FLUORESCENCE QUANTUM YIELDS OF SOME RHODAMINE DYES

Transcription

1 Journal of Luminescence 27 (1982) North-Holland Publishing Company FLUORESCENCE QUANTUM YIELDS OF SOME RHODAMINE DYES R.F. KUBIN and A.N. FLETCHER Chemistry Division, Research Department, Naval Weapons Center, China Lake, California, USA Received 10 February 1982 Fluorescence quantum yields of seven rhodamine dyes were measured relative to quinine sulfate dihydrate (QSH) in 1.0 N H 2S04. The values obtained were rhodamine 6G (0.95), B (0.65), 3B (0.45), 19 (0.95), 101 (0.96), 110 (0.92), 123 (0.90) at 25.0 C. Effects of temperature on the quantum yields of rhodamine B and QSH show a large temperature coefficient for rhodamine B and a significant one for QSH. Dye concentration was found to be critical in reporting observed fluorescence wavelength maxima. 1. Introduction Fluorescence quantum yields of several rhodamine dyes (i.e., B, 101, 6G) have recently been remeasured [1 3].For some of these dyes there has been some question of the value of the quantum yield. For example, it has been noted that the value for rhodamine B ranges from 0.69 to 0.97 [2a] in ethanol. Two values have been reported for rhodamine 6G, 0.94 [1] and 0.88 [3] in ethanol. At this laboratory we have at present a continuing need for reliable quantum yield determinations on new dyes; e.g., energy transfer agents and laser dyes. Against this background we have measured the fluorescence quantum yields of rhodamine B, 3B, 6G, 19, 101, 110, 123 using quinine sulfate dihydrate (QSH) in 1.0 N H2S04 as the primary standard. We have also employed rhodamine 6G as a second standard in order to be able to detect any change that might occur in the standard solutions with time. Our results are summarized in table 1. We have also investigated the effects of temperature and concentration. There are some differences in the literature between the reported fluorescence wavelength maxima and some of our observed values. We ascribe these differences to the various concentrations used in making these fluorescence measurements. Our feeling is that the reported maxima should be those observed for essentially infinite dilution /82/ /$ North-Holland

2 456 R. F. Kubin, A. N. Fletcher / Fluorescence quantum yields of some rhodamine dyes Table I Quantum yields at 25 C Compound (mol/l) 7) A 5 (nm) A5 (nm) peak ~ Remarks c(xl0 QSH Standard Rhodamine 6G Rhodamine B Basic EtOH Rhodamine 3B Rhodamine Basic EtOH Rhodamine Basic EtOl-1 Rhodamine Basic EtOH Rhodamine Experimental All rhodamine dyes were obtained from either Eastman Kodak or Exciton and were used as received. The QSH was recrystallized from water and dried as described earlier [4]. The QSH was dissolved in 1.0 N H 2S04 because that was the solvent used by Melhuish [5] when he determined the accepted value of for the quantum yield of QSH. The absorbances of all the dye solutions were determined on a Cary 14 recording spectrophotometer operated at a lowered dynode voltage setting. This procedure was used to widen the slits and hence the bandpass of the Cary in order to approach the 4 nm bandpass used on the Perkin Elmer MPF-44B Fluorescence Spectrophotometer. This was the narrowest bandpass used for a good signal to noise ratio. The MPF-44B is used in conjunction with a Perkin Elmer DCSU-1 computer for making corrected spectra. The excitation system (xenon lamp, optics, and grating) is calibrated by using an optically dense solution of rhodamine B in ethylene glycol for the wavelength region 200 to 600 nm. A triangular cuvette is used so that the fluorescence of the rhodamine B is centered in the detector field of view. All the exciting light is assumed to be reemitted with the same efficiency independent of wavelength. The instrument is in essence a quantum counter. The detector itself is calibrated using a secondary standard tungsten lamp. The computer has stored in a read only mode the spectral data for the tungsten lamp and is supposed to cover the nm range. However, we have found the data beyond approximately 700 nm unreliable. There is a blank in the memory from 724 to 736 nm and the output becomes unusually noisy and goes off scale beyond 800 nm. The manufacturer is presently working on resolving these problems. The detector response can also be calibrated using the internal xenon lamp. The latter procedure has a long wavelength limit of 600 nm. The quantum yield of

