J. Phys. Chem. 1992, 96,7988-7996
7988
intensities at 388 and 673 K (37 pages). Ordering information is given on any current masthead page.
Refereaces and Notes (1) (a) Friesen, D.; Hedberg, K. J. Am. Chem. Soc. 1980,102,3987. (b) Femholt, L.; Kveseth, K. Acta Chem. Scund. 1980, A34, 163. (c) Brunvoll, J. Thesis, Norges Tekniske H~gskole,Trondheim, Norway, 1962. (2) Kvescth, K. Acru Chem. Scund. 1974, A28, 482. Kveseth, K. Ibid. 1975, A29, 307. See also ref IC. (3) Huang, J.; Hedberg, K. J . Am. Chem. Soc. 1989,111,6909. Hagen, K.; Hedberg, K. Ibid. 1973, 95, 8263. (4) Almenningen, A.; Bastiansen, 0.; Fernholt, L.; Hedberg,K. Acro
Chem. S c u d . 1971,25, 1946. ( 5 ) Huang J.; Hedberg, K. J. Am. Chem. Soc. 1990, 112,2070. (6) Thomassen, H.; Samdal, S.;Hedberg, K. J. Am. Chem. Soc. 1992,114, 2810. (7) Iwasaki, M. Bull Chem. Soc. Jpn. 1958, 31, 1071. (8) (a) Gloclder, G.; Sage, C. J . Chcm. Phys. 1941 9.387. (b) Simpson, D.; Plyler E. K. J. Res. Nutl. Bur Srundards 1953, 50, 223. (c) Kagarise, R. E. J. Chem. Phys. 1957, 26, 380. (d) Brown, F. B.; Clagge, A. D. H.; Heitkamp, N. D.; Koster, D. F.; Danti, A. J. Mol. Specfrosc. 1967,24, 163. (9) (a) Gundersen, G.; Hedberg, K. J. Chem. Phys. 1%9,51, 2500. (b) Hagen, K.; Hedberg, K. J. Am. Chem. SOC.1973, 95, 1003. (10) Hedberg, L. Abstracts, Fifth Austin Symposium on Gas Phase Molecular Structure, Austin, TX, March 1974; p 37.
(11) (a) Elastic amplituca and phases: Ross A. W.; Fink, M.;Hilderbrandt, R. Inrernarionul Tublesfor Crysrullogruphy; International Union of Crystallography;Kluwer: Dordrecht, Holland, 1992. (b) Inelastic amplitdes: Cromer, D. T.; Mann, J. B. J . Chem. Phys. 1967,47, 1892. Cromer,D. T. ibid. 1969, 50, 4857. (12) Frisch, M. J.; Binldey, J. S.; Schlegel, H. B.; Raghavachari, K.; Melius, C. F.;Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; Defrees, D. J.; Seeger,R.;Whiteside, R. A.; Fox, D. J.; Fleuder, E. M.;Pople, J. A. Guussiun 86, Camegie-Mellon Quantum Chemistry Publishing Unit, Pittsburgh, PA, 1984. (13) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acra 1973, 28, 213. (14) Pulay, P.; Fogarasi, G.; Pongor, G.; Boggs, J. E.; Vargha, A. J . Am.
Chem. Soc. 1983,105, 7037. (15) Thomassen, H.; Hedberg, K. J. Phys. Chem. 1990, 94,4847. (16) Hedberg, L. Abstracts, Seventh Austin Symposium of Gas Phase Molecular Structure, Austin, TX, Feb 1978; p 49. ( I 7) (a) Kuchitsu, K.; Cyvin, S.J. In Molecular Structure and Vibrution; Cyvin, S.J., Ed.; Elsevier: Amsterdam, 1972; Chapter 12. (b) Robiette, A. G. In Molecuhr Sfrucrure by Di/frac?ion Merhods; Sim, G . A., Sutton, L. E. Eds; Specialist Periodical Reports; The Chemical Society: London, 1973; Vol I, Chapter 4. (18) Gallaher, K. L.; Yokozeki, A.; Bauer, S . H. J. Phys. Chem. 1974.78, 2389. (19) Kagarise, R. E.; Daasch, L. W. J . Chem. Phys. 1955, 23, 130. (20) Serboli, G.; Minasso, M. Specrrochim. Acra, Purr A 1968, 24, 1813. (21) Wiberg, K. B.; Murcko, M. A. J. Am. Chem. SOC.1987, 91, 3616.
Lasing Action in a Family of Peryiene Derivatives: Singlet Absorption and Emission Spectra, Triplet Absorption and Oxygen Quenching Constants, and Molecular Mechanlcs and Semiempirical Molecular Orbital Calculations Mahin SilBrai, W a Hadel, Ronald R. Saws,* Syeda Husain, Karsten Krogh-Jespemn,* John D.Westbrook, and George R.Bird* Department of Chemistry, Rutgers. The State University of New Jersey, New Brunswick, New Jersey 08903 (Received: August 12, 1991; In Final Form: May 1I , 1992)
We prescnt experimental and computational determinations of the excited-state properties in several perylene dyestuffs that are potential candidates for usc as laser dyes. Attention is focused on the following species derived from 3,4,9,10perylenetetracarboxylic acid dianhydride: the bis( (2,6-dimethylphenyl)imide) (lb, DXP); the bis(methy1imide)(IC, DMP); both 1,6,7,12-tetrachloro- (ld, Cl4DMP) and 1,2,5,6,7,8,11,12-octachloro-( l e , ClsDMP) derivatives. Soluble derivatives of seven-ringed or larger aromatic systems are produced by the introduction of relatively rigid out-of-plane substituents that prevent intermolecular close packing. Chiral distortions in ring-chlorinated perylene derivatives significantly alter the shape of the absorption bands, a consequence of symmetry breaking. Triplet-triplet absorptions and oxygen-quenching rates are observed under the conditions found in a dye laser cavity. Semiempirical molecular orbital calculations (INDOIS) provide detailed mapping of the singlet and triplet excited state manifolds of DMP, CIdDMP, and C18DMP. Computed transition energies and intensities are used in the interpretation of the spectral features, in particular the observed T1 T, and potential SI S, absorptions. We conclude that perylene-3,4,9,1O-tetracarboxylic acid diimide chromophores may be solubilized and utilized in laser materials exhibiting superior performance in terms of power output, tuning range, and light stability.
