)
-
-
fi,
0022-3654/94/2098-3762$04.50/0 0 1994 American Chemical Society
Excited State Interactions with Molecular Oxygen
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3763
A,
absorbances at 355 nm of -0.5. This procedure eliminates the requirement to apply corrections for differential fractional absorption of the exciting light. The concentrations of the substituted naphthalenes in these solutions are not sufficiently high to significantly influence the refractive index of the solvent, and therefore, it was not necessary to apply any refractive index correction factor.20 The use of aromatic ketones as sensitizers for the substituted naphthalene triplet states derives from the method used previously by ourselves26 and by Gorman et alaz7in which the naphthalene (N) triplet stateis populated withunit efficiency by energy transfer from the aromatic ketone (K)triplet state, viz.,
production from the first excited singlet state, varies from zero for anthracene to unity for 9,lO-dicyanoanthracene. In that work it was established that the magnitude of& is determined by the same factors which govern intersystem crossing yields for anthracene derivatives, i.e. the activation energy for intersystem crossing to higher triplet states, and does not depend critically on CT interactions. In contrast to many previous investigations of triplet state quenching by oxygen, our recent workg is characterized by the selective variation of one molecular parameter (so far as this is possible) in order to determine the influence of that parameter on the magnitudes of k& and In this paper we extend this approach to a range of substituted naphthalenes in acetonitrile, benzene, and cyclohexane in an attempt to further enhance understanding of solvent effects on oxygen quenching. The marked variations in k& and which we observe are interpreted as being due to variations in the energies of the CT states, 1.3(M+-.02-), in these different solvents brought about mainly by varying the nature of the substituents on the naphthalene ring.
fi.
fi
N
'K*-'K*
(&-= l)-'N*
9 A = 4T
-
(1)
The singlet oxygen yield ( 4 ~arising ) from triplet state quenching is given by eq 2.
Experimental Section Materials. Phenazine (Pz), acridine, 2-bromonaphthalene, 1-nitronaphthalene, and 2-methoxynaphthalene (Aldrich) were recrystallized from ethanol; biphenyl (Aldrich, 99%) was sublimed; 1-methylnaphthalene, 1-methoxynaphthalene, and 1-bromonaphthalene were purified by repeated vacuum distillation; naphthalene (N) (Aldrich scintillation grade, Gold Label), 1-ethylnaphthalene (Fluka, 99%), benzophenone (BP) (Aldrich, Gold Label), and p-methoxyacetophenone (pMAP) (Aldrich, 99%) were used as received; acenaphthene, 2-methylnaphthalene, 2-ethylnaphthalene, 1-fluoronaphthalene, 1-chloronaphthalene, and 1-cyanonaphthalene (Aldrich) were used as received; tetraphenylporphyrin (TPP) was a gift from C. Tanielian. Acetonitrile (Aldrich, spectrophotometric grade) was dried by refluxing over calcium hydride; benzene (Aldrich spectrophotometric grade), cyclohexane (Aldrich spectrophotometric and anhydrous grades), and carbon tetrachloride (Aldrich spectrophotometric grade) were used as received. Instrumentation. Kinetic absorption measurements were carried out using the third harmonic (355 nm) of a JK system 2000 Q-switched Nd:YAG laser (25 ns, 25 mJ pulse-') as described el~ewhere.2~ For singlet oxygen luminescence measurements, the third harmonic of a lumonics HY200 Q-switched Nd:YAG laser (8 ns, 15 mJ pulse-') was employed as the excitation source. Time-resolved singlet oxygen luminescence (1270 nm) was detected using a Judson germanium photodiode (J16-85PRO5M)-amplifier combination as described previously."J Steadystate absorption measurements were made on a Phillips PU8800 spectrophotometer. Triplet state energies (ET)were determined by employing the oxygen perturbation method developed by Evans.25 Briefly, a solution of the substituted naphthalene (-0.1 mol dm-3) in carbon tetrachloride was exposed to a high pressure of oxygen (-80 atm) in a