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Oxygen quenching of fluorescence of organic dye molecules
Volume 26, number 1
CHEMICAL PHYSICS LETTERS
OXYGEN QUENCHING OF FLUORESCENCE John
OLMSTED
1 May 1974
OF ORGANIC DYE MOLECULES 111
American University of Beirut, Beinct, Lebanon Received 1 August 1973 Revised manuscript received 5 Febuary 1974 Studies of the rate constant for oxygen quenching of fluorescence, previously confined to relatively simple condensed aromatic systems, have been extended to representative heteroaromatic molecules including uranine-type dyes. It is found that the quenching constant, which is diffusioncontrolled for aromatics lie 9,10_di~her_rylant_hracene. is reduced to 1.0 X 10” M-r set-’ for acridine. 0.21 x 10” Mm1 see-’ for unnine. 0.14 X lOI M ’ set * for quinine bisulphate, and is less than 7 X 10s M-t sk-’ for rhodamineB and eosin-Y. The causes of these reductions are suggested to lie in steric factors and in interaction processes involving non-bonding electrons that arc competitive with-pi-system charge-transfer complexation.
The quenching of fluorescence of aromatic molecules by molecular oxygen in liquid solution has been demonstrated to be, in general, virtually 100% efficient [l] ; the bimolecular rate constant k, is observed to be in reasonable agreement with that predicted by theory for a diffusion-controlled process when the experimental diffusion coefficient for oxygen is used [2] _Indeed, the generality of this behavior is sufficient that it has been suggested that fluorescence quenching ratios can be used, following a Stern-Volmer relationship
optical densities of about 0.3 in the excitation wavelength region. Gas saturation at 1 atm pressure was achieved by bubbling helium (Matheson) or oxygen (local), previously saturated with ethanol by passing through bubbling towers, through a solution in a spectrofluorimeter cell. Bubbling for a per’iod of one to two minutes was ascertained to provide saturation without causing any measurable concentration change due to solvent evaporation. Following bubbling, the cells were stoppered and fluorescence intensities measured immediately, using optimum excitation and fluorescence wavelength settings, on a Baird-Atomic model 1,/I= 1 i-r/c, [02] (1) SF-1 Fluorispec with recorder output. The fluorescence intensity from such a stoppered celi remained constant’ to obtain mean decay times of excited singlets to withfor a minimum of several minutes following the initialin2Q% [3]. Nearly all of the data reported thus far per&r to reading. Intensity measurements were made under idenaromatic systems containing relatively little polar chartical conditions for helium-, air-, and oxygen-saturated acter. When electron-withdrawingor ionizable substisolutions; the air and oxygen readings were found to .L be consistent in each case. The quenching rate constant tuents are present in the aromatic molecule, one might expect that variations in kq .woulh be observed, if the kq was then calculated from eq. (1), using values from. charge-transfe; theory of oxygen quenching due to the literature for oxygen solubihty [S] and fluorescence decay times [3] i Fractionally-distilled ethanol free of- .Mull&en is correct [4] ; 6rganic dye molecules having such substituents have be&studied in the current_detectable. fluoresce&impurities was used and the corn:. pounds we&of best commercially-avaiiabie:p~rity, veriwork, the resu+ofwhich’are reported here:..: ’ f!kd to give ‘&qrescence and excitation. sPectrb.in agree:. :. Quenching fate co~nstants collected m tat& 1‘v&e 1’:‘-!:T_ ;, ... :; : * .- _..’ ment:.~th.the:]iterature..‘.. obtained -fro.& me&.rre&nts.df &o &tensit$ r&s. t$ iate: &rsta+ fo’f a&imbe_ of-_ ; : -i’, fluorescence~.of heliu&fai&; “;ld,o~~~en-~~~~~:~.~ed’~‘.1, 1. j Qxygen~quenc&g .. .:.~~~riolic~~sol~tio~i.o’f .conce&aiion suffctent to give ; ‘~~~mpiesentative coinpounds‘are:.coliected’fo;g_t,~~~.in table -II.; .. ; _. , . ;_1.> _ ‘. I,.. . I __ ..,:... (, :,.‘I ‘: ., ;( -,__‘,“._, : ..I. ‘,I ..::.( .:,. _.- 2 ._ ..-:33;.;
Volume 26, number 1
Oxygen
quenching
Compound 9,lOdiiphenylanthracene carbazole chrysene acridine Fhydroxycoumarin acridine orange uranine (fluorescein dianion) fluorescein cation quinine bisuiphnte rhodamineS eosin-Y
CHEMICAL Table I of fluorescence, +sec)
9.4 15.2 44.7
a)
c)
*:z 4.4 8.1 6.4 c) 12.5 f) 3.2 4.5
ethanol
