Itoh, Takita, Matsumoto
/
1363
Quenching of Intramolecular Exciplexes
Quenching of Intramolecular Exciplexes by Pyridine and Methylpyridines Suggesting Triple Exciplex (D’DA)” Formation Michiya Itoh,* Nobuo Takita, and Momoe Matsumoto Contribution from the Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-machi. Kanazawa 920, Japan. Received March 20, I979
Abstract: Intramolecular exciplex (DA)* fluorescence of 1-(9,10-dicyano-2-anthryl)-3-(1- or 2-naphthy1)propane was quenched by pyridine and methylpyridines (D’). Double exponential decay of the intramolecular exciplex was observed at room temperature, while a single exponential decay of the exciplex in the 9,lO-dicyanoanthracene and naphthalene system was observed. The result in the former system implies the formation of a new type of triple exciplex, (D’DA)*, including pyridine molecule, though neither new fluorescence nor transient absorption spectrum due to the triple exciplex was detected. Association and dissociation rate constants of (D’DA)* obtained in the intramolecular system decrease in the following order of the quenchers: pyridine > 4-methyl- > 3,5-dimethyl- > 2-methyl- > 2,6-dimethylpyridine. These results suggest that the nonbonding orbital of a nitrogen atom in a pyridine ring may play an important role in the terelectronic interaction, (D’DA)*, in the excited state.
metal for 5 h. The solutions of samples contained in a rectangular Since Beens and Weller’ reported the triple complex forquartz cell (1 cm) with graded seals were purged by dry nitrogen gas mation in the excited state (triple exciplex) between 1,4-dior degassed by freeze-pump-thaw cycles. Both methods afford the cyanobenzene and two naphthalene molecules, some of the same experimental results. All absorption and fluorescence spectra investigations on the triple exciplex were r e p ~ r t e d . Grell~,~ were recorded on Hitachi 323 and MPF-4 spectrophotometers at rcom mann and Suckow3 reported the triple exciplex formation in temperature, respectively. The fluorescence decays and the timethe anthracene and N,N-diethylaniline system, and discussed resolved fluorescence spectra were determined by analyzing expothe ionization and intersystem crossing leading to the formanential decay curves measured by an oscilloscope (Tektronix 465) and tion of the triplet state of anthracene. Mimura and I t ~ h ~ -a photomultiplier ~ (HTV R666) and by an excitation with a coaxial N2 gas laser which has a maximum -20 kW photon peak intensity demonstrated the formation and reversible dissociation of the at 337 nm and 4-5-nsec duration. Observed fluorescence decay curves triple exciplex (D2A)* in the system of 1,4-dicyanobenzene were analyzed by a computer-simulated deconvolution.Il and alkylnaphthalenes by double exponential decay of the exciplex (DA)*, and also reported an intramolecular triple Results and Discussion exciplex formation between two naphthyl moieties of 1,3The tetrahydrofuran (THF) solution of 1-(9,10-dicyanodinaphthylpropane and 1,4-dicyanobenzene. The sequence of 2-anthryl)-3-( 1- or 2-naphthy1)propane ( P p - or P,P-DCAN) the electron donor and acceptor, (DDA)*, and the electronic exhibits intramolecular exciplex fluorescence in the 500structure in the triple exciplex were shown by experimental and 580-nm region a t room temperature, as reported previously.6 theoretical points of view. On the other hand, Caldwell et al. Figure 1 shows the fluorescence spectra of THF solutions of reported the exciplex quenching suggesting the termolecular P,P-DCAN containing several concentrations of pyridine a t interaction between the exciplex and the second electron donor room temperature (20-23 “C). The exciplex fluorescence a t or acceptor. 500-580 nm as well as 9,lO-dicyanoanthryl (DCA) moiety This paper8 describes the fluorescence quenching of the fluorescence a t 400-450 nm was remarkably quenched by intramolecular exciplex in 1-(9,10-dicyano-2-anthryl)-3-( 1pyridine. However, the exciplex and also DCA moiety fluoor 2-naphthyl)propane9 by pyridine and alkylpyridines in rescence were much less quenched by 2,6-dimethylpyridine comparison with that of the intermolecular system of 9,lOin T H F , as shown in Figure 2. In the fluorescence quenching dicyanoanthracene (DCA) and naphthalene.’O Double expoof the exciplex, there seem to be several mechanisms such as nential decay of the intramolecular exciplex (DA)* was oba complex formation both