1227
AGGREGATION OF ARYLAZONAPHTHOLS
The Aggregation of Arylazonaphthols.
11.
Steric Effects on Dimer Structure in Water by Alan R. Monahan,* Nicholas J. Germano, and Daniel F. Blossey Research Laboratories, Xerox Corporation, Xerox Square, Rocheater, New York 14603 (Received October 12, 1970) Publication costs assisted by The Xerox Corporation
The absorption spectra of three ionic arylazonaphthol dyes (3'-alkyl-4'-chloro-6'-sulfophenylazo-2-hydroxyl3-naphthoic acids) in aqueous acidic solution (pH 3.15) are analyzed in terms of monomer-dimer equilibria in the 10-8 to 10-4 mol 1.-1 concentration range. From the changes in the absorption spectra with dye concentration, the dimerization constant (Kes = cd/cm2), the pure monomer spectrum, and the pure dimer spectrum are calculated for each dye. The dimerization constants are (6.83 i 0.80) X lo4,(5.55 0.39) X IO4, and (3.40 ==I 0.71) x lo3 1. mol'' for the (I) methyl-, (11) ethyl-, and (111) isopropyl-, 3'mbstituted dyes, respectively. Each dimer spectrum consists of an intense H-band hypsochromic to the monomer band, and a weaker J-band, bathochromic to the monomer band. The J-band strength, relative to the H-band, increases with increasing size of the substituent thus indicating steric effects in the dimer structure. As calculated from the relative strengths of the split bands in the dimer spectrum, the twist angle between equivalent molecular axes of the molecules in the dimer increases from 64 ==I 1' in dye I, to 72 i 0' in dye 11,to 88 =k '1 in dye 111. Both in-plane and parallel-plane structural models are considered for the dimer with the in-plane model giving the most consistent results.
*
Introduction It has been known for many years that most ionic organic dyes in aqueous solution deviate from strict Beer's law behavioral These departures from normal solution spectra behavior are generally interpreted as being due to the formation of dimers and higher order aggregates. Thus, by measuring the changes in the observed extinction coefficient with changes in the dye concentration, the dye aggregation processes can be observed directly by spectrophotometric techniques.2 For example, the absorption spectra of most ionic hydroxy azo dyes in water show a decrease in the strength of the main absorption band accompanied by a hypsochromic shift (shift to higher energy) in the peak maxThe spectral ima with increasing dye concentration. changes of azo dyes with the degree of dye aggregation are generally small and cause only minor alterations in the absorption spectra of the azo dyes. This is in marked contrast to the phthalocyanines2 and cyanine dyes4 which are known to form new, easily resolved absorption bands upon aggregation. The differences between the degree of spectral change upon aggregation for the azo and cyanine are due in part to the differences between the absorptive strengths of the main absorption band of the isolated dye molecules. Azo dyes5 have transition dipole strengths on the order of erg cm3 and cyanine dyess on the order of erg cm3. From molecular exciton t h e ~ r ythe , ~ energy splitting of the bands in the dimer spectrum is roughly proportional to the absorptive strength of the monomer band which would explain the relatively small changes between the 213
monomer and dimer spectra of azo dyes as opposed to the large changes for cyanine dyes. Since the spectral changes upon aggregation are dependent in many ways on the properties of the isolated molecule, the degree of spectral change is not a direct indication of the degree of aggregation. I n fact, both ionic arylazonaphthol dyes5 and cyanine dyes4&have equilibrium constants for dimer formation, K,, = cd/cm2, on the order of lo4 1. mol-' in spite of the fact that their spectral changes are quite different. Thus, the spectral changes with dye concentration must be converted into association constants to fully evaluate the affinity for aggregation of the dye molecules. Also, in order to interpret the spectra in terms of structural dimer models,' the pure monomer and pure dimer spectra must be separated. For cases where there is small overlap between the monomer and dimer bands, as is the case for the cyanine dyes, this separation is accomplished quite easily. I n the case of azo dyes, however, the monomer and dimer bands are completely over(1) S.E. Sheppard, Proc. Roy. Soc., Ser. A , 8 2 , 256 (1909). (2) S. E.Sheppard and A. L. Geddes, J. Amer. Chem. SOC.,66, 2003 (1944). (3) E. Coates, J . SOC.Dyers Colour., 85, 355 (1969). (4) (a) W. West and S. Pearoe, J. Phya. Chem., 69, 1894 (1965); (b) W. West, S. P. Lovell, and W. Cooper, Photogr. Sci. Eng., 14,52 (1970). (5) A. R. Monahan and D. F. Blossey, J. Phys. Chem., 74, 4014 (1970). (6) E. S.Emerson, M. A. Conlin, A. E. Rosenoff, K. S.Norland, H. Rodriguez, D. Chin, and G. R. Bird, ibid., 71,2396 (1967). (7) M.Kasha, H.R. Rawls, and M. Ashraf El-Bayoumi, Pure AppZ. Chem., 11,371 (1965). The Journal of Physical Chemistry, Vol. 76, No. 9, 1971
1228 lapping which makes their separation more difficult.
