J . Phys. Chem. 1986,90, 2666-2669
2666
Photophyskal Properttes and Laser Performance of Photostable UV Laser Dyes. 2. Ring-Bridged p-Quaterphenyls Monika Rinke, Hans Gusten,’ and Hans J. Ache Kernforschungszentrum Karlsruhe, Institut fur Radiochemie, D - 7500 Karlsruhe, Federal Republic of Germany (Received: November 19, 1985)
The photophysical properties of nine ring-bridged p-quaterphenyls, such as the singlet and triplet absorption spectra, the fluorescence spectra, the fluorescence quantum yield, and fluorescence decay time, as well as the laser performance data, such as the tuning range, conversion efficiency, and photochemical stability, have been measured in ethanol and dioxane at room temperature. Compared to the methyl-substituted p-quaterphenyls, ring-bridging shifts the absorption maximum by 20-25 nm toward longer wavelengths which results in a better match of the absorption maximum with emission of the XeCl excimer laser. The ring-bridged p-quaterphenyls exhibit laser dye emission in the 363-385-nm range with conversion efficiencies (slope efficiency) between 3 and 17% and with low lasing thresholds. Ring-bridgingofpquaterphenyl with methylene, oxygen, and ethylene causes a considerable decrease of the conversion efficiency in the order given, while the photochemical stability in dioxane increases with the same order of ring-bridging. 2,7-Diphenyl-9,10-dihydrophenanthreneis one of the most stable UV laser dyes. Knowing the fluorescence quantum yield and decay time, one can predict the possibility of laser action and the conversion efficiency of a potential UV laser dye.
Introduction
Results and Discussion
In the preceding paper’ it was demonstrated that among the p-oligophenylenes, according to their photophysical and other chemical or physical properties, the pquaterphenyls offer the best compromise for photostable UV laser dyes. Among the important properties of a good laser dye are a broad tuning range, a high conversion efficiency, and a high photochemical stability. The low solubility of the large organic molecules often restricts their use as a laser dye. A proper match of the absorption maximum of the laser dye with the emission wavelength of the pumping laser is of importance for minimizing the necessary concentration. This paper describes a study on the photophysical properties and the characteristics of the laser performance of a number of ringbridged p-quaterphenyls. They represent a new class of photostable UV laser dyes which are particularly suitable for being pumped by the output of an XeCl excimer laser. We shall further demonstrate that knowledge of some photophysical properties allows one to predict the laser action and the conversion efficiency of the substituted and ring-bridged p-quaterphenyls.
(a) Photophysical Properties. The photophysical data of the ring-bridged p-quaterphenyls, such as the maximum of the the molar decadic absorption coefficient electronic absorption AB,, e at the maximum of absorption as well as at the wavelength of the XeCl excimer emission a t 308 nm, the maximum of the fluorescence spectrum Xc,, the bandwidth of the fluorescence spectrum at half-maximum (50%), the fluorescence quantum yield Qf,and the fluorescence decay time Tf have been summarized in Table I. To illustrate the relative position of the singlet absorption, fluorescence emission, and triplet absorption spectra of the ring-bridged p-quaterphenyls, these three spectra of 2,7-diphenyl-9,lO-dihydrophenanthrene(12)are given in Figure 1. The absorption and fluorescence maxima of the ring-bridged pquaterphenyls are blue-shifted by 2-4 nm when changing from dioxane to ethanol. With the exception of 18 and 19,the singlet absorption spectra of the ring-bridged p-quaterphenyls do not exhibit fine structures. Bridging of the phenyl rings gives rise to an increased rigidity of p-quaterphenyl. If