lsadore B. Berlrnan
562
the less exotliermie nature of the enthalpies measured in benzene; however, we are unable to completely rule out a more exothermic solvation of the organomercurial acid by benzene as the causa of this discrepancy. Since the differences in enthalpies between the two solvents differ for the two base systems studied, it does seem more probable {,hat h e difference is connected with the bases (and their solvation) rather than the acid which is constant to all systems. It is> mforranate that solubility limitations prevented us from using C6Hl2 as a solvent since this would have proviided an additional test of the hypothesis presented.
Coaclusioris B~sperflL~or~~p~en~~m ise rindicated cury to be a very weak Lewis acid, although noticeably stronger than
diphenylmercury. When complexed with pyridine in solution both are apparently examples of three coordinate mercury since only a 1:1 complex is observed. It appears probable from a comparison of heats of solution and the adduct formation enthalpies in C&i6 and CC14 that CC14 is the more nearly inert solvent for these systems.
Acknowledgments. Acknowledgment is made to the donors of the Petroleum Research Fund, Grant No. 874G, administered by the American Chemical Society, for partial support of this work. Our thanks are also due to the Graduate School Research Fund. W. I-I. P. expresses his appreciation for a Dupont Summer Fellowship.
y of Heavy-Atom Collisional Quenching of the
l u o r e ~ ~tate e ~of ~Aromatic ~ Compounds in Solution' isatdore B. Beriman' Radiological and EnvkonmentalResearch Division, Argonne National Laboratory, Argonne, Illinois 60439 Iiicceived August 28, 7972) Publication costs assisted by the Aigonne National Laboratory
The values of the rate constant k , for heavy atom quenching of 57 aromatic compounds, induced by bromine in bromobenzene solutions, have been measured and found to vary from 0.7 x PO7 to 3.0 x 109 sec-I. This quenching constant is composed of two independent rate constants: kt, a rate constant for irntcmnolecular singlet-triplet energy transfer (chromophore to quencher), and k I , a rate constant for induced intramolecular intersystem crossing (in the chromophore). Our results are consistent with the concept that collisional quenching is of short range and that a close encounter between c ~ r ~ m o p h o and re quencher is necessary. During their intimate collision, the p orbital of the halogen overlaps the 7 orbital of the chromophore and the perturbation produced by the bromine leads to a breakdown of the spin-selection rules. Steric hindrance is believed to interfere with this close approach and to reduce the efficiency of the quenching process. Substituents that are noncoplanar with the basic chromophore and produce a large spectral red shift are very effective in shielding the chromophore from being quenched. Finally, many planar compounds that are sensitive to heavy-atom quenching are also susceptible to concentration quenching (e.g., anthracene) or excimer formation (e.g.,naphthalene).
Pntroductiam High Z atoms, whether as components of a fluorescent compound or of a solvent, are assumed to perturb and quench the fluorescence state by increasing the spin-orbit coupling3 and possibly forming a charge-transfer complex.4 In orde- ttr learn more about fluorescence quenching, the relative yiejd, R , of each of a series of compounds in benzene, B aixd in bromobenzene, BrB, has been systematically measured, where R is the ratio of the fluorescence intensity from a benzene solution to that from a bromobenzene solution. Selected compounds were also measured i n ethyl bromide. The compounds that have The Journal of Physical Chumistry, Vol. 77, No. 4, 1973
been investigated are those on which spectroscopic information is already available5 and whose absorption spectra are of longer wavelengths than those of DrB.
(1) Work performed under the auspices of the 1J. S. Atomic Energy
Commission. (2) Present address: Racah lnstitute of Physics, Hebrew University, Jerusalem, Israel. (3) M. Kasha, J. Chem. Phys., 20. 71 (1952). (4) S. Lipsky, W. P. Helman. and J. F, Merklin in "Luminescence of Organic and lnorganic Materials," H. P. Kalimann and G . M. Spruch, Ed., Wiley, New York, N. Y . , 1962, p 83. (5) I. B. Beriman, "Handbook of Fluorescence Spectra of Aromatic Molecules," 2nd ed, Academic Press, New York, N. Y., 1971.
