2578
J. Phys. Chem. 1983, 87, 2578-2581
membrane electron self-exchange (P + P+ reactions3’ between porphyrin macrocycles.
-
P+ + P)
Acknowledgment. The author thanks Drs. Norman Sutin and Bruce Brunschwig for the use of the MPF-4 fluorescence spectrophotometer and Dr. Jack Fajer for the gift of both the porphyrin and the chlorin. The author also (31)Ford, W.E.;Tollin, G.Photochem. Photobiol. 1982,35,809.
thanks Dr. John Connolly for measuring the fluorescence lifetime of the chlorin. Finally, the help and encouragement of Dr. Stephen Feldberg during the course of this work are greatly appreciated. This work was supported by the Division of Chemical Sciences, US. Department of Energy, under Contract No. DE-AC02-76CH00016. Registry No. Magnesium octaethylporphyrin, 20910-35-4; trans-etiochlorin I, 65564-77-4; methyl viologen, 1910-42-5; magnesium octaethylporphyrin ion( l+),34606-97-8.
Fluorescence Spectroscopy and Kinetics of Triphenodioxazines Klrk W. Butz, Brian L. Justus, and Gary W. Scott Department of Chemistry, University of California, Riverside, Riverside, California 9252 1 (Recelvd: November 17, 1982: I n Final Form: January 17, 1983)
The photophysical properties of several fluorescent triphenodioxazine dyes are reported. Room temperature absorption and corrected emission spectra are reported. Low-temperature fluorescencespectra are also reported and analyzed. Fluorescence quantum yields at room temperature are measured and found to be in the range low4to 0.6. Fluorescence lifetimes from -0.3 to 3 ns are observed in solution. Powdered samples of two of these dyes are found to be diamagnetic, contradicting theoretical predictions of a triplet ground state for these dyes.
Introduction The triphenodioxazines (see Figure 1) are a class of dyes which have been shown to be excellent weathering stabilizers for cellulose organic plastics’ and poly cY-olefins.2 Although the triphenodioxazines have been reported to be fluorescent? no systematic studies of their photophysical properties are available. Characterization of the properties of these highly photostable, fluorescent dyes is, in part, motivated by their potential application in the solar energy field, for example, in the construction of luminescent solar
concentrator^.^ Molecular orbital calculations of the a-electron system of triphenodioxane have been r e p ~ r t e d .These ~ ~ ~ calculations yield the unusual result that the first triplet state of this molecule is predicted to be at an energy 500-9600 cm-’ below the ground-state singlet due to a large exchange energy contribution to the triplet energy. The present paper presents a preliminary investigation of the photophysical properties of the triphenodioxazine (TPD) dyes shown in Figure 1. We report here the room temperature absorption and corrected emission spectra of these compounds. Also, fluorescence quantum yields and lifetimes of these dyes were determined. Values for the room temperature magnetic susceptibilities of two of these molecules are also reported. (1) R. C. Harris and G. C. Newland, U.S. Patent 3031 321. (2) R. C. Harris and G . C. Newland, U.S. Patent 3391 104. (3)J. H.Chandet, G. C. Newland, H. W. Patten, and J. W. Tamblyn, SPE Trans, 1, 26 (1961). (4)C. F. Rapp, N. L. Boling, I. M. Thomas, G. L. Opdycke, R. B. Fletcher, J. Chrysochoos, and P. S. Friedman, Sandia Laboratories Report, SAND 79-7005,1979. (5)I. Chen, M.Lardon, and L. Weinberner. J. Chem. Phys., 47. 2873 (1967). (6) D. H. Phillips and J. C. Schug, Int. J. Quantum Chem., 4, 221 (1971). 0022-3654f83/2087-2578.$01.50/0
