KOTES
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Vol. 67
for a donor molecule A and an acceptor molecule B may be described by the equation3aB'Z
AE
=
rA- A~ + c
(2) where I A is the ionization potential of A (equal to the energy of the highest occupied molecular orbital) and A B is the electron affinity of B (equal to the energy of its lowest unoccupied orbital) and C is a constantrelating to the distance the electron has to travel and the dielectric constant of the medium. The change in the order of conductivities of iron benzylisonitrile complexes as compared to their reactivities suggests that the coiiductivity of these solids will he deteimiiied by AB, rather than by I.4.13 When go is plotted us. AE, a linear relationship is obtained (Fig. 2 ) . It is interesting that polyacener show a similar linear relationship (Fig 2 ) . The larger deviations for polyacencs from a straight line are probably due to slight variations in experimental techniques employed by various workers and the various states under which the specimens were investigated. A comparison between the isonitrile complexes and polyacenes demonstrates most clearly the wide ranges of activation energies, AE, and in the constant uo which may be achieved merely in one series of coordination complexes as compared to polyacenes Acknowledgments.-The authors wish to thank Drs. C. Eugene Coffey, Glen R. Kepler, and Charles F. Wahlig for helpful discussions, and Mr. Charles G. Wortz for numerous measurements. (12) RI. J. S. Dewar and A R. Leptey, J . Bm. Chem. Sac., 83, 4560 (1961). (13) An alternative explanation for the change in the order of reactiiities and conductivities IS that the former process proceeds in the transition state vza a a-complex and the latter process cza a r-complex. (See ref. l b for dlscussion.) (14) A linear relationship between the frequency factor and the activation energy in the .4rrhenzus equation has been noted for conductivlty,'6 diffusion,16"7 v i s c o s i t y , ~electron ~ emission 13 equilibria of all types,'b and numerous homogeneous16 and heterogeneous reactions 18 For discussions on the significance of this correlation the reader is referred t o the literature lb-l' (15) P. Ruetschi, 2. physzk. Chem., 14, 277 (1958). (16) J. E. Leffler, J . Ow. Chem., 20, 1202 (1955). (17) R. F. Brown, zbzd., 27, 3015 (1962)
RESONANCE TRANSFER OF EXCITATION EKERGY BETWEEN NUCLEOSIDES AND ACRIDINE ORANGE BY SADHAN BASUAND JOHN GREIST Chemical Laboratory, Indeana Unitersity, Bloornington, Indiana Received January 22, iQ68
I n a previous communication1 it has been shown that a fluorescence is excited in the region of 430-450 mp in a solution of inosine, adenosine, thymidine, or uridine in mater (0.02 M ) on irradiation with light of wave length 350 nip. This new fluorescence band of nucleosides partially overIaps with the absorption band of acridifie orange in aqueous solution (Fig. 1). Thus the condition for resonance transfer of excitation energy by a Forster mechanism exists in these systems and it was surmised that the excitation transfer might be detected by measurements of fluorescence intensities of acridine orange in solutions of dye and nucleosides in water. I n the region 300-400 mp where 0.02 ill solutions of nucleosides show absorption shoulders, a solution of (1) 9. Brtsu and J. Greist, J . chzm. phys., in press.
.04
a,
1
300
320
340
360
380
400
420
A (ma).
Fig. 1.-Absorption spectra: 0, adenosine ( orange ( M).
0.06
M ) ; A, acridine
t
i 0.06
.,.& 2
.
"r
n
-3
$
7 C.
0.04
0.04
... 01
O 0.02
0.02
g
aG
g
0.0
0.0
400
440
480
520
500
600
640
X (md.
Fig. 2.-Absorption spectra: ( a ) acridine orange (10' hf): a, absorption; A', fluorescence; (b) adenosine (10-2 M): 0,fluorescenee with 350 mp excitation; (e) acridine orange M ) adenosine ( M):A , fluorescence with 350 mp excitation.
