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(10) M. E. Jacox and D. E. Mllligan, J . Mol. Spectrosc., 58, 145 (1975). (11) W. J. Potts and R. A. Nyquist, Spectrocbim Acta, 15, 679 (1959). (12) C. B. Moore and G. C. Pimentel, J . Cbem. fbys., 38, 2816 (1963); 40, 342 (1964); W. H. Fletcher and W. T. Thompson, J . Mol. Spectrosc., 25, 240 (1968); D. C. McKean and J. L. Duncan, Spectrocbim. Acta, 27, 1879 (1970); C. Pouchan, A. Dargelos, and
A. R. Watkins M. Chaillet, Spectrocbim. Acta, far? A , 33, 253 (1977). (13) J. H. Schachtschneider, "Vibrational Anatysls of Polyatomic Molecules, V and VI", Technical Report No. 231-64 and 57-65, Shell Development Company, Emeryville, Calif. (14) K. Ramaswamy and R. Srlnivasan, Acta fbys. folon., A51, 139 (1977), and references therein.
Electronic Processes of Exciplexes of Anthanthrene with Substituted Dimethylanilines A. R. Watkins" Max-flanck-Institut fur Biophysikalische Chemie, 3400 Gottingen-Nikolausberg,West Germany (Received February 14, 1979) fubllcatlon costs assisted by Max-flanck-Institut fur Biophysikallsche Chemie
Rate constants for fluorescence emission, intersystem crossing,and radiationless deactivation have been measured for a series of exciplexes of anthanthrene in n-hexane solution as a function of exciplex energy. With decreasing exciplex energy fluorescence emission rapidly becomes inefficient; intersystem crossing within the exciplex shows little variation except when heavy atoms are present. No correlation could be found between exciplex energy and the rate of dissociation of the exciplex into the primarily excited molecule and the quencher.
Exciplexes, because of their importance as potential intermediates in photochemical and photobiological reactions and as possible chemical lasers, are becoming increasingly interesting as objects of study in their own right.' They are characterized by broad emission spectra, red-shifted with respect to the parent molecule, and generally possess highly polar structures. A good deal is already known about the thermodynamics and kinetics of their formation, and several theoretical studies have ap~ e a r e d . ~ItB has, however, proved more difficult to obtain information about the exciplexes themselves; the broad spectra preclude high-resolution spectroscopic studies, and the sparse information that we posses about their structure comes mainly from laser studies of exciplex absorption spectra.' Even more sparse is information concerning the way in which exciplexes decay;4there have appeared very few studies having as their aim the measurement of all the rate processes leading to the decay of exciplex systems. This paper attempts to partially remedy this deficiency. The rate processes which lead to the disappearence of an exciplex can be regarded as being of four kinds: dissociation into the excited parent molecule and quencher molecule (the so-called "feedback" step1), emission of fluorescence, intersystem crossing (ultimately to the triplet state of the parent molecule or quencher, depending on which has the lower energy triplet state), and radiationless decay to the (repulsive) ground state of the exciplex. In polar solvents a fifth step (dissociation into radical ions of the component molecules) may occur; we are concerned here with nonpolar solvents and with exciplex systems which do not lead to any irreversible photochemical step. The investigations t o be described here deal with exciplexes of anthanthrene with a number of derivatives of dimethylaniline, the resulting similarity in the structure of the exciplexes providing the basis for comparing the properties of these exciplexes with one another.
Experimental Section Anthanthrene (hereafter abbreviated to A) was purified by repeated recrystallization; 1,12-benzperylene (hereafter BP) was purified according to the method of Kajiwara et *Address correspondence to the author at CSIRO Division of Chemical Physics, P.O. Box 160, Clayton, Victoria 3168, Australia. 0022-3654/79/2083-1892$01 .OO/O
TABLE I: Oxidation Potentials, Exciplex Energies, and Quenching Kinetics for 1,12-Benzperylene and Anthanthrene Quenched by Substituted N,N-Dimethylanilinesa fluorescer
donor
1,12-benzperylene
p-Cl-DMA 3,5-DMDMA p-Me-DMA 2,4-DMDMA 3,4-DMDMA TMPD anthanthrene p-Cl-DMA 3,5-DMDMA p-Me-DMA 2,4-DMDMA 3,4-DMDMA TMPD
E ' , eV KL,M-' 2.96 2.88 2.77 2.65 2.58 2.28 2.66 2.58 2.47 2.35 2.28 1.98
328 504 1186 1840 2134 3000 127.5 152.0 183 187 217 213
k,, lo9 M-'s-' 1.73 2.65 6.24 9.68 11.23 15.8 17.2 20.5 24.7 25.3 29.3 28.8
a The anilines used, together with their abbreviations and oxidation potentials vs. SCE, are as follows: p-chloroN,N-dimethylaniline (p-C1-DMA, 0.84 V); 3,5-dimethoxyN,N-dimethylaniline (3,5-DMDMA, 0.76 V ) ; p-methylN,N-dimethylaniline (p-Me-DMA, 0.65 V ) ; 2,4-dimethoxyN,N-dimethylaniline (2,4-DMDMA, 0.53 V); 3,4dimethoxy-N,N-dimethylaniline(3,4-DMDMA, 0.46 V ) ; N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD, 0.16 V).