in the ground and excited states, served in the quenching by pyridine and methylpyridines (D’), photoionization, energy transfer, and so on. Pac and Sakurai while only single exponential decay of the exciplex was obreported the dynamic quenching of exciplexes in 2-naphthoserved in the DCA-naphthalene systems. The result of the nitrile-2-methylfuran and -2,3-dimethylbut-2-ene systems by intramolecular exciplex quenching suggests the formation and pyridine.I2 The electronic absorption spectra of a THF solution dissociation of a metastable triple exciplex, (D’DA)*, including exhibit no complex formation between P,P-DCAN (also pyridine or methylpyridine molecule (D’), though neither new P,a-DCAN) and pyridine molecule in the ground state. In the fluorescence nor a transient absorption spectrum due to intramolecular exciplex quenching by pyridine (several (D’DA)* is observed. The obtained association rate constants methylpyridines) reported here, the photochemical scheme of the triple exciplex are much smaller in the quenching by (Scheme I) involving the formation of a new type of triple 2-methyl- and 2,6-dimethylpyridines than by 4-methyl- and exciplex (D’DA)* is tentatively a ~ s u m e d , where ~,~ A = 3,5-dimethylpyridines. The triple exciplex formation seems 9,lO-dicyanoanthryl and D = naphthyl moieties and D’ = to be hindered by 2- and 6-methyl groups, and the nonbonding quencher. The quenching r a t i o of ( I E ’ I I E ) of the exciplex orbital of the pyridine nitrogen atom seems to play an imporfluorescence by the quencher is expressed by the equation tant role in the (D’DA)* formation, though the structure is obscure at this stage. IE’/IE= ( I E o / I E ) ( I A / I A o ) = 1 + pk77~’[D’] (1)
Experimental Section
= 1
Samples and solvents were purified by the methods described in the previous paper^.^.'^ Pyridine and methylpyridines (Nakarai GR grade) were purified by distillation after refluxing with potassium
0002-7863/79/1501-7363$01 .OO/O
+ ((k9 + kio)k7[D’l/(k4 + k s + k d ( k s + k9 + kio)J + kio)/(ks + k9 + kio), 7 ~ =’ (k4 + k5 + k6)-’
p = (k9
where I o and I are fluorescence intensities in the absence and 0 1979 American Chemical Society
Journal of the American Chemical Society
1364
M A D
+ hv,AM 4 D’ +
/
101:24
November 21, 1979
M
+ D’ + ?,
A
D AM D AM D, AM
+
/
+ D’
D
3.5-dimethyl-
-
4-methyl2 -met hy I
~
;PY 2.6-dimethyl-
1.2 ID’IIM Figure 3. Stern-Volmer plots ( I E ’ I I E ) of the exciplex quenching of p&DCAN (3 X M) by pyridine and several methylpyridines. Fluorescence intensity (IE) was monitored at 650 nm.
Figure 1. Fluorescence quenching of the THF solution of P,P-DCAN (2.8 X IOd6 M) by pyridine at room temperature (excitation wavelength, 380 nm).
~
r 0’1 a,
o
b.
18
C
32 63
d.
~IO-’M
2.6- dimetylpyridine
11
0
1
1
20
I
llns
I
LO
I
I
60
,
80
Figure 2. Fluorescence quenching of the THF solution of P,P-DCAN (2.5 X 1 O V M) by 2,6-dimethylpyridine at room temperature (excitation wavelength, 380 nm).
Figure 4. Fluorescence decay curves of the P,P-DCAN exciplex (monitored at 600 nm) in THF solutions containing several concentrations of pyridine (P,P-DCAN, 3.0 X M). The time indicated in an abscissa is after a peak of signal of which intensity was normalized at 0 ns, since a very short ns) of the exciplex fluorescence was observed, as mentioned rise time ( 4-methyl- > 3S-dimethyl- > 2-methyl- > 2,6-dimethylpyridine!j. Further, the ratio of association and dissociation rate constants (k7/kg) also decreases in almost the same order. This implies that the dissociation process of (D'DA)* to (DA)* is more significant in 2,6-dimethylpyridine than in 3,5-dimethylpyridine. In this exciplex quenching, a single exponential decay of (DA)* is apparently observed on condition that (kg k9 klo) is much greater (or smaller) than (k4 ks k6 k7 [D']), and that the longer lifetime component is much greater (or smaller) than the short-lifetime component, Le., C I >> c2 (or C I > k8, the quenching constant, k,, of DCA-naphthalene exciplex becomes approximately identical with k7 in the reaction scheme mentioned above. (17) H. Ohta. D. Creed, P. P. H. Wine, R. A. Caldwell, and L. A. Melton, J. Am. Chem. SOC., 98, 2002 (1976). (18) E. A. Chandross and H. T. Thomas, Chem. Phys. Lett., 9,393 (1971). (19) G.N. Taylor, E. A. Chandross, and A. H. Schiebel, J. Am. Chem. Soc., 96, 2693 (1974). (20) K. A. Zachariasse, W. Kuhnle, and A. Weller, Chem. Phys. Lett., 59, 375 (1978).
