J . Phys. Chem. 1986, 90, 2661-2665 In the bulk gas, the ~k(0)value monotonically changes from about 200 to 100 ns with increasing excitation energy (Table I). These values agree fairly well with the fluorescence lifetimes obtained in the supersonic jet.11,12 Accordingly, the slow fluorescence in the bulk gas obviously corresponds to the fluorescence which shows a single exponential decay in the jet. In the bulk gas also, the fast component is very difficult to detect in the decay curve for 338.7-nm excitation (see Figure 6a). However, the molecular level structure even at a very low vibrational level of SI cannot be identified as the small molecule limit. As already mentioned, the minute analysis of the experimental results on the quantum yield and decay of the fluorescence at various pressures in the bulk gas indicates that the fluorescence characteristics of acetaldehyde vapor for region-I excitation belong to the intermediate case. The electronic relaxation in S1 of acetaldehyde vapor after region-I excitation is thus characterized by reversible intersystem crossing to TI. The SI TI intersystem crossing yield in lower vibrational levels of SIacetaldehyde was reported by Gandini and
-
2661
Hackett5 to be unity at moderately high pressures. Therefore, the lifetime of the fast fluorescence can reasonably be related to the SI T, intersystem crossing rate. The possibility, suggested by Noble and that isolated acetaldehyde molecules excited to lower vibrational levels in Sl undergo internal conversion to So may be ruled out. Noble and Lee"J2 suggested further that the fast fluorescence, observed in a bulk gas by Atkinson et a1.: is to be emitted from levels with considerably high rotational quantum numbers. As mentioned previously, however, both the quantum yield and the decay of fluorescence seem to be almost independent of the rotational level excited. It is, therefore, very improbable that the fast component which can be observed in a bulk gas corresponds to fluorescence from high rotational levels.
-
Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research (59430003) from the Ministry of Education, Science, and Culture. Registry No. CH3CH0, 75-07-0.
Photophysical Properties and Laser Performance of Photostable UV Laser Dyes. 1. Substituted p-Quaterphenyls Monika Rinke, Hans Giisten,* and Hans J. Ache Kernforschungszentrum Karlsruhe, Institut fur Radiochemie, D- 7500 Karlsruhe, Federal Republic of Germany (Received: November 19, 1985)
The photophysical properties such as singlet absorption, triplet absorption, and fluorescence spectra, the fluorescence quantum yield and fluorescence decay time as well as the laser performance data such as the tuning range, the conversion efficiency, and the photochemical stability of nine methyl- or methoxy-substituted p-quaterphenyls have been measured in ethanol and dioxane at room temperature. The substituted p-quaterphenyls exhibit laser dye emission in the 345-393-nm range with conversion efficiencies up to 15%. Sterical hindrance through methyl substitution in p-quaterphenyl causes a blue shift of the absorption spectrum and reduces the conversion efficiency. Methoxy-substituted p-quaterphenyls display broader tuning ranges but substantially lower photochemical stability than methyl-substituted p-quaterphenyls. The photochemical stability of the best laser dyes investigated is better by a factor of up to 100 than that of a commercial UV laser dye. The photochemical degradation of the new UV laser dyes under XeCl excimer laser pumping is much slower in dioxane than in ethanol.