3 R. F Kubin, A.N. Fletcher / Fluorescence quantum yields ofsome rhodamine dyes 457 rhodamine 110 was measured relative to QSH using the two different calibrations of the detector and the results differed by about 8%, the upper error limit of the results reported here. The summary given in table 1 is for the tungsten lamp calibration. The instrument has a very good 4-cell turret head constant temperature cuvette holder as an accessory. This accessory is indispensable for making reasonably quick quantum yield measurements. Cuvettes in which the fluorescence area of QSH has been measured under identical conditions are used. The cuvettes are then used to hold QSH, dye solution, and pure solvents. It is necessary each time quantum yields are determined to keep the QSH standard in the instrument so that frequent checks can be made to compensate for small amounts of dynode voltage drift that occur. If this voltage changes by ±4V in 650, the observed change in the output signal is approximately 3 to 4%. Using fresh standard solutions, the quantum yield of rhodamine 6G was measured as 0.95 assuming a value of 0.55 for QSH. These same solutions, using fresh samples, were then used as comparison standards to determine unknown quantum yields. In this procedure the quantum yield of each standard was determined in terms of the other and provided a check on the integrity of the standard solutions. For QSH the range of values of 25.0 Cwas 0.53 to 0.56 assuming a value of 0.95 for rhodamine 6G. For rhodamine 6G the range of values of 25.0 Cwas 0.92 to 0.96 assuming a value of 0.55 for QSH. Thus, the reproducibility of this procedure was about ±5%. Repeat determinations of dye quantum yields using new solutions have a reproducibility of ±8%. Within a given set, the quantum yields of various dyes calculated using the two standards were observed to vary from agreement to ±5% difference. We have reported the average of these determinations. This difference, at least for the few dyes thus far determined, does not seem to depend on the excitation wavelength range or on the standard used. For instance, for rhodamine B the value of the quantum yield obtained using QSH or rhodamine 6G was the same. In the case of rhodamine 3B, the quantum yield value based on rhodamine 6G was about 4% lower than the value obtained using QSH as the reference standard. All dye solution dilutions were made by weight. The pure and basic ethanol solvents were stored under argon in inert atmosphere vessels. Withdrawal of the solvents was assumed to air saturate them. Absorption spectra taken over a period of time showed that dyes in pure ethanol were stable for at least 2 months. Dye solutions in basic ethanol had noticeably increased absorption in the UV region (220 to 260 nm) after as short a time as 2 days. A major fraction of this increase is due to the ethanol alone because once the basic ethanol is air saturated the increase in absorption becomes noticeable. When using the square cuvettes, dye concentrations were held to a value yielding an absorbance of ~ in a 1 cm path. At these absorbances self-absorption due to overlap of the principal (S 1 ~ S0) excitation band and