methylene, oxygen, and ethylene are used as bridges, the resulting ring-bridged p-quaterphenyls are actually derivatives of fluorene, dibenzofuran, and 9,l O-dihydrophenanthrene. The bridging effect is reflected in the photophysical properties and has consequences on the use of these molecules as laser dyes. In general, the absorption and the fluorescence spectra are shifted toward longer wavelengths, the fluorescence spectra exhibit more fine structures, and the Stokes’ shift is decreased. The latter two effects will decrease the tuning range of a potential laser dye. Compared to the methyl-substituted p-quaterphenyls with no steric hindrance,’ the light absorption of the ring-bridged p-quaterphenyls is red-shifted by 20-25 nm when one or two bridges are introduced consecutively. This effect allows a better match of the absorption maximum with the emission of the XeCl excimer laser. The rigidity decreases in the order of methylene - oxygen ethylene. This is indicated by the decrease of the fluorescence quantum yield and the molar decadic absorption coefficient emax of 11, 10,and 13. For practical applications, the solubility increases in the same ~ r d e r . ~ - ~ Contrary to 10 and 11, the spacious ethylene bridge in 12 or 13 allows the two inner phenylene rings to twist to a certain extent. From X-ray data on 9,lO-dihydrophenanthreneit is estimated that this interplanar angle is about 20°.6
Experimental Section
(a) Instrumentation and Techniques. A detailed description of the measurement techniques of the photophysical properties, such as the fluorescence quantum yields, the fluorescence decay times, and the triplet absorption spectra, as well as of the laser performance data, such as the tuning range, the conversion efficiency, and the photochemical stability has been given in ref 1. (b) Substances. The following ring-bridged p-quaterphenyls have been investigated:2 3,7-diphenyldibenzofuran(lo), 2,7-dim-tolylfluorene (ll), 2,7-diphenyl-9,10-dihydrophenanthrene(12), 2,7-di-m-tolyl-9,10-dihydrophenanthrene (13),[3,3’] bidibenzofuranyl (14), 8,8’-diisobutoxy[3,3’]bidibenzofuranyl (15), [2,2’]bifluorenyl (16),9,10,9’, lO’-tetrahydro[2,2’] biphenanthryl (17),dibenzo[b,b’lfuro[2,3-f;5,4-flbisbenzofuran (18),and 2,7di-m-tolylphenanthrene (19). All compounds had been synthesized previously by Wirth et al.3-5 (1) M.Rinke, H.Gosten, and H. J. Ache, J . Phys. Chem., preceding article in this issue. (2) As we use the photophysical data of the nine substituted p-quaterphenyls from the preceding paper’ together with the ring-bridged p-quaterphenyls of this paper in one plot (see Figure 6). the latter have been numbered consecutively here. (3) H.0.Wirth, K. H. Gonner, R. Stuck, and W. Kern, Makromol. Chem. 63, 30 (1963). (4)H.0.Wirth, K. H. Gonner, and W. Kern, Makromol. Chem. 63, 53 (1963).
( 5 ) H. 0. Wirth, G. Waese, and W. Kern, Makromol. Chem. 86, 139 (1965).
0022-3654/86/2090-2666$01.50/00 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 12, 1986 2667
Properties of Photostable UV Laser Dyes
TABLE I: Photophysical Data on Absorption and Fluorescence of Ring-Bridged p-Quaterphenyls in Ethanol and Dioxane at Room Temperature e,
compd no./solvent' 10/E
lO/D 11/E 12/E 12/D
13/E 14/E 14/D
15/D 16/E 16/D 17/E 17/D 18/E 18/D 19/E O E
nm
AB,
321 323 323 318 321 318 319 323 336 320 323 322 324 354 362 279
L.mol-'.cm-'
vrns
Xamax
A308
XCmam nm
bandwidth, nm
Qr
48 100 51 400 49000 42500 42600 43 150 58400 55 350 52500
38000 37400 42600 41 000 39 300 40500 46700 40500 33 300
0.82
0.74 0.72
0.85 1 .oo
0.68 0.75
1 .oo 0.90
0.91 0.80
0.86
40000 39000 39 500
0.59
1 .oo
0.78
1.65
88000
12500 34000
343-388 348-394 344-393 350-401 355-407 351-403 347-395 353-398 365-414 348-396 355-401 352-402 357-407 354-383 359-394 369-419
0.73
57500 49 500 49 200
350 371 369 374 378 376 371 375 386 372 376 376 380 358 378 395
0.07
16.0