PHYSICS
soIutions
X-~(Io’~M-‘SE&~)
2.3 c), 2.0 2.0 1.5 1.0 0.88 0.17 0.21 0.14 0.15 < 0.07 g) < 0.05 8)
a) From ref. [3] unless otherwise noted. b) hleasured in this work unless otherwise noted. c) Ref. [ 2 1. d) Ref. [6]. e) Computed from dianion lifetime - see text. f) Ref. [ 141. g) Detection limit of the method.
It will be readily noted that the remarkable invariance of this constant obtained for simpler aromatic hydrocarbon molecules is not maintained as one goes to systems of greater complexity. Acridine (fig. la), acridine orange (fig. 1b), and 7-hydroxycoumarin (fig. I c) have quenching rate constants about a factor of two smaller than diffusioncontrolled; uranine (fig. le), fhrorescein cation (fig. If) and quinine bisulphate (fig. Id) show an order of magnitude reduction in quenching; while rhodamine-B (fig. 1g) and eosin-Y (fig. I h) display no observable quenching_ The results reported for 7-hydroxycoumarin and for fluorescein cation must be considered to be less certain than others obtained in this study owing to uncertainties in the fluorescence lifetime values for these compounds. In the case-of 7-hydroxycoumarin exciplex emission has been observed with concomitant lifetime alterations [6]. The concentrations used in this work should be low enough to prevent exciplex formation, and the monomer lifetime has therefore been used; none the less, in some experiments with 7hydroxycoumarin unstable quenching variations were observed which indicate the possibility of more complicated chemical.interactions. For.the fluorescein cation; fluorescence-lifetime values have not been reported, but a semi-empirical lifetime could be-computed from the measured lifetime for the dianion. This @as
LETTERS
1 May 1974
done by computing the ratio of radiative lifetimes for the two species from the integrated areas of their first singlet absorption bands and then converting to natural lifetimes using fluorescence quantum yield measurements from the literature [7]. The computed lifetime must be considered to be more uncertain than measured values by at least the uncertainty in the quantum yield values. Several possible explanations may be posited for the rather Iarge deviations from diffusion-controlled quenching observed for these compounds: (a) the quenching rate could be determined by the stability of the contact-charge-transfer complex [S] ;(b) the complex-enhanced intersystem crossing could be prohibited or inhibited by energetic considerations [9]; (c) bulky substituent groups could be introducing sterichindrance factors; (d) other interactions between polar substituents and the approaching 0, molecule could be competitive with charge-transfer-comp!ex formation_ Contrary to the expectation from explanation (a) and to the experimental correlation between quenching constant and ionization potential for some aromatic molecules observed by Brewer in the gas phase [S], we find no simple correlation between reduced quenching efficiency and trends in predicted ionization potentials (an observation which also holds for the smaller variations in oxygen quenching observed by Stevens and Algar on a series of catacondensed aromatics in benzene solution [9] ). The acridine ionization potential, for example, should not be significant!y higher than those of naphthalene and anthracene; yet it is less efficiently quenched by oxygen. Most strikingIy, the difference in quenching rate constants for uranine (fluorescein dianion) and fluorescein cation is only about 50%, despite the large charge difference and the fact that these compounds lie-in the intermediate range of the quenching constant where small alterations in the ionization potential might be expected to cause large changes in the charge transfer complex stability_ Apparently the cause of variation in quenching rate constants for these compounds must be sought elsewhere. if intersystem crossing were prohibited for some reason in those compounds for which greatly reduced quenching is observed, the very low r&es could be due to other types of interaction [l] , in which case no cvidence. for triplet St&e formation would be’found; ln
CHEMICAL PHYSICS LE-ITERS
Volume 26, number 1
9
1
h
Fig. 1. Structures of molecules studied in this work. (a) acridine; (b) a&dine orange; (c) 7hydroxjvoumarin; (e) uranine (fluorescein dianion); (f) fluorescein cation;(g) rhodamine-B; (h) eosin-Y.