-
-
Iotrduction Perylene dyestuffs were introduced by Kardos in 1913 and Friedlaender synthesized the first bis-phenylimide dyes shortly thereafter.'*2 Modem investigations of perylene derivatives as electrically and optically active materials began with the work of Graser and Haedick2 on soluble and fluorescent dyes, especially for solar concentrators. Graser and Haedicke explored the development of highly chlorinated, soluble, and flwrescent derivatives and also conducted crystallographic investigations." Perylene derivatives were found to include some promising microcrystalline photoconductors for electrophotography' and have been incorporated in prototype solar photovoltaic cells by Tang8and by some of us.9 The synthesis and luminescence of perylene derivatives have been explored by Langhals et al., including the pre ration of the bis((2.5-di-tert-butylpheny1)imide) (DBPI, la).l l2 Ford and Kamat observed the triplet-triplet absorption of DBPI and noted a near coincidence of So SI and TI T, ( n > 2) absorption bands.13 The investigation of triplet properties is complicated by the need for triplet sensitizers and by the overlapping of the very
-
-
r
strong fluorescence bands with the most interesting region of T1 T,, absorption. Ford also investigated the luminescent behavior of free perylenetetracarboxylic acid, both the monoimide and the diimide from glycine, and DBPI for their potential use as biochemical probes.I4 The first high efficiency lasing of a perylene derivative, the bis((2,adimethylphenyl)imide) (DXP,lb), was reported by Sadrai and Bird.l5 Perylene itself produced only short lasing pulses of low efficiency because of the larger quantum yield of triplet formation and the overlap of T, T,, absorption with the f l u o r ~ c band.I6 e However, perylene has been used successfully as a singlet sensitizer by Pavlopoulos to enhance the lasing output of a longer wavelength rhodamine dye." Weak lasing of DBPI has been observed recently,I8and efficient lasii has been reported for DXP in "sol-gel" media by Reisfeld et al.19 The properties of a good laser dye are strong absorption, high quantum yield of fluorescence, photochemical stability under intense excitation, a region of strong luminescence well out of the absorption wing, very low quantum yield of triplet states, and preferably minimal overlap between both the regions of strong -L
0022-3654/92/2096-7988S03.00/00 1992 American Chemical Society
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The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 7989
Lasing Action in Perylene Derivatives
C1,27.76; N, 4.55. Attempts to remove the last traces of methylene chloride were u n s u d d . Mass spectral analysis indicated the prcaence of small amounts of C13 and CIS isomers. 2. Wydcrl M e a ” & Absorption spectra were obtained with a Cary 17D spectrophotometer feeding an Adam Smith Co. digital interface to a Tektronix 4052 computer. Fluorescence spectra, corrected to give absolute differential quantum yields (d@/dX)and absolute quantum yields by integration, were taken on a Perkin-Elmer Hitachi MPF3 fluorescence spectrometer. The sample and reference channels of this instrument were read into two Hewlett-Packard 3437A fast digital voltmeters interfaced with the computer. Rhodamine 101 (100% fluorescence quantum yield) was used as primary standrd. We have found this configuration of interfaces and programs accurate to better than AS% relative error in quantum yields, sometimes even to 1-2% when a reliable standard with fluorescence emission in the same spectral region is available (as here). The collection of triplet absorption and kinetic data was made on an excimer plus dye laser apparatus which has been described elsewhere.24 No valid data could be collected within the fluorescence band for 1W200 ns after the onset of the laser pulse because the brilliant fluorescence of the sample saturated the photomultiplier, even though attenuated in the monochromator. To determine the limits of valid observation, unsensitized DXP was used as a reference material having essentially zero triplet R1 yield and only the fluorescence overload as a false transient. By contrast, triplet concentrations up to about 26% of the starting l a : R, = 2,5-(t-C4H9)2C6H3;R2 = R3 = H; DBPl ground-state dye could be generated by direct excimer laser irl b : R1 = 2,6-(CH3)2C,H,; R2 = R3 = H; DXP radiation of C18DMP. Not having reliable data on the beam profde, we have not attempted to determine triplet quantum yields IC: R1 = CH3; R2 = R3 H; DMP from the laser pulse experiments. The large decreases in direct I d : R, = CH3; R2 H; R3 = CI; CI4DMP fluorescence quantum yields upon chlorine substitution give an 10: R1 = CH3; R2 R3 = CI; CIeDMP indirect appraisal of triplet yields. The measurements of lasing output were performed in the small It: R1 = CsHs; R2 = R3 = H laser geometry developed by Littman and Metcalf.” A frequency l g : R1 = C(CH3)3-CH2; R2 R3 = H doubled YAG laser (2XYAG) delivered 0.5-mJ pulses of 6-ns width at 10-Hz repetition rate to the side wall of a fluorescence TI T, and SI S, pumping absorption and the SI S, laser cuvette tilted some 20° out of the vertical to minimize unamtroUed dye emission. The 0’ 0” vibronic fluorescence band cannot lasing with the outer cuvette walls as 4% cavity reflectors. Power participate in laser action, since the ground and excited states measurements were made with a 99% and a 10% reflector in a cannot easily be inverted for this transition; thus one seeks to shift simple cavity without monochromator, after which a 2400 fluoresoence intensity away from this band, as we have done earlier lines/nm American Holographic Co. grating was inserted at with a family of rhodol or rhodamone dyes.20 It was the unusually grazing incidence to the orighd kam (full width of grating fded). high intensity of the 0’ 1” and 0‘ 2” vibronic bands of the A rotating 99+% mirror was used to return a monochromated perylene derivatives which suggested them as laser materials. We beam to the grating to create a wavelength-controlled cavity for report here on a comprehensive examination of some perylene spectral range maasurements. The output wavelength was dederivatives carried out in order to refine our understanding of the termined by a Bausch and Lomb small grating spectrograph. optimal experimental conditions needed for lasing and also to Since the lasing solvents had to carry dye concentrations up provide a computational analysis of the photophysics exhibited to 5 X 10“ M or higher, dimethylformamide (DMF) and chloby these materials. robenzene (C6H5CI)were used as laser solvents. Both solvents. ExperhenW and Coarputationnl Dewlip lack the large heat capacity and low derivative of refractive index with temperature (dn/dT) which make water and the simple 1. Synthesis and Purifhtion. The syntheses of the imides of alcohols preferred laser solvents for higher efficiency and power. the commercially available 3,4,9,10-perylenetetracarboxylicacid dianhydridewere performed amding to literature procedure~.~~J~ Chlorobenzene is a very unsatisfactory laser dye solvent but we were forced to use it with the alkali-sensitive chlorinated perylenes. Products were purified by recrystallization and/or chromatography Neither of the ring-chlorinated derivatives could be treated with with the exception of the bis((2,64imethylphenyl)imide) (DXP, bases, or even potential bases such as DMF, without suffering lb) which was purified by train sublimation.21Anal. Calcd for degradation. CMHXN2O4:C, 80.00, H, 4.30; N, 4.70. Found: C, 79.70; H, 3. Computrtiuaal DecIL. Semiempirical molecular orbital 4.50; N, 4.40. Purification of 1,2,5,6,7,8,11,12-octachlorocalculations based on the INDO model Hamiltonian parameterid perylene3,4,9,1(Ftetracarboxylicacid bis(methylirnide) (C18DMP, for spectroscopy (INDO/S) were camed out on DMP, CLDMP, le) was accomplished by column chromatography on silica with and CIBDMPusing the ESPPAC program?6 The backbone gebenzene as eluent (for X-ray structural data of le, see ref. 22). ometry for DMP was obtained for the X-ray structure of the Chlorination of DMP (3.36 g) with chlorosulfonicacid (36 g) N,”-bis(CH2CH2CH20CH3) derivative: averaged and symand iodine (0.56 g) was accomplished by heating at 60-70 OC metrized to exhibit inherent D2,, symmetry (C2,, after the bisfor 20 h.23 The reaction mixture was poured into an ice/water (methyls) were attached). The backbone of the recently determixture and the insoluble dye was removed by filtration. Conmined X-ray geometry for ClaDMP was used and a tinuous extraction with methylene chloride gave 3.0 g of geometry for C4DMP was generated by simply replacing the 1,6,7,12-tarachloroperylene-bis-methylimide(CLDMP, Id): mp 2,5,8,1 l-chlorines by hydrogens (at proper distances) in the 436442 OC; IH NMR (60 MHZ) 6 8.85 (s, 4 H) and 3.35 (5, ClaDMP structure. The bis(methy1) groups and the appropriate 6 H). This material showed a single spot on thin-layer chronumber of hydrogen atoms were attached to the backbones with matography. Anal. Calcd for C26H1oC14N204.0.25CH2C12: c , standard bond length and angles. The ground-statewavefunctions 54.56; H, 1.82; C1, 27.67; N, 4.85. Found: C, 54.39; H, 1.96;
I
I
0
-
--
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7990 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
Sadrai et al.