+ +
+
Hydrocarbon Analogues of the Type I1 Photoeliminations of Ketones. Photochemistry of 1-Substituted 4-Phenyl-4-pentenesl Joseph M. Hornback* and Gary S. Proehl Contribution f r o m The Department of Chemistry, Unioersity of Dencer, Denoer, Colorado 80208. Receiued September 19, 1978
Abstract: Direct irradiation of 1,4-diphenyl-4-penten-1-01 produces mainly 2-methyl-2,5-diphenyItetrahydrofuran, while benzophenone-sensitized photolysis gives a-methylstyrene, acetophenone, and I ,4-diphenyl- I -pentanone. The direct irradiation is postulated to proceed via the radical anion of the alkene. A mechanism for the inefficient triplet state reaction (a = 0.0005) is proposed which involves initial hydrogen abstraction by the methylene carbon of the excited alkene to give a 1.4 biradical, which then produces the observed products. The mechanism is analogous to the accepted mechanism for the type I1 photofragmentation of ketones. The mechanism is supported by solvent effects and deuterium-labeling studies. Two related alkenes, 4phenyl-4-penten- 1-01 and 1,4-diphenyl-4-pentene, show similar photochemical behavior, although they react even less efficiently than 1,4-diphenyl-4-penten-l-ol.
There are now a number of reports in the literature describing the photoinduced abstraction of hydrogen atoms by carbon which occurs in various molecules containing carbon-carbon double or triple bonds. Some examples include intermolecular abstractions by acyclic alkene^,^-^ cyclic alk e n e ~ ,a,P-unsaturated ~,~ ketone^,^^^ a#-unsaturated ester^,^ and acetylenes.I0 Intramolecular abstractions by alkenes,'.' ' - I 7 a,b-unsaturated enones,Is and a,P-unsaturated amidest9have also been observed. We report here the full details of our study of the photochemistry of substituted phenylpentenes. These compounds undergo intramolecular hydrogen abstractions very similar to the type I1 reaction of ketones,*Oalthough much less efficiently.
Results For our initial studies we chose to investigate the photochemistry of 1,4-diphenyl-4-penten- 1-01 ( l ) ,for two reasons. Based on the reported substituent effects on the type I1 reaction of ketones,2oa we felt that both the phenyl and hydroxyl groups on carbon 1 would increase the rate of hydrogen abstraction. In addition, if 1 were to undergo a type I I reaction, the 1,4 biradical which would result could also be generated by irradiation of 1,4-diphenyl-l-pentanone(6), and thus an independent check on the behavior of this biradical would be available. 0002-7863/79/1501-7367$01 .OO/O
The synthesis of 1 was readily accomplished by a Wittig reaction of 1,4-diphenyl-4-hydroxy- l-butanone21 (2), with excess methylenetriphenylphosphorane. The photochemistry of 1 depended on both the solvent employed and the multiplicity of the excited state. The photoproducts observed under various conditions are summarized in Table I. The major product from the direct irradiation of 1 in either hexane or benzene was 2-methyl-2,5-diphenyltetrahydrofuran (3), obtained as a mixture of stereoisomers. In addition, small amounts of emethylstyrene (4), acetophenone ( 5 ) , and an 0
OH
II
1
PhCCH&H,CHPh 2
-
Ph,PCH,
0
CH,
I1
I
PhCHCH2CH2CPh 6
CH,
OH
11
I
PhCCH2CHlCHPh 1
0
+
II
PhCCH,
hv +
+
5
II
PhCCH, 4
+ CH, 3
unidentified photoproduct were formed. Photoproduct 3 was identified by comparison of its spectral properties with those of an authentic sample prepared by acid-catalyzed cyclization of 1. Direct irradiation of 1 in tert-butyl alcohol gave the same products, although the relative percentages changed. The ef-
0 1979 American Chemical Society