order to verify that oxygen quenching leads to the triplet state, we have studied the fluoresce&sensitized
photo-oxidation of ! ,3diphenylisobenzofuran, a compound that is known to be extremely sensitive to the presence of singlet oxygen [ 101. Since fluorescein in ethanol has an unperturbed intersystem crossing yield of zero [l I] ;fluorescein-sensitized photo-oxidation can only occur if oxygen quenching of the excited sin.. glet leads to the triplet state and hence to production of singlet oxygen. W fmd that sensitized photo-oxida.ti& does occui wiih a r+ethat is compatible with _the observed rate of fluorescence quenckg; a finding .: which is.in a&e&&t with &nil& results observed by 1.
.,
May 1974
(d) quinine bisulphate;
Gollnick for fluorescein-sensitized photo-oxidation
of
2,Sdimethylfuran [H]. It has been suggested that a high quenching rate depends on the second excited triplet state lying below the charge-transfer state [12], but th&collected results on catacondensed aromatics do not bear this out [I], nor do the results of the present study. For example, the S1 -T, energy gaps in acridine orange, fluoresFein,and eosin-Y are dl in the vicinity of 2500 cm;1 1131, -, implying that T2 fo; each compound is && above S,, yet the quenching c&stants for these dye% diffe; by an order of magnitude. None thi: less, it appears that the quenching variationi observed &Steve@s a$ Alga! 19] : .. .: ..,_ .‘_. -,.. ,_. ._ .: ... : ..35:
Volume 26, number 1 must in some way
CHEMICAL PHYSICS LETl-ERS
be due to variations in the CT-triplet interaction matrix element, since none of the other possibilities posited above can appIy for their series of simple aromatics. Turning to possibility (c), the relatively poor quenching efficiency of quinine bisulphate by oxygen can be ratiorialized at least partially on steric grounds, since the very bulky saturated tertiary amine substituent group may be expected to screen off the aromatic pi-cloud very effectively from an approaching oxygen molecule. Berlman has suggested a similar screening effect as a possible explanation for reduced quenching efficiency in fluoranthene [3]. Similarly, the reduced quenching sensitivities of rhodamine-I3 and eosin-Y relative to fluorescein can be attributed to the addition of the relatively bulky diethy amino and brcmine substituents around the periphery of the aromatic system, thereby sterically hindering the approach of oxygen molecules from the side. The small reduction of quenching efficiency for acridine and 7-hydroxycoumarin cannot be attributed to steric factors, nor can the significant decrease of the rate constant for uranine be so explained when it is recalled that the phenyl groups in 9,lOdiphenyL anthracene are ineffective in this respect. The protective feature in the fluorescein-like system seems to be the carboxylic acid substituent, which could possibly tie up an incoming oxygen molecule in a five-membered ring with appreciable resonance stabilization, thereby competing effectively with charge-transfer complexation and reducing the likelihood of enhanced intersystem crossing. For nitrogen- and oxygen-heterocyclic aromatic compounds containing non-bonding electrons one can also speculate that a competition might be occurring between pi-cloud and n-electron interactions, but clearly additional data are needed to elucidate the details of the interactions in all these instances.