Experimental Procedures The compounds 6,13-dichlorotriphenodioxazine(C1,TPD), a sodium salt of a sulfonated derivative of C1,TPD (C12TPD(S03),),a sodium salt of a sulfonated derivative of 6,13-dichloro-3,10-diphenyltriphenodioxazine (Cl2PhZTPD(SO3),), 8,18-dichloro-5,15-diethyl-5,15-dihydrodiindolo[ 3,2- b:3’,2’-m] triphenodioxazine (C1,Et2TPD), a sodium salt of a sulfonated derivative of C12Et2TPD (Direct Blue log), 8,18-diphenyl-5,15-diethyl-5,15-dihydrodiindolo[3,2-b:3’,2’-m] triphenodioxazine (Ph2EhTPD),and a sodium salt of a sulfonated derivative of PhzEtzTPD(Ph2EhTPD(S03),)were synthesized and purified according to the procedure of Nachtigall and Zweig’ with some minor modifications. Mass spectral analysis of the ClzTPD thus synthesized gave a result consistent with the expected structure for this molecule (MR = 355.2). 6,13-Dichloro-3,10-diphenyltriphenodioxazine (C12Ph2TPD)was obtained as a gift from Dr. G. C. Newland of Tennessee Eastman Co. All of the solvents used were spectral grade. Room temperature absorption spectra were obtained with a UV-visible recording spectrophotometer (Cary 17D). All solution concentrations were adjusted to give a maximum absorbance between 0.5 and 1in the spectral range investigated. Corrected fluorescence spectra of ClzPhzTPD in toluene, ClzTPD in toluene, and Ph,EbTPD in chloroform were obtained with one or more of the following spectrofluorimeters: (1) SPEX, Ind. Model 211 Fluorolog 2 spectrofluorimeter equipped with a Datamate data acquisition system, (2) a Farrand spectrofluorimeter with autoprocessor, and (3) a SPEX, Ind. Model 222 Fluorolog system. These spectra were all corrected for the wavelength-dependent response of the de(7) G. Nachtigall and A. Zweig, NTIS report AD/A005082.
0 1983 American Chemical Society
Spectroscopy of Triphenodioxazines
The Journal of phvslcel Chemistry, Vol. 87, No. 74, 7983 2579
X
1. 0
6. 1
0.0 1.0
0. 0 3.1
X >
t; g
e
d I
z
0,
" z J 0
W V
-
z
W V
u, W
VI a W
0 ci
3 LL
~
0.0 6. 8
0.0 1.0
Y
C2HS
Figure 1. Molecular structures of the trlphenodioxazine derivatives studied: (a) X = Ci, R = H: Ci,TpD; X = CI, R = Ph: CI,Ph,TPD (b) Y = Ci: C1gt2TPQ Y = Ph: Ph&t,TPD. (Seetext for abbrevlatlons.)
18000
13000
23000
28000
WAVENUMBER (cm-1)
Flgure 3. Adsorption and corrected fluorescence spectra of (a) Ci2TPD In toluene, (b) C12Ph2TPDin toluene, and (c) Ph,Et,TPD In chloroform. The samples were optically dilute (c < lod M) for fluorescence. The left ordinate is in relative fluorescence intensity units, normalized to 1 at the maximum Intensity. The absorption spectra were obtained with the maxlmum absortmce between 0.5 and 1.O. The right ordinate is scaled to the molar extinction coefficient, t, in units of L mi-'cm-'.
,
'"-2
0
2
4 6 T I M E (nsec)
8
10
12
1. 0 r
Figure 2. Fluorescencedecay kinetics of Ph&TPD in CHCI, produced by excitation with a -lO-ps, 532-nm laser pulse. The detection wavelengths were >600 nm. The circles give the observed experimental decay, while the smooth curve is a least-squares best fit obtained by deconvoiutlon from the instrument response to the laser excitation pulse assuming a single exponential decay. The calculated fluorescence lifetime from this data was 2.71 ns.