A computer program, which was outlined in a previous paper,5 has been developed to separate the monomer and dimer contributions to the spectra and calculate the equilibrium constants for dimerization. I n this study, the effect of varying the alkyl substituent on the aryl portion of an ionic arylazonaphthol was investigated. The ionic arylazonaphthols have been chosen because their aggregation processes have not been studied before due to their small spectral changes with aggregation, and a more detailed knowledge of the aggregate structure is desired in view of the importance of these dyes in the dying of polymerss and in color reproduction.9 The object of this study was to determine the magnitude to which a structural variation in the dye molecule changes the stability and structure of the dimer. Similar studies have been carried out on chlorophylls10 and cyanine dye~.~a,BEmerson, et al.,e have shown that methyl substitution in the center of a cyanine dye results in an aggregate band hypsochromic (H-band) to the monomer band. Conversely, ethyl substitution results in a bathochromic (J-band) shift upon aggregation. I n this paper we report studies of the monomer-dimer equilibria of three molecules of the structure
in aqueous acidic solution (pH 3.15) where R = CH3, C2H.5, and i-C3H7. The methyl-, ethyl-, and isopropylsubstituted dyes will be referred to in the following as dye I, dye 11, and dye 111, respectively. The visible absorption spectra of the three dyes in aqueous solution were measured in the concentration range of to mol The spectra were analyzed using previously reported computer techniques5 which separate the monomer and dimer contributions to the spectra a t each concentration. The calculated monomer and dimer spectra were then analyzed in terms of molecular exciton theory’ to determine possible dimer structures. Analyses of the results suggest dimer structures which are consistent with dimer stability, which is defined in terms of the standard free energy for dimer formation.
Experimental Section Preparation of the Dyes. The three dyes were prepared by standard preparative proceduresll from 2hydroxy-3-naphthoic acid (Pfister) and the corresponding 2-amino-4-alkyl-5-chlorobenzenesulfonic acids. The 4-methyl- and ethylamines were commercial samples supplied by Eastman Organic Chemicals and American Cyanamid, respectively. The isopropylaminochlorobenzenesulfonic acid was prepared from cumene.12 The structure of the isopropyl compound The Journal of Physical Chemistry, Vol. 76, No. 9,1071