..36. : -.
‘.
1 hlay 1974
The results of the present study show clearly that oxygen quenching of the fluorescence of aromatic systems in solution has rate constants that vary over at least two orders of magnitude as the complexity of the molecule is increased and that the variations must be due to several factors, of which steric effects appear to be substantial and ionization potential variations minimal. This work was assisted byta research grant from the Arts and Sciences Research Committee of the American University of Beirut. References [l]
D.R. Kearns. Chem. Rev_ 71 (1971) 395, and references therein. [Z] W.R. Ware, J. Phys. Chem;66 (1962)4X. [3] 1.6. Berlman. Handbook of fluorescence spectra of aromatic molecules, 2nd Ed. (Academic Press, New York, 1971). [4 ] H. Tsubomura and R.S. Mulliien, J. Am. Chem. Sot. 82 (1960) 5966. [.5] Landoldt-Bijmstein, Zahlenwerte und Funktionen, 6. Auflage, II Band, 2. Teil, Bandteil b (Springer, Berlin, 1962). 16 ] A. Dienes, C.V. Shank and A.M. Trozzolo, Appl. Phys. Letters 17 (1970) 189. 171 H. Leonhardt, L. Gordon and R. Livingston, J. Phys. Chem. 75 (1971) 245. 181 T. Brewer, J_ Am. Chem. Sot. 93 (1971) 775. J. Phys. Chem. 72 (1968) 191 B. Stevens and B.E. A@, 2582. 1101 J_ Olmsted III and G. Karal, J. Am. Chem. Sot. 94 (1972) 3305. Pll K. Gollnick, Advan. Photochem. 6 (1968) 1. 1121 B. Stevens and B.E. Algar, Ann. N-Y. Acad. Sci. 171 (1970) 50. r131 R.W. Chambers and D.R. Keams, Photochem. Photobiol. 10 (1969) 215. t141 hi-D. Galanm, Dokl. Akad. Nauk SSSR 73 (1950) 925.
CHEMICAL PHYSICS LETTERS
OXYGEN QUENCHING OF FLUORESCENCE John
OLMSTED
1 May 1974
OF ORGANIC DYE MOLECULES 111
American University of Beirut, Beinct, Lebanon Received 1 August 1973 Revised manuscript received 5 Febuary 1974 Studies of the rate constant for oxygen quenching of fluorescence, previously confined to relatively simple condensed aromatic systems, have been extended to representative heteroaromatic molecules including uranine-type dyes. It is found that the quenching constant, which is diffusioncontrolled for aromatics lie 9,10_di~her_rylant_hracene. is reduced to 1.0 X 10” M-r set-’ for acridine. 0.21 x 10” Mm1 see-’ for unnine. 0.14 X lOI M ’ set * for quinine bisulphate, and is less than 7 X 10s M-t sk-’ for rhodamineB and eosin-Y. The causes of these reductions are suggested to lie in steric factors and in interaction processes involving non-bonding electrons that arc competitive with-pi-system charge-transfer complexation.
