J. Q. UMBERGER
2054
Solvent Proticity via Erythrosin Internal Conversion
by J. Q. Umberger E. I . du Pont de Nemours and Co., Inc., Photo Products Department, Parlin, New Jersey (Received October 24, lQ66)
Systematic solvent variations show erythrosin fluorescence is quenched in accordance with solvent H bond donor strength. Unlike solvatochromism, the quenching appears insensitive to solvent polarity; thus, common organic solvents-from alcohols through halogenated hydrocarbons like chloroform-are listed according to protic strength. The quenching involves nonadiabatic transition from excited state to “hot” ground state. Apparently the high energy of the “hot” ground state of symmetrical dyes results from resonance inhibition, e.g., by proton transfer from solvent to erythrosin, by intramolecular proton transfer in ultraviolet screeners, by rotation out of coplanarity in carbocyanines, and by skeletal folding in symmetrical anthracenes.
Past work1I2indicates that diminution of fluorescence accompanying iodine substitution in fluorescein is due not only to intersystem crossing but also to internal conversion from SI to So. I have measured erythrosin fluorescence in a wide variety of solvents and conclude internal conversion is encouraged by solvent proticity, a Le., H-bond-donor strength. Aside from fundamental energy conversion implications, these measurements arrange solvents according to proticity and correlate structures with activities. Trifluoroethanol and water, Table I, are quite protic, are active quenchers, and yield a short fluorescence lifetime and low fluorescence; they also dissolve gelatin -further indication of protic ~trength.~”Ethylene glycol and glycerol are surprisingly protic ; intramolecular H bonding of one H apparently activates the second H. The first H is weakened by chelation, as in glycol monoethers. Tertiary alcohols are less protic than secondary which are less protic than prim a r ~ . ~ bWith inductive activation, chloroform is more protic than n-butylamine and almost as protic as &butyl alcohol. Quenching appears independent of polarity, as shown by large r for polar aprotic solvents.
Experimental Section Spectrophotometer curves showed a plateau in the absorption of erythrosin at -500 mp. This spectral region, from a filtered incandescent projector, excited the fluorescence. Emission was measured at 90”
from excitation via an S13 photocathode, Dumont 7664 photomultiplier. Lifetimes confirmed the above intensity measurements. 7 was measured via a sub-nsec phase fluorometer5 of P. C. Hoell, DuPont Central Research Department. A precision check gave lo-’ M aqueous uranine r = 4.2 nsec, 4.3 published;6 M aqueous erythrosin 7 = 0.1 nsec, 0.08 published.6 A stock solution was prepared as follows: 236 mg of Erythrosin B, Fisher E513, CI 45430 (93% dye) in 50 ml of dimethylformamide (Fisher Certified D119); 2 ml of stock 98 ml of test solvent (Fisher Certified) formed M. Measurements in 100% Matheson Spectroquality solvents yielded the same r . The Ca4S0 desiccant (5 g/ 100 ml of solution) increased r -0.1 nsec for aprotic solvents, e.g., acetone. Erythrosin, Na+ was relatively insoluble in nonpolar solvents; replacing Na+ by (C4H9)4N+increased lipophilicity. The r values of Rose bengal (CI 45440) were 1.41 times those of erythrosin.
+
(1) A. H. Adelman and G . Oster, J . Am. Chem. SOC.,78, 3977 (1956). (2) L. S. Forster and D. Dudley, J . Phys. Chem., 66, 838 (1962). (3) A. J. Parker, Quart. Rev. (London), 16, 163 (1962); Intern. Sci. Technol., No. 44,28 (1965). (4) (a) J. Q . Umberger, Phot. Sei. Eng., in press; gelatin dissolves in strongly protic solvents and gels in polyprotic solvents; (b) A. K. Chandra and A. B. Sannigrahi, J . Phys. Chem.,69, 2494 (1965). (6) P. Pringsheim, “Fluorescence and Phosphorescence,” Interscience Publishers, Inc., New York, N. Y.,1949, p 10. (6) See ref 5, p 373.
