298
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978
Werner et
al.
Substituent Effects on the Fluorescence Properties of Aromatic Esters of 9-Anthroic Acid T. C. Werner,* William Hawkins, John Facci, Russell Torrisi, and Terry Trembath Department of Chemistry, Union College, Schenectady, New York 12308 (Received August 8, 1977) Publication costs assisted by the Petroleum Research Fund
The spectral properties of four aromatic esters of 9-anthroic acid, phenyl 9-anthroate (9-COOPh), phenyl 10-bromo-9-anthroate (10-Br-9-COOPh), p-nitrophenyl 9-anthroate (9-COO-p-N02Ph),and p-nitrophenyl 10-bromo-9-anthroate (lO-Br-9-COO-p-NO,Ph), have been investigated. Introduction of a nitro group onto the phenyl ring of 9-COOPh causes a large reduction of the fluorescence quantum yield (&) while substitution of a bromine at the 10 position of 9-COOPh and 9-COO-p-NOzPh causes a decrease in c$f for the former and an increase for the latter. The purpose of this study was to account for these observations. Arrhenius activation energies (EA)for the temperature dependence of c$f were calculated for all four esters by assuming that intersystem crossing was the only temperature-dependent process deactivating S1. These EAvalues when combined with the q5f values and measures of the mean frequency of fluorescene suggest that a new triplet level (T,) is introduced near S1 by nitro substitution. Efficient intersystem crossing to this new triplet results in a lower c$f for the nitro derivatives. Moreover, the data indicate that T, is primarily the n,r*triplet of the nitrophenyl portion. The lack of a normal heavy atom effect on $f for 10-Br-9-C00-p-N02Phis a result of the already large spin-orbit T,(n,T*) crossing. Substitution of bromine onto 9coupling interaction associated with this S1(*,r*) COO-p-NOzPhdoes cause a lowering of SI and an increase in EA by about 300 cm-l. Thus the greater c$f for the bromo derivative is a result of the greater EA for this molecule than for 9-COO-p-NOzPh.
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Introduction Over the past several years, we have been studying the spectral properties of carboxyl substituted anthracenes. We have found that the spectral properties of these molecules are very dependent on the position of substitution on the anthracene ring and on ~olvent.l-~ In the course of these studies we prepared several aromatic esters of 9-anthroic acid and it was observed that the spectral properties of these esters were quite dependent on the phenol derived portion of the ester. For example, the spectral properties of methyl 9-anthroate (9-COOMe) and phenyl 9-anthroate (9-COOPh) are similar. Both molecules possess a very high fluorescence quantum yield (4f 0.8-0.9). However when a nitro group is substituted onto the phenyl ring, as in p-nitrophenyl 9-anthroate (9COO-p-NO'Ph), the qbf value is greatly reduced (-0.14).3 Our initial suggestion to explain this quenching effect was that a triplet level of the nitrophenyl portion was situated in such a way that intersystem crossing was f a ~ i l i t a t e d . ~ Since we have not had success in locating the triplet levels of anthroate esters by direct spectral techniques,' we turned to indirect methods to check our suggestion. We will describe these methods in this paper. For most 9-substituted anthracenes, the only nonradiative process which competes with fluorescence is a thermally activated intersystem crossing to a triplet slightly above the lowest excited singlet We felt that if a new triplet level was introduced by the p-nitrophenyl portion, the activation energy (EA) derived from an Arrhenius plot of q5f vs. temperature data should be different for 9-COOPh and 9-COO-p-N02Ph. Thus, the energy gap between S1 and the accepting triplet (T,) should differ for the two esters and this difference could be probed by heavy atom quenchers. It has been shown that the efficiency with which a heavy atom quenches a fluorophore's fluorescence is inversely related to the gap between SI and T2.215We also recognized that another factor which could affect the efficiency of the heavy atom quencher is the nature of the accepting triplet. T, for 9-COOPh is undoubtedly a H,R* N
0022-3654/78/2082-0298$0 1.OO/O
triplet while for g-COO-p-NO,Ph, T, is likely an n , r * to triplet. In the latter case crossing is from S1(r,r*) Tx(n,r*)and this mechanism has an inherently significant spin-orbit coupling interaction. Therefore heavy atom enhancement of spin-orbit coupling may not be sufficient to increase the efficiency of intersystem crossing above that which already exists for 9-COO-p-NOzPh. To determine the effect which a heavy atom would have on qbf, we synthesized derivatives of 9-COOPh and 9COO-p-NOzPh which have bromines in the 10 position of the anthracene ring (10-Br-9-COOPh and 10-Br-9-COOp-N02Ph). We then determined the spectral properties, +f values, and EA values for thermal quenching of fluorescence for all four esters. We report here the results of this work.
Experimental Section I. Chemicals and Solvents. Spectroquality cyclohexane from Matheson Coleman and Bell and ethanol from U S . Industrial Chemicals were used for the spectral measurements. Methyl 9-anthroate was prepared from 9-anthroic acid (Aldrich Chemical Co.) and diazomethaneO6 Phenyl 9-anthroate and p-nitrophenyl9-anthroate were synthesized from 9-anthroic acid and phenol or p-nitrophenol according to the procedure of Parrish and Stock.7 The same procedure was used to synthesize phenyl 10bromo-9-anthroate and p-nitrophenyl 10-bromo-9anthroate except the starting acid was now 10-bromo9-anthroic acid. This acid was obtained from 9,lO-dibromoanthracene (Aldrich Chemical Co.) by the procedure of Mikhailov and Bronovitshaya.8 Proof of structures was obtained by comparison with known melting points where available and by a combination of infrared, proton NMR, and mass spectroscopy. 11. Instrumentation. UV visible absorption measurements were made on a Cary Model 118 spectrophotometer. Fluorescence emission measurements were recorded on a Perkin-Elmer MPF-2A spectrofluorometer 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978 299
Fluorescence Properties of Aromatic Esters of 9-Anthroic Acid
TABLE I: Spectral Data for the Aromatic Anthroates in Cyclohexane Compound @faT f , b ns k f b j f kisbsf 9-COOPh 10-Br-9-COOPh 9-COO-p-N02Ph 10-Br-9-C00-p-N02Ph
0.86? 0.04
0.434 f 0.005 0.139k 0.002 0.199 f: 0.002
12.6 f 6.3 ? 2.3 i: 3.0 f
0.5 0.5 0.5 0.5
6.8 6.9 6.0 6.6
1.1 9.0 37 26
&a,
b &?
yma,d,h
-1.6 1.64 i: 0.14(3)e 1.10 f 0.03(6Ie 1.31 f 0.07(4)e
2.10 2.09 2.05 2.02
cmh
-2.26 2.25 2.16 2.15
Ai 2.6 0.78 1.5
a Mean and 95%confidence interval. Mean T = 23 "C. Reference @f = 0.173 (methyl 9-anthroate in ethanol). frequency (see Results). e Number of determinations. f x ~ O - ' S - ~ . g X cm-'. x lo-'' cm-'. X lo-" s-'.
using excitation and emission band passes of 7 and 3 nm, respectively. The emission spectra were corrected for the response function of the instrument as described earlier.2 To record absorption and emission measurements as a function of temperature, ethanol at a given temperature was circulated through thermostated cell blocks in the above instruments using an FTS Systems Inc. Flexi Cool Unit. The temperature range for these measurements was 17-67 "C and the temperature was measured using a copper-constantan thermocouple. In the absorption work, the thermocouple, was placed directly into the sample cell above the light path. For fluorescence measurements, the sample cell was sealed off a t