Introduction Since the first reports on laser action of fluorescent organic molecules in solution by Schafer et ale1and Sorokin and Lankard2 in 1966, a sizeable number of organic laser dyes have become commercially available. Due to their wavelength tunability, wide spectral coverage, and simplicity, organic laser dyes used in optical systems, called dye lasers, become increasingly important in analytical chemistry, spectroscopy, and various other fields of applications. Organic dyes that show laser action were previously selected by trial and error. In the course of the past decade it has been realized that a good laser dye molecule should comprise the following photophysical and chemical properties: high fluorescence quantum yield, short fluorescence decay time, broad spectral region of fluorescence, little overlap of the fluorescence and triplet absorption spectral regions, large Stokes' shift, high molar absorption coefficient at the wavelength of the pump laser, M. photochemical stability, solubility better than Since by far the majority of chemical compounds are colorless, their electronic excitation requires laser light in the UV. Thus, for various applications of laser light in the UV there is an interest to search for better UV laser dyes than currently available. However, in the near UV spectral range under 400 nm the demand ~~
on the photochemical stability is high because the energy of the pumping laser light is on the order of the binding energy of covalent carbon-carbon and carbon-hetero atom bonds of the laser dye. Thus, the photochemical stability of the laser dye is one of the most important parameters. The most efficient source of coherent radiation in the UV range for dye laser pumping is the excimer laser operating on XeCl (308 nm) or KrF (248 nm) emission. In view of the photochemical stability of the laser dye it is preferable to use the longest wavelength for dye laser pumping. This requires a proper matching of the wavelength of the single absorption maximum of the laser dye with the emission line of the pump laser. Among the most efficient and photostable laser dyes in the near-UV range are p-terpheny13 and p-q~aterphenyl.~ In this and the following paper we will report on the laser performance of new tunable, photostable UV laser dyes of the class of substituted p-quaterphenyls which have been preselected on account of their photophysical properties using the criteria listed above. Experimental Section Instrumentation and Techniques. ( a ) Photophysical Parameters. All ultraviolet absorption spectra were recorded on a Cary Model 15 spectrometer. The absolute fluorescence spectra, the
~
(1) F. P. Schafer, W. Schmidt, and J. Volze, Appl. Phys. Left. 9, 306 ( 1 966). (2) P. P. Sorokin and J. R. Lankard, IBM J . Res. Deu. 10, 162 (1966).
0022-3654/86/2090-266 1$01.50/0
(3) H. Bucher and W. Chow, Appl. Phys. 13, 267 (1977). (4) P. Cassard, P. B. Corkum and A. J. Alcock, Appl. Phys. 25, 17 (1981).
0 1986 American Chemical Society
2662 The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
Rinke et al.
TABLE I: Photophysical Data on Absorption and Fluorescence of Substituted p-Quaterphenyls in Ethanol and Dioxane at Room Temperature e,
compd no./ solventa
Xamam
L.mo1-I.cm-l
nm
Xamax
X30S
nm
298 301 297 304 278 301 303 309 277 266 269 302
47 300 46 570 47 300 44 630 35 600 36 500 35 200 29 100 35 700 40 680 41 190 28 600
41 500 43 840 41 100 43 700 7 900 33 200 33 000 29 100 8 600 1450 2 770 26 300
367 372 367 374 355 370 374 373 354 353 357 378