TABLE I: Shglet S p d m a p k Ropartkr of Perykntttraerrboxylic Acid welds rad 9 , 1 0 - D i c m * dye (4 XI' (4 A2a (8) XfmC vi' hfiE
u
DXP DBPI' CI4DMP CIBDMP
1s If
2
527 (84600) 527 (96000) 522 (32500) 520 (31 600) 522 (99000) 525 407 (8800)
489 490 488 489 486
(45 500) (57600) (22500) (24000)
384 (9300)
460
579 578 592
627d
426 (7800) 450 (17 000) 456
540 536 557 564 538
572
622
364 (6200)
414
437
460"
solv C6HSCl CHClj C6HsCl CsHjCl CHzClZ DMF C6HjCl
@fl
0.98 0.99 0.85 0.55 0.99 0.70 0.49
"Am, XI. and X2 are the wavelengths (nm) of the absorption maximum and the first and the sccond blueshifted vibrational satellites, rspcctively. hf,, and A& are the wavelengths (nm) of the fluortscence maximum and the f m t and second red-shift4 bExtinction coefficients (M-I an-').'Xf-, vibrational satellites, respectively. dShoulder. cData from ref 13.
were found by standard closed shell restricted Hartree-Fock
calculations in the INDO/ 1 s parameterization~cheme.2'~Excited singlet and triplet states were then obtained from configuration interaction calculations involving approximately 400configurations singly excited relative to the closed shell ground state (SECI). The singlet transition energies and intensities were in addition calculated in the time-dependent Hartree-Fock (TDHF) app r o x i m a t i ~ nusing ~ ~ ~the ~ same elementary excitations as in the SECI calculations. Finally, triplet-triplet (TI T,) and excited singlet-singlet (SI S,) excitation energies were computed directly from SECI calculations (about 600 codigurations) based on restricted open shell HartreeFock calculations for TI and SI, Oscillator strengths for singlet-singlet (So S,,, SI S& and triplet-triplet (TI T,,) transitions were calculated in the dipole length approximation with the one-center sp polarization terms included. All calculations were carried out on a Convex C-220 minisupercomputer.
-
-
-
-
-
Results and IWBcussioa 1. Dye Solubility. The first variation to be examined is the great solubility increases which occurs from insoluble DMP to the readily soluble DXP and to the chlorinated derivatives (CLDMp, ChDMF'). The torsional angles available to the pendant phenyl rings in DXP and a comparison compound, the bis(phenylimide), have been modeled using molecular mechanics (MMX program).'O The ground-state energy wells for torsion are predicted to be flat around the minima with DXP having the global minimum at 90" only 200 cm-'deeper ( - k e n than the energies at 65" and 115" torsion. Steric interactions make the potential energy rise rapidly outside the 65"-115" interval. Unlike the two tert-butyl groups of DBPI, the 2,6-dimethyl group in DXP interfere only with the carbonyl oxygen and not with the central molecular plane. The smaller bis(pheny1imide) is indicated to have a shallow double potential well with minima at 60" and 120" and a central maximum at 90"only 200 cm-'(- k e n above these minima. Exterior kBTpoints on the torsional potential curve for this species occur at 44" and 136", and outside this range the potential again increases steeply. The near-perpendicular orientation of the phenyl substituents in these species prevents the packing at 3.4 A separation commonly occurring in large wnjugated systems and thus diminishes the strongly attractive intermolecular forces. The solubilization of C W M P and C18DMPproceeds by similar logic,but a very different sort of structural chemistry is at work3' The critical substituent positions are the 'bay" positions32 (1,6,7,12) at the center of the perylene residue and both derivatives owe their solubility to substitution at these sites. Bay hydrogens in planar DMP are already in van der Waals range, and the bay chlorines in C18DMP (and by inference also in CI.,DMP)" experience severe repulsive contacts and are thrust below and above the molecular plane (see X-ray structure in ref 22). producin a structure which can no longer fit into a plane-parallel 3.4 packing scheme. Substantial angular stress on these substituent bonds may provide an explanation for the instability of the chlorinated dyes in even the mildest bases. A likely degradation mechanism is displacement of Cl- by OH-. 2. Absorption .ad Flwnsceace Spectra. The experimental parameters pertaining to singlet absorption and fluorescence of the perylme derivatives under investigation here are listed in Table
w
Figure 1. Absorption (-)
and fluortscence spcctra of DXP (lb) in DMF. The first vibrational satellite (579-nm pcak) is the region for high efficiency lasing, but the sccond satellite is also active. The triplet extinction spectrum is shown in large dash- (---). The long wavelength isosbestic point (*= cs) lies at 540 nm, and the triplet half maximum point also lis at 540 M.I The spectrum of lasing action is shown by the horizontal bar: solid for efficient lasing from 569 to 585 nm and dashed for intermittent lasing from 586 to 605 nm. (as.)
3
2
WAVELENGTH (nm)
Figure 2. Absorption (-) and fluorescence spcctra of the CllDMP (la) dye in chlorobenzene. Note that the extinction minima between vibronic peaks are largely filled in by band broadening. The triplet extinction spectrum is shown in large dashes (---). It is redshifted and largely overlaps the fluorescence band. This is unfavorable for lasing action but dots favor the full development of the broad lasing range from 575 to 620 nm (solid horizontal bar). (so.)
- -
-
I; actual So SI,SI So, and TI T, spectra are shown in Figures 1 (DXP), 2 (CLDMP), and 3 (C18DMP). For a simplified view of torsional freedom vis-a-vis potential intersystem crossing, we will consider the points lying kBTabove the torsional The high fluoreicencequantum yield (en = 0.98, Table I) of DXP may be understood in termsof the greater locking of phenyl ring torsional motion, ca.f 2 P , by the pendant 2,ddimethylphenyl groups. The simple bis(phmylimi&) has much greater twisting freedom (ca.f 4 5 O ) relative to the perpendicular
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 7991
Lasing Action in Perylene Derivatives
TABLE Ik Dipok Stmgtb Moments .ad Radiative Lifetimed dyea DXP CI4DMP ClsDMP
VOW
b
DS' 66.1 49.5 56.1
19000 (527) 19 150 (522) 19200 (520)
DSWBC 20 000 (500) 20700 (483) 21 200 (472)
DSWHBW' 1820 1660 2060
P
7d'
0.62 0.48 0.56
4.0 5.6 5.2
'In chlorobenzene. bcm-' (nm). 'Dipole strength in debyes squared, calculated from eq 1. *Dipole strength-weighted band center in cm-'and in nm (parentheses) calculated from eq 2. 'Dipole strength-weighted root-mean-square half-band width in cm-I, calculated from eq 3. 'Oscillator strength calculated from DS and DSWBC without refractive index correction. *Radiative lifetime (ns) calculated from the dipole strength with n-* refractive index correction.
gives the dipole strength in debyes squared. The next moment is the dipole strength-weighted band center frequency DSWBC = (v) = s c d v / I ( c / v ) dv =
(wm/(wN= 1 2 / 1 1
(2)
Finally, the dipole strength-weighted half-band width (DSWHBW) has also been calculated from the square root of the following expression DSWHBW2 = ((v- ( v ) ) ~=)
I(.-
(v))2(dDS/dv)dv/ S(dDS/dv) dv
4.25
4.75
5.25
5.75
6.25
6.75
UAVELENGTH (nm X l c T 2 )
Figure 3. Absorption (-) and fluorescence spectra of the ClsDMP (le) dye in chlorobenzene. The triplet extinction spectrum is (.-e)
shown in circles and dashes ( 0 - 0 ) .