The quenching of fluorescence of aromatic molecules by molecular oxygen in liquid solution has been demonstrated to be, in general, virtually 100% efficient [l] ; the bimolecular rate constant k, is observed to be in reasonable agreement with that predicted by theory for a diffusion-controlled process when the experimental diffusion coefficient for oxygen is used [2] _Indeed, the generality of this behavior is sufficient that it has been suggested that fluorescence quenching ratios can be used, following a Stern-Volmer relationship
optical densities of about 0.3 in the excitation wavelength region. Gas saturation at 1 atm pressure was achieved by bubbling helium (Matheson) or oxygen (local), previously saturated with ethanol by passing through bubbling towers, through a solution in a spectrofluorimeter cell. Bubbling for a per’iod of one to two minutes was ascertained to provide saturation without causing any measurable concentration change due to solvent evaporation. Following bubbling, the cells were stoppered and fluorescence intensities measured immediately, using optimum excitation and fluorescence wavelength settings, on a Baird-Atomic model 1,/I= 1 i-r/c, [02] (1) SF-1 Fluorispec with recorder output. The fluorescence intensity from such a stoppered celi remained constant’ to obtain mean decay times of excited singlets to withfor a minimum of several minutes following the initialin2Q% [3]. Nearly all of the data reported thus far per&r to reading. Intensity measurements were made under idenaromatic systems containing relatively little polar chartical conditions for helium-, air-, and oxygen-saturated acter. When electron-withdrawingor ionizable substisolutions; the air and oxygen readings were found to .L be consistent in each case. The quenching rate constant tuents are present in the aromatic molecule, one might expect that variations in kq .woulh be observed, if the kq was then calculated from eq. (1), using values from. charge-transfe; theory of oxygen quenching due to the literature for oxygen solubihty [S] and fluorescence decay times [3] i Fractionally-distilled ethanol free of- .Mull&en is correct [4] ; 6rganic dye molecules having such substituents have be&studied in the current_detectable. fluoresce&impurities was used and the corn:. pounds we&of best commercially-avaiiabie:p~rity, veriwork, the resu+ofwhich’are reported here:..: ’ f!kd to give ‘&qrescence and excitation. sPectrb.in agree:. :. Quenching fate co~nstants collected m tat& 1‘v&e 1’:‘-!:T_ ;, ... :; : * .- _..’ ment:.~th.the:]iterature..‘.. obtained -fro.& me&.rre&nts.df &o &tensit$ r&s. t$ iate: &rsta+ fo’f a&imbe_ of-_ ; : -i’, fluorescence~.of heliu&fai&; “;ld,o~~~en-~~~~~:~.~ed’~‘.1, 1. j Qxygen~quenc&g .. .:.~~~riolic~~sol~tio~i.o’f .conce&aiion suffctent to give ; ‘~~~mpiesentative coinpounds‘are:.coliected’fo;g_t,~~~.in table -II.; .. ; _. , . ;_1.> _ ‘. I,.. . I __ ..,:... (, :,.‘I ‘: ., ;( -,__‘,“._, : ..I. ‘,I ..::.( .:,. _.- 2 ._ ..-:33;.;
Volume 26, number 1
Oxygen
quenching
Compound 9,lOdiiphenylanthracene carbazole chrysene acridine Fhydroxycoumarin acridine orange uranine (fluorescein dianion) fluorescein cation quinine bisuiphnte rhodamineS eosin-Y
CHEMICAL Table I of fluorescence, +sec)
9.4 15.2 44.7
a)
c)
*:z 4.4 8.1 6.4 c) 12.5 f) 3.2 4.5
ethanol
PHYSICS
soIutions
X-~(Io’~M-‘SE&~)
2.3 c), 2.0 2.0 1.5 1.0 0.88 0.17 0.21 0.14 0.15 < 0.07 g) < 0.05 8)
a) From ref. [3] unless otherwise noted. b) hleasured in this work unless otherwise noted. c) Ref. [ 2 1. d) Ref. [6]. e) Computed from dianion lifetime - see text. f) Ref. [ 141. g) Detection limit of the method.