r
4
tection equipment. After digitization and conversion to an energy scale, the corrected emission spectra were numerically integrated. The fluorescence quantum yields were then calculated by using the method of optically dilute solutions,8 utilizing the results obtained from a solution of rhodamine 590 as the standard (&, = 0.93).9 Fluorescence lifetimes were obtained by exciting the samples with a single pulse (At 10 ps) from a frequency-doubled (532 nm), mode-locked Nd3+:glass laser system.l0 The emission was observed with a fast photodiode (ITT FW4000-UV, S-20) and a 500-MHz oscilloscope (Tektronix 7904). The time base of this oscilloscope was calibrated in a separate experiment by passing a 532-nm pulse through a fixed-gap, partially reflecting etalon (R = 0.70, 1 = 54.7 cm) and observing the apparent time separation between peaks in the etalon-generated pulse train. The measured instrument response function of this detection equipment was found to have a temporal width for the excitation pulse of 0.87 f 0.12 ns (fwhm). The oscilloscope traces of the fluorescence decay curves were photographed, digitized, and deconvoluted with this instrument response function by a least-squares fit analysis with an assumed single exponential decay using the technique we have previously described.l" These fits produced
the experimentally observed fluorescence decay times. The low-temperature fluorescence spectrum of ClzTPD in toluene was obtained by cooling the sample to 10 K in a closed-cycle helium refrigerator (Air Products Model CSW-202). The sample was excited with the 440-nm line of a HeCd laser (Liconix Model 4050), and the emission was dispersed with a 1-m Czerny-Turner monochromator (Hilger-Engis Model 1000) and detected with a red-sensitive photomultiplier tube (Hamamatsu 1P28A). The wavelength calibration of the monochromator was obtained by recording the emission lines from a Fe/Ne hollow cathode lamp. The magnetic susceptibility measurements were performed a t room temperature with a Faraday magnetic susceptibility system (Cahn model 2000-RG).
(8)J. N. Demas and G. A. Crosby, J. Phys. Chem., 76,991 (1971). (9)P.R. Hammond,J. Chem. Phys.,70, 3884 (1979). (10)A.L.Huston,G. W. Scott, and A. Gupta, J . Chem. Phys.,76,4978 (1982).
Results A summary of the triphenodioxazine absorption and emission spectra, fluorescence quantum yields, and fluorescence lifetimes obtained are given in Table I.
-
0.0 ' i 6000
/ 19ooc
17900
1900G
2cooc
WAVENUMBE2 (cm-1)
-
Figure 4. Fluorescence spectrum of Ci2TPD in toluene at 10 K. Sample c o n m a t i o n was lo4 M. The intensities are not corrected for the spectral response of the detection equipment.
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Butz et al.
The Journal of Physical Chemistry, Vol. 87,No. 14, 1983
TABLE I : A Summary of the Observed Triphenodioxazine Absorption and Emission Data absorption, a,b
emission,
cm-' E max 19 700, 60 700 2 1 150, 22 500, 24 100 s 19 550, 7 000' 21 000 1 8 300, 30 900 19 650, 2 1 000 s 19 250 1 0 000' 16950, 68000 1 8 300, 19 800 s 16 600, 6000' 1 7 700 16550, 10000 1 7 850 1 7 450 3000'
cm-' 18 800, 1 7 750
0.57
i. 0.06
16900
0.62
t
0.05 0.041 i. 0.01
1 7 750, 1 6 450, 1 5 050 s 1 5 450 16100, 1 5 250 s
2.15
t
0.15 0.48
Vmaxl
molecule C1,TPD
solvent toluene
CI,TPD(SO,),
H,O
C1,Ph ,TPD
toluene
CI,Ph,TPD(SO,), Ph,Et :TPD
H,O CHCI,
Ph,Et,TPD(SO,),
H,O
C 1Et ~ ,TPD
CHCI,
(I,b umax,
-
Tfl,
14800
ns
i.
0.05
3.7
i.
ns 0.3
33 t 11 6.5 * 0.6
0.77 * 0.04 0.0058 c 0.003 23 c 8 2.71 e 0.25 0.57 t 0.05 4.0 t 0.4 , cm-' 1 9 568 19 293 1 9 204 1 9 037 1 8 915 1 8 703 1 8 616 1 8 561 1 8 425 1 8 262 1 8 133 1 8 019 1 7 932 17 884 1 7 754 1 7 601 1 7 460 a
AT, cm-'
assignment
- 364 - 89
0 167 289 501 588 643 779 942 1071 1185 1272 1328 1450 1603 1744
V , = 289 c m - '
U:
= 1450 cm-I
5,+ L2 + 5 c m - '
Refer t o Figure 4. Estimated uncertainty = i.10 cm-I.