fluorescence quantum yield, and the fluorescence decay time were measured with a recording spectrofluorometer and a fluorometer relying on the pulse-sampling technique. The spectral sensitivity of the spectrofluorometer was determined in the spectral range of 275-800 nm with a mercury arc UV standard and a quartziodine lamp of known intensity distribution combined with a white reflectance standard of Teflon powder5which replaced the sample. Details about the instrumentation built at this laboratory and about the measurement technique have been published previ~usly.~,’ In order to determine the fluorescence quantum yield according to the method of Parker and Rees,* quinine bisulfate in 0.1 N HISO, was used as a reference standard. A fluorescence quantum yield of 0.55 at room temperature was a s ~ u m e d . ~Refractive indices for dioxane and ethanol at room temperature were taken into account for calculating the fluorescence quantum yields. The concentrations of the dye solutions and of the fluorescence standard were adjusted so that their absorbances were equal at the exciting wavelength. To avoid errors due to reabsorption, low concenmol dm-3 were used for the determination of trations of the fluorescence quantum yield. Considering the reproducibility and the various sources of error, the values of the fluorescence quantum yields given below are deemed to be accurate to within at least 5%. M The fluorescence decay times were determined at concentration by impulse fluorimetry using the sampling technique. The decay curves showed a single-exponential shape and have been evaluated with a computer program using the method of successive approach. The triplet-triplet absorption spectra were investigated at room temperature with a home-built conventional flash photolysis spectrometer.6 The concentrations of the degassed solutions were about M. Degassing was performed by five freeze-evacuTorr. ation-thaw cycles to a pressure of less than ( b ) Laser Performance Data. All parameters for evaluating the laser dye properties such as tuning range, conversion efficiency, and photochemical stability have been measured with an excimer laser (EMG 102, Lambda Physik, Gottingen) operating on the laser emission of XeCl (308 nm) in combination with a dye laser (Fl2000, Lambda Physik, Gottingen). 40% of the available pump energy of the eximer laser, separated via a beam splitter, was used for transverse pumping of the dye laser oscillator which is a Hansch-type resonator. An optical grating was used as the wavelength selective element operating in the sixth or seventh order. The narrow-band output of the dye laser was measured with a pyroelectric detector (ED 100, Gen-Tec Inc.). For the determination of the tuning range and the energy conversion efficiency of the laser dyes tested, the pump laser was operated
-
(5) V. R. Weidner and J. J. Hsia, J . Opt. SOC.A m . 71, 856 (1981). (6) H. BIume and H. Giisten, in Ultraoiolette Strahlen, J. Kiefer, Ed., W. de Gruyter Verlag, New York, 1977, Chapter 6. (7) S. Schoof, H. Giisten, and C . von Sonntag, Ber. Bunsenges. Phys. Chem. 82, 1068 (1978). (8) C. A. Parker and W. T. Rees, Analyst (London) 85, 587 (1960). (9) W. H. Melhuish, J . Phys. Chem. 65, 229 (1961).
bandwidth,
Ti3
nm
Qr
ns
342-395 348-399 343-396 350-400 332-382 344-400 349-402 348-401 332-380 331-382 334-386 355-412
0.77
0.74
0.80
0.74
0.71 0.76
0.75 0.83
0.68 0.71 0.76
1 .oo 0.78 0.78
0.69
1.oo