It will be readily noted that the remarkable invariance of this constant obtained for simpler aromatic hydrocarbon molecules is not maintained as one goes to systems of greater complexity. Acridine (fig. la), acridine orange (fig. 1b), and 7-hydroxycoumarin (fig. I c) have quenching rate constants about a factor of two smaller than diffusioncontrolled; uranine (fig. le), fhrorescein cation (fig. If) and quinine bisulphate (fig. Id) show an order of magnitude reduction in quenching; while rhodamine-B (fig. 1g) and eosin-Y (fig. I h) display no observable quenching_ The results reported for 7-hydroxycoumarin and for fluorescein cation must be considered to be less certain than others obtained in this study owing to uncertainties in the fluorescence lifetime values for these compounds. In the case-of 7-hydroxycoumarin exciplex emission has been observed with concomitant lifetime alterations [6]. The concentrations used in this work should be low enough to prevent exciplex formation, and the monomer lifetime has therefore been used; none the less, in some experiments with 7hydroxycoumarin unstable quenching variations were observed which indicate the possibility of more complicated chemical.interactions. For.the fluorescein cation; fluorescence-lifetime values have not been reported, but a semi-empirical lifetime could be-computed from the measured lifetime for the dianion. This @as
LETTERS
1 May 1974
done by computing the ratio of radiative lifetimes for the two species from the integrated areas of their first singlet absorption bands and then converting to natural lifetimes using fluorescence quantum yield measurements from the literature [7]. The computed lifetime must be considered to be more uncertain than measured values by at least the uncertainty in the quantum yield values. Several possible explanations may be posited for the rather Iarge deviations from diffusion-controlled quenching observed for these compounds: (a) the quenching rate could be determined by the stability of the contact-charge-transfer complex [S] ;(b) the complex-enhanced intersystem crossing could be prohibited or inhibited by energetic considerations [9]; (c) bulky substituent groups could be introducing sterichindrance factors; (d) other interactions between polar substituents and the approaching 0, molecule could be competitive with charge-transfer-comp!ex formation_ Contrary to the expectation from explanation (a) and to the experimental correlation between quenching constant and ionization potential for some aromatic molecules observed by Brewer in the gas phase [S], we find no simple correlation between reduced quenching efficiency and trends in predicted ionization potentials (an observation which also holds for the smaller variations in oxygen quenching observed by Stevens and Algar on a series of catacondensed aromatics in benzene solution [9] ). The acridine ionization potential, for example, should not be significant!y higher than those of naphthalene and anthracene; yet it is less efficiently quenched by oxygen. Most strikingIy, the difference in quenching rate constants for uranine (fluorescein dianion) and fluorescein cation is only about 50%, despite the large charge difference and the fact that these compounds lie-in the intermediate range of the quenching constant where small alterations in the ionization potential might be expected to cause large changes in the charge transfer complex stability_ Apparently the cause of variation in quenching rate constants for these compounds must be sought elsewhere. if intersystem crossing were prohibited for some reason in those compounds for which greatly reduced quenching is observed, the very low r&es could be due to other types of interaction [l] , in which case no cvidence. for triplet St&e formation would be’found; ln
CHEMICAL PHYSICS LE-ITERS
Volume 26, number 1
9
1
h
Fig. 1. Structures of molecules studied in this work. (a) acridine; (b) a&dine orange; (c) 7hydroxjvoumarin; (e) uranine (fluorescein dianion); (f) fluorescein cation;(g) rhodamine-B; (h) eosin-Y.
order to verify that oxygen quenching leads to the triplet state, we have studied the fluoresce&sensitized
photo-oxidation of ! ,3diphenylisobenzofuran, a compound that is known to be extremely sensitive to the presence of singlet oxygen [ 101. Since fluorescein in ethanol has an unperturbed intersystem crossing yield of zero [l I] ;fluorescein-sensitized photo-oxidation can only occur if oxygen quenching of the excited sin.. glet leads to the triplet state and hence to production of singlet oxygen. W fmd that sensitized photo-oxida.ti& does occui wiih a r+ethat is compatible with _the observed rate of fluorescence quenckg; a finding .: which is.in a&e&&t with &nil& results observed by 1.