Figure 2 shows the fluorescence decay kinetics of Ph2EhTPD with the fit of a calculated exponential decay to the experimental data. The absorption and corrected emission spectra of C12TPD,C12Ph2TPD,and Ph2EhTPD are shown in Figure 3. The low-temperature emission spectrum of C12TPDin toluene is given in Figure 4, and the corresponding vacuum wavenumbers of the band maxima are given in Table 11. The room temperature magnetic susceptibilities of both ClzTPD and ClzPhzTPD were obtained and showed these compounds to be diamagnetic. The values obtained were x(C1,TPD) = -3.3 X lo4 f 2.1 X lo4 and x(C12Ph2TPD)= -0.86 X lo4 f 0.34 x IO+ cm3/g. Discussion All of the triphenodioxazines investigated have strong visible absorption bands, and an additional absorption system in the UV. The maximum visible extinction coefficients, given in Table I, range from -0.3 X lo4to 7 X lo4. This absorption into the first excited singlet state is certainly due to a l m * transition. These absorption spectra exhibit distinct vibronic structure with an average vibrational spticing of about 1400 cm-' (see Figure 3). This
observed frequency spacing is probably due to a C=C stretch mode associated with the fused ring structure. The observed decrease of the Franck-Condon factors for this mode with increasing excited state vibrational quantum number indicates little disdacement in the excited state along this normal mode coordinate. The fluorescence spectra also exhibit vibronic structure, but the vibrational splittings, 1050 cm-' (C12TPD),1300 cm-' (Cl2Ph2TPD),and 850 cm-' (Ph2Et2TPD),are less than those found in the absorption spectra. The Stokes shift, measured between the maxima of the first bands in the absorption and fluorescence spectra, ranges from 550 (Cl2PhZTPD)to 900 cm-I (C1,TPD) for these three molecules. These values, although small, are typical of similar red dyes? again indicating little change in the equilibrium geometries of the two states involved. The fluorescence spectrum of C12TPDtaken at 10 K in toluene (see Figure 4 and Table 11) indicates vibronic activity, mainly, in two modes-vl (289 cm-') and v2 (1450 cm-'). The former, low-frequency mode is probably a puckering or twisting motion of the fused ring chromophore while the latter is likely a C=C stretch mode. The vibrational splitting observed in the room temperature fluorescence spectrum is probably just determined by the inhomogeneous overlapping of bands due to these two modes. No long-lived emission (phosphorescence)was observed from this low-temperature sample of Cl,TPD, and no emission could be detected at all between -600 and 700 nm. Thus,we were not able to detect any phosphorescence from this molecule under these conditions within the limits of our detection equipment. The quantum yield values for the nonsulfonated TPD dye molecules (see Table I) indicate that one or more nonradiative decay mechanisms compete effectively with emission for relaxation back to the ground state. Internal conversion from S1 to So is a good possibility for the nonradiative process in these molecules due to their relatively small S1-So energy gap. The possibility of intersystem crossing to the triplet manifold as a competing mechanism is an interesting one in the light of molecular orbital calculation predictions that these molecules possess low-energytriplet leveh6f' From our measurements of the magnetic susceptibilities of C12TPD and C12PhzTPDat room temperature, we conclude that the ground states of
J. Phys. Chem. 1083, 87, 2581-2584
these molecules are, in fact, singlet in nature. This measurement does not, however, rule out the possibility of the existence of low-lying triplet mr* (or nr*) levels that could sph-orbit couple to SI.However, the triphenodioxazines are quite photochemically stable: indicating that triplet-state photochemical reactions do not occur with any significant quantum yield. In addition, it is known that the oxazines, which are closely related to the triphenodioxazines, have low triplet quantum yields." Further experiments are planned to determine the nature of the nonradiative decay process. The measured excited-state lifetimes of the triphenodioxazines are typical of dye molecules with strongly allowed So S1transitions. The observed radiative lifetimes, calculated from the measured lifetimes and quantum yields for C12TPDand C12Ph2TPD, agree relatively well with the radiative lifetimes calculated from integration of the absorption and emission spectral2 (seeTable I). The observed radiative lifetime for C12Et2TPDis somewhat smaller than the calculated value, but, as indicated in Table I, the calculated value has a rather large experimental error associated with it. The fluorescence properties of the triphenodioxazine dyes are changed dramatically following incorporation of the negatively charged sulfonate group into the ring structure. The addition of the sulfonate group causes a red shift of the absorption and emission bands, and the spectra of these derivatives are broader than those of the parent compounds. The fluorescence quantum yields of the sulfonated derivatives are much smaller, and the extinction coefficients considerably less, than those of the parent compounds. I t then follows that the fluorescence lifetimes of the sulfonated derivatives are much less than those of the corresponding parent compounds. These properties could be due to an accelerated internal conversion via a reversible transfer of charge from the a
-
(11)K. H.Drexhage in "Dye Lasers", F. P. Schafer, Ed., SpringerVerlag, Berlin, 1973,Chapter 4. (12)S. J. Strickler and R. A. Berg, J. Chem. Phys., 37,814 (1962).