orientation. It has a considerably lower fluorescence quantum yield (afl= 0.70)than DXP and also a solubility too low for use in sidepumped hem, as the external phenyl group can twist more nearly into coplanarity with the perylene core.34 Despite reduced fluorescence yields, at least one chlorinated dye, CLDMP, is a surprisingly broad-banded lasing material. The contradiction will be explained below through consideration of triplet yields and TI Tn absorptions. Here we note that the cleanly rtsolved progressions of vibrational peaks in spectra of DXP (1400 cm-l in SI (absorption) and 1250 cm-' in So (fluorescence), Figure 1) are nearly lost in the chlorinated derivatives (cf. Figures 2 and 3). This progression is attributed to the symmetrical long-axis breathing mode of the perylene core which is strongly coupled to the long-axis polarized lowest singlet transition. In the chlorinated compounds, the planar perylene core has been twisted into a "lock-washer" codiguration.22 The band broadening displayed in Figures 2 and 3 can be understood qualitatively in terms of a coupling between the chlorine outof-plane bendmg vibrations (ca. 180-390cm-')and the long-axis breathing vibration (1400 cm-I) through the van der Waals repulsions between the chlorines and their opposing motions in the aromatic stretching vibration. Also,lowering of molecular symmetry apparently lifts a very weak or forbidden electronic transition up to appreciable intensity in the spectrum of CLDMP: the bands at 427 and 408 nm (1091-cm-l vibrational separation) of Figure 2 simply do not fit into the vibrational progression of the first singlet system. To gain a fuller understanding of these absorption spectra, the first three moments of the dipole strength of the transitionm have been calculated in frequency space, as is appropriate for understanding vibrational couplings. The first moment is just the dipole strength
-
DS = ( m ) 2 = c I ( e / X ) dX = c I ( e / v ) dv = cAXZc/X
(1)
The last variant expression in (1) is suitable for calculating the dipole strength with any wavelength recording digitized spectrometer. With c in the usual M-' cm-' units, c = 9.185 X
13/11
- (12/ZJ2
(3)
where Z3 = &/A3 and Z2 and 1' are defined in eq 2. The values obtained for these moments (Table 11) show remarkably small differences among the three species DXP, CLDMP, and C18DMP. The torsion of the perylene plane by chlorine substitution leads to only modest reductions (10-2096) in the So-S, dipole strength. From the assumption that the dipole strength is equal for absorption and fluorescence in mirror image, the radiative lifetime for DXP is calculated to be 4.0 X 1V s in chlorobenzene. The lifetimes of the chlorinated dyes in chlorobenzene are likewise estimated as 5.6 ns (C14DMP) and 5.2 ns (CIRDMP). Note that the dipole strength-weightedband center frequencies should not agree with the observed absorption peak frequencies and that the difference (v- - vDswec) is a significant parameter characterizing the asymmetry in the band shape. The value of this parameter increases from lo00 cm-l in DXP to 1550 cm-' in C4DMP and to 2000 cm-' in C18DMP, suggesting that relatively more intensity is shifted into the vibrational satellites with increasing chlorination. The relative invariance in the So SI transition properties to even rather radical substitution and s"l distortion does not extend to the triplct-triplet absorption system in the region of lasing action. The triplet wavelength shifts observed upon chlorination will do much to explain the variation of lasing bandwidths and efficiencies among the dyes. 3. ~ ~ ~ S n d ~ ~ T n A b s o r p TheINDO/S t i o r a . results for the lowest singlet and triplet states in DMP, CLJlMP, and C18DMP are shown in Table 111. DMP belongs to the C, point group and thus only transitions from the ground state ( ) to singlet A,, (outsf-plane polarized) and B, (in-plane p o d ) states will be electric dipole allowed. The So SItransition is computed near 22 850 cm-' and is in-plane, long-axis polarized with substantial oscillator strength (f = 1.3). It is principally described as the HOMO ( r ) LUMO ( r * )excitation and clearly corresponds to the first observed absorption band (19000 cm-l; Table I and Figure 1). There is a window region of about 6000-cm-' width before numerous forbidden or weakly allowed transitions to higher lying excited singlet states emerge. At 29450 cm-'lies a B,, state with low oscillator strength (Sa,f = 0.04) and in-plane, short axis polarization that may be matched to a weak ultraviolet transition observed at 370 nm (27000an-',e = 4200) in DXP.I5 A strongly allowed transition, in addition to S, SI, does not appear before nearly 37 OOO cm-l above So (So Si3 270 nm, f = 0.37,long axis polarization). The So SIabsorption band in both chlorinated species is computed at higher energy than the So S1transition of DMP, more so for ClsDMP than for C14DMP. This is in full a c " c with the experimental data (Table I; Figures 2 and 3) although the computed blue shift relative to DMP (-800-1OOO cm-I) is
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7992 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
TABLE ID: Computed Singlet (S, .ad Q
D r V w b
-
S,) and Triplet (S,
-
T,) Excitatioa Eaergieu (E, c d ) and Odlhtor S&" (f) far DMP, O M P ,
DMP stateC Sl(B,) Sz(A8)
S,(B,) S4(Au) S,(B,)
Sadrai et al.
CldDMP
ClaDMP
E
f
E
f
22850 (22 100) 29050 (28600) 29450 (29000) 30350 (30300) 30400(30300)
1.281 (0.999)
23 650 (23000) 27750 (27300) 27900 (27450) 30 150 (30 100) 30750 (30400)
0.983 (0.787) 0.017 (0.017) 0.061 (0.055) 0.002 (0.001) 0.001 (0.001)
d 0.035 (0.035) 0.002 (0.002)
d
11 200 16 550 19 600 22 700 22 950
E 23 800 27550 27650 28750 29700
(23 200) (27 100) (27 200) (28700) (29650)
f 0.954 (0.755) 0.015 (0.015) 0.060 (0.055) 0.002 (0,001) 0.001 (0.001)
11 400 16 750 19 900 22 650 22 950
aBoth SECI and TDHF (parentheses) data from INDO/S calculations are reported for each excited singlet state. bOscillator strength from S, to any triplet state is zero due to orthogonality of the spin functions. 'Schoenflia symmetry labels for the electronic states classified according to the C, point group, appropriate for DMP. dOscillator strength in the electric dipole approximation is identically zero for symmetry reasons.
TABLE iV: Triplet Spectroscopic Propeftiea and Wbg Range of Perykaetetr8arboxyMc Acid D h k h a d 9,l&~chbroanthr8cene dye DXP DBPI~ CI,DMP Cl8DMP
1I3
2
hTmO(8)
XTl/ZC
excitation mode
519 (47 100) 520 (46000) 525 (30000) 580 (30000)
529 530 540-606 618
C14H8C12T-T transfer C14HloT-T transfer direct direct
427 (36300)
435
'nm. bM-l cm-I. JShoulder.
-
- -
-- -
-
-
-- -
lasing range 569-585, 575 (max), intermittent to 605 nm
C6H6
lased in CHCld 575-620, 595 (max) no lasing superfluoresced
C~HSCI C~HSCI CH2C12
- of half-peak triplet absorption (nm). dData from ref 13. e N o wavelength given, ref 18. .,- is the red-shifted wavelength
overestimated compared to the experimental value (-200 cm-I). We note that most of the intensity in the So S1 transition is preserved upon chlorination, but also that So S2and So S3 transitions in the structurally distorted species have increased computed intensities relative to DMP. So S2was symmetry forbiddm in DMP but has gained weak allowadness as a primarily out&-plane potarizedtransition (f 0.015). The predominantly short axis polarized transition to S3roughly doubles its intensity (f 0.06) in CbDMP and C18DMP. Both S2and S3have moved to lower energies and hence closer to SI (-4000 cm-') but they are only separated from each other by about 200 cm-'.The additional absorption observed at 427 nm (23400 cm-')and 408 nm (24500cm-')in CIflMP (Figure 2) must be associated with transitions to one or both of thae excited states. The experimental separations between peak maxima in the two band systems (427 nm - 522 nm = 4250 an-')and the computed S2-S1 (27750cm-l - 23 650cm-'= 4100 cm-')or S3-SI energy differences (27900 cm-' - 23 650 cm-'= 4250 cm-l) are virtually identical. The So S3transition is computed approximately 4 times more intense than the S, S2transition, however. It thus appears likely that the 427-nmpeak contains both electronic transitions to S2and S3with the short axis polarized So S3transition being by far the major intensity contributor. The higher energy 408-nmpeak is vibronic in origin according to this assignment, probably corresponding to a fundamental excitation of a C-C stretching mode. These two additional electronic transitions are predicted in the same spectral region of C18DMP as well but they are not cleanly resolved in the experimental spectrum. However, careful inspection of F w r e 3 does indicate the presence of a small shoulder near 435 nm, as expected from comparison of the computed data for ClsDMP with that of C1,DMP. We tentatively assign this band as the predominantly short-axispolarized S, S3transition. The particular virtue of the TDHF method is its ability to generate reliable spectral response properties, such as absorption intensities. Not only the absolute intensity values but also their relative distribution among the various electronic absorption bands are generally superior in TDHF relative to SECI. We were thus hoping that the results from the TDHF calculations could help firm the assignments of the 427/408-nmpeaks in CbDMP. Inspection of the data in Table 111shows a duction in the intensity of the So SIband. However, the energetic separations between the So S2and So S3 absorptions and their absolute as well