SOLVENT PROTICITY via ERYTHROSIN INTERNAL CONVERSION
Table I: Solvent Proticity from Quenching of Erythrosin Fluorescence (24’)
Solvent
Trifluoroethanol Water Glycerol Ethylene glycol Formamidea Methanol Primary alcohols Secondary alcohols Glycol monomethyl ether &Butyl alcohol Trichloroethylene, chloroform Methylene dichloride, methylchloroform, n-butylamine Acetonitrile Dimethylformamide Acetone
Fluorescence lifetime, 7, x lo* sec
Relative fluorescence yield
Dissolves gelatin
2 3 8 8-10 14-15 14-15 15-16 18-20 22
Yes Yes Yes Yes Yes
1.7 -2.0
23 -24
No No
-2.5
-30
No
-0.1 0.1
0.7 -0.8 -0.8 1.o 1.3
No No No No
2.8 2.9 3.5
5 Formamide, though no more protic than methanol, dissolved gelatin on long stirring a t 200°F. This is ascribed to formamide chelation with gelatin backbone, converted to cisamide. Trans-amides, e.g., N-methylacetamide, are gelatin precipitants; cis-amides, e.g., acetamide and 2-pyrrolidinone, are compatible with gelatin.4 That formamide is no more protic than methanol suggests that resonance is secondary to induction in proticity activation.
Discussion The interpretation of the decreased fluorescence yield in protic solvents as due to increased SI + So internal conversion is strengthened by the work of K ~ c h t a who , ~ found decreased erythrosin triplet yield in protic solvents such as water, and Gollnick and Schenck,*who found that the quantum yield of tripletstate formation for erythrosin in methanol is as high as 0.6. It thus seems improbable that increased intersystem crossing is the cause of the greatly reduced fluorescence when the solvent is changed to water. Hbond activated fluorescence in aromatic aldehydes, ,~ Table with close-lying q , x * , and x , x * ~ t a t e sconfirm I. But in view of the long wavelength x + T* absorption of erythrosin, a close-lying q + T* absorption appears improbable. Indeed, both the peak (546 r n M in HCCI,, 526 mp in HZO) and the plateau (512 mp in HCC13, 495 mp in H,O) in the absorption of erythrosin and also its fluorescence emission show hypsochromic shift with increase in solvent polarity. Unlike the above aldehydes, erythrosin is a symmetrical dye;
2055
the present data, combined with recent work on carbocyanines’o and Moffitt’s theory of formamidinium ion,” suggest that the internal conversion be rationalized in terms of destruction of symmetry. Judging from the carbocyanines,l0 hypsochromic absorption shift with increase in solvent polarity is typical of symmetrical ionic dyes. From this and the essentially unchanged shape and height of the optical density curve in the various solvents, it is assumed that the dianion is the predominant species of erythrosin in the ground state. Internal conversion apparently is encouraged by transfer of a proton from solvent to dianionic erythrosin, after its excitation, to form an asymmetric or monoanionic transient which is nonfluorescent. In protic solvents, the proton transfer apparently is so rapid as to compete with fluorescence and cause quenching in -10-9 sec. This time allows proton transfer in phenols but is too fast for rearrangement of the nuclear skeleton in pseudo-acids.12 Thus, proticity or “isoskeletal” acidity is determining. H bridging must exist prior t o excitation for quick protonation. Consistent with this view, it was observed that rigid glycerol at Dry Ice temperature did not lessen fluorescence quenching relative to glycerol a t room temperature. Apparently, viscous glycerol does not hamper protonation as it, with FranckCondon factors, might hamper other quenching mechanisms involving greater molecular rearrangement, e.g., carbon-iodine bond rupture or gross bending or twisting of the molecular frame. For completeness, it is desirable to rationalize differences in erythrosin and uranin; the fluorescence yield of uranin is relatively high and independent of solvent proticity. In erythrosin, iodine might increase proton affinity via electron donation to the adjacent oxygen. Iodine also brings S1 and So states closer, as evinced in its bathochromic absorption shift, and thus might encourage Si + So internal conversion on pr~tonation.’~ (7) A. D. Kuchta, private communication. (8) K. Gollnick and G . 0. Schenck, Pure Appl. Chem., 9,507(1964). (9) K. Bredereck, et al., “International Conference on Luminescence,” New York University, 1961, John Wiley and Sons, Inc., New York, N.Y., 1962, p 161. (10) J. Q. Umberger, Phot. Sei. Eng., in press. (11) W. E. Moffitt, Proc. Phys. SOC.,A63, 700 (1950). (12) M. Eigen, Angew. Chem. Intern. Ed. Engl., 3 , 1 (1964), O H reacts with phenols, k ,- 1.4 X 10’0, and with pseudo-acid acetylacetone, k 4 X 104. Also see S. H. Maron and V. K. La Mer, Ann. N . Y . Acad. Sci., 39, 355 (1940); J. Hine, J . Org. Chem., 31, 1236 (1966). (13) It is conceivable that symmetry-destroying processes, e a , single protonation, also encourage very rapid SI + TIintersystem crossing followed by very rapid TI-+ So nonadiabatic transition t o asymmetric ground state. Invention of 0.01-psec flash photolysis apparatus would permit detection of such short-lived triplets if they exist.