aL5 Purified samples of anthanthrene or 1,12-benzperylene
showed no indications of impurities in their fluorescence and flash photolysis spectra, and were used for all subsequent experiments. The amines used were all derivatives of N,N-dimethylaniline; they were prepared by methylation of the corresponding substituted amines followed by purification by vacuum distillation or by successive recrystallization. The abbreviations by which the amines are referred to in this paper as well as their oxidation potentials can be found in Table I. The solvent used in this work, n-hexane, was purified by distillation. All measurements were carried out at room temperature on samples which had been degassed by successive freezepump-thaw cycles. Fluorescence quenching measurements were carried out in a cuvet without stopcocks6with a Hitachi-Perkin-Elmer MPF2A spectrofluorimeter with digital readout; a typical fluorescence spectrum is shown in Figure 1. Corrected @ 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 14, 1979
Excipiexes of Anthanthrene
17
16
18
19
600 550
500
X
450
408
(nm)
Figure 1. Fluorescence spectrum (uncorrected) of anthanthrene quenched by 0.144 M p-methyidimethyianiiine in n-hexane. The structured emission at shorter wavelengths is due to unquenched anthanthrene (the primarily excited molecule); the broad emission to longer Wavelengths is due to the anthanthrene-p-methyidimethyianiiine excipiex.
fluorescence spectra and quantum yields were measured on a Fica 55 corrected spectrofluorimeter (SociBtB Francaise d'Instruments de Contr8le et d'Analyses); this was a grating instrument which gave spectra (corrected for the spectral response of photomultiplier and emission monochromator) linear in wavenumber. Since gratings give a dispersion linear in wavelength, the experimental spectra, which had been measured at constant slit width, had to be further corrected7 by multiplying by a factor X2. Absolute fluorescence quantum yields of the anthanthrene-amine samples were obtained by comparing the integrated corrected emission spectrum of the sample with that from a solution of fluorescein in 1 M NaOH; the fluorescence quantum yield of the latter solution was taken to be 0.90.8 Details of the fluorescence quantum yield measurements are as follows: prior to the measurements the optical density of the fluorescein solution at the exciting wavelength was made equal to that of the sample to be measured.8 This procedure had to be modified for the systems A 3,4-DMDMA and A 2,4-DMDMA, where the exciplex emission was too weak to allow a direct comparison with the fluorescein standard. The fluorescence quantum yields of these two systems were obtained by comparing them with anthanthrene-amine systems of known exciplex fluorescence quantum yield and having the same anthanthrene concentration. Since, in the wavelength range where only anthanthrene absorbed, the spectral properties of the two solutions were identical, the quantum yield measurements could be carried out with arbitrarily wide excitation slits, ensuring that the exciplex fluorescence emission from the weakly emitting sample was sufficiently intense to permit reliable measurements. The exciplex emission from A TMPD was too weak to be measured even by these methods. Apart from these modifications, the methods used here were essentially those advocated by Parker7 and Demas and Crosby.6 Each experiment was carried out at two excitation wavelengths; no variation in fluorescence quantum yield with wavelength was found for
+
+
+
21
(crn-' x IO-' )
WAVENUMBER
700
20
1893
Figure 2. Transient absorption spectrum obtained on flashing 7.5 X M anthanthrene with 0.10 M 3,4-DMDMA in n-hexane. Optical path length is 10 cm.
I
I
0
I'
/' /
.:
I
.