-
solvent DMF
as relative intensities from the TDHF calculations are very similar to the results obtained from the SECI calculations. These data thus support an interpretation of the 427-nmpeak as the combined excitation to S2and S3with the 408-nmpeak associated with an excited-state vibration. A large window region (-8000 cm-')is also present between the two lowest triplet states in DMP, and inspection of the wavefunctions shows that TI and T2 are the triplet analogues of the S1and S, states. The computed position of the low-lying TI state (10 200 cm-')is in very good accord with the measured value of 9600 & 700 cm-I for the DBPI deri~ative.'~The lowest triplet state in C14DMPand in C18DMP is computed higher in energy (1000-1200cm-')than that of DMP, following the pattern established by the lowest singlet states, but the window region to T2damases in magnitude to a.SO00 cm-'.The sequence of states in the triplet manifold above T2 differs from that in the singlet manifold for all three DMP species. The density of triplet statca increases quite rapidly at higher energies; for example, 12 triplet states are computed for DMP in the region 24000-30 OOO cm-' above So. 4. Tripbt-TMpled Abmption. The conditions of measurement and the key parameters of the T-T absorption bands are given in Table IV. The dye solutions were usually deoxygenated to extend the lifetimesof the pulsepduced triplet states well beyond the time interval for fluorescence overload of the detecting photomultiplier. Aerated samples3swere also examined to obtain oxygen triplet quenching constants.14 F ' i i a series of quantitativetransmittance changesare obtained by repeated single flashes made with succdapivcwavelength Settings of the detector monochromator. These data yield immediately the quantity, AA(X) (At4 = Ae AC1= (*- 9)ACI), extraplated back to pulse initiation, evaluated here every 5 nm and at selected absorption maxima and minima. The change in tnrnsient concentration AC+ was then calculated taking into aocount the following considerations. In every cdcctcd transient spectrum there are two or more isosbestic points (+= tS, Table V), so that some local values of the triplet extinction coefficient become known from the local absence of absorption transients. One must connect these isasbestic points to interpolate into a nonisocbestic region and then determine the concentrations in that regiax This bridging or interpolation proteas was greatly facilitated by the absence of intense vibrational structure in the
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 7993
Lasing Action in Perylene Derivatives TABLE V Shgk-Triplet ISO&S~~C Points U d ta tb Dct-tloa -
dye DXP QDMP ClaDMP
2
of Triplet-Triplet A a e o r p t i ~
isosbtstic points' (8) 543 (11 400) 530 (27300, 533 (22300) 405 (8600)
519 (46700) 490 (22300j 410 (9000) 393 (4000)
triplet bands of the chlorinated species. Absence of fine structure was particularly conspicuous in the spectrum of Cl,DMP, where the triplet band extended more than one of the usual vibrational intervals beyond the shoulder of the singlet absorption band without showing structure (Figure 3). This was the simplest case and the molecule with the largest absorption transients, obtained without use of a triplet sensitizer. For C18DMP we used the two relatively remote isosbestic points (Table V) and combined these with the absorbance changes at 520 and 450 nm to tit a second-order equation in *(A) and a single concentration change to all four points. This procedure gave a concentration of ACT = 6.44 X lod M from a starting ClsDMP (singlet) concentration of 2.47 X M. The mathematical stability of this system was demonstrated by using a linear interpolation between the isosbestic points and obtaining only a small change in the value of ACT. C4DMP exhibits vibrational structure in both singlet and triplet transitions (Figure 2), but here five isosbestic points facilitate the analysis (Table V). In one sense these points make up a crude spectrum of *(A) but a finer analysis complete with a value of CTturns out to be difficult. The singlet e&) and triplet *(A) spectra do show sufficient vibrational fine structure to make the assignment of CT imprecise. Two limits can be set on CT by showing that negative t values are obtained outside one limit and that an implausible connection of the long wavelength triplet absorption (A > 530 nm) to the shorter wavelength absorption occufs outside the second limit. The connection of isosbestic points is indicated in Figure 2. These interpolation procedures give 6 X loF7M < CT C 1.5 X lo4 M. Hence, the long wavelength values of *(A) for C14DMP are, at best, semiquantitative. The spectrum (Figure 2) shows unambiguously the descending long wavelength tail of d A ) , which interacts strongly with lasihg action for this dye. C14DMPhas a aflof 85% and a much lower direct triplet yield than ClsDMP (afl= 0.55). As expected, smaller absorption transients and a smaller concentration change were hence obtained. In addition, C14DMP was pumped with the 450-nm output of the dye laser at 5-8 mllpulse, much less than the 23 mJ/pulse from the 308-nm excimer pulse pumping C18DMP. The decrease in photon flux applied to C14DMP also contributes to the small concentration change observed with this dye. Since the triplet state of DXP could not be produced by direct excitation, it was formed by T-T energy transfer from 9,lO-dichloroanthracene (2). 2 has a triplet energy of 40.2 kcal/mol (equivalent to 711 nm or 14000 c ~ n - l ) ?higher ~ than both the reportedtriplet energy of perylene, 36 kcal/mol, and that measured for DBPI (27.5 kcal/m0l).l3 The triplet-triplet absorption spectrum of 2 (Figure 4), obtained at a concentration of C = 1.0 X lo4 M, resembles that of anthracene with a strong, narrow absorption peak near 430 nm and only featureless absorption in the 500-600 nm region of interest. The peak triplet extinction is = 36 300 at 427 nm. 2 is itself a fluorescent material with a "or-image spectrum (Figure 4) and af,= 0.55 f O.lO?' In the transfer experiments, this singlet fluorescence dies away well before the triplet-triplet energy transfer to the perylene derivative is completed. The maximum concentrationchange for pure 2 was ACT = 8.74 X l p M in a solution with C,= 1.05 X 10-4 M. Pure solutions of 2 in chlorobenzene showed a close progression of three isosbestic points at 405, 393, and 363.5 nm (Table V). 2 does share with anthracene one serious disadvantage as a sensitizer for triplet absorption measurements, namely the relatively featureless triplet absorption from 450 to 550 nm (Figure 4). It is essential to eliminate this more rapidly decaying triplet background from the acceptor triplet spectrum.
497 (42100) 440 (6300) .
425 (8000)
380 (2000)
364 (6100)
e 4. Absorption (-) and fluorescence (---)spectra of 9,lO-dichloroanthracene (2). The triplet extinction spectrum is shown in large dashes (---). m
TABLE VI: Rate Con~tmtsfor Triplet Qwnching
donor CWMP ClaDMP 2
quencher O2 (air satd)
02 (air satd) DXP
k1' 2.7 X 106 2.0 X 106 15.7 X lo3
k,b 1.7 X lo9 1.2 X lo9 18 X 1@
Pseudefirst-order rate constant (s-I). Sccond-order rate constant (M-*s-l), derived from d(CT)/df = k 2 [ 0 2 ] [ C & Pure DXP as control gave no observable absorption transients under d h t Punpingwith 97-ml pulses Of 351-nm radiation. This excluded false abscnption transients arisii from secoI1(IBry effects such as local heating and refractive index gradients. Maximum change in sensitized triplet concentration, ACT, for DXP occurred at 20 ps, The triplet extinction values (q)from 480 to 620 nm were obtained by first noting the locations of the ~ k t i points c (543, 519, and 497 nm; Table V). The region between the isosbestics at 519 and 497 nm has a relatively featureless triplet absorption region covering a peak-to-trough-to-peak singlet a b sorption region. We obtained a best fit with concentrations of G(2) = 1.0 X 10-4 M,G(DXP) = 7.45 X 106 M, andD(A m .C ), = 8.7 X lO-' M. The triplet concentration can be adgned within &lo% in this region and the raw transmittances processed accordingly. It was not possible to obtain reliable data for the acceptor triplet absorption in the region of strong donor triplet absorption below 490 nm. Representative triplet absorption decay curves (for C18DMP) are shown in Figure 5 and derived quenching rate constants are shown in Table VI. The pseudo-fmt-order triplet quenching rate for C18DMPin air-equilibrated chlorobenzene ([O,] = 1.6 X lW3 M at 25 OC and 1 atm) is 2.0 X lo6 s-l, giving a second-order rate constant of 1.2 X 109 M-' P I . This is approximately the value for a pure diffusion controlled reaction (k, 5 X lo9 M-I s-l 1. 5. cplcuhfed Si S, d TI T, Absorpbiol& AS indicated earlier, the INDO/S calculations predict a large number of electronic states at higher energies. In DMP, TI has B,symmetry and therefore any strong TI T, absorption must be in-plane (bJ, preferably long axis polarized, thus terminating in a higher triplet state (T,,) of A, symmetry. The open shell S E I calculation predict '4 states for DMP at the following energiea (cn-') above TI (oscillator strengths in absorption from TI are in parenthcs&: T2 at 9600 (0.008); T3 at 10900 (-0.0); TSat 15 200 (0.007);
-
+
+
-
7994 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 a751
Sadrai et al.