at a repetition of 1 H z with a pulse energy of 20 mJ. The concentration of the laser dyes was adjusted to 99.9% absorption of the pump energy at the wavelength of 308 nm. A sufficient number of pulses were sampled to reduce the influence of the fluctuation of the pump energy. The energy of the pump laser was measured with a second pyroelectric detector (ED 200, Gen-Tec Inc.) in combination with an oscilloscope ( R M 547, Tektronix). Due to fluctuations of the pump laser energy the energy of the pump and the dye laser can be measured with an accuracy of &5%. Thus, this accuracy is believed to be valid also for the value of the energy conversion efficiency, which is derived from the slope of the plot pumping energy vs. dye laser energy. This so-called slope efficiencyL0is then converted to a quantum efficiency. The photochemical stability of a laser dye was determined as the half-life energy which is the amount of the total absorbed pump energy until the dye laser energy has dropped to 50% of its initial value. With the known concentration of the laser dye and the energy of the photons of the excimer laser used, the half-life energy can be expressed as the number of photons absorbed per laser dye molecule when the output energy of the dye laser has dropped to half of its initial energy output.” For the photodegradation experiments the dye flow cell with the 120-mL reservoir in the dye laser was replaced by a quartz cuvette (2 X 1 cm) of 8 mL volume. The solution was stirred with a magnetic stirrer. The repetition rate of the excimer laser was generally 10 Hz with a pulse energy of 20 mJ per pulse which corresponds to an energy density of 0.4 J/cm2 within the area of the laser beam. The concentration of the laser dyes was such that in all cases at least 99% absorption of the pump laser light was achieved. From the reproducibility of the laser dye degradation experiments we deduce an error of f 2 0 % for the half-life energy values. Substances. The following substituted p-quaterphenyls have been investigated: 3,3”’-dimethyl-p-quaterphenyl(l),3,3”‘-ditert-butyl-p-quaterphenyl (2),3,3”’-dimethoxy-p-quaterphenyl (3), 2,2”’-dimethyl-p-quaterphenyl (4), 2,2”’-dimethoxy-pquaterphenyl (5), 2’,3’’-dimethoxy-p-quaterphenyl (6), 3,2’,3’’,3’’’-tetramethyLp-quaterphenyl (7), 3,3’,2’’,3’”-tetramethyl-p-quaterphenyl (8), and 3,3’,2’”3’’’-tetramethoxy-pquaterphenyl(9). All compounds have been synthesized by Wirth et a1.I2 Results ( a ) Photophysical Properties. The photophysical data of the substituted p-quaterphenyls, such as the maximum of the electronic absorption Xam,, the molar decadic absorption coefficient t at the wavelength of maximum absorption as well as at the wavelength of the XeCl pump laser at 308 nm, the maximum of the fluorescence spectrum Xem,,, the full width at half-maximum of ( I O ) G. Marowsky, Opt. Acta 23, 855 (1976). (11) V. S. Antonov and K. L. Hohla, Appl. Phys. B 830, 109 (1983). (12) H. 0. Wirth, K. H. Gonner, R. Stuck, and W. Kern, Makromol. Chem. 63. 30 (1963)
The Journal of Physical Chemistry, Vol, 90, No. 12, 1986 2663
Properties of Photostable UV Laser Dyes
TABLE II: Laser Performance of Substituted p-Quaterpbenyls under XeCl Excimer Excitation in Ethanol and Dioxane at Room Temperature compd no./ xcnMx, tuning range, cc, 7, Ell29 lifetime T, solvent” nm nm cm2 % 104 JIL photons/molecule 1ID 372 365-379 2.49 15.0 327 17800 2.75 14.1 11.4 560 363-376 368 2)E 373 355 362 373 374 354
31D
4 1 ~ 4 1 ~ 5 1 ~ 6/E 71E
7/D 357 81D 381 9 1 ~ ‘E = ethanol, D = dioxane. Wavelenath / nm 350
250
I
L
,
10000
2.08
11.5 6.2
359-384 359-383 345-360
2.16 1.76 1.99
11.3 9.5 5.9
347-366 370-393
1.99 1.75
4.5 5.2
100
300
’
366-380 347-364
150
500
fl
/ \
,
,
36000
,
, x ,
,
32WO ZllOOO Waenumbar Icm-’
,
A \ ,
11NO
I
10.8 41.9 532 3.5 1.6 17.1 78.6 346 1.5
566 403 7780 136 76 183 1610 1260 49
WAVELENGTHhm
550 11.0
400
350
1
20000
Figure 1. Singlet absorption,fluorescence, and triplet absorption spectra of 2,2”’-dimethyl-p-quaterphenyl(4) in ethanol.