stainless steel cell with fused silica windows (path length 5 cm). Following vigorous agitation, the absorption spectrum was recorded. Oxygen-enhanced absorption was observed due to contact CT absorption as a broad structureless band on top of which enhanced singlet-triplet absorption with pronounced vibrational structure was apparent. The triplet state energy was evaluated from the position of the (0,O)band. Methods. Singlet Oxygen Quantum Yield Measurements. The procedure for determination of singlet oxygen yields was as follows. Air-equilibrated solutions of the substituted naphthalenes (0.050.1 mol dm-9, each containing an aromatic ketone sensitizer, were optically matched (fO.O1 absorbance units) at the laser excitation wavelength to a standard reference solution for which the singlet oxygen yield is published (vide infra). Solutions using the same solvent were prepared in 1-cm-square quartz cells with
0 2
(lOO%)-Of('Ag)
e,,
where 4~ is the quantum yield of triplet state production of the molecule of interest under the conditions of the experiment, Po2 is the fraction of triplet states quenched by oxygen, and is the fraction of these triplet states quenched by oxygen which yield O;('Ag). Since the method we employ results in Qr = 1 (with one exception) and also for all the naphthalene derivatives studied @ = 1, then In the case of 1-nitronaphthalene, which absorbs strongly at 355 nm, direct excitation of optically matched solutions was employed which yielded a measurement of @A and notfT. Since nitronaphthalenes are nonfluorescent, it is likely that their singlet lifetimes are so short that no excited singlet states are intercepted by oxygen under our experimental conditions. Thus, to determine a value of for l-nitronaphthalene, we used eq 2 (with @ = 1) and the published value28 of Qr (=0.63), which we assumed to be solvent independent. For the other naphthalene derivatives, which were all sensitized using aromatic ketones, we assumed energy transfer from the triplet ketone to be 100% efficient. Support for this assertion is given in ref 29a,b. To further support this assumption, we monitored the triplet absorption of 1-methoxynaphthalene, at 440 nm, in degassed acetonitrile at low laser intensities ( in benzene, hexane, and cyclohexane were approximately equal to (1/9)kd (cf. eq 3), implying exclusive quenching via the energy-transfer channel (with k,, >> k 4 and ki, > 0, Table 2) quenching via the enhanced intersystem crossing channel is absent. Thus we may assume that quenching via the triplet channel depends exclusively upon electron-transfer quenching and that non-CTmediated quenching may, for these molecules, be neglected. From Scheme 1 and eq 3, the following expression for k i 2is obtained.
k i 2 3kdkiE/9(k4 + kia)
(9)
The electron-transfer quenching may be visualized as proceeding according to eq 10.
-
kcr 3(~...0~,3~p)*
-40
I
...oil
3 ( ~ +
kb
3
+
( . . . ~0 3 , ~ - ) 2
8
M
02(32p)(10)
If the assumption is made that the primary electron-transfer step ( k m ) is irreversible and that the subsequent back-electron transfer (kb)to form ground-state products is fast relative to kcT, then k h = k m . The rateconstant kmmay be expressed according to transition-state theory by eq 11.
k,, = Z exp(-AG*/RT)
(11)
where Z is the frequency factor and AG* is the free energy of activation for electron transfer. Substitution of eq 11 into eq 9 gives
k i 2=
[-3k-d exp(AG*/RT) + -19 kdz
3kd
-I
(12)
which gives a limiting value of k& = (3/9)kd = 1Olodm3 mol-' s-l when k m = Ze(-AG*/RT) >> k4. Various methods for the estimation of AG* exist of which the most widely used are by Marcus4' (see eq 13) and by Rehm and Weller.35 Due to the limited range of AGm values available in this study, we find that our data are satisfactorily described by
Figure 6. Dependence of the rate constant (k$ for enhanced intersystem crossing on A@. The k& data were evaluated using eq 8. The curve passing through the data is the predicted dependence of ki2 on A P T from eqs 12 and 13 with X = 78 kJ mol-'; see text.