+
acridine orange M ) shows little absorption (Fig. 1). On exciting a solution of acridine orange with 350 mp light, a weak fluorescence with a peak a t 540 m p is observed. In the presence of 0.02 fl!l adenosine the fluorescence intensity of acridine orange a t 540 mp excited by 350 mp light is increased by a factor of three over that observed in the absence of adenosine (Fig. 2 ) . Evidently, the light energy at 350 mp absorbed by adenosine is being transferred t o the acridine orange from which it is being emitted as fluorescent light. The efficiency of excitation-energy transfer from the donor XIl (nucleoside) to the acceptor Mz (acridine orange), i.e., the number of M2 molecules excited by transfer calculated for one excited 31, molecule, was determined by comparison of the fluorescence intensity of 1 1 2 on excitation by ultraviolet radiation in the absorption band of the donor (350 mp) and in a band where the acceptor absorbs but the donor is transparent (450 mp). If the fraction of 350 mp light absorbed in MI equals all and that in MZequals aZ1,then the transfer efficiency, T12,is given by the relation
Tiz
=
I?1//€22
-
a21
0111
where 1 2 1 and IZ2are the respective fluorescence intensities of >I2 under equal flux of exciting photons. The efficiencies T12calculated for four different nucleosideacridine orange systems are summarized as
COMMUNICATIONS TO THE EDITOR
June, 1963 Nucleoside
Ti2 (%I
lnosine Adenosine Uridine Thymidine
40 35 31
28
According to Forster's theory there is a critical concentration for each system a t which the transfer efficiency reaches a value of 100%. I n the present study, the highest possible concentrations of nucleosides in water have been used. The efficiency values in the range of 30-4OyG as reported above, therefore, do not represent the most efficient situation. It may be asked if the efficiency could be increased by increasing the concentration of dye. When the dye concentration M , some peculiar was increased from to observations were made. There was no increase in the intensity of fluorescence of acridine orange &I) in the presence of nucleosides when excited with 350 mp light compared to the intensity of the dye alone. The absorption intensity in the region of 300-400 mp could be accounted for by the absorption of dye alone, ie., at higher dye concentration, nucleoside absorption in the region of 300-400 mH disappears. Further, when the solution was excited with 450 mp light, the fluorescence intensity of dye-nucleoside mixture WRS higher than that of dye alone, although optical density a t 450 mp was not different in the two cases. This suggests that there is some kind of association between dye and nucleoside a t higher dye concentration. Pn
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fact acridine orange is known to undergo self-assocjatioii a t higher concentration and its association with nucleosides would not be surprising. This association, however, can bring about a degradation of the associated nucleoside molecules which are believed to be responsible1 for the increased light absorption a t 350 mp. It is evident, therefore, that in order to study the energy transfer phenomenon between the associated nucleosides and the dye molecule, the dye concentration cannot be increased much Jvithout affecting the very nature of the associated species. The lorn transfer efficiency for these systems may also be due to the operation of a cascade transfer mechanism; i.e., instead of a resonance transfer of energy, the photon emitted by a nucleoside may be reabsorbed by a dye molecule which in turn will emit its own fluorescent light. Whereas the resonance transfer depends only on the distance between two molecules, Le., on the concentration, the cascade transfer depends on the distance over which the emitted photon can travel inside the solution before encountering one of the absorbing species, i e . , on the geometric shape of the sample. Thus resonance transfer depends only on the concentration of the molecules and is independent of the shape of the sample; the cascade transfer depends on the thickness of the solution: the greater the thickness, the more efficient is the transfer. Acknowledgment.-This work is part of a program supported by the Office of Naval Research.
COMMUNICATIONS TO THE EDITOR REVERSIBLE PHOTOBIKDING OF RIBOFLAVIN TO PhlKACROMOLECdLES I N AQUEOUS SOLUTION
Sir: I n the course of a stiildy of the enhanced rate of photofading of riboflavin in the prewence of polymers such as polyvinylpyrrolidone and the non-ionic surface active agent polyoxyethylene (20) sorbitan monooleate, direct evidence was obtained for reversible photobinding of riboffavin to the macromolecules. Although there was no detectable binding of riboflavin by either polyvinylpyrrolidone or the surlace active agent in absence of light, as determined by dialysis and solubility studies, reversible binding of riboflavin to the polymers was readily detected during illumination of the solutions with visible light. Oster and Wasserman' recently reported photo-induceld binding of fluorescein dyes to solid substrates such as zinc oxide and alumina, but there has been no previous demonstration of photobinding in solution. The photobinding wm detected initially by comparing the quantity of riboflavin in the aqueous and polymer phases after heat precipitation of polyoxyethylene (20) sorbitan monooleate a t 95' in the presence and absence (1)
G. Oster and M. Wasserman, J . Phys. Chcm., 66, I636 (1962).
of light. The photobinding was demonstrated a t room temperature by determining the effect of irradiation on the distribution of riboflavin between an ion-exchange resin, Amberlite 20Q2acid form, and an aqueous solution of either polyvinylpyrrolidone or polyoxyethylene (20) sorbitan monooleate. Visible light had no effect on the distribution of riboflavin between the aqueous phase and the ion exchange resin in the absence of polymer. Table I presents data from a typical photobinding study. Approximately 1.5-2.0 g. of resin was placed in a cuvette containing 50 ml. of polymer solution and sufficient riboflavin to provide a convenient absorbance in the aqueous phase after equilibration with the resin in the dark. The sample was illuminated with light, from a 500-watt tungsten projection lamp, fitted with Corning filters 3-75 and 5-57 to eliminate light below 410 mp and provide maximum transmission of wave lengths near the absorption maximum of riboflavin, 445 m p . The sample mas maintained a t constant temperature during irradiation and samples of the aqueous polymer phase were removed for assay by means of a syringe fitted with a hypodermic needle. The increased concentration of riboflavin detectable ( 2 ) Amberlite 200 is a sulfonated styrene-divinylbenzene copolymer Rohm & Haas Co., Philadelphia. Pa.