2581
system of the chromophore to the sulfonate group following excitation. Additionally, the solvent used, water, may influence the photophysical properties of these dyes due to ionization of the sulfonate group.
Conclusions The triphenodioxazine dyes are fluorescent dyes which, additionally, possess good photochemical stability and exhibit excellent polymer photostabilizing characteristic~.'-~The observed fluorescence quantum yields range from to -0.6, the lower values occurring for the sulfonated dyes in water. It appears likely that competing nonradiative internal conversion processes are responsible for these lower fluorescence efficiencies. Additional studies of the excited-state properties of these molecules are planned to elucidate the mechanisms of these processes. Acknowledgment. We thank Mr. H. Davis, Dr. F. Purcell, and Mr. R. Kaminski, SPEX, Ind., and Professor J. N. Pitts, Jr., and Dr. D. Lokensgard of the UCR Chemistry Department for assistance in obtaining the quantum yields with their respective spectrofluorimeters. We also thank Mr. A. Johnston of this department for advice regarding the synthesis and purification of the dyes, and Dr. G. C. Newland of Tennessee Eastman Co. for the gift of some C12Ph2TPD. The HeCd laser used in this research was provided by Biomedical Sciences Grant 5507RR-07010-12from the National Institutes of Health and National Science Foundation Grant CHE77-09163. We also thank Ms. Linda DeLucci for typing the manuscript. This work was supported by a grant from the Universitywide Energy Research Group of the University of California and, in part, by the Energy Sciences Program of the University of California, Riverside. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Registry No. Cl,TPD, 4794-44-9; C12Ph2TPD,13437-05-3; PhzEhTPD,85614-24-0; C12EbTPD,6358-30-1; Direct Blue 108, 1324-58-9.
Pressure Dependence of Sound Absorption in an Aqueous Solution of CaSO,+ C. C. Hsut and F. H. Fisher' Marine physical Laboretciy of the Scrlpps Institution of Oceanography, University of Caiifornk, Sen Diego, Califomis 92152 (Received November 29, 1982)
Sound absorption has been measured in a 0.011 M aqueous solution of C&04 at 25 "C. The values of maximum absorption per wavelength, l@(ax),, are 10.3 f 0.1 at 1atm and 7.7 f 0.1 at 307 atm. The relaxation frequency is 177 f 9 kHz at 1 atm and, within experimental error, is independent of pressure. This corresponds to an inner coordination sphere substitution rate of about lo6, 2 orders below that of 2 X lo8 quoted by Eigen and Ta". At 1 atm, the CaS04 absorption is about 30% that of the same concentration of MgSO,.
Introduction Fisher1 recently pointed out that aqueous solutions of CaS04should exhibit acoustic absorption with a relaxation frequency around 200 kHz and with a magnitude about that of an equivalent concentration of MgS04. This Contribution of the Scripps Institution of Oceanography, new series. *Departmentof Electrical Engineering, Chung Cheng Institute of Technology, Tahsi, Taoyuan, Taiwan 335,Republic of China.
conjecture was based on experimental acoustic data published by Kurtze and Tamm2 for aqueous solutions of MgS04,MgCrO,, and CaCr04, as seen in Figure 1. Wilson and Leonard3had reported that no sound absorption was (1)Fisher, F. H.Acustica 1981,48, 116-7. Acustica (2)Kurtze, G.;Ta", K. Nature (London) 1951,168,346; 1953,3,33. (3) Wilson, 0 . B., Jr.; Leonard, R. W. J. Acoust. SOC.Am. 1954,26, 223-6.
QQ22-3654/83/2Q87-2581$01.5O/O0 1983 American Chemical Society