A. R. MONAHAN, N. J. GERMANO, AND D. F. BLOSSEY was confirmed by direct comparison with the infrared and nmr spectra of the methyl and ethyl analogs. Anal. Calcd for C9H12C1N03S: C, 43.3; H, 4.9; C1, 14.1; N, 5.6; S, 12.9. Found: C, 43.0; H, 5.1; C1, 14.1; Tu’, 5.4; S, 12.3. The amines were recrystallized twice from a sodium acetate deionized water solution and 2-hydroxy-3-naphthoic acid was recrystallized twice from isopropyl alcohol. After coupling a t pH 9, each dye was washed several times with distilled water and recrystallized twice from an isopropyl alcohol-water mixture to a constant extinction coefficient. The disodium salts of the dyes were obtained in each synthesis. The compounds were dried under vacuum for 12 hr a t 60”. Anal. Calcd for dye I : C1sH11N2Na2OBSC1: C, 46.5; H, 2.4; N, 6.0; Na, 9.9; S, 6.9; C1, 7.6. Found: C,46.3;H,2.6;N,6.2;Naj9.6;s) 6.8;C1,7.4. Anal. Calcd for dye 11: C1sH13N2Na2O6SCl: C, 47.7; H, 2.7; N, 5.9; Na, 9.6; S, 6.7; C1, 7.4. Found: C, 48.0; H, 2.4; N, 6.2; Na, 9.6; S, 7.0; C1,7.7. Anal. Calcd for dye 111: C20H~JS2iYa20eSC1:C, 48.8; H, 3.1; N, 5.7; Na, 9.3; S, 6.5; C1, 7.2. Found: C, 48.8; H, 3.3; K, 5.9; Na, 9.6; S, 6.5; C1,7.5. Preparation of Solutions. The spectroscopic measurements were performed using dye-water solutions having a p H of 3.15 0.05 M HC1). The p H was chosen such that both the hydroxy and carboxylate groups of the dye molecules would be protonated whereas the sulfonic group would be ionized. For calculation of the proper conditions for protonation of the carboxylate group, the dissociation constant of @naphthoic acid was used. The dissociation constant for p-naphthoic acidlSis 7 X and is an upper limit for the dissociation constant of the carboxylate group on the dye molecules because of the electronic character of the other substituents on the naphthol group.I4 Therefore, at pH -3, the fraction of carboxylate groups protonated to those ionized is at least 1OO:l. The hydroxy group is also protonated because to ionize the hydroxy portion of the molecule, a p H of about 10 would be r e q ~ i r e d . ~The sulfonate group is definitely ionized because, as Reeves and KaiserI5 have recently shown, the azo dye sulfonates can be converted into protonated species only a t very low pH (0.1 to 3 M aqueous HzS04). (8) E. J. G. Bailey, J .
SOC.Dyers Colour., 85, 571 (1969). (9) J. A. C. Yule, “Principles of Color Reproduction,” Wiley, Nev York, N. Y., 1967, p 193. (10) R. Sauer, J. R. Lindsay Smith, and A. J. Schultz, J . Amer. Chem. Soc., 88, 2681 (1966). (11) H. Zollinger, “Azo and Diazo Chemistry,” Interscience, New York, N. Y., 1961, pp 311-360. (12) F. H. Adams, U. S. Patent 2,598,483 (1952). (13) “Handbook of Chemistry and Physics,” 48th ed, The Chemical Rubber Co., CleveIand, Ohio, 1967-1968, p D-90. (14) C. D. Ritchie and R. E. L’schold, J . Amer. Chem. SOC.,90, 2821 (1968). (16) R. L. Reeves and R. 8. Kaiser, J . Phys. Chem., 73, 2279 (1969).
1229
AGGREGATION OF ARYLAZONAPHTHOLS For each dye, five solutions were prepared in the to M concentration range. The water was obtained from a RIillipore Super-& system (18 megohms) and acidified to pH 3.15 using HC1. The pH of all solutions was within 10.05 pH unit before and after spectrophotometric analysis. Spectra were run on a Cary Model 14R automatic spectrophotometer using 0.1-, 1-, 5-, and 10-cm matched quartz cells.
