J. Phys. Chem. 1987, 91, 5192-5795
5792
Molecular Association In Flexlble Dloleflnic Dyes El-Zeiny M. Ebeid* Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt
and Alistair J. Lees Department of Chemistry, University Center at Binghamton, State University of New York, Binghamton, New York 13901 (Received: January 5, 1987; In Final Form: May 12, 1987)
The emission characteristics, solution lifetimes, and molecular association of the methyl iodide derivative of the laser dye 1,4-bis(@-pyridyl-2-vinyl)benzene (P2VBeMeI) together with the isoelectronic dye 1,4-bis(4-pyridy1-2-vinyl)benzene(P4VB) are reported. Both compounds undergo molecular aggregation with subsequent excimer formation. P4VB is characterized by relatively high fluorescence quantum yields and increased solution photostability. The spectral behavior of P4VB is also reported as a function of micellization effect and medium acidity.
Introduction Excimeric emission in many flexible diolefinic systems has been reported both in the crystalline state'-3 and in solution^.^" Cationic species of some diolefinic laser dyes also give excimer-like emission in concentrated solutions7 and in the crystalline state.*-1° Ground-state molecular aggregation in diolefinic compounds has also been previously reported as a precursor of excimer formation. For example, it is known that there is a close contact between aromatic rings and electron-rich moities, e.g., the carbonyl and the oxygens in crystalline diethyl 1,4-phenylenediacrylate1' pyridyl rings in 1,4-bis(fl-pyridyl-2-vinyl)benzene(P2VB)I2*l3 crystals. Conductometric and surface tension techniques also revealed ground-state aggregation in concentrated P2VB-HCl sol~tions.~ In this article we report the molecular association of the methyl iodide derivative of 1,4-bis(P-pyridy1-2-viinyl)benzene(P2VB.MeI) as well as 1,4-bis(4-pyridyl-2-vinyl)benzene (P4VB), using solution
P P V B . Me1
P4VB
(1) Cohen, M. D.; Elgavi, A,; Green, B. S.;Ludmer, Z.; Schmidt, G. M. J. J . Am. Chem. SOC.1972, 94, 6776-6779. (2) Yakhot, V.; Cohen, M. D.; Ludmer, Z. Adu. Phorochem. 1979, 11, 489-523. (3) Ebeid, E. M.; Bridge, N. J. J. Chem. Soc., Faraday Trans. I 1984,80, 11 13-1 122. (4) Sakamoto, M.; Huy, S.; Nakanishi, H.; Nakanishi, F.; Yurugi, T.; Hasegawa, M. Chem. Lett. 1981, 99-102. (5) Ebeid, E. M.; El-Daly, S.A.; Hasegawa, M. Laser Chem. 1985, 5, 309-319. (6) Ebeid, E. M.; Kandil, S. H. J . Photochem. 1986, 32, 384-393. (7) Ebeid, E. M.; Issa, R. M.; Ghoneim, M. M.; El-Daly, S. A. J. Chem. SOC.,Faraday Trans. 1 1986, 82, 909-919. (8) Ebeid, E. M. J. Phys. Chem. Solids 1986, 47, 945-948. (9) Swiatkiewicz, J.; Eisenhardt, G.; Prasad, P. N.; Thomas, J. M.; Jones, W., Theocharis, C. R. J. Phys. Chem. 1982, 86, 1764-1767. (10) Nakanishi, H.; Parkinson, G. M.; Jones, W.; Thomas, J. M.; Hasegawa, M. Isr. J. Chem. 1979, 18* 261-265. (11) Hasegawa, M. Pure Appl. Chem. 1986, 58, 1179. (12) Sasada, Y.; Shimanouchi, H.; Nakanishi, H.; Hasegawa, M. Bull. Chem. SOC.Jpn. 1971, 44, 1262-1270. (13) Nakanishi, H.; Jones, W.; Thomas, J. M.; Hasegawa, M.; Rees, W. L. Proc. R . Soc. London, A 1980, 369, 307-325.
