PETERM. FROEHLICH AND HARRY A. MORRISON
3566 binding between benzoic acid and an excited molecule. I n cases where there is no possibility of hydrogen bonding, an eltxtron transfer interaction occurs, but if the molecu1t:s have the ability to hydrogen bond then this interaction leadn to the fluorescence quenching. The results also lead to the conclusion that in the electron transfer oases all three species of benzoic acid (monomer, ring dimer, and linear dimer) may act as
quenchers. I n the hydrogen bonding case only the monomer and linear dimer can quench.
Acknowledgment. The authors are grateful to the Research Corporation for their financial support of these investigations. We would also like to acknowledge the assistance of the Nuclear Engineering Department of Lowell Technological Institute.
A Study of‘ Alkylbeazene Luminescence*l2
y Peter M. Froehlich and Harry A. Morrison* Department of Chemistry, Purdue University, Lafayette, Indiana 47907
(Received March 17, 197%)
The relative (to toluene) fluorescence (at room temperature and 77’K) and phosphorescence (77”K) intensities as well as excited singlet lifetimes of a number of alkylbenzenes are tabulated. Empirical correlations of the data include the previously observed “xylene effect” (whereby ortho and para substituents enhance # f ) and a new “a-substitution effect” (whereby alkyl substituents on a carbon a to the ring reduce 4f). In the latter ease, it can be shown that for monosubstituted alkylbenzenes, relative fluorescence intensity decreases as a ljnear fuiiction of the number of @-hydrogens. The ‘‘xylene” effect is shown to be a consequence of increased 7cf whereas the “a-substitution” effect is a result of decreased kt plus increased k d (two a-hydrogens) or a particularly large increase in k d (one or zero a-hydrogens) ( k d represents internal conversion and intersystem crossing). Though the “xylene effect” is insensitive to temperature, those compounds with enhanced nonmdi:ative decay show dramatic enhancements in fluorescence relative to toluene a t 77°K.
Introduction In the course of a study of the quenching of alkylbenzene luminescence by diene^,^ it was necessary to examine the fluorescence and phosphorescence emission of a number of alliylloenzenes, including many for which such spectral data have not yet been reported. The resulting quantum efficiency and lifetime data prove mefuI, not only in their own right, but also in that they permit valid csmpirical correlations to be discerned. Such an approach has been quite successfully demonstrated for I2enzenoid aromatics in the compendia by Berlmaq4 and further references to these works will be made throughout this paper.
Experimental Section Chemicals, Aromatic hydrocarbons were obtained from commercial sources and were tested for purity by vpc using either an Aerograph Autoprep A-700 or a Variaxi-Aerograph 90-P instrument equipped with a 20 f t x 0.25 in. 15% Carbowax on Chromosorb W DAICS-treatrd column or a 20 ft X 0.25 in. 30% Carbowax on Ghroaiosorb Y column. If the reagent contained impurities, it was purified by preparative vpc with one of the above columns. All aromatic compounds m r e molecularly distillchd and rechecked for The Journal of Physical Chemistry, Vol. 7 6 , N o . 14,1971
purity by vpc with a 5 ft X 0.125 in. 3’% SE-30 column before use. Burdick and Jackson “distilled in glass” hexane was used as received. The uv spectrum of each lot (in a 1-cm cell us. air) was recorded to ensure that there was no absorption above 230 nm. Phillips “instrument grade” isopentane (99.5 mol %) was passed through two chromatography columns containing either NIerck alumina or RJatheson Coleman and Bell silica gel. The uv spectrum of each lot (in a 1-cm cell us. air) was recorded to ensure that there was no absorption above 230 nm. Spectroscopy. Ultraviolet spectra were recorded using 1-cm quartz cells with a Cary 14 spectrophotometer. Absorbance measurements were obtained with the Cary instrument or a Gilford photometer (Rlodel 222A) coupled to a Beckman DU monochromator. (1) Organic Photochemistry, Part XX; Part XIX: P Froehlich and H. Morrison, Chem. Commun., 184 (1972). (2) Abstracted from the doctoral dissertation of P. Froehlich, Purdue University, Aug 1971. Presented, in part, a t the 162nd National Meeting of the American Chemical Society, Washington, D. C., Sept 1971, (3) P. Froehlich arid H. Morriuon, to be published. (4) I. B. Berlman, “Handbook of Fluorescence Spectra of Aromatic Hydrocarbons,” Academic Press, New York, N. Y . , 1965: (b) I. B. Berlman, ibid., 2nd ed, 1971.
p)
ALKYLBENZFSE LUMIXESCENCE
3567
---)___
Table 4 : R,elatiyre Fluorescence Intensities of Alkylbenzenes ?
-Room
Re1 fluorescencea
ALkylbensene
Benzene Tolueiie Ethyl7%-Propylisopropyln-B utylIsobuLyl.. sac-Buty itert-Butyl%-HexylGyclohexyln-Decyln-Nonadecyl1,2-Dimethyl1,3-Dimethy 11,4-Dimc?thyl1,4-Di-tert-buty 11,3,5-Trimethyl1,2,3-Trime18hyl1,2,4-Trimetbyl-. 1,3,5-Tri-terl-loul,yl112,3,4-Tetrarnet,hyl1,2,3,5-Tet1ramethyl1,2,4,5-Tetramet,hy-L Pentamethyl-, Hexameihyi-, Hexaeth yi-
0.26 (1.00) 0.77 0.82 0.52 0.82 0.86 0.62 0.23 0.88 0.64 0.85 0.82 1.21 0.94 1.60 0.44 0.63 0.58 2.03 -0.07 0.81 0.84 2.12 0.54 -0.01 -0.01
temperatur----
77OK re1 fluorescenced
+fb
0 . 058,e 0.071 0. 14,60.23' 0.15" 0.128 0.10e
0.036 (0.14) 0.11 0.11 0.076 0.11 0.12 0.088 0.032 0.12 0.089 0.12 0.11 0.17 0.13 0.22 0.061 0 088 0.081 0.28
0.126
(1.OO)
0.86 1.or
0.82 0.85 1.02 0.72 0.58 0.91
0.13$ 0.16," 0.20' 0.14,80.18' 0,33,$0.24f 0.14," 0.14f 0.08' 0,34,$0.31
1.25 0.95 1.55 1.48 Q.61 0.64
NO.01
0.11 0.13 0.30 0.075
0.32' 0,028
