J. Phys. Chem. 1083, 87, 4351-4353
4351
orbital would tend to leave the positive charge of the core localized on the heteroatom, The dipole moment for an excited state depends upon the location of this resultant positive charge of the core and the electron distribution in the excited state. From these data, the electron distribution of thiirane is nearly symmetrically distributed about this positive charge on the sulfur atom resulting in a near zero dipole moment in this excited state. Oxirane, however, has its electron density displaced from the oxygen toward the carbon atoms.
the oxygen end of the molecule to the carbon moiety. The dipole moment for the s Rydberg band for thiirane was determined to be ca. -0.9 D. The highest occupied molecular orbital of oxirane and thiirane has been assigned from photoelectron and spectroscopic data as the bl molecular orbital localized in the lone pair orbital of the heteroatom perpendicular to the molecular plane. Population analyses for oxirane and thiirane for this molecular orbital indicate that the electron density is approximately 80% localized on the oxygen atom in oxirane26and 90% localized in thiirane.’Jg Excitations from this molecular
Acknowledgment. The authors gratefully acknowledge the Robert A. Welch Foundation and North Texas State University Faculty Research Fund for support of this investigation. Registry No. Ethylene oxide, 75-21-8.
(18) M. B. Robin, ‘Higher Excited States of Polyatomic Molecules”, Vol. 1, Academic Press, New York, 1974. (19) E. J. McAlduff and K. N. Houk, Can. J. Chem., 55,318 (1977). (20) G. L. Bendazzoli, G. Gottarelli, and P. Palmier, J. Am. Chem. SOC.,96, 11 (1974). (21) G. L. Cunningham, A. W. Boyd, R. J. Meyem, and W. D. Gwinn, J. Chem. Phys., 19, 676 (1951). (22) J. Applequiet, J. Am. Chem. SOC., 94, 2952 (1972). (23) W. J. Potta, Spectrochim. Acta, 21, 511 (1965). (24) J. M. Freeman and T. Hemhall, Can. J.Chem., 46,2135 (1968).
(25) T. K. Lie and A. B. F. Duncan, J. Chem. Phys., 17, 241 (1949). (26) H. Basch, M. B. Robin, N. A. Kuebler, C. Baker, and D. W. Turner, J. Phys. Chem., 51, 52 (1969).
Amine Dimer Cation Radical Formation upon Quenching of trans-Stilbene-Amine Excipiexes by Amines Waiter Hub, Slegfrled Schnelder, Friedrich Dorr, Instltut f i r physikallsche und Theoretische Chemie, Technlsche Unlversitiit Miinchen, 0-8046 Qarching, West Germany
Joe D. Oxman, and Frederlck D. Lewis* Department of Chemistry, Northwestern Unherslty, Evanston, Illinois 60207 (Received: June 1, 1983; I n Final Form: August 1, 1983)
The interaction of singlet trans-stilbene with trialkylamines in benzene solution yields fluorescent exciplexes for which the predominant decay pathway is intersystem crossing to yield triplet trans-stilbene. Both the fluorescence and intersystem crossing processes can be quenched by added tertiary and primary amines. Exciplex quenching is subject to a pronounced steric effect, being inhibited by both N and a-C alkylation. Intramolecular exciplex quenching by the free amino group of a,w-alkanediaminesis highly dependent upon the diamine chain length. The observed steric and chain length effects on exciplex quenching are indicative of the formation of a triplex of trans-stilbene anion radical and a u-bonded amine dimer cation radical.
The through-space interaction of two nitrogen lone pairs is destabilizing for neutral amines, but stabilizing when one amine is oxidized to the cation radical due to the formation of a three-electron u bond (eq 1).l The strength
of the three-electron u bond in several cyclic and bicyclic diamines has been estimated to be 0.7 f 0.2 eV, based upon the cation radical proton affinities.’J While there is no available evidence concerning the formation of amine dimer cation radicals from monoamines or conformationally mobile diamines, the commonly observed phenomenon of quenching of singlet arene-amine exciplexes (contact radical ion pairs) by ground-state amines (eq 2)3 might (1) (a) Alder, R. W.; Arrowsmith, R. J.; Casson, A.; Sessions, R. B.; Heilbronner, E.; Kovac, B.; Huber, H.; Taagepera, M. J. Am. Chem. SOC. 1981,103,6137-42. (b) Alder, R. W.; Sessions, R. B. In “The Chemistry of Amines, Nitroso, and Nitro Compounds”; Patai, S., Ed.; Wiley: New York, 1982; Chapter 18. (2) Transient absorption spectra of u-bonded cyclic diamine cation radicals have recently been obtained Scaiano, J. C., private communication.
lArH
+ R3N
‘(ArH-.R,N+-)*
RSN k,ex
ArH
+ R3N
(2)
result from the specific interaction of the exciplex amine cation radical with neutral amine to form an amine dimer cation radical. We report here the preliminary results of our investigation of the quenching of trans-stilbeneamine exciplexes by amines. The pronounced steric effects observed for quenching by monoamines and chain-length effect for quenching by acyclic diamines provide evidence for a specific through-space interaction between amine lone pair orbitals.