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lifetime and other spectroscopic techniques. The fluorescence and photochemical quantum yields of P4VB are also reported. The excimeric phenomenon in flexible diolefinic molecules, besides its own importance, results in a significant emission quenching and a substantial change in the course of photochemical reactivity compared with dilute solution^.'^^'^ These factors are particularly important in laser dyes.16J7 The laser activity of P2VB-Me1 and P4VB is currently under investigation.
Experimental Section 1,4-Bis(/3-pyridy1-2-vinyl)benzene(P2VB) was prepared as described earlier.3 The methyl iodide derivative, P2VB-Me1, was prepared by stirring (for 12 h at room temperature) an equimolar mixture of P2VB and methyl iodide in acetone. In this reaction, the pyridyl moiety reacts as a nucleophile with methyl iodide giving an N-methylpyridinium iodide product similar to pyridine.ls Unreacted P2VB was filtered off, and the filtrate was left in the dark where reddish crystals separated upon solvent evaporation. The material was then recrystallized from ethanol by slow evaporation in the dark at room temperature. Differential scanning calorimetry (DSC) shows a broad exothermic decomposition band starting at 160 O C followed by a broad endothermic band starting at 260 OC that is assigned to melting of P2VB-Me1 compared with sharp melting of parent P2VB at 232 OC. P2VB.MeI also gives a different I R spectrum and X-ray powder diffraction pattern compared with the parent P2VB. Conductometric titration of P2VB.MeI vs. silver nitrate standard solution showed that isolated P2VB.MeI comprises P2VB and Me1 in an equimolar ratio. Unlike P2VB, the methyl iodide derivative is highly soluble in water, and this is important when it is used as a laser dye. The preparation of the hydrochloride derivative P2VB.HCl has been described earlier.7 Anionic and cationic micelles have been prepared with sodium dodecyl sulfate (SDS; Fluka, puriss) and cetyltrimethylammonium chloride (CTAC; Kodak), respectively. P4VB has been kindly provided by Professor Masaki Hasegawa of Tokyo University. Emission spectra were recorded on a Shimadzu R F 510 spectrofluorophotometer. Fluorescence quantum yields (&) were measured relative to 9,lO-diphenylanthracene (DPA) as a reference standard.19 Photochemical quantum yields (dC)have been (14) Nakanishi, F.; Nakanishi, H.; Hasegawa, M. J. Chem. SOC.Jpn., Chem. Ind. Chem. 1976, 10, 1575-1578. (15) Suzuki, Y.; Tamaki, T.; Hasegawa, M. Bull. Chem. SOC.Jpn. 1974, 47., _.. 210.
(16) Ebeid, E. M.; Sabry, M. M. F.; El-Daly, S. A. Laser Chem. 1985, 5 , 223-230. (17) Ebeid, E. H.; Issa, R. M.; El-Daly, S. A.; Sabry, M. M. F. J . Chem. SOC.,Faraday Trans. 2 1986.82, 1981-1989. (18) Allinger, N. L.; Cava, M. P.; De Jongh, D. C.; Johnson, C. R.; Lebel, N . A.; Stevens, C. L. Organic Chemistry; Worth: New York, 1971; p 742. (19) Morris, J. V.; Mahaney, M. A,; Huber, J. R. J . Phys. Chem. 1976, 80, 969-974.
0 1987 American Chemical Society
Molecular Association in Flexible Diolefinic Dyes
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
5793
12
12
A (nm)
Figure 1. Emission and excitation spectra of P2VB-Me1 in methanol at 20 OC: ( X - X ) emission (X, = 365 nm) and (---) excitation (&, = 415 mol dm-’ and (---) emission (A, = 365 nm) and nm) spectra of (-) excitation (A,,, = 500 nm) spectra of lo-’ mol dm-’.