Empirical Study of Heavy-Atom Collisional Quenching
~
~
~
c
e
~
~
~
~
e
Certain fluorescence parameters of aromatic compouiids in cyclohexane have been measured,5 among these are the decay time 7 and i,he quantum yield Qy. It is assumed herein that the values of these parameters (and of those derived below) are e s entially ~ unchanged in benzene solutions. From these calues of 7 and Qy, the rate constants for emission k,, for nonradiative transitions k,, and for induced (by heavy atom) intersystem crossing k I , have been calculated according to the equations r-* = Re + h r
&," = h,i(h, QL'
-t h,)
- kAh, + k, +
h,)
1 -t- 7 k X
fi -= QY/QY' Or
b , = (X
-
1)T-l
The rate constant k , is composed of two independent components, a rate constant for transfer K t and a rate constant for induced intersystem crossing hI. An average wave number, vaV, of each fluorescence spectrum was also available.5 The quantity vaV is defined as
where f(v) is the photon flux per unit wave number increment, vmln is the smallest wave number of the fluorescence band, and vmaX is the largest. From these values a quantity, Av,, representative of the shift in the spectrum in wave numbers was calculated according to Av, =
-
v,:
v,,
where vav@ is the average wave number of the fluorescence spectrum of a basic chromophore, and vBV that of the compound listed. Since bSis intended to represent the shift in the fluorescence spectrum, its value will be useful only when the same type of transition i s measured. It is for this reason that the compounds in Table 1 are listed as groups containing the same type of fluorescence transition. A quantity, bph, is used to indicate the energy gap between the fluorescence state and the next lower triplet state, and was computed from l?VFlil
=
v,,
Vph
where vph is the valiie of the 0-0 transition of the pbosphorescence In a few cases, several triplet states may be below the first excited singlet state, e.g., anthracene,'i and tIvph is then taken as the energy gap between the average fluorescence wave number and the higher triplet state. Although Parisera has predicted the existence of various triplet levels below the first excited singlet level for gome of the compounds studied, their energy values are in question. For many compounds. the energy of the first triplet level is not available. Each compound, i.1 dilute ( < l o - 3 mol) B and BrB solutions, was excited by monochromatic uv radiation, and the fluorescence emission was reproduced on a Beckman DK-2 spectrophotometer. An attempt was made to keep the concentralion of the solute in both B and RrB the same, so as to avoid having to make a geometrical correction in the yield ~ ~ a s u r e m e n t s ~ g
563
Because BrB is a polar aromatic solvent, the spectra are often red shifted (extreme cases -40 A) relative to those in B. Ethyl bromide has a tendency to blue shift the spectra. No adjustment has been made to correct for the wavelength sensitivity of the spectrometer, nor has a correction been made to the intensity ~7aluesto take into account the effect produced by a difference in index of refraction among the solutions on the light collection efficiency of the apparatus. The corrections are assumed to be about the same small value for each compound and are neglected. Finally, an attempt was made not to use an exciting wavelength that fell at the onset of the absorption spectrum so as to avoid the possibility of the exciting radiation being more readily absorbed in the spectrumshifted solutions and the necessity of making a geometrical correction for the light collection efficiency of the equipment. The estimated error in k , is about &15%. Our experimental techniquesl* are very sensitive to the presence of impurities in compounds. A new procedure for detecting the presence of an impurity can now be added to those listed in the above reference. If an impurity contributes peaks to the recorded spectrum in benzene, the spectrum in BrB will be modified because the degree of quenching will usually be different for the two species. In the present work, any compound that contained a perceptible impurity was omitted.
