Dee., 1962
BYDROGEX BONDING OF EXCITED STATES
have been found by means of photoflash techniques in stilbenes.35736 A maximum of potential energy a t a twisting angle of 90” is inconsistent with the experimental result that cis FI trans-isomerization occurs even in rigid media a t low t e m p e r a t ~ r e . ’ J ~ , * ~These ,~~ (35) G. Porter, private communication. (36) K. Breitschwerdt and A. Weller, Z. physik. Chem. (Frankfurt), 20, 353 (1959). (37) E. Lippert and W. Llider, J. Phus. Chem., 66, 2430 (1962).
2491
results can be understood if a torsion force is assumed effective in the triplet state. This torsion forw mists if the triplet, state has a minimum potential energy a t 90 degrees twisting angle. Accordingly we consider the activation energy of 2-3 kcal./mole found by FischerZ0for the trans + cis-isomerization of stilbene as being the activation energy of the transition from the first excited singlet to the triplet state in the trans-isomer.
HYDROGEN BONDING OF EXCITED STATES BY E. J. BOWEN,N. J. HOLDER, AND G. B. WOODGER Physical Chemistry Laboratory, Oxford University, Oxford, England Received Mag 26, 1962
Measurements have been made of the quantum yields of fluorescence of acridine dissolved in mixtures of water (alkaline) and certain organic solvents. The results are interpreted on the assumption that hydrogen bonding with the solvent, and particularly with water, takes place rapidly after the molecule is excited by absorbing a light quantum, and that such complexes represent the fluorescent entity; thermal breaking of the hydrogen bonds leading to degradation of electronic excitation energy. The acridinium ion shows a different behavior; its fluorescence changes little in solvent muctures of different composition and is much less dependent on temperature.
TABLE I Acridine is classed as a “n-electron deficient” heterocyclic’ and as such would be expected to -Quantum yield of fluorescence - F show poor fluorescence properties. It is in fact Volume % Dimethyl’ Ethanol Formamide Dioxane formamide of water non-fluorescent in the crystalline state, or when 0.37 0.37 0.37 0.37 dissolved in organic solvents such as benzene or 100 .31 .33 .3’2 .34 ethyl acetate, but it fluoresces blue in solution in 90 .23 .24 .29 water or aqueous alcohol. The acridinium ion 80 .28 .13 .I8 .18 .24 fluoresces a green color in whatever solvent it may 70 ,086 .12 .19 60 .ll be formed. For example, the addition of proton,084 .15 ,055 donors such as the chloroacetic acids to benzene 50 ,079 ,036 ,053 .10 ,062 40 solutions of acridine causes the development of the ,019 .022 .073 characteristic green fluorescence.2 Acridine is a 30 .048 ,012 .000 .047 weak base, and the molecule in water changes over 20 .036 ,006 .000 ,023 to the ion at about pH 5.45, as shown by change of 10 ,034 .000 ,000 .032 .012 absorption spectrum. The fluorescence change0 over from blue to green, however, occurs a t about pH 10.35. The explanation of this effect has TABLE I1 been given by F6rster3 and Weller.4 The electronic DIOXANE-WATER MIXTURES excitation of acridine makes it a much stronger -Water % by volumebase, and between the pH limits given above light 90 70 50 is absorbed by Ihe molecule, and this in its excited E = 6190 E = 5900 E 5750 state very rapidly acquires a proton, so that the log K = 4.96 log K = 5.21 log K = 5.51 -F-FTemp., -Fgreen fluorescence emission occurs from the excited OK. Exptl. Calcd. Exptl. Calcd. Exptl. Calcd. ionic form. The blue fluorescence of aqueous acri283 0.38 0.385 0.17 0.173 0.07 0,075 dine solutions observed a t pH values greater than .30 .299 .13 ,128 .05 .054 293 10.35 is the chief subject of this paper. Bowen .24 .230 .10 303 .091 .04 .039 and Sahu described measurements of the variation 313 .18 ,179 .07 .071 .03 ,029 of the fluorescence of acridine in water-aclohol .05 .054 .02 .022 .14 .137 323 mixture^.^ Their results, however, require some .11 .lo7 .04 .04 .02 .017 333 revision since, as pointed out, in neutral solutions .08 ,083 .. ... .. ... 343 the solute fluoresces partly as the molecule and partly as the ion. In the work now presented 1.3 A T ammonia or 10.02 N sodium hydroxide solutions almost entirely in the molecular form even in its were used instead of neutral water for making up excited state. Acridine concentrations of about mixtures, to ensure that the acridine was kept 10-4 A i mere used. The solutions were contained in a small transparent dewar vessel and the tem(1) A. Albert, “IIeterocyolic Chemistry,” Chap. 3, Univ. of London, perature adjusted by the insertion of hot or cold 1959. (2) N. Mataga and S. Tsuno, Bull. Chem. Soe. J a p a n , SO, 368 glass “fingers.” The fluorescence was excited by (1957). 3660 A. radiation, and a calibrated photomulti(3) Th. Fdrster, Z. Elektrochem., 84, 42 (1950). plier-spectrograph combination, corrected for in(4) A. Wrller. ibid. 61,956 (1957). strumental wave length sensitivity differences, (5) E.J. Bowen and J. Snliu, J. Ciiem. SOC.,3716 (1958).