Experimental Section trans-Stilbene (Aldrich) was recrystallized once from benzene and twice from absolute ethanol. All amines (Aldrich) were distilled prior to use. Spectroquality (3) (a) Grellmann, K. H.; Suckow, U. Chem. Phys. Lett. 1975, 32, 250-4. (b) Yang, N. C.; Shold, D. M.; Kim, B. J. Am. Chem. SOC.1976, 98, 6587-96. (c) Saltiel, J.; Townsend, D. E.; Wataon, B. D.; Shannon, P.; Finson, S. L. Ibid. 1977,99,884-96. (d) Van, S.-P.; Hammond, G. S. Ibid. 1978, 100, 3896-902. (e) Caldwell, R. A.; Creed, D.; DeMarco, D. C.; Melton, L. A.; Ohta, H.; Wine, P. H. Ibid. 1980,102, 2369-77.
0 1983 American Chemical Society
4552
llm Journal of Physical Chemistty, Vol. 87, No. 22, 1983
Letters
'TS*+ R,N
\
3.53
0
34[ 2,6
It
(TS: R,N:)*
pmine]
Flgure 2. Amine concentration dependence of the trans -stilbeneethyldiisopropyiamine (A)and -triethylamine (0)exciplex lifetimes.
TABLE I. Quenching of Singlet trans-Stilbene and trans-Stilbene- Amine Exciplexes by Amines Flgure 1. Energy level diagram for the trans-stilbene locally excited singlet and triplet states and the trans-stilbene-trlalkylamine exciplex and triplex.
benzene (Aldrich) was refluxed over sodium and distilled. Solutions were purged with prepurified nitrogen prior to fluorescence or product quantum yield measurements. Vacuum line degassing led to no increase in quantum yields. Fluorescence spectra were recorded on a PerkinElmer MF'F-44A spedrofluorimeter. Nanosecond exciplex lifetimes were determined via a single-photon-counting technique with excitation wavelength at 315 nm and observation a t 455 nm. Quantum yields for cis-stilbene formation were measured with a merry-go-round apparatus immersed in a water bath.4b Monochromatic 313-nm irradiation was provided by a potassium chromate filter solution. Light intensities were measured by trans-stilbene actinometry. The extent of trans-stilbene isomerization was determined with a Varian 3700 dual-flame ionization gas chromatograph with a 6 ft X f 8 in. column containing 5% SF-96 on Chromasorb G.
ky$
amine i-Pr,NEt Et,N
3.6 2.0
Me,N i-Pr,NMe- t-BuNH, i-Pr,NMe-i-PrNH, i-Pr,NMe-n-PrNH, Me,N(CH,),NMe, Me,N(CH,),NMe,
in nonpolar solvent is summarized in Figure l.4 In the case of the trans-stilbene-ethyldiisopropylamineexciplex, exciplex fluorescence (vex= 21000 cm-', @ptex = O.02,7,, = 14 ns) and intersystem crossing (ast= 0.93 f 0.1) are the only observed exciplex decay pathways.4b Neither the lifetime (Figure 2) or fluorescence quantum yield nor the quantum yield for cis-stilbene formation via isomerization of triplet trans-stilbene5 are dependent upon the amine concentration (0.05-1.2 M). Thus exciplex quenching by a second molecule of amine (Figure 1)is not a significant decay process for this exciplex. Weakly fluorescent exciplexes (vex = 21100 f IO0 cm-' = 2.6 eV) are also formed upon quenching of singlet trans-stilbene by triethylamine and trimethylamine. Stern-Volmer constants for quenching of singlet transstilbene fluorescence (KqS7J by monoamines increase with increasing amine alkylation (Table I), reflecting the effect of substitution on amine ionization potentials.4 In the case of the exciplexes of trans-stilbene and triethylamine or trimethylamine, the quantum yield for exciplex fluorescence, the exciplex lifetime,5*and the quantum yield for ~~~~
~
~
(4) (a) Lewis, F. D.; Ho, T.-I. J.Am. Chem. SOC.1977,99,7991-6. (b) Leww, F. D.; Simpson, J. T. J. Phy8. Chem. 1979,83,20169. (c) Aloisi, G. G.; Bartocci, G.; Favaro, G.; Mazzucato, U. Zbid. 1980, 84, 2020-4.
2.1 2.4
M-1