= ethanol, D = dioxane. Wnvelennlh I nm
Wavelength / n m
250
300
350
LOO
L50
I
Wovenumber / tm-1
Figure 1. Singlet absorption, fluorescence, tuning range, and triplet absorption spectra of 2,7-diphenyl-9,10-dihydrophenanthrene(12) in
ethanol. Introducing three oxygen bridges in p-quaterphenyl as in di-
benzo[b,blfuro[2,3-f;5,4-j'] bisbenzofuran (18) results in a completely planar molecule. Nevertheless, 18, in contrast to 2,7di-m-tolylphenanthrene (19), is not a polynuclear aromatic hydrocarbon. A comparison of the absorption and fluorescence spectra as well as of their photophysical properties (see Table I) reveals that 18 is still a ring-bridged p-quaterphenyl while 19 is a phenanthrene derivative with a low fluorescence quantum yield and a long fluorescence decay time typical of phenanthrene'^^ and other polynuclear aromatic hydrocarbon^.^ The absorption spectrum of 19 which is juxtaposed to the methylene-bridged p-quaterphenyl 11 in Figure 2, displays the typical features of a phenanthrene derivative.' The absorption maxima of the ringbridged p-quaterphenyls occur slightly at longer wavelengths than the 308-nm emission of the XeCl excimer laser. This gives a better match with this wavelength than for the para-substituted p quaterphenyls.' The molar decadic absorption coefficient of the ring-bridged p-quaterphenyls a t 308 nm is about 4 X lo3 L. mol-l-cm-'. As the light absorption of the ring-bridged p quaterphenyls is red-shifted by approximately 20-25 nm in comparison to the substituted p-quaterphenyls,' the former class of laser dyes can also be pumped with a nitrogen or XeF excimer laser as well as with the UV emission lines of high-power argon or krypton ion lasers (see Figure 2). Ring-bridging has little effect on the fluorescence quantum yield or the fluorescence decay time which is less than 1 ns. Only in the case of 18, where three oxygen atoms bridge the p-quaterphenyl, coplanarity prolongs the fluorescence decay time. The (6)G. H. Beavans, D. M. Hall, M. S.Leslie, and E. E. Turner, J . Chem. SOC.854,(1952). (7) I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, Academic Press, New York, 1971. (8)G. Heinrich and H. Giisten, Z . Phys. Chem. (Frankfurt am Main) 118, 31 (1979). (9) G. Heinrich and H. Giisten, in Polynuclear Aromatic Hydrocarbons: Chemistry and Biological Effects, A. Bjorseth and A. J. Dennis, Eds., Battelle Press, Columbus,OH, 1980,pp 983-1008.
10000
36000
28000
32000
21000
Wavenumber / cm-1
Figure 2. Absorption and fluorescence spectra of 2,7-di-m-tolylfluorene (11) and 2,7-di-m-tolylphenanthrene(19) in ethanol. WAVELENGTH/nm
I
c
I6
28ooo 2 m WAVENUMBER/cml Figure 3. Tuning range spectra of ring-bridged p-quaterphenyls in dioxane as a function of the slope efficiency (XeCI laser pumping). 3"
28ooo
fluorescence quantum yields and decay times of 11 and 13 compare reasonably well with those measured in cyclohexane by Berlman? Kawski et al.1° have recently reported the fluorescence decay time of 10 and 18 in different solvents. Within the limits of error of (10) A. Kawski and M. Alicka, Z . Naturforsch. 3% 520 (1983).
2668 The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
Rinke et al.
TABLE Ik Laser Performance of Ring-Bridged p-Quaterphenylsunder XeCl Excimer Excitation in Ethanol and Dioxane
1 O/D
nm 371 370
121D 13/E
compd no./solvent’
11/E lllD
A,,
tuning range, nm
be,
lo+
cm2
?,
365-377 363-377
2.54 2.69
11.9 12.7
317 376
371-384 370-382
1.77 1.81
4.3 3.1
374 386 376 380 378
369-380 380-391 370-382 376-385 375-381
2.41
11.2 11.3 16.6 3.1 3.6
180 4.0 4.5 >>40b 4.8 >>49b 78 5.3 6.6 104 20
131D 141D
15/D 16/D 17/D 18/D
2.72 1.77 1.69
lifetime T, photons/molecule
lo4 J/L
%
8770 210 237 >>1880b 224 >>2450b 4100 220 316 4609 912
‘E = ethanol, D = dioxane. b N o photodegradation up to the adsorbed energy given (see Figure 7). 2.0
Wavelength I n m
1
350
150
LOO
a b
o r--7--
T-
7
1.5
/ ./.