-
2492 TABLE I11 ETHANOL-WATER MIXTURES
_ _ _ ~ 90 E = 659J log K = 5.16
Temp.,
-F-
K.
Exptl.
Calod.
273 283 293 303 313 323 333 343
0.49
0.51
0
.'20 .33 .2G .31 .1G
.12 .09
80 E = 6410 log K = 5.16 --F----. Exptl. Calcd.
0.42
,4.2 .35
. 34
.37
.21 .l6
.21 .1G .12 .09
. '97 .12 .09 .07
0.45 .3G . 2T .21 .1F .12 .09
Wat,er % by volume-----------------70 E = 6050 log K = 5.16 -F-Exptl. Calcd.
0.30 .23 .17 .13 .10
.07
.os
.OG .04
0.31 .23 .17 .I3 .IO .08
60
E
log K = 4.69 -F------Gxptl. Crtlod.
0.17 .11 .ll
00 07 ,05
.OG
.04
.05
.03
0.19 .I4 .11 .08 ,07 0d .04 .04
-
50 4750 log K = 4.60
E
= 5090
-F-Exptl.
0.12 .I0 08 . 06 05 0.4 03 .02
Calod.
0.13 .10 .08 , 06 ,05 0.1 .0:1 ,@2
collected and dispersed into a spectrum the alkaline solutions) and that the excitted state, which fluorescence emerging a t right angles. Absolute is much more strongly basic, rapidly hydrogenfluorescence quantum yields F were determined by bonds with the solvent, particularly with water, comparing the area of the corrected fluorescence and that these entities either fluoresce or dissociat,e spectra with that from a solution of quinine sulfate with energy degradation. The frequency factor of in 0.1 N sulfuric acid made up to an equal optical this latter process is given by the product of kdensity, taking F for this standard a t 18" as 0.55.6 values of Tables I1 and I11 and the reciprocal of Table I gives the values of quantum yields of the mean radiational lifetime of the excited fluorescence F a t 18" for mixtures of (alkaline) state (= 6.7 X lo7 see.-' from absorption band area'), and reaches the high value of about water with four organic liquids. Tables I1 and I11 give values of F at different' If the excited state is hydrogen bonded to the soltemperatures for water-dioxane and water-thanol vent and its thermally activated state is released from this bonding the high value of the frequency mixtures of different compositions. If a degradatory process requiring an energy of factor of the degradation reaction may be explained activation E is assumed to compete with fluores- in terms of an entropy of activation.8 The green fluorescence of the acridinium ion cence emission, it follows that shows a different behavior in solvent mixtures. E Table IV gives fluorescence yields for various waterlog (1/F - 1) = log k - ethanol compositions a t 18", the water being acidi2.3RT fied to 0.02 N with sulfuric acid. The yields fall The experimental F values in Tables 11 and 111 off very little as the alcohol concentration is inare compared with the calculated values. Values creased, and the effect of temperature is also small, of E range from 6600 to 4750 cal./mole, diminish- E being about 950 cal./mole. 'l%ese results ining as the water content is decreased, but more so dicate that there is no large difference in the for ethanol than for dioxane mixtures. Table I degree of hydrogen bonding between the ground shows that values of F reach zero for pure dioxane and excited states of t,he acridinium ion. or dimethylformamide but are non-zero for ethanol TABLE IV or formamide. These facts point strongly to the Volume % Volume % interpretation that the blue fluorescent entity of of aqueous of aqueous N HSHOI Quantum yield N 11zS01 Quantum yield acridine is a state hydrogen-bonded t'o the solvent, and that the E value represents the heat of such 5 of fluorescence 5 of fluorescence 100 0.54 50 0.52 bonding. However, the absorption spectra of acridine in water, alcohol, or benzene are remarkably 90 .54 40 .51 similar, and the solubility of acridine in water is 80 .54 30 .50 70 .54 20 .4H very low ( = 2 X mole/l.), indicating negligible 60 .53 10 .49 hydrogen bonding of the ground state. It would Seem that light is absorbed by the molecule (in (7) T h . Forster, "Fluoresaenz Organischer Verbindiingen," Van(6) C. A. Parker and W. T. Rees, Analyst, 86, 596 (1960); 87, 83 (1962); C. A. Parker, Anal. Chem.. 34, 502 (1962).
denhoeck and Ruprecht, Gottingen, 1951, p. 158. (8) C . Steel and K. .I. Laidler, J . Chem. I'hys., 34, 1827 (1961)
