H. Schmidt, Robert Zellhofer
Jul 1, 1974
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Abstract
The dependence of the ESR triplet spectra of some acridine dyes (3,6diamino-10-methylacridinium chloride, 3,6-diaminoacridine hydrochloride, 3-aminoacridine hydrochloride, 3,6-diamino-2,7-dimethylacridine hydrochloride, 3,6-bis-diethylaminoacridine Perchlorate, 9-aminoacridine hydrochloride and 3,9-diaminoacridine hydrochloride) upon the concentration has been investigated in methanol—water solutions at 90 °K. Compared with the monomers the zero-field splitting energies calculated from the spectra change very little at lower dye concentrations up to about 10 4 M. This is consistent with the assumption of sandwich-type dimers or lower aggregates. With the exception of 9-aminoacridine and 3,9-diaminoacridine the spectra, however, change strongly at higher dye concentrations. Using triplet exciton theory these results can best be explained by the existence of higher aggregates with the single molecules oriented in a translationally non-equivalent manner. Introduction In a previous paper1 it has been reported that the ESR triplet spectra of acridine orange in a solid matrix are strongly dependent upon the dye concentration. Zero-field splitting (zfs) calculated from these spectra decreases substantially with increasing acridine orange concentration. This effect is understandable on the assumption that the spectra at low and high dye concentrations are caused by different species. The spectra at low dye concentrations originate from monomers and/or dimers with a sandwich type structure. On the other hand using triplet exciton theory it can be demonstrated that the spectra H. Schmidt, Z. physik. Chem. Neue Folge 80 (1972) 44. ESR Triplet Spectra of Acridine Dyes and Their Aggregates 205 at higher concentrations (above 10-2 M) have to be attributed to extended aggregates of which the molecular planes differ distinctly from a parallel arrangement1-2 Other workers assume sandwich-like structures almost entirely for aggregates of acridine dyes and related compounds. The purpose of the present study therefore is to investigate whether the highly aggregated species mentioned above are restricted to acridine orange or whether changes of the zfs on varying concentration can also be observed with other acridine dyes. Experimental Most of the experimental details have been described in a preceding paper1. Samples were illuminated by a high pressure xenon lamp (Osram: XBO 900 or XBO 1600 W) at about 90°K in a glassy methanol (Merck: p.A.)/water (triple distilled) mixture (90°/o w/w). All measurements were carried out in presence of oxygen. Sharp cut filters (Schott & Gen.) were used to cut off short-wavelength light. To prevent heating of the samples a water cell (6 cm pathlength) and two IR absorbing filters (Schott & Gen. : type KG 3) were employed. All the acridines were purified carefully by recrystallisation and Chromatographie methods. The purity of the dyes has been tested by thin-layer chromatography (in the dark!), by measuring of the molar extinction coefficients and as far as possible by their melting points. Acridines not commercially available were synthesized as described in the literature. For the ESR measurements the following compounds were used [melting points (stem corrected) : free acridine bases] : 3-aminoaeridine hydrochloride3-4-5 m.p. 226—227 °C, 9-aminoacridine hydrochloride (Merck p.A.) m.p. 245—246°C, 3,6-diaminoacridine hydrochloride (proflavine: . & ., p.A.) m.p. 297—298°C, 3,9-diaminoacridine hydrochloride5 m.p. 140—160°C, 3,6-diamino-10methylacridinium chloride (acriflavine: Fluka, purum; purification5'6), 3,6-bis-diethylaminoacridine Perchlorate5-7, 3,6-diamino-2,7-dimethylacridine hydrochloride (acridine yellow: Schuchardt). The last three dyes do not have an exact melting point. 2 H. Schmidt, Z. Naturforsch. 26a (1971) 763. » A. Kaufmann, Ber. 42 (1909) 3482. 4 A. Albert, J. ehem. Soc. [London] 1948, 1228. 5 A. Albebt, The Acridines, E. Arnold Ltd., London 1966. 6 M. Gailliot, Quant. J. Pharmac. Pharmacol. 7 (1934) 63. 7 G. Löbeb and G. Achtest, Biopolymers 8 (1969) 595. 206 Hartmut Schmidt and Robert Zellhofer Results The theory of the ESR triplet spectra used for the evaluation of these spectra has been reported in a previous paper8. The order of the zero-field splitting energies , , has been chosen by using the terminology of Kottis and Lefebvre 9 and at first does not correlate with any molecular axes system. a) Triplet spectra of the acridine dyes Acriflavine The detailed zero-field splitting energies at high and low dye concentrations are summarized in Table 1. As in the case of acridine orange1, a distinct concentration dependence of the ESR triplet spectra can be observed for acriflavine, too (Fig. 1). Within the experi2500 3000 3500 4000 lOauss! Fig. 1. ESR triplet spectra of acriflavine. Concentration (a) 1 10—^M,"excitation with unpolarized light; (6) 1 · -4 M, unpolarized light; (c) 1 · -4 M, polarized light, E parallel to H; (d) 1 · IO-4 M, polarized light E perpendicular to « . Schmidt, Photochem. Photobiol. 11 (1970) 17. 9 P. Kottis and R. Lefebvre, J. chem. Physics 41 (1964) 379. ESR Triplet Spectra of Acridine Dyes and Their Aggregates 207 Table 1. Dependence of the zero-field splitting energies (cm,-1 he) of the acridine dyes upon the concentration (M l_1). (Maximal experimental error: normally 0.0005, * between 0.0006 and 0.0015, for D*m=2 smaller than 0.001 cm'1 he) Compound Concentration