the fluorescence band, the fluorescence quantum yield Qa and the fluorescence decay time Tf have been summarized in Table
I. The relative positions of the singlet absorption, fluorescence emission, and triplet absorption spectra of the substituted pquaterphenyls are illustrated by the example of 2,2”’-dimethylpquaterphenyl(4) in ethanol (see Figure 1). With the exception of p-quaterphenyls 6 and 9 all substituted p-quaterphenyls show the same broad absorption bands as 2,2‘”-dimethyl-pquaterphenyl (4) in Figure 1. While the absorption maxima of the substituted p-quaterphenyls are red-shifted by 2-3 nm when going from ethanol to dioxane, the fluorescence maxima are red-shifted by 4-5 nm. Within the experimental error the molar decadic absorption coefficients are the same in ethanol and dioxane. With the exception of 1,2, and 3, all fluorescence bands in Table I are structureless as shown in Figure 1. The fluorescence quantum yields of the substituted p-quaterphenyls in Table I range from 0.68 to 0.80; the fluorescence decay times vary between 0.74 and 1.O ns. The fluorescence quantum yields and fluorescence decay times of 1 and 8 in ethanol compare reasonably well with those which have been measured previously in cyclohexane by Berlman.13 The triplet-triplet absorption spectrum of 2,2”’-dimethyl-pquaterphenyl (4) (Figure 1) is very similar to that of 3,3”’-ditert-butyl-p-quaterphenyl (2) in ethanol with a A, of 518 nm and a bandwidth at half-maximum of 473-541 nm as well as similar to the T-T absorption spectrum of the parent compound p-q~aterpheny1.I~Thus, substituents which cause sterical hindrance obviously do not alter the energetic position of the T-T absorption spectrum in p-quaterphenyls. ( 6 ) Laser Performance. The data of the laser performance of the substituted p-quaterphenyls such as the maximum of the laser the tuning range as bandwidth at spectrum (tuning curve) A,, half-maximum, the cross section for stimulated laser dye emission uo the slope efficiency 7,the half-life energy value El/*, and the (13) I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, Academic Press, New York, 197 1. (14) T. G . Pavlopoulos and P. R. Hammond, J. Am. Chem. SOC.96,6568 (1974).
30000
28000
26000 WAVENUMBER/cm I
24000
Figure 2. Tuning ranges and slope efficiencies of substituted p-quaterphenyls in ethanol and dioxane.
mean lifetime T of the laser dyes are summarized in Table 11. The cross sections for stimulated laser dye emission a, were calculated according to eq 1 where X, = emission wavelength, n = refractive
A:E(A) ae=--
8ncon2
Qf Tf
(1)
index, co = velocity of light, and E(X) = the normalized fluorescence line-shape function.15 Since the data of Qf and T f are connected with the natural fluorescence lifetime T~ via T~ = rr/Qf and, in turn, T~ is connected with the molar decadic absorption coefficient the Qf and 71values in Table I measured in ethanol have been used to calculate the values for the cross section of stimulated emission ue in dioxane. The conversion efficiency is given here as a quantum efficiency derived from the slope of a plot of the pump energy vs. the dye laser output energy (slope efficiency).I0 In Figure 2 the tuning ranges of the substituted p-quaterphenyls in ethanol and dioxane are plotted, normalized to the slope efficiency measured at the maximum of the tuning range. While the slope efficiencies of 1 and 2 are about the same, methoxy substitution in 3 slightly lowers the energy output of the laser dye. Sterical hindrance of p-quaterphenyls, such as in 4,7,8, and 9, lowers the conversion efficiency. Parallel to the longer wavelength fluorescence of the methoxy-substituted pquaterphenyls in comparison to the methyl-substituted ones there is a bathochromic shift of the tuning range spectra of the methoxy-substituted p-quaterphenyls (compare 4 and 3 as well 8 and 9 in Table 11). The photochemical degradation of the substituted p-quaterphenyls in ethanol and dioxane under lasing conditions is shown in Figure 3. In general, with the increase in absorbed energy of the pump laser, an exponential decay of the dye laser energy (15) 0. G. Peterson, J. P. Webb, W. C. McColgin and J. H. Eberly, J. Appl. Phys. 42, 1917 (1971). (16) S.J. Strickler and R. A. Berg, J . Chem. Phys. 37, 814 (1962).