TABLE I: Lifetimes of Laser Dye Solutions at 20 O C (&. = 365 nm)
dye P2VB.MeI
dil s o h dm-’) A,, nm 414 513
P2VB.HCI P4VB
423 400 426
mol 7,ns
0.68 (3) 4.50 (6) 0.27 (4) 4.50 (6) 0.833 (4) 0.568 (5) 0.575 (5)
concd s o h (lo-’ mol dm-’) A, nm T , ns
513
0.32 (2)
500
0.927 (10)
426
1.009 (9)
measured according to a method that was described earlierS7 Emission lifetimes were calculated from data collected on a Photochemical Research Associates (PRA) System 3000 time correlated pulsed single photon counting apparatus. Samples were excited a t 365 nm with light from a PRA Model 510 hydrogen flash lamp that had been transmitted through a H-10 (Instruments SA, Inc.) monochromator. Emission was detected from the sample at 90° by means of a H-10 monochromator and a thermoelectrically cooled Hamamatsu R 955 photomultiplier tube. The photon counts were stored on a Tracor Northern TN-7200 microprocessor-based multichannel analyzer. The instrument response function was deconvoluted from the fluorescence data to obtain an undisturbed decay which was then fitted by a leastsquares technique. UV-visible absorption spectra have been recorded on a Unicam SP 8000 spectrophotometer. Absorption spectra of concentrated solutions have been recorded on a Bausch & Lomb Spectronic 2000 instrument with a 0.2-cm absorption cell.
Results and Discussion Photoassociation of P2VB.MeZ and P2VB.HCl. Dilute (ca. mol dm-3) P2VB.MeI solutions give molecular emission at 415 nm (Aex = 365 nm) and an excitation maximum a t 370 nm. The emission is red shifted t o ca.500 nm in concentrated (ca. lo-’ mol dm-3) methanolic solutions and yields an excitation peak at 440 nm as shown in Figure 1. The lifetimes of dilute and concentrated P2VB.MeI solutions are summarized in Table I. Two lifetime values are recorded, and the different values are obviously associated with aggregated and nonaggregated molecules. Molecular aggregation of P2VB.MeI has also been confirmed by electrical conductivity measurements. A plot of the specific conductance u (in siemens per centimeter) versus P2VB.MeI concentration yields a break in the curve a t a concentration value
10
I
I
500
400
Figure 2. Effect of temperature on the excimeric emission intensity of 5 X lo-* mol dm-’ solutions of (-.-) P2VB.MeI in D M S O and P2VB.HC1 in (-) glycerol and in (X - X) DMSO.
3.0
3.2 3.4 3.6 1000/T(K-’) Figure 3. Plot of logarithmic excimeric emission intensity at 500 nm vs reciprocal temperature of ( 0 ) P2VB.MeI in DMSO, ( 0 )P2VB.HCI in DMSO, and (0)PZVB-HCI in glycerol.
of ca. 3.75 X lo4 mol dm-3 that is the critical concentration for molecular aggregation.20 The size (dimers, oligomers, high polymers) of the aggregated assemblies is currently investigated by using fluorescence polarization and laser scattering techniques. However, the critical concentrations for the appearance of the excimer emission and/or molecular aggregation is not too high compared with those of planar aromatic compounds.21 It seems that small aggregates are formed whose size is presumably dependent upon concentration. The smooth inflection in the concentration-conductance plot around the break supports this view. The lifetimes of P2VBeHCl solutions are also given in Table I. It is generally observed that excimeric species give rise to short lifetimes, and this is diagnostic of, for example, low molecular interaction, thermal vibrations, photochemical reactivity, and enhanced radiationless processes. Short excited-state lifetimes (2U) Eicke, H. F. Top. Curr. Chem. 1980,87, 1-85. (21) Stevens, B. Adv. Photochem. 1971, 8, 161-226.