. I
w
E, cm-l
transition'
f
X, nm
DMP U
a
.ooo
-
.20 v w
z
a
-
-\
i
C
t M ICROSECONGS
Figwe 5. Pseudo-first-order decay curves of the transient long wavelength triplet absorption of ClsDMP in dichlorobenzene. (a) The solution is aerated ([O,] = 1.6 X lo-)M) and gives a first-order rate constant kl = 2.0 X 106 s-I. (b) The solution is deoxygenated to not more than 5% of air-equilibrium O2and gives a firstsrder decay constant kl = 8.8 x 10' s-l. The improved signal-to-noise ratio of this trace represents the
--
----
SI (ref)
SI SI SI SI SI
s6 SI0
-------
SO
-
-
s26
T6
TI T7 TI Ts
conditions under which T-T absorptions were measured.
T6 at 18800 (0.11);Ts at 20300 (1.34);Tg at 20800 (0.18);Ti6 at 27400 (-0.0). Transitions to three states in the range 19000-21 OOO cm-' above TI thus carry appreciable oscillator strengths. The transitions should be preceded and s u d d by spectral -window" regions of substantial widths. The central, long-axis polarized transition at 20 300 cm-'has by far the largest intensity, and its vibrational envelope will probably hide any flanking electronic transitions of considerably less intensity. The observed T-T maximum of DXP in chlorobenzene occurs near 520 nm (19200 cm-I) with a secondary peak at 567 nm (17600 cm-I), whereas the maximum T-T absorption of DBPI is in the range 495-505 nm (20200-19 800 cm-I) with a very large extinction coefficient (e > 60000 M-' cm-').I3 These experimental data are in apparent agreement with the computed data, although no signs of several absorbing triplet states were reported for DBPI. A triplet spectrum similar to ours has recently bcen reported for a bis(alky1) derivative of DMP.3s We would expect that the absorbing triplet states in C1,DMP and ClsDMP have their parentage in the absorbing triplet state(s) of DMP, but the lowering of molecular symmetry in the chlorinated species complicates matters. There are many triplet states with absorption intensity from TI computed in the spectral regions of observed maximum absorbance. For example, in CWMP there are four transitions computed in the region 18 5 W 2 0 300 cm-l with oscillator strengths larger than 0.1. The mmt intense of these transitions is computed at 18 900 cm-'(f = 0.34), whereas the observed maximum occurs at 18 200 cm-'with a prominent long wavelength shoulder from 550 to 600 nm. In ClsDMP, the most intense triplet-triplet transition is computed at 18650 cm-' (f = 0.38)and observed at 580 nm (17250 cm-I). Comparing to the values given above for DMP (20 300 cm-',f= 1.34),the calculations for the chlorinated, nonplanar species thus reproduce the observed trend toward red-shifting of the maximum TI T, absorption with a concomitant reduction in intensity relative to the planar species. Considering unlrnown facton such as the actual geometry of C4DMP and the specific effects of the polar solvent-solute interactions on state energies and intensities, the overall agreement in trends is highly satisfactory. We also lack any knowledge of the Franck-Condon factors that will govern the detailed experimental band shape and the local extinction coefficients for the several absorption processes possible. Taken in this light, the triplet calculations provide guidance indicating that several molecular triplet states may overlap the desired region of useful lasing. The triplet bands actually observbd arc in reasonable
SI1 SI,
No further SI
TI TI
T9 Tio
s7
SI0
SI1 SI3 No further SI
TI TI
T6 7'7
TI TI
T8
TI
23 650 7 300 10 100 10600 13300 20 600
423 0.98 1370 0.22 990 0.36 943 0.34 752 0.06 485 0.06 S, transitions below 26 OOO cm-' 18 500 541 0.21 18900 529 0.34 19550 512 0.12 20 250 494 0.11 21 200 472 0.06 ClsDMP
SI (ref)
6s'
SI SI SI
438 1.28 1087 0.37 885 0.43 763 0.63 493 0.08 460 0.07 S, transitions below 28 000 cm-I 18 800 532 0.11 20 300 493 1.34 20 800 481 0.18 CldDMP
SO
TI
22 850 9200 1 1 300 13 100 20 300 21 750
-
T9 TI0
23 800 7 100 7 350 9 700 10 500 12 400
420 0.95 1409 0.10 1361 0.07 1031 0.35 952 0.34 807 0.08 S, transitions below 25 000 cm-' 18 100 552 0.08 18650 536 0.38 19 450 514 0.10 19 850 504 0.21 20 850 480 0.06
a Transitions listed include only those having oscillator strengths f > 0.05 and energy less than 25000 cm-I. The state indices are a simple ranking in terms of increasing energy within each manifold. The So SItransition is listed for reference. *Oscillator strength.