0.5
I
I
I 13
Faash lntensi t y
Figure 3. Extrapolated triplet absorbance E o plotted against incident flash intensity for 9.0 X IO" M anthanthrene in n-hexane (dashed line) and 9.0 X IO" M anthanthrene with 4.94 X lo-' M TMPD in n-hexane (solid line). The experimental points are also indicated.
any sample. The resulting exciplex quantum yields were corrected for refractive index differences between sample and ~ t a n d a r dand , ~ for incomplete quenching by the added amine.4 Transient absorption spectra and quantum yields of triplet formation were measured in a conventional microsecond flash photolysis apparatus; the transient spectra observed in these experiments confirmed that the exciplex decayed in all cases to give the triplet of anthanthrene. A typical spectrum is shown in Figure 2. The kinetics of the decay of the anthanthrene triplet were analyzed'O at a number of different incident flash intensities (appropriate cutoff filters ensured that only anthanthrene was excited), first without and then (by opening a break-seal in an appropriately constructed cuvet) with a known concentration of added amine. Plotting the triplet absorption extrapolated back to time zero, Eo, against incident flash intensity gives, in the linear portions, two straight lines the slopes of which are in the ratio of the triplet quantum yield of anthanthrene to that of the exciplex.ll The triplet quantum yield of anthanthrene was taken to be 0.23;12an example of the method is shown in Figure 3 for A + TMPD. Triplet quantum yields measured in this way were corrected for incomplete quenching of the primarily excited anthanthrene by the added amine. The reproducibility of this method has been established from
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The Journal of Physical Chemistry, Vol. 83, No. 14, 1979
1.5‘
I
I
I
20
2.2
2.4
I
2.6
E (D/Df) - E(A‘/A)
I
I
26
(V)
Figure 4. Excipiex emission energy hu as a function of €(DID+) €(A-/A) for (0) exciplexes of anthanthrene and (0)exciplexes of 1,12-benzperyiene. The solid line corresponds to eq 2.
duplicate experiments as being about &3%. It was noticed that the rate of decay of the anthanthrene triplet became slower on adding amine; a possible reason for this is that the amine scavenges adventitious triplet quenchers, particularly residual oxygen.13 Exciplex lifetimes were measured on a single-photon counting apparatus, except in the cases of A + 2,4DMDMA, A + 3,4-DMDMA, and A + TMPD, where the emission intensity was too low to allow reliable measurement. These measured exciplex lifetimes can be shown to be a good approximation to the true exciplex fluorescence lifetime^.^ The lifetime of anthanthrene in the absence of any quencher was measured with a pulsed nitrogen laser.
Results and Discussion Exciplex energies are given with tolerable accuracy by the relation1* E’ = E(D/D+) - E(A-/A) + 0.13 (1) where, in the present instance, E(D/D+) is the oxidation potential of the amine; E(A-/A) is the reduction potential of the aromatic, and was taken to be -1.69 V vs. SCE for 1,lZbenzperylene and -1.99 V vs. SCE for anthanthrene.15 It is well known16 that the energy corresponding to the exciplex emission maximum is given by hu = E(D/D+) - E(A-/A) - 0.15 (2) and the exciplex spectra obtained here conform satisfactorily to this relation, as Figure 4 shows. The scatter in the points is probably due to uncertainties in the redox potentials used. Table I lists values of the Stern-Volmer quenching constants, KL,for the systems investigated; the quenching rate constants, k,, were calculated by using the measured fluorescence lifetime of anthanthrene (7.4 ns) and a value of 190 ns for 1,12-benzperylene.17 It is noteworthy that, as the exciplex energy E’ decreases, k increases, reaching a maximum value of about 3.0 X laoM-’ s-’. In fact, exciplex formation is reversible, and inclusion of the “feedback” step leads to the (simplified) reaction scheme k,
‘M* t D +
1:
k2
A. R. Watkins
l(M-D+)*
(3)
1;
and to a quenching rate constant given by k, = k1/(1 + K~T’)
(4)
The rate constants hl and k 2 can only be obtained directly by following reaction 3 on a nanosecond time scale.18 The increase in k, with decreasing E’ suggests, however, that
+
AG ( e V ) Figure 5. Plot of log k2 as a function of AG(see text) for the following systems: (0)1,lP-benzperyiene with substituted dimethylanilines; (+) anthanthrene with substituted dimethyianiiines; (0)various excipiex system^;'^ (m) pyrene-dimethylaniiine in c y ~ i o h e x a n e ; ~(A) ~ pyrene-dimethyianiiine in benzene;” (A) pyrene-tributylamine in nhexane;36 (0) pyrene-diethyianiiine in methyicy~lohexane.~~
at low exciplex energies k2r’