The Journal of Physical Chemistry, Vol. 90, No. 12, I986
2664
1.o 0.9
Rinke et al. Wavelength I nm
250
300
350
100
150
0.6 0.7 ~
0.6
F
E
0.5
I-
1u
13 0.1
d
6.9 (tl
0
2000
1000 6000 pump laser energy / J
BO00
I
Figure 3. Photochemical degradation of substituted p-quaterphenyls in ethanol and dioxane as a function of the total absorbed pump laser energy (XeCI excimer laser).
is observed. The laser dye stability varies over a wide range from 49 to 1.78 X lo4 photon absorption processes per molecule. Comparing the photochemical stability of the different substituted p-quaterphenyls in Table I1 we have to bear in mind that the photochemical stability is generally much higher in dioxane than in ethanol (see Figure 3, compare 4 and 7 in both solvents). Alkyl-substituted p-quaterphenyls such as 1,4, and 8, are more photostable than the corresponding methoxy-substituted p quaterphenyls 3, 5, and 9.
Discussion (a) Photophysical Properties. Excimer lasers currently provide the most efficient source of coherent radiation in the UV; they permit efficient and convenient pumping of short wavelength dye lasers with high pulse energy and high repetition rates. Furthermore, excimer lasers operating on XeCl(308 nm) and K r F (248 nm) offer the shortest pump wavelength and can be used to pump laser dyes over the entire visible range to the near I R spectral range. Although the efficiency of the discharge-pumped KrF is higher than that of XeCl, in general, when the absorption cross section of the laser dye allows pumping with both emission wavelengths, 308 nm is the wavelength of choice. The following factors are in support of the XeCl excimer laser as pump laser: Some of the organic solvents considerably absorb the pump light of the KrF emission. Photochemical degradation of the laser dye is more pronounced a t 248 nm.” Most laser dyes in the near-UV give a better match of their absorption maximum with the 308-nm emission than with 248-nm excimer emission. Since laser dyes are generally large organic molecules with an extended *-electron system, it is difficult to achieve the necessary to 5 X 10-3 M for use in a dye concentration range of 5 X laser. Thus, in order to minimize solubility problems, the proper matching of the wavelength of the absorption maximum of the laser dye with the emission of the pump laser is of importance. In the series of the p-oligophenylenes from p-terphenyl to p sexiphenyl, the pquaterphenyls offer the best compromise between photophysical and other physical or chemical properties. In accordance with the general properties of a good laser dye given in the Introduction, the following properties are in favor of p quaterphenyl: The absorption maximum in the series ofpterphenyl(278 nm), p-quaterphenyl (300 nm), p-quinquephenyl (3 10 nm) matches reasonably well with the 308-nm emission of the XeCl excimer laser. (17)
E.A. Stappaerts, Appl. Opt. 16, 3079 (1977).
kO000
32000 28000 Wnvenumber I c m - ’
36000
21000
Figure 4. Absorption and fluorescence spectra of 3,3”’-dimethyl-pquaterphenyl (1) and 2,2’”-dimethyl-p-quaterphenyl (4) in ethanol.
The fluorescence decay time decreases from p-terphenyl to higher p-oligophenylenes while the molar decadic absorption coefficient increases.13 In the sequence of the p-oligophenylenes, the triplet-triplet absorption spectra are shifted bathochromically more than the corresponding fluorescence spectra.14 Among the p-oligophenylenes, the fluorescence emission of p-quaterphenyl gives still rise to tuning ranges in the near-UV under 400 nm. The photochemical stability increases with the number of phenyl rings. The solubility decreases rapidly in the series of p-oligophenylenes. An even number of phenyl rings allows an easier synthetic access from subunits like biphenyls. Kern et a1.18 have shown that the solubility of p-quaterphenyl can be drastically increased when substituted by methyl groups. Substitution, however, can affect the photophysical properties. Introduction of a methyl group in the ortho position introduces a strong hypsochromic shift of the absorption spectrum due to sterical hindrance which is particularly evident in the p-quaterphenyls 1 and 4 (see Figure 4). Furthermore, sterical hindrance makes the fluorescence spectrum become less structured, as shown in Figure 4. A less structured fluorescence spectrum offers the possibility for the laser spectrum to display a broader tuning range. Thus, introduction of methyl groups in the ortho positions (2,2”’) of the terminal phenyl rings in p-quaterphenyl and in the meta or ortho position (2’,3’,2”,3”) of the inner phenylene rings disturbs the planarity of p-quaterphenyl. Since steric effects drive the long-wavelength absorption band toward shorter wavelengths increasing the Stokes’ shift (see Table I), proper substitution can be used to create a better match of the absorption maximum of a laser dye with the emission of the pump laser and can control the tuning range. A comparison of 4 and 5 as well as 8 and 9, however, shows clearly that introduction of a methoxy group does not give rise to this sterical hindrance. Nevertheless, methoxysubstituted p-quaterphenyls display broader tuning ranges than methyl-substituted p-quaterphenyls as is illustrated in Figure 5. ( b ) Laser Performance. Contrary to the photophysical properties in Table I which are material constants, the data for the laser performance of the substituted p-quaterphenyls in Table I1 are dependent on the pump laser/dye laser system used. The tuning range and the efficiency of a laser dye depend very much on the entire dye laser system as well as on the mode of dye laser operation. In this work we used narrow-band emission of the dye (18) W. Kern, M. Seibel, and (1959).