-
accord with the most intense of the calculated transitions. We conclude that triplet concentrations can become large in these perylene derivatives and triplet-triplet absorption can interdict lasing action. The higher SI S,transitionscanoccur as soon as a potentially useful lasing population is generated. Although several excited singlet transitions could overlap in the lasing region of interest, excited singlet-singlet absorption does not represent a serious problem in these perylene derivatives according to the results of the electronic structure calculations. The computed SI S, spectrum is remarkably void of strong absorption in the 20 OOOcm-l region above SI,despite a high overall density of excited singlet states. In DMP, for example, a total of nearly 30 singlet states are computed in the region 6000-25 000 c m - I above SIbut only five transitions from SIcarry oscillator strength above 0.05, viz., SI ss at 9200 cm-',f = 0.37;SI SI,,at 1 1 300 m-', f = 0.43;SI SI1at 13 100 cm-',f= 0.63;SI S19at 20300 cm-',f= 0.08;SI Szlat 21 750 cm-I; f = 0.07. The tetra- and octachlorinated species follow the pattern set by DMP closely with the possibility of strong absorption from SIpredicted only in the near-infrared (7000-11OOO cm-I). The excited singlet spectrum should be transparent in the lasing region sincc no SI S, valence transitions with oscillator strengths larger than 0.1 are predicted from the near-infrared up to 25 OOO cm-' above SI.Certainly, low-lying Rydberg orbitals can be expected in this high-energy region (-4OOOO cm-'above So) but transitions to these diffuse orbitals are expected to be weak. The measurement of higher SI S, transitions and even their extinction coefficients by twophoton, two laser jet spectroscopy is a well developed art even for
-
-
--
-
--
-
-
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 7995
Lasing Action in Perylene Derivatives large However, this technique is likely to become exceedingly difficult in the region of greatest need, precisely where SI Sogain is expected. Thus the molecular transition calculations undertaken here become especially sisnifcant for mapping interfering higher singlet-singlet absorption bands, a potential trouble we can now fortunately dismiss for this class of chromophores. The computed absorption data from SIand TI are summarized in Table V for the three DMP species. 6. L a s i i Performance. We see immediately from Figure 1 that DXP shares with DBPI13 the desirable property of having the strong TI T, absorption shifted into near-coincidence with the So SI absorption band. This placement leaves the fluorescence and lasing region relatively free from Q-spoiling by rising TI T, absorption, though it is a less complete solution than Pavlopoulos and Hammond's special triplet placement.@ In addition, SI TI intersystem crossing is highly improbable for these two unhalogenated molecules, so the location of TI T, bands is of secondary importance.. Both are expected to be efficient laser dyes even under flashlamp excitation with pulse widths on the order of a microsecond (vs 5-10 ns for 2XYAG excitation). The broad band lasiig of DXP can be understood in these tenns. As soon as the laser is tuned beyond the laser-inactive 0' 0" fluorescence region, strong output (10% lumped energy efficiency) and steady emission from 569 to 585 nm is obtained. The high lasing efficiency is an actual impediment to exploiting the full wavelength lasing band of this dye unless the dye laser has unusual optical selectivity against parasitic oscillations. The intermittent laser pulses of red light from 586 to 605 nm seemed to occur only when shorter wavelength runaway parasitic oscillations did not occur. It is a common belief that dyes with small Stokes shifts do not make good laser dyes, but that is an oversimplification inapplicable to materials like the unchlorinated perylenes, where the strong vibrational progressions give efficient lasing from both the 0' 1" and 0' 2" components. The more controllable lasing behavior of CbDMP in the region 575-620 nm is now understandable in terms of the encroachment of the TI T, band (Figure 2). This overlap of a triplet state, produced in significant yield, does still allow short-pulse lasing, as from the 6-11s pulse excitation of the 2XYAG pumping laser. However, the efficiency is at least an order of magnitude lower than that of DXP. In this situation, parasitic oscillations are suppressed and the wavelength limit of substantial gain coupled with lower TI T, absorption at 620 nm can be reached. ClsDMP does not lase because intersystem crossing is large (alsc 0.40)and the strong triplet absorption is red-shifted well beyond the fluorescence band. Ford and Kamat considered the lasing potential of DBPI and suggested the need for a quencher to remove triplet states.13 This may be unnecessary given the exceedingly high fluorescence quantum yield (99%) of both DBPI and also DXP, plus the lack of strong TI T, absorption in the lasiig region. DBPI has shown weak lasing action under pumping with a very high power N2 laserI8 (perylene materials actually have a pronounced absorption minimum near 337 nm, the N2laser output line).I5 By contrast, CbDMP needs a quencher to perform. The first criterion for a quencher is that it should quench with a reciprocal rate constant shorter than the duration of the exciting pulse. Triplet lifetime observations on CbDMP in chlorobenzene equilibrated with ambient air gave an exponential decay time of t l l z = 0.693/k1 = 2.6 X lO-'s. We note that this quenching reaction is too slow to reduce triplet population signifcantlyduring a 6 4 s duration of a typical 2XYAG driving pulse. One could equilibrate a flashlamp laser solution with pure O2at 1 atm and further improve the quenching situation to tl12= 5.0 X s. The stability of these dyes to oxidative conditions makes it possible to consider this unusual procedure. In contrast to the demonstrated aggregation of DBPI,I3*l8we believe that DXP is relatively free from aggregation and 'excimer-like" effects. It is more efficiently blocked on both sides against close approach of a second molecule. No shift toward a red color is noted as concentration is raised with constant concentration times optical path length in DMF or chlorobenzene up to M concentration.
-
--
-
-
- -
-
-
-
Concluding Remarks We share with other investigat~rs~~J~ the belief that the basic
perylene3,4,9,104etracarboxylic acid d i i i d e chromophore may be utilized to give laser materials superior in power output, tuning range, and light stability to the best of the rhodamines, notably rhodamine 6G. The perylene chromophore can be adequately solubilized either by well-selected bulky imide groups or by ring-warping the perylene backbone with bulky 1,6,7,12 sub stituents. We have reported on a methanol-soluble material and a suitably hindered and twisted but nonchlorinated perylene derivative!' If the solubilizing substituents are no longer chlorines, the lowered cumulative heavy atom effect may reduce the undesirable intersystem crossing yield to acceptable values, and the triplet location may also satisfy Pavlopoulos and Hammond's criterion.40 We are currently exploring new synthetic variants which may bring the parent twisted chromophore into the preferred hydroxylic laser solvents without compromising either the chemical stability and the strengths of this unusual chromophore or the relative freedom from triplet absorption overlap with the lasing region. The triplet-triplet absorption measurements were completed before the electronic structure calculations were undertaken. For the excited singlet-inglet manifold we have advanced theoretical calculations for absorptions of vital concern and very difficult observation. Calculations of this type and complexity should become a major tool for prediction of new phenomena and the success or failure of molecular dye designs.
Acknowledgment. We gratefully acknowledge financial support by a succwion of grants from 3M Corporation. The concluding stages were supported by the U.S. Army Research Office (Grant No. DAAL03-89-K-0168). A sample of Paliogen Red L. 4120 dye (C.I. 71130$2 alias DMP) received from Mr. J. Dayan of BASF-Wyandotte Corp., Parsippany, NJ, was very useful in the synthesis of the chlorinated derivatives. We thank Professor M. Littman of the Electrical Engineering Department, Princeton University, for his essential help with the lasing measurements. Registry No. Ib, 76372-76-4;Id, 106342-00-1; le, 117685-28-6;If, 128-65-4;lg, 117685-27-5; 2, 605-48-1;Olr 7782-44-7;chlorosulfonic acid, 7790-94-5;iodine, 7153-56-2.
References and Notes (1)Kardos, M. German Patent 1913,No. 276,956. (2)Friedlaender, P. Forrschr. Teerfarbenfabr. 1924,12, 493; 1926,14, 484. (3)Gram, F. Bundesrepublik Deurschland Offenlegungsschnf?1973,No. 2,139,688. (4)Graser, F.; Haedicke, E. Liebigs Ann. Chem. 1980, 1994. (5) Graser, F.; Hadicke, E. Liebigs Ann. Chem. 1984,483. (6)Hadicke, E.; Graser, F. Acra Crysfallogr. 1986,C-42, 189. (7)Popovic, Z.D.; Loutfy, R. 0.; Hor, A.-M. Can. J. Chem. 1985,63,134. ( 8 ) Tang, C.W. US.Patent 1981,No. 4,281,503.; Appl. Phys. Left. 1986, 48, 183. (9)Panayotatorr, P.; Parikh, D.; Sauers, R. R.; Bird, G.R.; Picchowski, A,; Husain, S. Solar Cells 1986, 18, 71. (10) Lukac, I.; Langhals, H. Chem. Ber. 1983,116,3524. (11) Rademacher, A.; Markle, S.; Langhals, H. Chem. Ber. 1982, 115, 2927. (12)Langhals, H. Chem. Ber. 1985, 118, 4645. (13) Ford, W. E.; Kamat, P. V. J . Phys. Chem. 1987,91, 6373. (14)Ford, W. E. J . Phorochem. 1986,34,43. (15) Sadrai, M.; Bird, G. R. Opr. Commun. 1984,51,62. (16)Lidholt, L. R.; Waldimiroff, W. W. J . Opto-Electron. 1970,2, 21. (17) Pavlopoulos, T. G.Opr. Commun. 1981, 38, 299. (18) El-Ebeid, Z.M.; El-Daly, S. A.; Langhals, H. J. Phys. Chem. 1988, 92, 4564. (19) Reisfeld, R.; Brusilovsky, D.; Eyal, M.; Muon, E.; Burshstein, Z.; Ivri, J. SPIE Vol. 1989,1182,230. Paper presented at the French-Israeli Workshop on Solid State Lasers, December 12-14, 1988,in Jerusalem, Israel. (20) Sauers, R. R.; Husain, S.N.; Picchowski, A. P.; Bird, G. R. Dyes Pigm. 1987,8, 35. (21)Wagner, H. J.; Loutfy, R. 0.;Hsiao, C. K. J . Mater. Sci. 1982,17, 2781. (22)Sadrai, M.; Bird, G. R.; Potenza, J. A.; Schugar, H. J. Acra Crysraltogr. 1990,C-46,637. (23) Seybold, G.;Wagenblast, G. Dyes Pigm. 1990,11, 303. (24)Mow, R. A.; Shen, S.;Hadel, L. M.; Kmiccik-Lawrynowicz, G.; Wlostowska, J.; Krogh-Jespersen,K. J . Am. Chem. Soc. 1987,109,4341.A Lambda Physik FL 2002 Dye laser pumped by a Lambda Physik EMG 101 excimer laser has been added to the apparatus described in this reference.
