2712
Communications to the Editor
Electron Spin Resonance Study of Photoinduced
TABLE I: Esr Data
from Dye Molecules Giving
a Triplet Signal
Triplet States from Organic Dye Solutions Dyea Publication costs assisted by the Air Force Materials Laboratory
Sir: The development of tunable organic dye lasers is proving to be a significant advancement in the field of phot0chemistry.l . 2 A major difficulty of organic dye laser systems is that quenching of stimulated emission may occur when the singlet state undergoes intersystem crossing to a triplet state. The triplet state of dye molecules is detrimental to laser action because it depletes the population of the singlet lasing level, as well as allowing for the possibility of laser quenching arising from T T absorpt i ~ n . We ~ . ~have undertaken an esr investigation of a number of dyes to determine the factors which affect intersystem crossing to a triplet state and to determine what class or type of dye gives a A M = 1 2 esr signal. Glazkov, e t a / . , 5 had looked a t concentrated (10P2 M ) solutions of some xanthene dyes and attributed a A M = &2 esr signal to a triplet of dye aggregates and not to the monomer. We have also obtained the esr spectra of these and other dyes in concentrated alcoholic solutions and have found the intensity of the half-field signal to be both concentration and wavelength dependent. A 5 x M EPA solution of Rhodamine 6G also gave a signal at 1637 G, however, Selwyn and Steinfeld6 found no spectrophotometric evidence for aggregate formation a t this concentration of Rhodamine 6 6 a t 57°K in EPA. This shows that the triplet esr signal is due to the monomer rather than an aggregate of the dye. Besides the xanthene dyes, Crystal Violet and Ethyl (Diquinoline) Red also gave strong esr signals between 1612 and 1637 6. The half-lifes and zero-field splitting parameter D* were measured and are listed in Table I. Some oxazine dyes were also investigated and Oxazine 1. Cresyl Violet Chloride, Acetate, and Nitrate, and Nile Blue gave no triplet esr signal. No phosphorescence was observed for these oxazine dyes, indicating very little triplet formation and short lifetimes. The chloride and oxalate salts of Malachite Green also gave no triplet esr signal, although phosphorescence was detected. As shown in Table I, Fluorescein gave a signal at 1612 G but Erythrosin B with iodide substituents gave no esr signal and very weak phosphorescence. The lack of an esr signal for the Sbf = 1 2 transition is most likely due to the heavy atoms in which the triplet lifetime is too short and the steadystate concentration too low to detect a triplet esr signal.
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1-heJournal of Physical Chemistry. Vol. 77. No. 22, 1973
Rosamine Rhodamine B CI -
OAc Rhodamine 6G Acridine Red Fluorescein Ethyl (Diquinoline)
Red
1634
28
0.7
0.0293
1626 1627 1637 1628 1612
18 18
0.5 0.5
14 26
0.8 0.7
9
0.6
0.0336 0.031 5 0.0244 0.0299 0.0478
19 22
0.1 0.2
0.0324 0.0208
1628 1636
Crystal Violet
UThe dyes (10-3-10-2 M ) in ethanol-methanol ( 4 : l ) glass at 77°K were photolyzed in the cavity with an Ostram 1-180-200W mercury lamp. Degassing the sample had little effect on the signal. Line width from peak to peak on derivative. Half-lifes were measured from the dark decay of the AM = f 2 esr transition intensity. Calculated from D* = / [ 3 ( h ~ ) ~ , ’ 4 ] 3(g,!3H,,)2~3’2where v is the klystron frequency and H m is the field position of the low-field maximum of the b M = 1 2 esr transition derivative spectrum.8
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Solvent radical signals a t g = 2.00 were detected in all the dyes studied indicating dye sensitization of the solvent.7 As indicated in Table I, the anion has little affect upon the field position and intensity of the triplet signal for Rhodamine B. The anion also had no affect on producing a signal in the Cresyl Violet or Malachite Green cases.
Acknouledgment. The authors would like to acknowledge Dr. H. M. Rosenberg for helpful discussions and Dr. R. N. Steppe1 for providing purified samples of Cresyl Violet and Rhodamine dyes. References a n d Notes (1) 8 . B. Snavely. Proc. / € E € , 57, 1374 ( 1 9 6 9 ) . ( 2 ) K. H. Drexhage, Laser Focus. 9,35 ( 1 9 7 3 ) . (3) P. P. Sorokin, J. R . Lankard, V. L. Moruzzi, and E. C Hammond, J. Chem. Phys., 48,4726 (1968). ( 4 ) W. Schmidt and F. P. Schafer, 2. Naturforsch.. 22, 1563 (1967) (5) Y. V. Glazkov, N . I . Zotov, and E. K. Kruglik, Izv. Akad. Nauk SSR, Ser. Fiz., 32, 1500 ( 1 9 6 8 ) . (6) J . E. Selwyn and J. I . Steinfeld, J. Phys. Chern., 76, 762 (1972). (7) S. Siegzl and K. Eisenthal, J , Chem. Phys., 42,2494 ( 1 9 6 5 ) . (8) C. Thompson, J, Chem. Phys., 4 1 , 1 ( 1 9 6 4 ) .
Air Force Maferiais Laboratory A F M L I LPH Wright-Patterson Air F o r c e Base Ohio 45433 Received August 10, 1973
F. R. Antonucci* L. G. Toller