5794 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
Ebeid and Lees
I
*
h
(nml
'
\
h inm)
Figure 4. (a) Emission and excitation spectra of P4VB solutions in DMSO: emission (Aex = 365 nm) and (-.-) excitation (Aem = 420 nm) of concentration lo-' mol dm-'; ( X - X ) emission (Aex = 365 nm) and (-) excitation (Aem = 500 nm) of concentration 5 X lo-* mol dnr3. (.-) 5 X lo4, and lo-* (b) Absorption spectra of ( X - X ) 5 X (e-)
(-e-)
mol dm-' solutions in DMSO. were previously reported for excimeric species in crystalline diolefin~.~ The effect of temperature on the excimeric emission intensity is shown in Figure 2. The enthalpy of photoassociation (AHa) has been calculated from the plots between the logarithmic excimeric emission intensity and the reciprocal temperatures' as shown in Figure 3. For P2VB.MeI in DMSO, AHa = -13.8 kJ mol-', a value that is close to that for the isoelectronic dye P2VB.HCl previously estimated as7 AH, = -1 1.0 kJ mol-' in DMSO. Both the excimeric maximum and AHavalues are solvent dependent. For P2VB.HC1 in glycerol, the excimeric emission maximum occurs at 490 nm and AH, = -32.2 kJ mol-' compared with the emission maximum at 510 nm and AHa -1 1.0 kJ mol-' in DMSO. The increased AHa values in glycerol are due to the added potential barrier due to diffusion limitations. This is also accompanied by a blue shift of the emission maximum toward molecular emission. 4, and 4c of P4VB. P4VB has a relatively high fluorescence quantum yield +f = 0.65 (Aex = 370 nm) in DMSO. The photochemical quantum yield (4,) of P4VB in DMSO has been evaluated as 4, = 0.005 (Aex = 365 nm) and 4, = 0.07 (kx= 337 nm), indicating a reasonable dye photostability. Photoirradiation (Aex = 365 nm) of dilute P4VB solutions causes a decrease in the absorption maximum at 370 nm accompained by a slight increase in absorbance at wavelengths below 300 nm. An isosbestic point is obtained at 320 nm. In such dilute P4VB solutions unimolecular trans-cis photoisomerization is expected as previously reported for similar diolefins.6 In concentrated (ca. mol dm-3) solutions, however, bimolecular processes are expected, giving photoligomers. Molecular Aggregation in P4VB. The effect of concentration on emission, excitation, and UV-absorption spectra of P4VB is shown in Figure 4. In dilute (ca. mol dm-3) P4VB solutions in DMSO, molecular emission is observed at 420 nm (Aex = 365 nm) and an excitation maximum at 370 nrii appears. In concentrated solutions (ca. 5 X lo-* mol dm-3) excimeric emission at 500 nm and an excitation peak at 420 nm are obtained (see Figure 4a). The UV-absorption spectral pattern also changes as a function of concentration as shown in Figure 4b. In dilute solutions (ca. mol dm-3) an absorption maximum is obtained at 370 nm that coincides with the excitation peak at 370 nm. In concentrated DMSO solutions (ca. mol dm-3) an absorption maximum is obtained at 410 nm that coincides with the excitation peak at 410 nm. Both the excitation and absorption spectra at high concentrations are quite different from the corresponding spectra in dilute solutions. This in fact supports the existence of molecular aggregates already formed in the grond state. The size and shape of these aggregates are currently under investigation. Excimer-like emission at ca. 490 nm is also displayed by P4VB crystals (Aex = 365 nm) that is shifted by ca. 3688 cm-' from
A
(nm)
Figure 5. Emission (Aex = 365 nm) and excitation (Aem = 470 nm) of P4VB crystals. The effect of photoirradiation ageing is also shown: (-.-) fresh; irradiated for 20 and (--) 40 min. (e-)
.n.