-
/
I
c , . , ,, , , , , ,
5.0
10.0 15.0 PUMP LASER E N E R G Y h J
,
20.0
Figure 5. Variation of the dye laser output energy as a function of the XeCl laser pumping energy for [2,2’]bifluorenyl (16), [3,3’] bidibenzofuranyl (14), and 9,10,9‘,10’-tetrahydro[2,2‘]biphenanthryl(17) in dioxane.
29000
21000 ?5000 Wavenumber /cm
23000
Figure 4. Fluorescence spectrum of [3,3’]bidibenzofuranyl(14) in dioxane at 1.2X M and 4.8 X lo4 M with the tuning range spectrum
measured at the latter concentration. the solvent dependency of 7ffor 10 and 18,our fluorescence decay times agree with those of Kawski et al.1° A comparison of the triplet-triplet absorption spectrum of 2,7-diphenyl-9,1O-dihydrophenanthrene (12),with the T-T absorption spectrum of p-quaterphenyl” and substituted p-quaterphenyls’ indicates that ring-bridging does not shift the energetic levels of the T-T absorption. ( b ) Laser Performance. The measured data of the laser performance of the ring-bridged p-quaterphenyls, such as the maxthe imum of the laser emission spectrum (tuning curve) A,, tuning range at half-maximum (50%), the cross section for stimulated laser dye emission ue, the quantum efficiency (slope efficiency) q, the half-life energy value Ell2, and the mean lifetime 7 in absorbed photons per molecule of the laser dyes, are summarized in Table 11. In Figure 3 the laser emission spectra of the ring-bridged p-quaterphenyls in ethanol and dioxane (Table 11) are normalized to their corresponding slope efficiencies measured at the maximum of the laser emission spectrum. Compared to the laser emission spectra of the substituted pquaterphenyls the tuning ranges are about the same, with the exception of the two- and threefold ring-bridged p-quaterphenyls 16* 173 and 18* Due to the fine structure Of the fluorescence spectra of the planar laser dyes 16, 17,and 18 and the reduced Stokes’ shift at the concentrations used for laser action ((1 1) T. G. Pavlopoulos and P. R. Hammond, J . Am. Chem. SOC.9 6 , 6 5 6 8 (1974).
M), the tuning range is curtailed, as shown in Figure 4 for 14. In Figure 4 the fluorescence spectra of 14 in dilute solution and at the concentration used for the lasing experiments are displayed together with the laser emission spectrum. The most planar molecule 18 displays a very small Stokes’s shift and a high molar absorption coefficient (see Table I). Unfortunately, the light absorption is shifted so far toward longer wavelengths that the match of the 308-nm emission of the XeCl excimer with the absorption maximum is poor and high concentrations are needed to obtain 99.9% light absorption of the pump laser light. The quantum efficiencies of the ring-bridged p-quaterphenyls vary from 3 to 17%. In the order of ring bridges with CH2 > 0 > CH2CH2groups there is a considerable decrease in the conversion efficiency (slope efficiency) from 16 via 14 to 17,as shown in Figure 5. The lasing thresholds for the ring-bridged pquaterphenyls are low (see Figure 5). The nonlinearity at pumping energies of over 10 mJ might be attributable to the absorption of pump radiation by excited singlet states of the laser dyes. Tomin et al.12 have discussed the pump intensity dependent energy conversion of p-terphenyl pumped with a KrF excimer laser as a result of SI-S, absorption processes. They observe a deviation from linearity between the dye laser output and the pump laser input energy at a pump energy of 0.3 J/cm2 which is in accordance with our value of about 0.25 J/cm2. As the T-T absorption spectrum of ring-bridged p-quaterphenyls does not interfere with the tuning range (see Figure l), the nonlinearity in Figure 5 is not caused by the triplet-state population. Table I1 shows that the slope efficiency q is proportional to the cross section of stimulated emission ue of the laser dye. As, in turn, u, is connected via eq 1 with the fluorescence quantum yield Qr and decay time X;E(A) Qr gc=--
8?rcon2 7f,13 the
(1)
7f
slope efficiency of a potential laser dye can be estimated
(12) V. 1. Tomin, A. J. Alcock, W. J . Sarjeant, and K. E. Leopold, Opt. Commun. 26, 396 (1978). (13) 0. G. Peterson, J. P. Webb, W. C. McColgin, and J. H. Eberly, J . Appl. Phys. 42, 1917 (1971).
