The Concentrational Changes of the Decay Time of B in * C. Bojarski, G. Zurkowska, and J. Tyrzyk Research Group for Luminescence, Institute of Physics, Technical University, Gdansk, Poland

Z. Naturforsch. 37a, 74-77 (1982); received October 13, 1981

The photoluminescence decay time r of neutral solutions of rhodamine B in ethanol was mea- sured for concentrations c from 10~5 mol l-1 to 10-2 mol l-1 at 295 K. The T(C) curve observed is atypical, showing two regions of decreasing r. The drop over the range 4 X 10-4 to 10~3 mol 1_1 was explained by the presence of two ionic forms of rhodamine B, and that at concentrations greater than 4 X 10-3 mol l-1 — by rhodamine B dimers.

1. Introduction 0.1, where £max is the maximum value of the extinc- tion coefficient, c is the concentration and t is the Rhodamine B is a xanthene whose solutions sample thickness. According to Ketskemety et al. have been widely used as active media of dye [7] the effect of reabsorption and secondary fluores- lasers. In the last decade several papers devoted to cence on t may be neglected under these conditions. the explanation of concentrational changes of the The relative quantum yield was measured in the absorption spectra of rhodamine B in ethanol spectral range of (575 ± 8) nm using a method (RB/Et) at low concentrations have appeared described by Förster [8]. The absorption spectra [1 —4]. It was shown in that the changes mentioned were measured with a VSU2-P spectrophotometer, are not the result of dimerization [5] but occur due and the fluorescence spectra and the quantum yield to changes of the acid-base equilibrium. — with a device described elsewhere [9]. In the present paper we report on concentra- tional changes of the photoluminescence (PL) decay time of neutral RB/Et solutions. 3. Results and Discussion Figure 1 shows electronic absorption and fluores- 2. Experimental cence spectra of RB/Et at various dye concentra- tions. Clearly, with increasing concentrations the Rhodamine B (C28H31CIN2O3, m.w. 479.03) absorption and fluorescence spectra are red shifted. produced by GMBH & Co. (Schuchardt) has been The absorption curves for concentrations 6.3 X10-6 purified by repeated crystallization from ethanol < c < 6.3 X 10~3 mol 1_1 go through isosbestic and vacuum evaporation. Analytically pure ethanol, points, indicating two species in equilibrium within manufactured by POCH-Gliwice, was used as the this range of concentrations. The pure spectrum solvent without additional purification. The mea- for each of the species and also the equilibrium surements of the mean PL decay time r were done constant K were obtained numerically using the at 295 K with a phase fluorometer as described method described in [10], which is a modification elsewhere [6]. To excite the fluorescence, frontal of earlier methods developed by Förster and illumination with an HBO-200 lamp, providing, Levshin [11-13]. via an SPM-2 monochromator, light of (546 ± 8) nm Figure 2 shows resolved spectra of both species, wavelength was adopted. At the receiving end an exhibiting maxima at 543 nm and 555 nm (curves 1 OG-3 cutoff filter (A > 580 nm) was employed, x was and 2). Shown are also the absorption spectra for measured using samples so thin that 2.3emaxct < RB/Et with NaOH trace and HCl trace (curves 1' and 2'). They are almost identical with the resolved * This work was carried out under the Research Project curves for the species 1 and 2. Hence the shift MR. 1.9. shown in Fig. 1 is caused by changes of the acid- Reprint requests to Prof. Dr. C. Bojarski, Politechnika Gdanska, Instytut Fizyki, ul. Majakowskiego 11/12, 80-952 base equilibrium. The slight difference in £max for Gdansk, Poland. curves 2 and 2' may be caused by the fact that the

0340-4811 / 82 / 0100-0074 $ 01.00/0. — Please ord er a reprint rather than making your own copy. C. Bojarski, G. Zurkowska, and J. Tyrzyk • Fluorescence Decay Time of Rhodamine B in Ethanol 75

Mnm] Fig. 1. Absorption (left) and fluorescence (right) spectra of rhodamine B in ethanol at different dye concentration at 295 K. Concentrations in mol l-1.