DYE I ' I N WATER CT=3,26 x 10-6
Results Concentrafion Dependence. The concentration dependence of the absorption spectra of dye I is shown in Figure 1. Isosbestic points occur at 17,700 and 20,100 cm-1. At the most dilute concentration of 3.26 X 10-6 M , the main peak (monomer peak) occurs a t 19,500 em-l. Successively higher concentrations result in a blue shift of this peak and a measurable decrease in apparent extinction coefficient. The concentration dependencies of the spectra of dye I1 and dye I11 are qualitatively similar to dye I in that both show dilute solution maxima near 19,500 cm-l. However, isosbestic points appear at 17,900 and 20,600 cm-1 for dye I1 and at 18,100 and 21,600 cm-l for dye 111. The shift of these isosbestic points with molecular structural changes is due to the differences in physical size of the substituents (see Discussion). Monomer-Dimer Equilibrium. The monomer-dimer equilibrium of dye I1 in aqueous solution has been reported previously for pH 6.00 and the same concentration range.5 Under these conditions, the carboxylate group, as well as the sulfonate group, is ionized. I n this paper, the three dyes are analyzed at pH 3.15 so that both structural and pH dependencies could be compared. It has already been noted that a t pH 3.15, the carboxylate group is protonated whereas the sulfonate group is ionized. The calculation of the pure monomer spectrum, pure dimer spectrum, and equilibrium constant is accomplished using a computer procedure which was reported previously.6 In the analysis, it is initially assumed that the monomer concentration cm and dimer concentration c d follow the law of mass action
Keq =
Cd/Cm2
=
(Ct
- crn)/2
w
0 1.20
1.60
2.00
2.40
2.80
3.20
3.60
5 (cm-1) I( 10-4 Figure 1. Concentration-dependent spectra of dye I in water. All spectra obtained a t 22'.
!!
0 DYEI:
Id8' 10-6
Keq~(6.83'0.80)x104 LITER MOLE'' .
I
16
I
I
1
I
I
I
1
16~
MONOMER CONCENTRATION
Figure 2. Plot of log cd vs. log cm for three dyes in water. Solid lines drawn with theoretical slopes of 2.
(1)
where K,, is the association (equilibrium) constant for dimer formation. If the total concentration ct is small enough that no higher aggregates are present, then cd
20.50
(2)
Assuming that eq 1 and 2 hold, a best-fit monomer spectrum and dimer spectrum can be calculated for a given K,,. By varying K,, systematically, the best em, E d , and Keq are established where em and Ed are extinction coefficients of monomer and dimer, respectively. To make the calculation self-consistent, the best Em and Ed are used t o calculate K,, at each concentration and thereby establish limits of errors on Ke, and any resulting parameters.
The results of this analysis for dyes I, 11, and I11 are shown in Table I . Each dye was measured at five concentrations in the 10-6 to 1. mol-' concentration range. For each concentration, the total concentration ct, monomer concentration cm, dimer concentration c d , and equilibrium constant K,, are listed. The fit of the concentrations cm and Cd to eq 1 is demonstrated graphically in Figure 2 . The solid lines are drawn with a theoretical slope of 2.
Discussion Substituent Effects on Dimer Structure. Analysis of a, homologous series of ionic arylazonaphthol dyes allows direct interpretation of steric effects on dimer structure. The Journal of Physical Chemistry, Vol. 76,No.9,1971
1230
A. R. MONAHAN, N. J. GERMANO, AND D. F. BLOSSEY
Table I: Monomer-Dimer Equilibrium of Dyes in Water (pH 3.15) a t 22O Dye I: K 8 , = (6.83 f 0.80) X 1041. mol-' Total dye concentrations, mol 1.-1 ( X 108) Monomer concentrations, mol L-1 (x 106) Dimer concentrations, mol 1.-1 ( x 108) Equilibrium constants, 1. mol-1 ( X 10-4)
3.26
8.15
2.44
5.01
7.35
0.408
1.57
4.48
9.87
6.82
6.27
8.29
5.97
16.3
32.6
163.0
12.9
30.8
66.1 6.96
Dye 11: K,, = (5.55 i 0.39) X 1041. mol-' Total dye concentrations, mol 1.-1 ( x 108) Monomer concentrations, mol I.-' ( x 108) Dimer concentrations, mol L-1 ( X 108) Equilibrium constants, 1. mol-' (x 10-4)
2.14
5.35
1.79
3.80
6.40
0.173
0.777
2.55
5.49
5.40
5.39
6.24
5.05
Dye 111: Total dye concentrations, mol L-1 ( x 106) Monomer concentrations, mol 1.-1 ( X 108) Dimer concentrations, mol 1.-1 ( x 108) Equilibrium constants, 1. mol-1 ( X 10-4)
K,, =
16.5
21.4
107.0
10.4
26.7 40.1 5.62
(3.40 i 0.71) X 1031. mol-'
2.09
5.40
10.8
21.6
108.0
2.05
5.21
10.3
18.9
72.5
0.0188
0.0973
0.249
1.37
0.447
0.359
0,234
0.384
The three substituent groups, methyl, ethyl, and iso* 0 5 * 2 propyl, are equivalent in terms of electronic interaction with the ring structure but are different in terms of d ' 2.00 physical size. Thus, i j steric bulk appreciably affects dimer stability, the three dyes should show an inverse relationship between substituent size and dimer stability which indeed they do. The equilibrium constants and free energy for dimer formation are given in Table 11. Note that the equilibrium constant for dimer formation deg 1.00 creases as the steric size of the alkyl substituent ina W creases.