+
AG* = A/4(1 A@T/A)2 (13) any of the commonly used models for electron transfer. The dependence of k i 2 on AGm predicted from eq 12, using the Marcus equation (eq 13) to estimate AG* with the solvent reorganization energy term (A) equal to 78 kJ mol-1 and taking 2 = 10" s-l and kd/kd = 1 mol dm-3, is shown as the curve passing through the data in Figure 6. Our data suggest that changing the solvent from polar (acetonitrile) or polarizable (benzene) to nonpolar (cyclohexane) influences AGCT for each of the substituted naphthalenes studied by 14.5 kJ mol-I since this adjustment makes the quenching rates observed in cyclohexane at a particular AGCT equivalent to the quenching rates observed in benzene and acetonitrile at the same AGCT(see Figure 4). The main effect of the participation of charge-transfer interactions is to render the catalytic enhanced intersystem crossing channel of Scheme 1 more competitive, resulting, for these molecules, in an increase in k& and a decrease in& This result contrasts with our recent study12 of O;('A8) formation from the triplet states of anthracene derivatives (low ET) in cyclohexane, for which we found and k: to be independent of AGm. However, in the case of all these anthracene derivatives, AGCT> 0 because of the low ET values. Thus, both these studies are in agreement with the original suggestion by Gijzeman et al.293that CT interactions are more important for molecules with high triplet energies.
fif
Conclusions The efficiency of singlet oxygen O;(IAg) formation, and k i 2 ,the rate constant for triplet state quenching by 02(3Z;), have been measured for a range of substituted naphthalenes in acetonitrile, benzene, and cyclohexane. An inverse correlation between k& andfi exists which is almost independent of solvent (Figure 5 ) . Both k: andfif are strongly dependent on A@, the free energy change tor charge-transfer-state formation from the triplet state of the substituted naphthalene to 02(3X;), which is evidence for the participation of charge-transfer interactions during the quenching. The data are consistent with quenching showing a free energy of activation AG* for the formation of the charge-transfer state during quenching or for the formation of exciplexes in which there is substantial charge-transfer character. Finally, our measurements in acetonitrile and cyclohexane show clearly thatfi for naphthalene in these two solvents, as in the case of the solvent benzene, is less than unity.
fif,
fi
Acknowledgment. The authors are grateful to Professor T. G. Truscott for the loan of the high-pressure absorption cell, to
Excited State Interactions with Molecular Oxygen Professor C. Tanielian for the gift of a sample of tetraphenylporphyrin, and to the U.S.Army, Fundacion Andes, and the British Council for financial support.
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3769
(23) Darmanyan, A. P.; Arbogast, J. W.; Foote, C. S . J. Phys. Chem. 1991, 95,7308. (24) Wi1kinson.F.; WorraU,D. R.;McGarvey,D. J.;Goodwin,A.;Langlcy, A. J. Chem. Soc., Faraday Trans. 1993,89,2385. (25) Evans, D. F. J. Chem. Soc. 1957,1351. References and Notes (26) Gamer, A.; Wilkinson, F. InSinglet Oxygen,Reactions with Organic CompoundsandPolymers; Ranby, B., Rabek, J. F., Eds.;Wiley: New York, (1) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Inter1976;p 48. science: London, 1970;Chapter 10,pp 492-527 and references therein. (27) Gorman, A. A.; Hamblett, I.; Lambert, C.; Prtscott, A. L.; Rodgers, (2) Gijzeman, 0.L. J.; Kaufman, F.; Porter, G. J. Chem. Soc.,Faraday M. A. J.; Spence, H. M. J. Am. Chem. Soc. 1987,109, 3091. Trans. 