SPONTANEOUS AND STIMULATED EMISSION FROM DYES. SPECTRO- SCOPY OF THE NEUTRAL MOLECULES OF ACRIDINE ORANGE,

PROFLAVINE, AND RHODAMINE B

By J. FERGUSON* and A. W.-H. MAU*

[Manuscript received 2 March 19731

Abstract

The absorption and fluorescence spectra of acridin? orange, proflavine, and rhodamine B have been studied with particular attention being paid to the acid-base properties of these dyes in polar and non-polar solvents. These studies show that the neutral acridine dyes do not form dimers but they are readily protonated even in non-polar solvents due to traces of water which are difficult to remove. Attempts to achieve laser emission form the acridine dyes were unsuccessful, probably because of their low fluorescence quantum yields. Suggestions are made for optimizing the laser efficiency of rhodamine B in various alcohol solutions at low temperatures.

The development of dye lasers and mode-locked lasers has opened many new areas of research. Dye lasers are useful because of their tunability and mode-locked lasers are desirable for their high power and/or for ultrafast experiments. Dye molecules play an important role in both of these techniques and it is therefore important to understand the spectroscopic properties of such dyes. Our aim is to study these, particularly acid-base and monomer-dimer equilibria, with respect to temperature, concentration, solvent, and pH in order to arrive at a better description of the micro- and macro-scopic parameters of these systems.

In an earlier communication1 we showed that the correct interpretation of the spectroscopic properties of two acridine dyes in ethanol solutions involves the under- standing of the role played by water in the ethanol. Previous interpretationszg3 of these properties were shown to be incorrect. The present paper continues the study of the same dyes and we show that traces of water in "water-immiscible" hydrocarbon solvents also modify the spectroscopic properties.

Solutions of rhodamine B (rb)? in ethanol involve an acid-base equilibrium and the interpretation of their spectroscopy must recognize this, with conclusions which differ from those of Selwyn and Steinfeld.4 A study of this equilibrium has led to ways of optimizing conditions for laser emission and the present paper reports these.

* Research School of Chemistry, Australian National University, Canberra, A C T . 2600. t Abbreviations used in this paper: rb, rhodamine B; r b ~ , its acid form; r b ~ , its basic

form; daa, 3,6-diaminoacridine; ao, 3,6-bis(dimethy1amino)acridine. Ferguson, J., and Mau, A. W.-H., Chem. Phys. Lett., 1972, 17, 543. Mataga, N., Bull. chem. Soc. Japan, 1957, 30, 375. Levshin, V. L., J. exper, theor. Phys. USSR, 1955, 28, 213. Selwyn, J. E., and Steinfeld, J. I., J. phys. Chem., 1972, 76, 762.

Aust. J. Chem., 1973, 26, 1617-24

J. FERGUSON AND A. W.-H. MAU

The neutral acridines were purified by chromatography and recry~tallization.~ Rhodamine B was used without purification for the laser experiments but it was purified for the quantum yield measurements.

Acridine dyes tend to be adsorbed on the walls of the sample cells, particularly with non-polar solvents. Care was therefore taken to avoid prolonged standing of solutions and measurements were always made within a few hours after preparation.

Absorption spectra were measured with a Cary 17 spectrophotometer. Temperature variation was achieved by placing the cell in a gas flow tube and cooling it with controlled boil-off of liquid nitrogen.

Fluorescence spectra were measured with a 0.75-m Spex monochromator using EM1 9659QB and Philips 56UVP photomultipliers. Excitation was provided by a 150-W xenon sealed beam lamp, dispersed by a Zeiss MM12 double monochromator. A Hewlett-Packard radiant flux meter was used for calibration of light intensities.

Lifetimes of the fluorescence were measured by means of dye lasers pumped by an Avco C950 nitrogen laser. POPOP in toluene, optimized for high gain and short pulse width, was the most useful for the present dyes. The pulse duration was 2 . 8 ns (FWHM) with a short decay of 1 ns. Attenuation of the laser pulses was needed to avoid saturation of the detector and stimulating emission from the solutions. Decay traces were displayed on a Tektronix 7704 or a 564B sampling- storage oscilloscope.