17.7 0.337
--
DYE I IN WATER
-0-
- €DIMER MONOMER
20.50 w
Table 11: Equilibrium Constants and Free Energies for Dimer Formation Keqt
Dye
I I1 I11
0
- AGOtos,
1. mol-]
koa1 (mol dimer) -1
(6.83 ztO.80) X lo4 (5.55 +C 0.39) X lo4 (3.40 =k 0.71) X lo3
6.60 st 0.07 6.47 f 0 . 0 5 4 . 6 5 0.10
The relative stability of the dimer species of the t,hree dyes is reflected in their free energies of dimer formation which indicate that the dimer of dye I is more stable than the dimer of either dye I1 or dye 111 and that the dimer of dye I1 is more stable than the dimer of dye 111. The Journal of Physical Chemistry, Vol. 76,No. 9,1971
1.20
-1.60
1
I
2.00
V
2.40
2.80
3.20
3.60
(cm-1) x 10-4
Figure 3. Calculated absorption spectra of pure monomer and dimer species of dye I in water. The symbols on curves in this figure and following figures for notation purposes only.
The calculated pure monomer and dimer spectra for the three dyes are shown in Figures 3, 4,and 5 . The spectra are interpreted in terms of molecular exciton theory which predicts that the energy levels of the monomer specie are split in the dimer specie.' The splitting is a function of the geometrical structure of the
1231
AGGREGATION OF ARYLAZONAPHTHOLS I
I DYE
I
I
I
SI IN WATER
The amount of information available in the pure monomer and dimer spectra is not enough to specify exactly the dimer structures, but insight can be gained by examining the simpler models. The inputs are the monomer oscillator strength fm, the dimer J-band strength f ~ the , dimer H-band strength f ~ and , the splitting AT. From these inputs, it is possible to calculate an angle a between dipoles of the adjacent molecules in the dimer and a separation distance R between the molecules. If more structural variables are included, then the problem becomes insolvable. The splitting AT from molecular exciton theory’ is given by
i; (cm-1) X I O - 4 Figure 4. Calculated absorption spectra of pure monomer and dimer species of dye I1in water.
DYE J E IN WATER
(3) where D is the monomer dipole strength and G is a geometry factor which, for the simplest dimer models, is only dependent on the angle a. The two models which allow transitions to both J- and H-bands are a parallelplane twist angle model and an in-plane oblique angle model. The angle for both models is the angle a which is defined as the angle between the transition dipoles of the two molecules in the dimer. For these two simple dimer configurations, the geometry factor G of eq 3 is given by
G
= cos a
G = cos a
+ 3 sin2
(parallel-plane model)
(4a)
-
(4b)
(3
(in-plane model)
The models are chosen so that there are only two variables R and a whereby a solution is tractable. The angle a is independent of the model chosen and is given by a = 2 tan-1 (jHfj/~jfH)”~
(5) and fJ are the peak positions and os-
dimer. Each of the dimer spectra in Figures 3,4, and 5 show a strong H-band on the high-energy side of the monomer band and a weaker J-band on the low-energy side of the monomer band. The peak positions and splittings of the spectra for the monomer and dimer species are summarized in Table 111. Table 111: Spectral Peak Positions and Splitting, cm-1, for Monomer and Dimer Species -----Dimez----~ Dye
Monomer
iE
0
I
19,500 19,400 19,400
19,900 19,700 19,300
18,000 18,000
I1 I11
18,000
Splitting A?, BE - BJ
1,900 1,700 1,300
The resolution of the instrument is about 100 em-l for the spectral range of interest so that the numbers in Table 111are accurate within this limit.