The Journal of Physical Chemistry, Vol. 90, No. 12, I986 2669
Properties of Photostable UV Laser Dyes
16
1
d
’
I
8
1,0 1,2 1,4 1,6
1,8 2,O 2,2 2,4 2,6 2,8 3,O 3,2 CROSS SECTION FOR STIMULATED EMlSSION/lO-lscm2
Figure 6. Dependence of the slope efficiency 7 of substituted’ and ring-bridged p-quaterphenyls on their cross section for stimulated emission a,.
and T~ (eq 1) and, furthermore, the 11 value is prone to errors due to SI-S, absorption a t the wavelength of dye laser emission, the linearity in Figure 6 is satisfactory. It has been discussed previously that for high-power operation of dye lasers the photochemical stability of the laser dye is as important as the conversion efficiency and the tuning range.I4-l7 In particular for long-term irradiation experiments with high photon fluxes, the laser dye stability can be the determining factor. The photochemical degradation of the ring-bridged p-quaterphenyls under XeCl excimer laser irradiation is shown in Figure 7. The ethylene-bridged p-quaterphenyls such as 12 and 13 are unusually photostable laser dyes. Up to a total absorbed pump energy of 40 000 and 50 000 J/L, respectively, no photochemical degradation is noticeable (see Table 11). The photostability of the ring-bridged p-quaterphenyls decreases in the order CHICHz > 0 > CH2. With increasing number of ring bridges the photochemical stability decreases (see Figure 7 and Table 11). 2,7Diphenyl-9,lO-dihydrophenanthrene(12)and its methyl-substituted derivative 13 are the most stable UV laser dyes among those whose photochemical stability have ever been measured.” Unfortunately, the most photostable laser dyes such as 12,13,and 17 have poor conversion efficiencies of 3-4% only (see Table 11). The unusual photochemical stability of the ethylene-bridged pquaterphenyls is probably caused by the flexible ethylene group which gives rise to a rapid radiationless deactivation of the SIstate in competition with photodegradation and other deactivation processes of electronic excitation energy from the SIstate. 2,7Di-m-tolylfluorene (ll), [3,3’]bidibenzofuranyl (14), and [2,2’]bifluorenyl (16)are now commercially available UV laser dyes. The trademarks are RDC 370, R D C 374 and RDC 376, respectively.’* Acknowledgment. We thank Dr. H. 0. Wirth for samples of the ring-bridged p-quaterphenyls. Registry No. 10, 5834-24-2; 11, 31158-40-4; 12, 3419-47-4; 13, 31158-41-5; 14, 4499-66-5; 15, 101934-17-2; 16, 39168-58-6; 17, 95398-78-0; 18, 5834-19-5; 19, 95398-77-9.
pump Laser energy I J
Figure 7. Photochemical degradation of ring-bridged p-quaterphenyls in ethanol (E) and dioxane (D) as a function of the total absorbed pump laser energy (XeC1 excimer laser).
from its photophysical properties. In Figure 6 the slope efficiency of substituted’ and ring-bridged p-quaterphenyls is plotted vs. the cross section of stimulated emission. As a, is calculated by using the quotient of the two experimentally determined values of Qf
(14) (15) (16) there. (17) (18)
E. A. Stappaerts, Appl. Opt. 16, 3079 (1977). E. Sahar and D. Treves, Opr. Commun. 21,20 (1977). A. N. Fletcher, Appl. Phys. B31,19 (1983) and previous papers cited
V. S.Antonov and K. L. Hohla, Appl. Phys. B32, 9 (1983). Gallard-Schlesinger Chemical Corp., Carle Place, NY, and Radiant Dyes Chemie, P.O. Box 1462, 5632 Wermelskirchen, F.R. Germany.