.,
May 1974
(d) quinine bisulphate;
Gollnick for fluorescein-sensitized photo-oxidation
of
2,Sdimethylfuran [H]. It has been suggested that a high quenching rate depends on the second excited triplet state lying below the charge-transfer state [12], but th&collected results on catacondensed aromatics do not bear this out [I], nor do the results of the present study. For example, the S1 -T, energy gaps in acridine orange, fluoresFein,and eosin-Y are dl in the vicinity of 2500 cm;1 1131, -, implying that T2 fo; each compound is && above S,, yet the quenching c&stants for these dye% diffe; by an order of magnitude. None thi: less, it appears that the quenching variationi observed &Steve@s a$ Alga! 19] : .. .: ..,_ .‘_. -,.. ,_. ._ .: ... : ..35:
Volume 26, number 1 must in some way
CHEMICAL PHYSICS LETl-ERS
be due to variations in the CT-triplet interaction matrix element, since none of the other possibilities posited above can appIy for their series of simple aromatics. Turning to possibility (c), the relatively poor quenching efficiency of quinine bisulphate by oxygen can be ratiorialized at least partially on steric grounds, since the very bulky saturated tertiary amine substituent group may be expected to screen off the aromatic pi-cloud very effectively from an approaching oxygen molecule. Berlman has suggested a similar screening effect as a possible explanation for reduced quenching efficiency in fluoranthene [3]. Similarly, the reduced quenching sensitivities of rhodamine-I3 and eosin-Y relative to fluorescein can be attributed to the addition of the relatively bulky diethy amino and brcmine substituents around the periphery of the aromatic system, thereby sterically hindering the approach of oxygen molecules from the side. The small reduction of quenching efficiency for acridine and 7-hydroxycoumarin cannot be attributed to steric factors, nor can the significant decrease of the rate constant for uranine be so explained when it is recalled that the phenyl groups in 9,lOdiphenyL anthracene are ineffective in this respect. The protective feature in the fluorescein-like system seems to be the carboxylic acid substituent, which could possibly tie up an incoming oxygen molecule in a five-membered ring with appreciable resonance stabilization, thereby competing effectively with charge-transfer complexation and reducing the likelihood of enhanced intersystem crossing. For nitrogen- and oxygen-heterocyclic aromatic compounds containing non-bonding electrons one can also speculate that a competition might be occurring between pi-cloud and n-electron interactions, but clearly additional data are needed to elucidate the details of the interactions in all these instances.
..36. : -.
‘.
1 hlay 1974
The results of the present study show clearly that oxygen quenching of the fluorescence of aromatic systems in solution has rate constants that vary over at least two orders of magnitude as the complexity of the molecule is increased and that the variations must be due to several factors, of which steric effects appear to be substantial and ionization potential variations minimal. This work was assisted byta research grant from the Arts and Sciences Research Committee of the American University of Beirut. References [l]
D.R. Kearns. Chem. Rev_ 71 (1971) 395, and references therein. [Z] W.R. Ware, J. Phys. Chem;66 (1962)4X. [3] 1.6. Berlman. Handbook of fluorescence spectra of aromatic molecules, 2nd Ed. (Academic Press, New York, 1971). [4 ] H. Tsubomura and R.S. Mulliien, J. Am. Chem. Sot. 82 (1960) 5966. [.5] Landoldt-Bijmstein, Zahlenwerte und Funktionen, 6. Auflage, II Band, 2. Teil, Bandteil b (Springer, Berlin, 1962). 16 ] A. Dienes, C.V. Shank and A.M. Trozzolo, Appl. Phys. Letters 17 (1970) 189. 171 H. Leonhardt, L. Gordon and R. Livingston, J. Phys. Chem. 75 (1971) 245. 181 T. Brewer, J_ Am. Chem. Sot. 93 (1971) 775. J. Phys. Chem. 72 (1968) 191 B. Stevens and B.E. A@, 2582. 1101 J_ Olmsted III and G. Karal, J. Am. Chem. Sot. 94 (1972) 3305. Pll K. Gollnick, Advan. Photochem. 6 (1968) 1. 1121 B. Stevens and B.E. Algar, Ann. N-Y. Acad. Sci. 171 (1970) 50. r131 R.W. Chambers and D.R. Keams, Photochem. Photobiol. 10 (1969) 215. t141 hi-D. Galanm, Dokl. Akad. Nauk SSSR 73 (1950) 925.