where TH, TJ, fH, cillator strengths for the H-band and J-band in the dimer spectra. Equation 5 is the same regardless of the dimer structural model chosen. The oscillator strengths of the bands and the calculated structural parameters for the dimer are given in Table IV. The two dimer models which allow transitions to both excited dimer states and depend only on R and a are considered for the three dyes. Schematic diagrams of the two models are shown in Figure 6 . A hypochromic effect is noted for each of the dye systems upon dimerization. This is a common feature for many dye dimers. l6 Independent of the model, the dimer structure shows a pronounced steric effect. The ratio of fJ t o fH increases as the substituent size increases. From eq 5 , this also implies that the angle between the transition dipoles of the molecules in the dimer increases as the substituent size increases; indeed, the angle increases (16) K . K. Rohatgi, J . Mol. Spectrosc., 27, 545 (1968). The Journal of Physical Chemistry, Vol. 76,No. 9,1971
1232
A. R. MONAHAN, Tu'. J. GERMANO, AND D. F. BLOSSEY
Table IV : Oscillator Strengths and Dimer Structure
Monomer strength, f m Dimer J-band strength, f~ Dimer H-band strength, fa fJ/fH
2, i? (i) In-plane oblique angle model (ii) Parallel-plane twist angle model
I
I
I
R
I
/
Dye I
Dye I1
Dye I11
0.34 f 0.02 0.08 f 0.01 0.23 f. 0.01 0.35 64 f. 1
0.38 f. 0.02 0.09 & 0.01 0.21 f 0.01 0.43 72 f 0
0.34 f 0.00 0.15 f 0.00 0.18 f 0.00 0.83 88 f 1
6 . 3 f 0.1 4.4 f0 . 1
6.6 f0.1 4.0 & 0.0
7.4 f0.0 2 . 1 f 0.0
,
/
/
/ /
B I
PARALLEL PLANE DIMER MODEL
in going from dye I to dye 111. It seems reasonable that a larger twist angle and reduced dimer stability as reflected in a smaller Kegor larger AG indicates a reduction in the binding of the two dye molecules in the dimer. For this reason, the separation distances obtained for the in-plane oblique transition dipole model are the most reasonable since R and a increase as K,, decreases. The values of R are severely model dependent; and, since the exact model is not known, only the trends are significant and valuable in choosing the most consistent model. p H Effects on Dimer Structure. The monomer-dimer equilibrium of dye I1 in aqueous solution at p H 6.00 has been previously reporteda6 For approximately neutral conditions, both the carboxylate and sulfonate groups are ionized, whereas at pH 3.15, the carboxylate group is protonated and the sulfonate group is ionized. By comparing the results at these two values of pH, a qualitative picture of Coulombic repulsion effects on dimer structure should evolve. The pH dependencies of several monomer and dimer parameters are shown in Table V.
Table V : pH Dependence of Dye I1 Aggregation Parameters
--
K,, X 10-4, 1. mol-1 - AGo20~,kcal (mol dimer)-- l Monomer strength, f m Dimer J-band strength, j " ~
kR+ 28=+-a
fJ/fH 01, deg Splitting A:, cm-l
R, i i IN-PLANE
DIMER MODEL
Figure 6. Schematic diagrams for parallel-plane and in-plane dimer geometries.