(a) Acridine Dyes

There are aspects of the spectroscopy of 3,6-diaminoacridine (daa) and 3,6-bis- (dimethy1amino)acridine (ao) which are incompletely understood and, as these materials are possible laser dyes, a detailed spectroscopic study of them is needed. The polar nature of the molecules and the ease with which the ring nitrogen can be protonated make interpretation of their spectra far from straightforward. We therefore measured the absorption and fluorescence spectra of the vapour of acridine orange to obtain "solvent free" spectra for comparison with the solution spectra.

A vapour cell, similar to that described by Pappalardo and Ahmed,6 was used to measure the spectra and the results are shown in Figure 1, along with corresponding spectra taken in a polar solvent and a non-polar solvent. The fluorescence excitation spectrum of the vapour was also measured and it is given by the broken curve.

The vapour absorption and fluorescence curves show very good mirror symmetry and both spectra, as expected, occur at higher energies than the corresponding solution spectra. The decay time of the vapour fluorescence was measured with a POPOP dye laser and a value of 3.0 ns was obtained. This value is a little less than the solution value (Table 1).

The similarity between the band shape of the vapour absorption spectrum and those in solution is added proof of the incorrect assignments made by earlier workers.334 The solution absorption bands represent the neutral monomer form, not the dimer.

Pappalardo and Ahmed6 have suggested the possibility of laser action in organic vapours and the neutral acridine dye molecules are candidates. However, qualitative indications show that the quantum yield is low so that attaining laser action in the vapour of acridine orange would be extremely difficult.

Albert, A., "The Acridines," (Edward Arnold: London 1966). Pappalardo, B., and Ahmed, S., J. chem. Phys., 1972, 56, 5135.

EMISSION FROM DYES 1619

Our earlier report1 drew attention to the role of water, as a source of protons, in ethanol. In order to investigate the possible dimerization of ao and daa we turned to non-polar solvents. As these are immiscible with water, protonation is unlikely and we found no spectral changes over a large concentration range.' However, we observed changes in the spectrum when solutions are cooled to low temperatures.

Wavelength (nm) 340 360 380 400 420 440 460 480 500 540 580

I 1 I I I I I I I I I l l 1

x wavenumber (an-')

Fig. 1.-Absorption and fluorescence of neutral ao in various media. (a) 1 x 1 0 - 5 ~ ethanol solution at 295 K. (b) 2 x 1 0 - 5 ~ 3-methylpentane solution at 295 K. (c) Vapour at 488 K; vapour pressure c. Torr; the arrow points to the monitoring emission frequency used to

record the excitation spectrum (broken curve).

TABLE 1 FLUORESCENCE QUANTUM YIELDS (4) AND LIFETIMES (z) OF ao AND daa

In ethanol solutions (5 x 1 0 - 6 ~ )

ao base ao acid daa base daa acid

295 K z (ns) 3.6 3.4 3.3 3 a0 4 0.17 0.37 0.19 0.49

270 K 4 0.21 0.25 230 K 4 0.26 0.30 150K 4 0.36 0.32

A typical observation is shown in Figure 2(a) for a 4 x 1 0 - 5 ~ solution of ao in 3-methylpentane. There are corresponding changes in the fluorescence spectra and, at first sight, it would appear that the changes indicate association of the ao. Similar effects also occur in other glass-forming hydrocarbon solvents. As the bands which

1620 J. FERGUSON AND A. W.-H. MAU

appear at low temperature are very similar to the protonated form we decided to thoroughly dry the solvent to eliminate any traces of water.

Wavelength (nm) 360 380 400 420 440 460 480 500 540 580600 650

I I I I I 1 1 1 , 1 1 1 1 1

x wavenumber (an-')

Fig. 2.-Effect of water on the absorption (left) and fluorescence (right) spectra of ao in a non-polar solvent: (a) 4 x 1 0 - 5 ~ a0 in 3-methylpentane; (b) 4 x 1 0 - 5 ~ a0 in "dried"

3-methylpentane.