H.0.Wirth, Makromol. Chem. 29, 164
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2665
Properties of Photostable UV Laser Dyes Worelmgfh lnrn
W I * l l l “ ~ l hI n m
(50
150
150
(w
i
~
29000
21000
25000
WPvewmber l r m
23000
29000
21000
25000
21000
Wlllnmbei i ~ m
Figure 5. Fluorescence and laser emission spectra of 2,2”’-dimethyl-pquaterphenyl (4) and 2,2”’-dimethoxy-p-qlIaterphenyl(5) in ethanol.
laser light19*20 which gives rise to smaller tuning ranges and lower conversion efficiencies of the laser dye compared to a dye laser operating in the broad-band mde.3921922 Differences in conversion efficiencies between the two operation modes up to 100% have been o b s e r ~ e d . ~The ~ , ~conversion ~ efficiencies (slope efficiency) of the substituted p-quaterphenyls range between 4.5 and 15%. Substitution in meta positions of the terminal phenyl rings, as in 1,2, and 3, gives the highest energy conversion efficiency 7 (Table 11). Sterical hindrance through methyl groups, as in 4, 7,and 8, reduces 7 considerably. A comparison of 4 and 5 indicates that the methoxy group obviously does not cause sterical hindrance, because the latter molecule has a 7-value of 11.3% as compared to 6.2% for 4. As will be shown in the following paper, the conversion efficiency (slope efficiency) of a substituted p quaterphenyl can be estimated from its photophysical properties such as fluorescence quantum yield and fluorescence decay time.z5 The permanent bleaching of laser dyes and its effect on dye laser performance have been subjects of a number of investigations. Most investigations have been done on flash-lamp pumped laser dye d e t e r i o r a t i ~ n , ~some ~ , ~ ’ on excimer p ~ m p i n g . l~, IJ7 A comprehensive investigation of visible and UV laser dye stabilities under excimer laser pumping has recently been published by Antonov and Hohla.** It was pointed out earlierz9that there is no simple correlation between the bleaching of the laser dye and the decrease in dye laser energy. Again, the photochemical sta(19) L. D. Ziegler and B. S.Hudson, Opt. Commun. 32, 119 (1980). (20) D. M. Guthals and J. W. Nibler, Opt. Commun. 29, 322 (1979). (21) W. Zapka and U. Brackmann, Appl. Phys. 20,283 (1979). (22) 0.Uchino, T. Mizunami, M. Maeda, and Y . Miyazoe, Appl. Phys. 19,35 (1979). (23) H. Telle, W. Hiiffer, and D. Basting, Opt. Commun. 38, 402 (1981). (24) M. Rinke and H. Giisten, unpublished work. (25) M. Rinke, H. Giisten, and H. J. Ache, J . Phys. Chem., following article in this issue. (26) E. J. Schimitschek, J. A. Trias, P. R. Hammond, R. A. Henry, and R. L. Atkins, Opt. Commun. 16,313 (1976). (27) A. N . Fletcher, Appl. Phys. B B31, 19 (1983) and previous papers cited there. (28) V. S. Antonov and K. L. Hohla, Appl. Phys. B B32, 9 (1983). (29) A. N . Fletcher, Appl. Phys. 16,93 (1978).