from 64" for the methyl substituent to 72" for the ethyl substituent to 88" for the isopropyl substituent a t the 3' position on the dye molecule. The model must account for this marked substituent effect on the dipole twist angle as well as the reduced stability of the dimer The J O U T n d of Physical Chemistry, Vol. 76, No. $, 1971
(i) In-plane oblique angle model (ii) Parallel-plane twist angle model
pH 3.15 (dye 11-)
pH 6.00 (dye II*-)
5 . 5 5 f 0.39 6.47f0.05 0.38 f 0.02 0.09 f 0.01 0.43 72 f 1 1700 f 100
1.25 f 0.00 5.58 f. 0.00 0.35 f 0.00 0.07 f 0.00 0.32 63 f 0 2200 f 100
6.6=t0.1
5 . 7 =I=0 . 0
4.0 f0.0
4 . 3 f0.0
The equilibrium constants and free energies for dimer formation are reasonable in that the doubly ionized molecule has a less stable dimeric state than the singly ionized molecule. From separation distance R, the parallel plane model is most consistent with the pic-
PHOTOPHYSICAL PROCESSES OF WZ-DIFLUOROBENZENE ture of the doubly ionized molecule having a less stable dimeric state than the singly ionized molecule. Since the pH and steric dependencies suggest different dimer models, an intermediate case is probably the most likely. The trends in K,, and a are certainly meaningful in this study since they are independent of the dimer model. However, R is strongly dependent on the geometry factor G and is not dependable for data interpretation. Taking all things into account, the inplane oblique angle dimer model gives the most satisfactory results in that it is consistent with the picture of
1233
sterically hindered dimer formation. The parallelplane twist angle dimer model is most consistent with the pH dependencies, but interpretation is not as straightforward as with the substituent effects. The inconsistencies point up inadequacies in the simple molecular exciton theory when used to evaluate aggregate structure. AcknowledgmenZ. The authors would like to thank Allen F. DeLuca for solution preparations and spectrophotometric measurements.
Photophysical Processes of rn-Difluorobenzene by Terry L. Brewer University of Texas, Austin, Texas 78712
(Received September 30,1970)
Publication costs asaisted by the Camille and Henerg Dreyfus Foundation
The absorption and fluorescent spectra (of rn-difluorobenzene) are reported. The 0-0 band is at 263.5 nm. The quantum yield of fluorescence has been found to increase with increasing wavelength and reaches a maximum of 0.16 0.02 at the 0-0 band. Vibrational relaxation studies using butane as the quenching gas indicate that relaxation is a multicollisional process. The amount of energy lost per collision within a large error is 66 em+. The present data in conjunction with other data indicate that internal conversion from the first excited singlet to the ground state is at most a minor process.
Introduction
Experimental Section
Benzene, although the most thoroughly studied of the single-ring aromatics, is a poor theoretical standard for comparative fluorescent studies in aromatic compounds, since the relatively long radiative lifetime of the first singlet-singlet transition makes it a poor competitor with radiationless processes. This is exemplified in the comparison of the fluorescent yield of benzene with the yields from alkyl-substituted benzenes with allowed first singIet-singlet transitions.' However, when fluorobenzene is brought into the comparison it becomes obvious that factors other than radiative lifetimes are important in determining relative quantum yields of fluorescence. Fluorobenzene has a relatively short radiative lifetime of 100 nsec (ca. the lifetime of benzene), but its fluorescent quantum yield of 0.21 is only slightly higher than that of benzene.2 It thus becomes important to know the effect of additional fluorines attached to the benzene ring. The fluorescence of m-difluorobenzene has been previously studied. However, its unusual wavelength dependence and quantum yields suggest further investigation.
Chemicals. The m-difluorobenzene was purified by use of preparative gas chromatography and degassed by three freeze-thaw cycles before use. The butane was Matheson Co. instrument grade, which was degassed via the freeze-thaw process before use. Equipment. The optical train consisted of a 1000W xenon point source lamp, a Bausch and Lomb monochromater with grating of 600 lines mm.-' Slit widths were set a t 0.5 mm. This gave a band pass of 8 A. The wavelength settings were checked with a 500-W, high-pressure mercury arc. The light from the monochromator was collimated by a lens and passed through the T-shaped cell 4-cm long with windows of 2 cm in diameter. The windows were optically flat Suprasil quartz. The center of the side window was 1.3 cm from one end of the cell. At the end of the optical train was an RCA 935 phototube operated a t 90 V with (1) W.A. Noyes, Jr., and C. 8. Burton, Ber. Bunsenges. Phys. Chem., 72, 146 (1968). (2) K. Nakamura, J. Chem. Phgs., 53, 998 (1970). (3) F.W.Ayers, F. Grein, G. P. Semeluk, and I. Unger, Ber. Bunsenges. Phys. Chem., 72, 282 (1968). The Journal of Physical Chemistry, Vol. 76, No. 9, 1071