4 458 R. F. Kuhin, A.N. Fletcher / Fluorescence quantum yields of some rhodamine dyes the emission band is greatly minimized. This is very important for small Stokes shift dyes such as the rhodamines. Because the exciting light is attentuated by only about 3%, the assumptions of constant light flux in the detector solid angle and isotropic emission are valid. In the MPF-44B, the fluorescence is viewed at 90 to the incident excitation light. Solution concentrations are the molarities corrected to 25 C. The absorbance criterion above applied to the largest absorption band, generally the S 1 ~ S0 band for the dyes studied here. Because of the problems encountered in basic ethanol, a standard procedure was used to calculate absorbance of other than the principal band. The absorbance of the principal band which is stable for long periods even in basic ethanol was measured using 10 cm cells on the Cary. An excitation spectrum of the dye at the concentration used for the quantum yield determination was then used to obtain peak height ratios for the much smaller peaks at other wavelengths. These ratios were then used to calculate the absorbances at these other wavelengths from the one measured value. It is recognized that this procedure in fact assumes that the quantum yield is constant as a function of wavelength. For the temperature studies a Neslab RTE 4 circulating bath was used. This bath has the capability of holding to better than ±0.05 C.Initially the thermal cell holder was probed with a copper constantan thermocouple as were the individual cuvettes. Temperatures throughout the block were constant to better than 0.1 C,the reading limit of our thermocouple readout, a Doric DS-lOO-T3. In the range 0 to 60 Cthermal gradients at equilibrium were small between cell block and sample. At 60 C the gradient was approximately 4 C. In making the measurements, equilibrium was defined as 5 mm with no temperature change. Temperatures recorded were those measured in the dye solutions. The effects of concentration were studied using triangular cuvettes. These cuvettes insure that the exciting beam strikes the center of the detector solid angle. The cuvette is so oriented that the fluorescence is filtered by the solution. Fluorescence wavelengths were read directly from the MPF-44B corrected dye spectrum. The excitation wavelengths used were the observed maxima read from the Cary 14. Under the run conditions used, using a steel rule, the maximum reading error in these wavelengths was 4.7 A. Comparison of absorption spectra and excitation spectra taken on the Cary 14 and the MPF-44B, respectively, showed a precision of ±1 nm for reading the MPF-44B.x y recorder. 3. Results and discussion Table 1 gives the quantum yields found for the standard rhodamine dyes. The data are in good agreement with the previous recent literature values and

5 I R.F Kubin, AN. Fletcher / Fluorescence quantum yields ofsome rhodamine dyes 459 thus add substantiation to these values. For rhodamine 6G, the value of 0.95 agrees with Drexhage [6] (0.95) and Butenin et al. [1] (0.94 ±0.01) and is the recommended value over the value of 0.88 [3]. However, the precision of quantum yield values of ±5to 10% does not make the lower value that far out of line. The quantum yield determined for rhodamine B is in excellent agreement with the results of Schwerzel and K.losterman [2a]. From their graph we read a value of 0.64 at 25 C.The quantum yields for rhodamine 101 and 3B again agree with Drexhage s results [6]. Only in the case of rhodamine 110 is there a large difference. Our results indicate a higher quantum yield of 0.92 ±0.03 over the value of 0.85 [6]. The values differ by the maximum error observed in the reproducibility of our results. These results, using in essence two comparison standards, demonstrate that there is no difficulty in spectral range compatibility in using QSH in 1.0 N H 2S04 as a primary standard, a fear that has been expressed [2b]. Rhodamine 6G is an excellent candidate as a I O 0.60 o - 0..El... ~~-0-~~ ~~~~0~ RHODAMINE B - 0 QUININE SULFATE HYDRATE 020 I T ~ Fig. I. Temperature variation of the fluorescence quantum yield of quinine sulfate dihydrate and rhodamine B.

6 460 R. F. Kuhin, A. N. Fletcher / fluorescence quantum vield,~of arnie rhodamme ]ve.v 100 I I I I I I I I 90 - CONCENTRATION + - lx1o~m ~\t 1~1 I ~xio~ 80 - i~\~ \ \ sxio 4 - I j I 5X105-2X1 II! 1 I 70 / - II \ \ ~ 1.5X10 ii II 1 I 60 I 1 \ (~) I~II I 2 I I w H. I 2 I _50 - i ~ - :1 I 40 I ~ I I I 0 ~. 30 fi I 20- / i 1/ I I I.4L- I WAVE LENGTH rn, Fig. 2. Concentration dependence of the fluorescence spectrum of rhodamine 6G. in a triangular cell except for 1.5 X l0 M solution which was in a square cell. Spectra taken primary standard. Its carboxyl group is esterified: thus the dye is not subject to the acid-base equilibria in polar solvents [6] that we also have observed with those dyes having a nonesterified carboxyl group. Schwerzel and Klosterman [2a] made a study of the temperature dependence of several rhodamine dyes. We were particularly interested tn the temperature dependence of rhodamine B and QSH. Fig. I shows the observed temperature dependence of these dyes. The results point up the necessity for good temperature control in order to make precise measurements. Rhodamine B has a strong temperature dependence. The results are in essential agreement with those of Schwerzel and Klosterman. The temperature dependence of QSH, while less marked, is still significant. Over the 50 range of the