2 1973,69,708. (28) Hurley, R.; Testa, A. C. J. Am. Chem. Soc. 1968, 90, 1949. (3) Gijzeman, 0.L. J.; Kaufman, F. J. Chem. Soc., Faraday Trans. 2 (b)Carmichael, (29) (a)Land,E. J.Proc.R.Soc.London1968,A305,457. 1973,69,721. I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 1. (4) Garner, A.; Wilkinson, F. Chem. Phys. Lett. 1977,45,432. (30) Gorman, A. A.; Rodgen, M. A. J. In Handbook of Organic (5) Saltiel, J.; Atwater, B. W. Ado. Phorochem. 1988,14, 1. Photochemistry; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989;Vol. (6) McLean, A. J.; McGarvey, D. J.; Truscott, T. G.; Lambert, C. R.; 2,p 229. Land, E. J. J. Chem. Soc., Faraday Trans. 1990,86,3075. (31) Redmond, R. W.; Braslavsky, S . E. In Photosensitization; Moreno, (7) Ogilby, P. R.; Sanetra, J. J. Phys. Chem. 1993,97,4689. G., Pottier, R. H., Truscott, T. G., Eds.; NATO AS1 Series, Vol H15; (8) McLean, A. J.; Rodgers, M. A. J. J. Am. Chem. Soc. 1993,115, Springer: Berlin, 1988;p 93. 4786. (32) Gorman, A. A.; Krasnovsky,A. A.; Rodgen, M. A. J. J. Phys. Chem. (9) McGarvey, D.J.; Szekeres, P. G.; Wilkinson, F. Chem. Phys. Lett. 1991, 95,598. 1992,199,314. (33) Rossbroich, G.; Garcia, N. A.; Braslavsky,S.E. J. Photochem. 1985, (10) McGarvey, D. J.; Wilkinson, F.; Worrall, D. R.; Hobley, J.; Shaikh, 31,31. W. Chem. Phys. Lett. 1993,202,528. (11) Logunov, S.L.; Rodgers, M. A. J. J. Phys. Chem. 1993,97,5643. (34) Scurlcck, R. D.; Ogilby, P. R. J. Photochem. Photobiol. A . Chrm. (12) Wilkinson, F.; Olea, A. F.: McGarvey, D. J. J. Am. Chem. SOC.1993, 1993,72, 1. 115, 12144. (35) Rehm, D.; Weller, A. Isr. J. Chem. 1970,8,259. (13) Kikuchi,K.;Sato,C.; Watabe,M.;Ikedo,H.;Takahashi,Y.;Miyashi, (36) Wilkinson, F.; Tsiamis, C. J. Am. Chem. Soc. 1983,105,767. T.J. Am. Chem.Soc. 1993,115, 5180. (37) Kavarnos, G. J.; Turro, N. J. Chem. Reu. 1986,86,401. (14) Darmanyan, A. P.; Foote, C. S.J. Phys. Chem. 1993,97,4573. (38) Marcus, R. A. Angew. Chem. Int. Ed. Engl. 1993,32, 1 1 11. (15) Darmanyan, A. P.; Foote, C. S.J. Phys. Chem. 1993,97,5032. (39) Knibbe, H.; Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. (16) Krasnovsky, A. A,; Foote, C. S.J. Am. Chem. SOC.1993,115,6013. 1969,73,839. (17) Usui, Y.; Shimizu, N.; Mori, S . Bull. Chem. Soc.Jpn. 1992,65,897. (40) Ware, W. R. J. Phys. Chem. 1962,66,455. (18) Wilkinson, F.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref Data (41) Marcus, R. A. J. Chem. Phys. 1957,26,867. 1993,22, 113. (42) Mann, C. K.; Barnes, K. K. ElectrochemicalReactionsinNonaqueous (19) Redmond, R. W.; Braslavsky, S.E. Chem. Phys. Lett. 1988,148, Systems: Marcel Dekker: New York. 1970. 523. (43) Utsunomiya, C.; Kobayashi, T.; Nagakura, S . Bull. Chem. Soc. Jpn. (20) Kristiansen, M.; Scurlock, R. D.; Iu, K.-K.; Ogilby, P. R. J. Phys. 1975,48,1852. Chem. 1991, 95,5190. (44) Pysh, E.S.;Yang, N. C. J. Am. Chem. Soc. 1963,85,2124. (21) Chattopadhyay, S . K.; Kumor, C. V.; Das, P. K. J. Photochem. 1985, on JV, 01. (45) Neikam, W. C.; Dimeler, G. R.; Desmond, M. M. J. Electrochem. (22) Murov, S.L.;Carmichael, I.; Hug, G. L. Handbook ofPhotochemSOC.1964,111,1190. istry; Marcel Dekker, Inc.: New York, 1993. (46) Smith, G. J. J. Chem. Soc., Faraday Trans. 2 1982,78,769. ~~
01