A grease-free vacuum system was set up and 3-methylpentane was first treated with sodium metal under vacuum, then distilled twice before filling the evacuated cell containing a known weight of neutral ao. The cell containing a solution of ao was then sealed off in a vacuum and the spectrum was taken immediately. The solution was cooled and absorption and fluorescence spectra were recorded, as shown in Figure 2(b). The spectra do not show the changes observed for the solutions which were not carefully dried (Fig. 2(a)). This result indicates that traces of water in non- polar hydrocarbon solvents are responsible for protonation of ao in the cooled solutions, not aggregation of ao. This conclusion applies equally well to observations of low temperature spectra of other molecules which can be protonated. For example, the reported dimerization of 9,lO-diazaphenanthrene7 and the study of formaldehydes involve low temperature spectral changes which could result from protonation by water in the solvents used. There is, therefore, no evidence for dimerization of the neutral forms of these acridine dyes in either polar or non-polar solvents.

' De Vries Reilingh, D. N., Rettschnick, R. P. H., and Hoytink, G. J., J , chem. Phys., 1971, 54. 2722.

~ercbvici, T., King, J., and Becker, R. S., J. chem. Phys., 1972, 56, 3956.

EMISSION FROM DYES 1621

The fluorescence properties of ethanol solutions of the monohydrochloride of acridine orange and proflavine have been well characteri~ed.~"~ It is of interest to study the properties of the neutral molecules. Figure 3 shows the absorption and fluorescence spectra of ao and daa in ethanol solutions, treated with anhydrous potassium carbonate, at various temperatures. The increased intensity of absorption on cooling is larger than expected from solvent contraction and probably results from an interaction between the polar solutes and the polar solvent. The increased emission

Wavelength (nm)

x wavenumber (cm-I)

Fig. 3.-Absorption and fluorescence spectra of neutral acridine dyes in ethanol solutions (cell path length 1 cm): (a) 1 x 1 0 - 5 ~ of ao; (b) 1 x 1 0 - 5 ~ of daa. The arrows show the

wavenumber of excitation for the fluorescence spectra.

intensity is most important because of the possibility of laser action. The fluorescence quantum yields were measured, an anthracene solution (4 = 0.27)" being used for calibration, and they are given in Table 1. The values of 4 and lifetimes at room temperature for the protonated molecules are also given for comparison. Attempts to achieve laser emission were unsuccessful, even with the maximum power density of the nitrogen laser. The quantum yield data indicate that the efficiency of the materials as lasers would be so low as to be not practical.

(b) Rhodamine B

We have shown in the earlier report1 that the 554 and 542 nm bands, previously assumed to involve a dimer-monomer eq~il ibrium,~ actually are related to the acid and base form of the dye, respectively (see structural formulae). The relevant spectra are given in Figure 4. Even though the fluorescence was observed from the illuminated face in each case, reabsorption of the fluorescence leads to a change of spectral

Zanker, V., Z. phys. Chem., 1952, 199,225; 1954, (N.F.) 2, 52. lo Parker, C. A,, Hatchard, C. G., and Joyce, T. H., J. molec. Spectvosc., 1964, 14, 311. l1 Parker, C. A., "Photoluminescence of Solutions," (Elsevier: Amsterdam 1968).

1622 J. FERGUSON AND A. W.-H. MAU

distribution at higher concentrations. The acid form of rhodamine B ( r b ~ ) in 6 x 1 O W 6 ~ solution was obtained by simply adding one drop of IN HC1 to 10 ml of solution, and the base form ( r b ~ ) in 1.0 x 1 0 - 3 ~ solution by adding two drops of 2~ NaOH. The horizontal lines indicate the range of laser tuning by means grating inserted in the cavity.

acid form, r h ~

Wavelength (nm)

lo base form ,i; I/\'\,

10-3~ wavenumber (cm-I)

hase fonn. r b ~

Fig. 4.-Absorption and fluorescence of (a) rhodamine B acid form ( r b ~ ) ; (b) its base analogue ( r b ~ ) in ethanol at 295 K. I, Absorption; 2, fluorescence of 6 x 10-6~ solution; 3, fluorescence of 1 x 1 0 - 3 ~ solution. The bars show the range of external tuning of laser frequencies of the 1 x 10-3~ solutions.

The temperature dependence of quantum yield (4) is of particular relevance to laser experiments1' and these are given in Figure 5. There is a large temperature dependence for both forms in ethanol solution (curves I and 4). The quantum yields

lZ Ferguson, J., and Mau, A. W.-H., Chem. Phys. Lett., 1972, 14, 245.