7 R. F Kubin, A. N. Fletcher / Fluorescence quantum yields of some rhodamine dyes 461 measurements the slope of the line is /deg. Thus at 25 Cthe quantum yield changes by approximately 0.37% per degree. This result is somewhat higher than the range given by Melhuish [5]. It is also noted that the limited data suggest that the least squares line drawn for QSH should be a curve. The final point is that the quantum yield should be determined on as dilute a solution as possible. This consideration is especially important for small Stokes shift dyes such as the rhodamines. Absorption of fluorescence can become a serious problem. This is shown dramatically in fig. 2 for rhodamine 6G. As the concentration is increased there is an apparent shift in the peak fluorescence of almost 30 nm over the concentration range 1.5 x l0~ to I x i0 3 M. Lowering the concentration below about 1 X 106 M does not further affect the apparent peak fluorescence of the rhodamines studied. The values for the maximum peak fluorescence given in table 1 are somewhat shorter than reported elsewhere in the literature [1,3,7] and are for concentrations in the l0~ M range as shown. Differences in quantum yields reported in the literature can be ascribed not only to temperature but to concentration errors as well. Unless measurements of relative quantum yields at higher concentrations; i.e., where absorbances greatly exceed , are made under the conditions of matched absorbance of the test material and the standard at the exciting wavelengths, the quantum yield values will not be valid. When large amounts of the excitation light are absorbed, fluorescence within the cuvette will not be uniform. The emission will be high at the entrance of the excitation light into the cuvette and decrease with distance from the entrance face. In addition, if the Stokes shift is not large, reemission errors will occur. For small Stokes shift dyes, if the quantum yield is low, absorption of fluorescence would further lower the measured value. However, as is the case with the rhodamine dyes, if the quantum yield is high, then anomalously large values can be obtained due to reemission of light within the detector field of view from volumes not directly illuminated by the exciting light beam. the quantum yields in this work were calculated using the formula given by Demas and Crosby [8] including the index of refraction term and using the fraction of light absorbed rather than the absorbance. References [II A.V. Butemn, B. Ya. Kogan and NV. Gundobin, Opt. Spectrosc. (USSR), 47 (1979) a] RE. Schwerzel and N.E. Klosterman. NatI. Bur. Standards, SP 526, (Private communication. Complete data to be published.) [2bl T. Korstens and K. Kobs. J. Phys. Chem. 84 (1980) (3] J. Olmsted III, J. Phys. Chem. 83 (1979) [4] AN. Fletcher, Photochem. Photobiol. 9 (1969) 439. [5] W.H. Melhuish. J. Phys. Chem. 65 (1961) 229.

8 462 R. F. Kuh,n. A. N. Fletcher / Fluorescence quantum yields ofsome rhodamine dye.s [6] K.H. Drexhage. Structure and Properties of Laser Dyes, in Dye Lasers. F.P. Schafer. ed., 2nd edition (Springer, 1977). [7a] Laser Dyes. Exciton Chemical Company, Inc.. Dayton. OH. [7b3 Kodak Laser Products. Pub. No. JJ-169. Eastman Kodak Company, Rochester. NY. [8] J.N. Demas and GA. Crosby. J. Phys. Chem. 75 (1971) 991.