1137
Communications to the Editor
COMMUNICATIONS TO THE EDITOR
Downloaded by UNIV OF SUSSEX on September 13, 2015 | http://pubs.acs.org Publication Date: May 1, 1975 | doi: 10.1021/j100578a019
Exciplex Formation between 2-Amlnopyridine and p-Nitroaniline
Sir: Interest in exciplex phenomena has been increasing since the initial reports by Leonhardt and Weller1-3 that fluorescence quenching via charge transfer in the excited state can lead to a new chemical species with its own individual spectral properties. In the course of studying the fluorescence of 2-aminopyridine, 2-AMP, we have observed that p-nitroaniline, PNA, is a singlet quencher but more importantly leads to a new fluorescence emission, which we believe to be exciplex emission. The fluorescence of aminopyridines has been characterized in previous reports from M 2-AMP in EPA this l a b ~ r a t o r y .Excitation ~,~ of 4 x with 254-nm excitation leads to a fluorescence with a wavelength maximum at 340 nm and a quantum yield of -0.2. Upon addition of PNA the fluorescence of 2-AMP decreases and leads to a new emission, which is unstructured and shifted to longer wavelengths from the normal molecular fluorescence. p-Nitroaniline is nonfluorescent in all solvents studied, when excited with 254- or 366-nm excitation. The fluorescence behavior of 2-AMP in the presence of p nitroaniline is presented in Figure 1,where a new emission is seen to grow in at -410 nm concomitantly with decreasing 2-AMP fluorescence upon addition of PNA. The reversal in the exciplex emission intensity when comparing spectra (3) and (4) is due to increased absorption of PNA, which leads to fewer excited singlets of 2-AMP available
for exciplex formation. The uv absorption spectra of the two molecules exhibit no interaction, so that a ground-state complex can easily be excluded. Since p-nitroaniline has a lower lying singlet state than 2-AMP, we have also excited PNA directly with 366 nm, a wavelength which is not absorbed by 2-AMP; however, no exciplex emission is observed. The exciplex emission is also observed in ethyl ether and in acetonitrile solutions with emission wavelength maxima of 396 and 405 nm, respectively. This shift of exciplex emission with solvent is quite normal and indicates a species of strong polar character in the excited-state complex. It is quite evident that 2-AMP is the electron donor in this new exciplex system and that the ground state rather than the excited singlet state of PNA is involved in the charge-transfer process which results in exciplex formation. The decreasing fluorescence observed for 2-AMP, corrected for the strong PNA absorption in the 250-nm region, and the increasing emission upon addition of PNA indicate that the excited singlet state of 2-AMP is required for exciplex formation. The following scheme indicates the origin of two emissions observed
+
(2-AMP)*'
4 hv,
PNA
T-
[(2-AMP)*-**(PNA)'IC
.1
hVF'
The fluorescence yield of 2-AMP is not affected by degassing; however, the exciplex emission yield is reduced about 15% in air-saturated solutions. This exciplex system has the interesting property that the electron acceptor, p-nitroaniline, is nonfluorescent and thus provides an interesting case of perturbation of a nonfluorescent molecule to produce luminescence. In addition PNA is a bifunctional molecule which possesses an electron donor and acceptor group. The fact that the exciplex emission is also observed in acetonitrile, whereas charge-transI .o (2-AMP1
=4 x
16 'M,
, ,
0.8
t
0
360
420
480
WAVELENGTH, nm
,
0 0
Figure 1. Fluorescence spectra of 4 X M 2-aminopyridine in EPA at room temperature as a function of added pnitroaniline conM, (3) 2.9 X M, centration: (1) no p-nitroaniline, (2) 1.4 X and (4) 7.2 X M (A,, 254 nm).
2
._ e--:--*-----
4 (PNA) x
6
8
1
, 0
M.
Figure 2. Stern-Volmer plot for the fluorescence quenching of 4 X M 2-aminopyridine in EPA by pnitroaniline. The Journal of Physical Chemistry, Vol. 79, No. 11, 1975
Communications to the Editor
I 138
Downloaded by UNIV OF SUSSEX on September 13, 2015 | http://pubs.acs.org Publication Date: May 1, 1975 | doi: 10.1021/j100578a019
fer complexes such as anthracene-diethylamine in the same solvent lead to cation and anion production,6 indicate that in (2-AMP-PNA)*I there is significant resonance interaction which stabilizes the exciplex. The Stern-Volmer fluroescence quenching plot of 2-AMP with p -nitroaniline is shown in Figure 2 and the half-concentration value of M. This value is indicative of PNA is seen to be 1.5 X a relatively stable complex. Fluorescence quenching of the absorbing monomer through exciplex formation gives a nonlinear Stern-Volmer plot, which results from an equilibrium between the monomer and exciplex species. The ordinate in Figure 2 is normalized to the fluorescence yield M solution of 2-AMP in the absence of PNA, for a 4 X and the values are corrected for the percentage of light absorbed by PNA at 254 nm. Although the large PNA absorption a t 254 nm complicated the determination of the exciplex emission yield, we estimate that its fluorescence yield is approximately 1%of the value for 2-AMP or 0.002. The new exciplex system described here provides a basis
The Journal of Physical Chemistry. Vol. 79, No. 11, 1975
for generating many new and novel exciplex systems. Further studies are in progress and will be discussed in a future report.
References and Notes H. Leonhardt and A. Weller, 2.Phys. Chem. (Frankfurt am Main), 29, 277 (1961). H. Leonhardt and A. Weller, "Luminescence of Organic-lnorganlc Materials", H. P. Kallmann and 0. M. Spruch, Ed., Wiley, New York, N.Y., 1962, p 74. H. Leonhardt and A. Weller, Ber. Bunsenges, Phys. Chem., 67, 791 (1963). A. Weisstuch and A. C. Testa, J. Phys. Chem., 72, 1982 (1968). A. C. Testa, A. Welsstuch, and J. Hennessy in "Molecular Lumlnescence", E. C. Lim, Ed., W. A. Benjamin, New York, N.Y., 1969, p 863. H. Knibbe, D. Rehm, and A. Weller, Ber. Bunsenges. Phys. Chem., 72, 257 (1966).
Department of Chemistry St. John's University Jamaica, New York 11439 Received January 8, 1975
J. Wolleben A. C. Testa'