EMISSION FROM DYES 1623

also depend on the solvent and values measured at 295 K are given in Table 2. The literature values for rhodamine B in water solution have been reported as 0.25,13 and in ethanol solutions as 0.42,13 0.60,14 0.64,15 0.69, '~ 0.73," and 0.97.'' In dilute ethanol solutions (< 1 O P 5 ~ ) rhodamine B is largely in its base form and our value of 0 a71 agrees well with the value obtained by Parker et a1.l ' I6

for

Fig. 5.-Temperature dependence of 4

rhodamine B (5 x 1 0 - = ~ ) . I, r b ~ in EtOH;

2, r b ~ in ethylene glycol; 3, r b ~ in EtOH;

4, r b ~ in glycerol.

Corn- Viscosity 4 Relative pumping power

Solvent pound (cp) to reach threshold

r b ~ ethanol 1.2 0.71 1-1 r b ~ ethanol 1.2 0.49 1.4

ethylene glycol 17.5 0.56 1.2 glycerol 1260 0.78 1.0

The temperature dependence of the quantum yield of rhodamine B in glycerol has been studied earlier by pringsheim13 who found a threefold increase on cooling from 358 to 289 K. The results in Figure 5, over a wider temperature range and different solvents, show a similar trend. Assuming that the temperature dependence involves an activation barrier, an Arrhenius plot was made. Good linear plots were obtained and the activation energies are 7.0 kcal mol-' and 7.2 kcal mol-' for r b ~ and r b ~ , respectively, in ethanol solution.

Previous work has not taken into account the existence of acid and base forms of rhodamine B in ethanol solution and it was therefore decided to study the laser characteristics of both forms. The relative laser efficiencies are best compared by determining the minimum pumping power needed to achieve the threshold conditions for laser operation. The actual numerical figures depend on the laser configuration so that we must use arbitrary values and these are shown in Table 2. These values

l3 Pringsheim, P., "Fluorescence and Phosphorescence," (Interscience: New York 1949). l4 Peterson, 0. G., Webb, J. P., and McColgin, W. C., J. appl. Phys., 1971, 42, 1917. l5 Goncharov, V. A,, Zverev, G. M., and Martynov, A. D., Optika Spectrosk., 1971, 30, 78. l6 Parker, C. A,, and Rees, W. T., Analyst, 1960, 85, 587. l7 Weber, G., and Teale, F. W. J., Trans. Faraday Soc., 1957, 59, 646.

1624 J. FERGUSON AND A. W.-H. MAU

correlate well with the quantum yield results and they show that the base form has a lower threshold in ethanol solution. However, prolonged standing of the basic solution leads to the formation of a microcrystalline suspension which is damaging to the laser action.

The results of Figure 5 and Table 2 lead to the conclusion that r b ~ in glycerol should have the best dye laser characteristics. However, it was found that r b ~ solutions in glycerol and ethylene glycol ceased laser action after a few pulses and a higher pumping power is needed to sustain laser action for any length of time. It seems reasonable that the viscosity of these two solvents inhibits the movement of dye molecules so that replenishment of photobleached molecules cannot occur quickly enough to maintain laser oscillation. Photobleaching of the xanthene dyes has been considered recently.''

The temperature dependence of the quantum yield of r b ~ in ethanol and the resistance of xanthene dyes to photodegradation at low temperatures in generallg make it desirable to study the temperature efficiency of laser action of r b ~ . Pre- liminary results indicate that this is true.20

Contrary to previous publications there is no evidence to support the occurence of dimerization of the neutral acridine dyes in ethanol or non-polar solvents. Water plays an important role as a proton donor in polar solvents at room temperature and in non-polar solvents at low temperature. Efforts must be made to eliminate this water before carrying out spectroscopic measurements with molecules of this type (acridine and related compounds).

Measurements of quantum yields of acridine orange and 3,6-diaminoacridine suggest that it will be extremely difficult to stimulate emission from these molecules in their neutral forms.

Measurements of the fluorescence properties of acid and base forms of rhodamine B indicate that better laser efficiency is expected from this dye by cooling, and pre- liminary experiments have confirmed this.

ls Koizumi, M., and Usui, Y., Molec. Photochem., 1972, 4, 57. l9 Fork, R. L., and Kaplan, Z., Appl. Phys. Lett., 1972, 20,472. 2 0 Ferguson, J., and Mau, A. W.-H., unpublished data.