calculations of the resolved spectra 1 and 2 were Concentrational changes of RB/Et absorption based on the absorption spectra for the concentra- spectra were investigated earlier [3, 5, 14, 15]. tion range (3 X 10~6 — 10_1) mol l-1, where the Selvyn and Steinfeld [5] measured the absorption assumption of two species only is not satisfied spectra for RB/Et at much lower concentrations. strictly. They determined an equilibrium constant K = 204001 mol-1 and also gave the absorption spectra of two different species, exhibiting maxima at 542 nm and 554 nm. They assigned these spectra to RB monomers and dimers, respectively. Fer- guson and Mau [1, 2], and also Levshin et al. [3] showed that concentrational changes of RB/Et absorption spectra as observed in [5] reflect changes of the equilibrium between ionic forms of rhodamine B, namely between the base form (bipolar ion RB COO+") and the acid form (RB COOH+ cation). In [3] it was also shown, based on measurements of the mean molecular weight of RB, that for con- centrations up to 10-2 mol l-1 dimers are absent. However dimers are likely to exist in solutions of high concentrations. The slight bathochrome shift of the absorption band maximum accompanied by the appearence of a short-wave band at A = 520 nm (cf. the dashed curve in Fig. 1) evidences this pos-

X[nm] • sibility. Fig. 2. Resolved distinct species absorption spectra for Figure 3 shows the concentrational dependence rhodamine B in ethanol (solid lines 1 and 2); absorption of the PL decay time r(c) of RB/Et neutral solu- spectra for 1.58 X 10~5 mol l"1 RB/Et (dashed lines); 1' — trace of NaOH added to solution, 2' — trace of HCl added tions (empty circles). Evidently two regions of r to solution. decrease exist: 4 X 10-4 < c< 10-3 mol 1_1 and 76 C. Bojarski, G. Zurkowska, and J. Tyrzyk • Fluorescence Decay Time of Rhodamine B in Ethanol 76

c [mol/l ]—• Fig. 3. Fluorescence decay time of rhodamine B in ethanol plotted against concentration. c > 6 X 10-3 mol l-1. The PL decay time was also measured for the same system containing a trace of In NaOH (black circles). For the latter case the typical r (c) dependence was observed; the shorten- ing of r for c > 4 X 10-3 mol l-1 may be explained by PL quenching by non-luminescent dimers. The clmol/l] • explanation of the drop of r for neutral solutions of Fig. 4. Mole fraction X of the base form of rhodamine B in RB/Et within the range of highest concentrations ethanol vs concentration. is similar. On the other hand the decrease of r for moderate concentrations may be explained by Such a process may occur as the fluorescence spec- changes of the acid-base equilibrium with rising trum for the base form and the absorption spectrum concentration, tending to increase the contribution for the acid form of RB overlap considerably. For of the acid form exhibiting a lower absolute yield neutral solutions of RB/Et also the graph of the [rjo equal to 0.71 and 0.49 for the base and acid concentrational dependence of the relative quantum form, respectively [2]). They exhibit also different yield exhibits a similar atypical shape as the r(c) values of the decay time To. The measured values curve. This was observed by Levshin et al. (see were 2.7 ns and 2.15 ns for the base and acid Fig. 1 in [3]) and also confirmed by our investiga- form, respectively, which are somewhat less than tions. analogous values 3.1 ns and 2.5 ns as determined by Sadkowski and Fleming [4], or 3.5 ns and 2.8 ns 4. Concluding Remarks as determined by Levshin et al. [3]. Figure 4 shows the fraction X of the base form The t (c) curve for neutral solutions of RB/Et plotted against the dye concentration. For the cal- provides a direct proof of the presence in the solu- culations an equilibrium constant K = cRH+/c|+- = tion of two ionic forms of rhodamine B exhibiting 16431 mol-1 was assumed, where cRH+ and cR+- different PL decay times. In order to quantitatively denote the concentrations of the acid and base explain the experimental r (c) curve, a nonradiative forms, respectively. It is evident from Fig. 4 that excitation energy transfer between molecules of significant changes of X occur within a relatively two ionic forms should be taken into consideration. wide range of concentrations compared to the range For high concentrations also the energy transfer to in which the drop of r has been observed. However, nonluminescent dimers must not be neglected. the shape of the PL decay time curve r(c) is This problem seems to be difficult to solve, especial- affected not only by values of To and rjo for ly as the energy transfer may occur not only from both forms, but also by nonradiative excitation the base to the acid form, but also in the opposite energy transfer between molecules of these forms. direction.

[1] J. Ferguson and A. W. H. Mau, Chem. Phys. Letters [3] L. V. Levshin, T. D. Slavnova, V. I. Jushakov, N. B. 17, 543 (1972). Zorov, and V. Z. Pastchenko, Zh. Fiz. Chim. 48, 84 [2] J. Ferguson and A. W. H. Mau, Australian J. Chem. (1974). 26, 1617 (1973). [4] P. J. Sadkowski and G. R. Fleming, Chem. Phys. Letters 57, 526 (1978). C. Bojarski, G. Zurkowska, and J. Tyrzyk • Fluorescence Decay Time of Rhodamine B in Ethanol 77

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