7996
J . Phys. Chem. 1992,96, 7996-8001
(25) Littman, M.; Metcalf, H. Appl. Opt. 1978, 17, 2224. (26) Krogh-Jmpersen,K.; Westbrook, J. D.; Potenza, J. A,; Schugar, H. J. J. Am. Chem. Soc. 1987,109,7025. Westbrook, J. D.; Krogh-Jespersen, K.ESPPAC, an electronic structure program for the calculation of excited stare properties; Rutgers University: New Brunswick, NJ, 1989. (27) (a) Ridley, J.; Zerner, M. C. Theor. Chim. Acra 1973.32, 1 1 1. (b) Edwards, J. D.;Zerner. M. C. Theor. Chim. Acra 1987, 72, 347. (28) Linderberg, J.; Ohm, Y. Propagators in Quantum Chemistry; London: Academic Press, New York, 1973. (29) Krogb-Jespcrscn, K.; Ratner, M. A. Theor. Chim. Acta 1978,47,283. (30) MMX(89) may be. obtained from Serena Software, c/o K. Gilbert, P.O.Box 3076, Blwmington, IN 47402. (31) The twisted nature of a 1,6,7,12-tetrachlorinatedperylene is demonstrated by the X-ray structure of the NJV-di-n-butyl derivative: Iden, R.; Seybold, G.; Stange, A.; Eilingsfeld, H. Forschungsberichr T 84-164; Bundesministerium fuer Forschung und Technologic, 1984; see also ref 23. (32) Cosmo, R.; Hambley, T. W.; Sternhall, S.Tetrahedron Leu. 1987, 28, 6329. (33) It is not possible to obtain direct information about torsional energies in SIby molecular mechanics modeling in MMX, but presumably the torsional
potentials for the pendant groups are very similar in So and Si. (34) The fluoreacein dianion (a"= 91%)and the decarboxylatedhomolq 9-phenylfluorananion (0" = 20%) stand in a similar relationshipwith regard to internal conversion promoted by the released torsion of a conjugated aromatic group: Lindquist, L.; Lundeen, G. W. J . Chem. Phys. 1966,44,1711. (35) IUPAC Solubility Dara Series: Battino, R., Ed.;Pergamon Press: New York, 1981; Vol. VII, 312. (36) Hadley, G. S.;Keller, R. A, J . Phys. Chem. 1969, 73, 4351. (37) Engel, P. S.; Monroe.,B. M. Complications in Photosensitized Reactions. In Advances in Photochemistry; Pitts, J. N., Hammond, G., Noyes, W. A., Eds.; 1971; Vol. VIII, p 245. (38) Lobmannsroben,H. G.; Langhals, H. Appl. Phys. 1989, B-48,449. (39) Ito, M.; Ebata, T.; Mikami, N. Annu. Rev. Phys. Chem. 1988, 39, 123. (40) Pavlopoulos, T. G.; Hammond, P. R. J . Am. Chem. Soc. 1974, 96, 6568. (41) Bird, G. R. Unconventional Approaches to Efficient Loser Dyes; Paper N-6at Laser 90, San Diego, CA, 1990. (42) Colour Index, 3rd ed.; Society of Dyers and Colourists: Bradford, Yorkshire, England, 1971; Vol. IV, Chemical Constitutions, see p 4590.
Effect of Sdvent on Nonradiatlve Processes in Xanthene Dyes: Pyronln B In Alcohols and Alcohol-Water Mlxtures Yavuz Onganer and Edward L. Quitens* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409 (Received: August 13, 1991; In Final Form: May 18, 1992)
The fluorescencelifetime rf and quantum yield & of pyronin B in water, n-alcohols, and alcohol-water mixtures were measured as a function of temperature. The nonradiative rate constant k, was calculated from rf and &. At 25 O C , k, was qual to (4.1 f 0.2) X lo8 s-l in water and ranged from (1.3 f 0.3) X lo8 to (3.9 f 0.4) X lo8 s-l in alcohols and from (3.6 f 0.7) x lo8 to (6.0 f 0.5) X lo8 s-I in mixtures. The natural logarithm of k, increased linearly with the solvent polarity parameter ET(30) but with a steeper slope for alcohols than for mixtures. The nonradiative activation energy E,, which was obtained from Arrhenius plots of k,, was qual to 4.0 f 0.8 kcal mol-' in water, 5.1 f 0.3 kcal mol-' in alcohol-water mixtures, and 6.0 f 0.3 kcal mol-I in alcohols. The dependence of the nonradiative rate on solvent polarity can be explained by a model involving a planar-to-pyramidal change at the xantheneamine bond. On the basis of this model, the dependence on solvent polarity is due to specific solute-solvent interactions at the amino group.
Introduction Although the photophysics of rhodamine B and related xanthene dyes in solution has been well studied, the mechanism for &-So internal conversion, the main nonradiative process in these dyes,'V2 is still contr~versial.~ Internal conversion in these dyes is associated with the rigidity of the xantheneamine bond.'.2 This rigidity controls the temperature dependence of the dye fluorescence. For example, the fluorescence quantum yield is unity and independent of temperature when the diethylamino groups are rigidly fixed to the xanthene ring by methylene bridges, as in rhodamine 101, but decreases with temperature when the groups are not constrained, as in rhodamine B.e7 Several models have been proposed to explain the effect of solvent and molecular structure on internal conversion in xanthene dyes: the intramolecular rotation the twisted intramolecular charge-transfer (TICT) model,8-10 and the umbrellalike-motion (ULM) In the intramolecular rotation model, internal conversion is mainly governed by solvent viscosity, whereas in the TICT solvent, internal conversion is determined mainly by solvent polarity. In the ULM model, internal conversion is determined by specific solutesolvent interactions. In this paper, we describe a study of the nonradiative rate of pyrOnia B (PyB) in water, n-alcohols,and alcoholwater mixtures. PyB differs from rhodamine B in that the substituent group at the 9-carbon atom on the xanthene ring is an H atom instead of a carboxyphenyl (PhCOOR) group (Figure l).I**9l7 The goal of this study, in part, was to see which of these models beat explains internal conversion in xanthene dyes. The solvents were chosen 0022-3654/92/2096-7996$03.00/0
to compare with the previous work on the acid form of rhodamine B (FtBH+).18J9The fluorescence lifetime rfand the quantum yield t$f of PyB in the alcohols and alcohol-water mixtures were measured as a function of temperature. The nonradiative rate constant k,, for PyB was calculated from rf and t$fi Activation energies E, were obtained from Arrhenius plots of A two-state kinetic mechanism, which was previously developed in our laboratory to analyze the photophysics of RBH+ in a l c o h o l ~ , l ~ ~ ' ~ provides a mathematical framework to obtain quantitative relationships between k,,, E,, and ET(30). The molecular rationale for these relationships is provided by the ULM model, which attributes the solvent polarity dependence to specific soluttsolvent interactions at the diethylamino groups on the xanthene ring.
Experimental Details w o n i n B (Fluka, Standard grade) showed a single spot on a TLC plate and was used without further purification. The solvents (except water) were distilled and dried over molecular sieves prior to use. Distilled water that was passed through deionizing filters was used. The solvent viscosities were obtained from the literature or measured with a viscometer. Alcohol-water mixtures were prepared so that their viscosities were all equal to 1.4 cP. Values of the polarity parameter ET(30) for pure solvents were obtained from the literature.*O Values of E ~ ( 3 0 )for the alcoholwater mixtura were calculated as described previ~usly.'~ PyB was stored in the dark as a concentrated stock solution ( 2 5 mM) in methanol. Samples were prepared by evaporating 10-20 pL of the stock solution and then redissolving with 5 mL of solvent. The final 0 1992 American Chemical Society