Discussion The compounds investigated and their calculated and measured parameters are assembled in Table I. They are grouped according to certain fluorescence features of the basic chromophore. particularly according to a similarity in their fluorescence transition. As an example, naphthalene is assumed to be the basic chromophore in the first group, and the remaining compounds of the group are substituted analogs of naphthalene. The fluorescence transition of each of these compounds is relatively weak so that the decay time i s relatively long. Fluoranthene and 3-phenylfluoranthene are included because spectroscopically these compounds can be considered as substituted naphthalenes. l1 J Some general remarks can be made before each of the groupings is considered separately. The character of the transition is not of major importance in determining the value of k , because the fluorescence transitions in naphthalene, anthracene, and p-terphenyl are ' L b - I k i , lL,JA, and lBb-lA, respective1y,l3J4 and these compounds each have large and approximately equal values ofh,. Two major mechanisms appear to be enhanced by the breakdown of the spin selection rules and are responsible for large values of k,. One is a singlet-triplet energy transfer process whose rate constant is ht and can be represented as follows where SD1 and Suo represent the donor molecule in its (6) Reference 5, p 425. (7) R. E. Kellogg, J. Chem. Phys., 44, 411 (1966). (8) R. Pariser, J. Chem. Phys., 24, 250 (1956) (9) Reference 5, p 25. (10) Reference 5, p 29. (11) Reference5. p 8 l . (12) E. Heilbronner, J. P. Weber, J. Michl, and 9. Zahradnick, Theor, Chim. Acta, 6 , 141 (1966). (13) J . R. Platt, J. Chem. Phys., 17, 484 (1949). (14) I. 8.Berlman,J. Chem. Phys., 52, 5616 (1970). The Journal of Physical Chemistry, Vol. 77, No. 4, 1973
564
lsadore 8. Berlman
TABLE I: Parameters of Compounds in Bromobenzene Decay time nsec
Ouantum yield
ke,107 sec-'
k,,,107
96 46 76 114 57 90 76 44.7 36.6 53 34.5
0.23 0.60 0.38 0.26 0.13 0.17 0.12 0.14 0.08 0.30 0.65
0.23 1.32
0.77 0.88 0.82 0.65 1.5 0.9 1.1 1.9 2.5 1.3 1.o
1.4 3.0 1.5 6.4
0.03 0.77 0.33 0.94
2.1 26 22 14.7
0.36 0.35 0.49 0.76 1.o
9, IO-Dlchloroarthracene
4.9 4.6 6.5 10.1 9.4 8.5
0.55
7.3 7.6 7.5 7.6 11 6.6
letracene Rubrene
6.4 16.5
0.21 1 .o
3.2 6.1
Fluorene Dibenzofuran Carbazole N-Phenylcarbazole Triphenylbenzene
10 7.3 16.1 10.3 42.6
0.80 0.53 0.38 0.37 0.28
a
2-Phenylindole 1,&Diphenylindale 2.3-Diphenylind3le
2.0 2.0 3.8
0.86 0.90 0.64
43 45 17
p-Terphenyl 4-Methyl-p-terpiienyl
0.95 1 .o 1 .o 1.5 1.4 1.8 0.95 0.8 1.05 0.88 0.8 0.65
0.93 0.94 0.90 0.91 0.84 0.97 0.89 1.o 0.92 1 .o 0.94
98 94 90 61 60 56 102 111 95 104 125 145
1.35 1.2 1.4 2.06 1.15 1.26
0.89 1.o 1.o 0.94
66 83 71 46
8.2 (7
0.93
74
5
1.5 1. I 1. I
0.93 0.94 0.74
62 86 67
4.7
2.67
0.90
34
1.1 1.76 12.4 6.2
0.60 0.80
34 6.5 1.5
Compound Naphthalene Acenaphthene 2.3-Dimethylnaphthalene 2-Phenylnaphthniene Phenanthrene 2-Phenylphenanthrece 3.4-Benzophenanth~ne Chrysene Triphenylene Fluoranthene 3-Phenylfluorariihene Azuiene 1 .l'-Binaphthyl
1.4,5,8-Tetraphi?nylnaphthalene Perylene Anthracene 9-Methylanthracene 9-Pheny1anthra::ene 9 -Vinyl anthracene
9,IO-Diphenylarithracerw
3.3'-Dimethyl-p-terplienyl 2,2'-Methylene-~-terpm?nyl 2,2'.-Ethylene-p ,terphenyi 3-Phenyldibenzofuran
4,4"-Dihexahyc!rofarnesoxy-p-terphenyl p-Ouaterphenyl
3.3"'-Dimethyl~-3',2''-me~hylene-p-quaterphenyl 2'2' ' ' ' - D iet hy I-p-qu i na ue p heny I 3,3' ' ' ' - D i (ethylhep ty I ) -p -q u i nq uep heny I Tetramethyl-p-sexiphenyl (PPD) .2.5-Diphenyloxadiazole (PPF) 2,5-Diphonylf~.iraii (PPO) 2,5-Diphenyloxazole (nNPO) 2 - ( I-Naphthyl) -5-phenyloxazole (BBO) 2,5-Diphenylyioxazole (POPOP) p-Bis{2-(5-phenyloxazolyl)]benzene (Dimethyl POPCIP) 1.4-Bis-2-(4-methyl-5phenyloxazolvl) benzene (BPSB) Bis(isopropylstyry1) benzene
(RBOT) 2,5-Bis~5-tert-butylbenzoxazoly(2)]thiophene 1,3,6,8-Tetraphmylpyrene Diphenylstilbene ( 8 ) Tetraphenyl butadiene 1,6-Diphenylheratrieiie
1.8-Diphenyloctatetraene The Journal of Physical Chemistry, Vol. 77, No. 4 , 1973
1.o
0.09
0.5 0.23 0.23 0.2 0.2 0.3 0.22 0.57 1.9
7.3 2.4 3.6 0.6
sec
sec
1
295 44.4 68.6 19.4 16.8 14.2 13 31 28.3 19 8.1
3.1 0.94 0.88 0.16 0.28 0.15 0 15 0.67 0.74 0.34 0.21
69 7 45 0.9
1.9 2.3 1.3 1.4
0.64 0.43 0.18
13.1 14.4 7.8 2.3