-
Volume 7 1 , Number 7 June 1967
J. Q. UMBERGER
2056
vent. Broad applicability to symmetrical molecules is suggested by ultraviolet stabili~ers’~‘~ and carbocyanines’O where intramolecular proton transfer and rotation out of coplanarity, respectively, destroy symmetry. That rotation brings S1 and So together is evinced by bathochromic absorption shift in sterically crowded symmetrical dyes.15 Figure 1 is inapplicable t o the asymmetric aldehydes. Protonation-destructive of erythrosin symmetry-increases aldehyde symmetry, i.e., equalizes resonance-form energies Ar-
2; a
w z w
+
+
CH=OH
HIGH
LOW
SYMMETRY Figure 1. Internal conversion in symmetrical molecules. If skeletal folding is added to protonation and internal rotation as a symmetry-lessening process, the viscositydependent internal fluorescence quenching observed in symmetrical or vibrationally deficient anthracenes (E. J. Bowen and J. Sahu, J . Phys. Chem., 63, 4 (1959); Advan. Photochem., 1, 34 (1963)) also might be rationalized by Figure 1. Though recent work (R.G. Bennett and P. J. McCartin, J. Chem. Phys., 44, 1969 (1966)) indicates that both TI and TI states contribute to anthracene internal fluorescence quenching, fluorescence enhancement by viscous solvent still suggests some skeletal deformation process. For example, Bennett and McCartin found 9,lO-dichloroanthracene yields 80% fluorescence in solid poly(methy1 methacrylate) but only 52% in liquid ethanol a t 25”. A folded triplet state could undergo nonadiabatic transition to a similarly folded So state of energy close to said triplet. Indeed, skeletal deformation processes are not uncommon in excited electronic states: ethylene, coplanar in So, rotates 90” out of plane in the excited state; formaldehyde, with sp* coplanarity in So, becomes tetrahedral after excitation; and acetylene, colinear in So, relaxes to trans- structure after excitation (N. J. Turro, private communication). Anthracene has symmetrical, out-of-plane, skeletal deformation modes of vibration (S. Califano, J . Chem. Phys., 36, 903 (1962)) which, on excitation by light absorption or heat absorption (2-4 kcal/mole), might encourage molecular folding along an axis through the 9- and IO-positions. Thus, planarity and resonance in the outer rings would be preserved in the folded molecule as in 9,10-dihydroanthracene (J. D. Roberts and M. C. Caserio, “Basic Principles of Organic Chemistry,” W. A. Benjamin Inc., New York, N. Y., 1964, p 817). An alternate view of the effect of viscosity on fluorescence in symmetrical anthracenes is that the rigid media “freeze out” a skeletal deformation mode of vibration which, in the fluid media, promotes intersystem crossing.
I n Figure 1, So absorbs protons to form SI; then protonation of one oxygen lessens erythrosin symmetry and resonance, increases So energy, and encourages nonadiabatic transition to So. The high energy of “hot” So thus formed resides in electron localization14a and/or in atomic vibration. Deprotonation restores So symmetry as energy goes to solT h Journal of Physical Chemistry
A-CH-OH. Proton transfer from solvent to oxygen after excitation apparently stabilizes ++
the fluorescent T,T*states in aldehydes.9 Resonance differs in So and SIstates of symmetrical dyes. So stabilization is strong; symmetry-destroying processes, e.g., rotation or bond formation, are resisted by energy increase, Figure 1. Delocalization is encouraged by stepwise electron migration16into adjoining ( +)
unfilled orbitals or “positive holes’’ Q=CH-
(+)
Qtt
(+)
Q-CH--&
tt Q-CH=Q. Thus, an analogy exists between liquids where flow results from holes and light absorbers where chromophores, oxidation, etc., provide unfilled orbitals. Such a “hole theory”l0 integrates light absorption in dyes, radicals, metals, etc. SI stabilization is lessened by exclusion” of the middle (+)
resonance form Q-CH-Q; rotation and localization of electrophilicity and nucleophilicity, at opposite ends of the molecule, resemble classic nonresonating (+I
structures. I n the absence of Q-CH-Q, charge migration or delocalization along the total conjugation path becomes improbable, occurring only via synchro-
(+A
n
(+)
nized electron motion Q=CH-Q + Q-CH=Q. Thus, nonemissive So appears associated with stepwise (incoherent) electron migration analogous to diffusion and emissive S1 with synchronized (coherent) electron migration analogous to antenna oscillation. Though synchronized motion of the .rr electrons might readily be induced in SI as in laser emission, the spontaneous process should be relatively slow due to the general improbability of synchronized events. Thus, fluorescence should occur by correspondence with (14) (a) W. Kauzmann, “Quantum Chemistry,” Academic Press, New York, N. Y., 1957, pp 536-541; (b) J. R. Merrill and R. G. Bennett, J . Chem. Phye.. 43, 1410 (1965). Also see A. A. Lamola and L. J. Sharp, J . Phys. Chem., 70, 2634 (1966). These workers suggest that in the molecules possessing the internal H bond, w r y fast radiationless decay (perhaps from the singlet state precluding intersystem crossing) in the form of a tautomerism takes place. (15) M. J. Dewar in “Steric Effects in Conjugated Systems,” Academic Press, New York, N. Y., 1958,p 46. (16) L. Pauling, Proc. Nail. Acad. Sci. U.S.,25,579 (1939).