bility depends on the pump and dye laser system and the mode of its operation. Generally, two values are given for the lifetime of a laser dye.28 For practical reasons in dye laser operation, the value is given as the total absorbed pump laser energy per liter of laser dye solution when the output energy of the dye laser has dropped to 50% of its initial value. A more physical interpretation of the photochemical stability of a laser dye is the notation of the number of pump photons absorbed by a laser dye molecule when the output energy of the dye laser has dropped to 50% of its initial value. The Ell: value given in watts per hour is often quoted by laser dye suppliers as a measure of the laser dye stability. The two data for the photochemical stability of the substituted p-quaterphenyls are given as Ellaand 7 in Table 11. The most remarkable difference in these values can be noticed when the photochemical stabilities of 3,3”’-dimethyl-p-quaterphenyl (1) and 3,3’,2”,3’”-tetramethyl-p-quaterphenyl (8) are compared. While the E I I Zvalues are approximately the same, the lifetime in absorbed photons per molecule differs by a factor of 14. Due to the enormous difference of their molar absorption coefficients at the pumping wavelength at 308 nm (see Table I), the concentration of 8 to achieve 99.9% light absorption of the 308-nm XeCl emission has to be raised by a factor of 16. Due to the four methyl groups this high solubility of 8 ( 5 X M) can be obtained in ethanol as well as in dioxane (Table I). In this respect, the absorption spectrum of 3,3”2’”3”’-tetramethyl-p-quaterphenyl (8) (see Table I) matches the KrF (248 nm) excimer emission or that of the fourth harmonic of an Nd:YAG laser (266 nm)18 better than the 308-nm emission. Thus, for practical applications the p-quaterphenyls 1 and 8 in dioxane (Table 11) are the most stable laser dyes. However, a comparison of bleaching of 4 and 7 in ethanol and dioxane (Figure 3) reveals that the substituted p-quaterphenyls are much more stable in dioxane than in ethanol. The photochemical stability of methyl-substituted p-quaterphenyls is substantially higher than for methoxy-substituted p-quaterphenyls (Figure 3 and Table 11). Since laser performance data depend on the pump laser/dye laser system it is difficult to compare our data for the photochemical degradation of laser dyes with published values. The photochemical stability of 2,2”’-dimethyl-p-quaterphenylin dioxane, pumped by an XeCl excimer laser and with a dye laser resembling ours, was given as 9400 absorbed photons per molecule.28 Within the experimental error for this type of experiments the agreement with our value of 7780 photons per molecule (see Table 11) is good. Compared to the well-known laser dye stilbene 3 for which we measured a photochemical stability value of 85 absorbed photons per molecule, the photochemical stability of some of the substituted p-quaterphenyls is better by a factor of up to 100 and more. 2,2’”-Dimethyl-p-quaterphenyl(4) and 3,3’,2”,3’’’-tetramethyl-p-quaterphenyl (8) are now commercially available UV laser dyes. The trademarks are BMQ and TMQ, r e s p e ~ t i v e l y . ~ ~ Acknowledgment. We thank Dr. H. 0. Wirth for samples of the substituted p-quaterphenyls. Registry No. 1, 21470-20-2; 2, 4499-68-7; 3, 95398-79-1; 4, 449985-8; 5, 95398-76-8; 6, 4499-74-5; 7, 90247-99-7; 8, 4575-13-7; 9, 4499-73-4. (30) Lambda Physik, Gottingen, Federal Republic of Germany.