2057
THEREACTION OF H WITH 02
linear radio antennae only when the conjugated a electrons finally happen to move coherently or synchronously, apparently after a gestation period of sec. Stark effect datal’ suggest that symmetrical dyes have no permanent dipole in the So and S1 states and that S1 is more polarizable than So. This is quite consistent with the present view that electron delocalization and resonance stabilization are less in SI than in So. In SI, electrons and positive hole apparently can be localized at opposite ends of a molecule under the influence of an electric field. But So is relatively resistant to polarization by an external field, because
The Reaction of H with
0 2 .
such polarization would localize the electrons and reduce resonance stabilization. The high polarizability of SI states might contribute to “photochemical forces,”18 e.g., the relatively high binding energy in “excimers” such as an excited helium atom associated with a normal helium atom.lg ~~
~
(17) J. Kumamoto, J. C. Powers, Jr., and W. R. Heller, J. Chem. Phya., 36, 2893 (1962),and Chem. Eng. News, 41, 89 (1962); also see J. R. Platt, J . Chem. Phys., 34, 862 (1961). (18) J. Q. Umberger, Can. Chem. Process Id., 29, 108 (1945). (19) J. 0.Hirschfelder, C. E’. Curtiss, and R. B. Bird, “Molecular
Theory of Gases and Liquids,” John Wiley and Sons, Inc., New York, N. Y., 1954, p 1098.
The Dissociative Lifetime of H0,’
by R. L. Wadlinger and B. deB. Darwent The Maloney Chemistry Laboratory, The Catholic University of America, Washington,D . C. 60017 (Received October 64, 1366)
The reactions of H with H2Sand 02,the dissociative life of H02*, and the deactivation of H02*by C02, C R , and SF6 have been investigated by measuring the effect of the concentration of inert gas on the rate of formation of H2 in the photolysis of HzS mixed with Oz. The photochemically produced H has been shown to be hot, but the technique used in these measurements is sound provided that the concentrations of HzS and O2 are much less than that of the inert gas. The efficiencies of COZ,CF4, and SF6 in deactivating HOz* are identical. The rate constant for the dissociation of chemically activated HOz is approximately 2 X 1O’O sec-I. The reaction of H with HzSrequires E = 2.7 kcal mole-’, if H 0 2
+
requires zero activation energy.
I. Introduction
HOz*
The photolysis of HzS has been used as a source of H atoms for investigations of the kinetics of their reactions with a variety of hydrocarbons2a and with oxygen.2b The mechanism
+ h~ +H + HS H + HzS *Hz + HS
(1)
+ J_ HOz* HOz* + M +HOz + M
(3)
HZS
H
0 2
(2)
(4)
+
0 2
+H
+ 202
(5)
was suggestedzbfor the reactions of H with 0 2 , Reaction 5 is unusual in that it represents the collisioninduced decomposition of a vibrationally excited (1) This work was supported by Project SQUID, which is supported by the U. S. Office of Naval Research, Department of the Navy, under Contract N6 ori-10bTask 3. ( 2 ) (a) B. deB. Darwent and R. Roberts,Discuasions Faraday Soc., 14, 55 (1953); (b) B. deB. Darwent and V. J. Krasnanaky, Symp. Combust., 7th, London,1368, 3 (1958).
Volume 71. Number 7 June l3bY