I I
.-*n a
2'
.-e
-.e
2 .o
b
x
g
.-C
E
.-
.-*
-Z
1.5
0 D
E
w
1.c
c Figure 6. (a) Effect of temperature on the excimeric emission intensity of mol dm-3 solution of P4VB in DMSO. Temperatures at increasing intensities are 45,40, 30, and 20 OC. (b) Representation of the
logarithmic excimeric emission intensity at 500 nm vs the reciprocal temperature for mol dm-' P4VB solutions in (0) DMSO and ( 0 ) equivolume mixture of DMSO + glycerol. molecular emission at 415 nm. Figure 5 shows both the fluorescence and excitation spectra of P4VB crystals. Irradiation ageing (Aex = 365 nm) of the crystals causes a decrease in the excimeric emission intensity indicating a photoconsumption of the excimeric species. The nature of P4VB solid-state photoreaction is not clear. Earlier reportsI3 indicated that, within the series of closely related monomeric molecules P2VB, PSVB, and P4VB having similar molecular dimensions and comparable reactivity in solution, only P2VB photopolymerizes in the solid-state. The effect of temperature on the excimeric emission of P4VB concentrated solutions is shown in Figure 6 . The effect of increasing solvent viscosity on the vlaues of AHa is also shown. The value of AHa changes from -19.3 kJ mol-' in DMSO to -36.0 kJ mol-' in an equivolume mixture of DMSO and glycerol. The emission lifetimes of P4VB solutions are summarized in Table I. The results obtained illustrate that P4VB undergoes molecular association more efficiently compared with P2VB. This presum-
5795
J. Phys. Chem. 1987, 91, 5795-5800
(e),' but in P4VB, protonation does not result in a decrease in E values as shown in Figure 7b. The emission spectrum of acidified P4VB decreases in intensity (A, = 365 nm) and is red shifted compared with parent P4VB as shown in Figure 7a. The decrease in emission intensity in acidic medium is partly due to increased association of protonated P4VB compared with P4VB, as previously reported for P2VB.' Both cationic and anionic micellar media cause significant emission enhancement together with a slight blue shift as shown in Figure 7a. This is brought about by dye solubilization with a subsequent decrease in molecular association.
1 (nm)
A (nm)
Figure 7. Effect of protonation and micellization on (a) emission (Aex = 365 nm) and (b) absorption spectra of 1.7 X mol dmd3P4VB solutions in ( X - X ) water, in (---) lo-* mol dm-) SDS, in lo-* mol dm-3 (..e)
CTAC, and (-) after flushing with HCI gas in water. ably indicates higher planarity of P4VB compared with P2VB. It seems that the lone pair on the heteroatom in P2VB being close to the olefinic double bond causes a slight rotation. This view is also supported by the effect or protonation on the absorption maxima of the P2VB and P4VB compounds. In P2VB protonation is accompanied by a significant decrease in the molar absorptivity
Conclusion The ground-state molecular aggregation and excimeric emission in P2VB.MeI and P4VB dyes are obtained in concentrated solutions. This behavior has a significant effect on both photophysical and photochemical characteristics of both dyes, which have recently shown laser activity.22 Acknowledgment. We thank Professor Masaki Hasegawa of the University of Tokyo for providing P4VB sample, Dr. David Pinnick of the State University of New York for helping in lifetime measurements, and Samy A. El-Daly of Tanta University for helping in experimental work. Registry No. P2VB-Me1, 110144-21-3; P4VB, 110144-22-4. (22) Ebeid, E. M., Sabry, M. M. F., unpublished results.
Extended Huckel Calculatlons on Defect States In the
7r
System of Polyazine
William B. Euler Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881 (Received: November 14, 1986)
The extended Hiickel method is used to calculate the P electronic structure of polyazine, -(N=CH-CH=N-),. The band structure of the polymer is deduced from long-chain model compounds by using monomer molecular orbitals as the basis set. This gives results in agreement with tight-binding calculations. Polyazine is found to have a valence bandwidth of 1.4 eV, a conduction bandwidth of 1.3 eV, and a band gap of 2.3 eV. The effect of a single defect or a double defect centered on either carbon or nitrogen was considered. For single atom defects, whether centered at carbon or nitrogen, one midgap state is created, the band gap is decreased, and the wave function of the new state is delocalized over four to five monomer units. In contrast, when two defects are formed, two midgap states appear, the band gap increases, but the new wave functions are still delocalized over four to five monomers. The various kinds of defects can be formed from one another by simple one- or two-bond shifts in the P system. The importance of these kinds of shifts to charge transport is discussed.
Introduction The field of conducting polymers has attracted a great deal of research.' Progress has been made both in terms of synthesizing environmentally stable and useful materials and in terms of understanding the fundamental physics of the conducting state. Typical examples of polymers that can be doped into a highly conducting regime include polyacetylene,2 p~lypyrrole,~ poly( 1 ) (a) Diaz, A. F.; Kanazawa, K. K. In Extended Linear Chain Compounds; Miller, J. S., Ed.; Plenum: New York, 1983; Vol. 3, pp 417-441. (b) Baughman, R. H. In Contemporary Topics in Polymer Science; Vandenberg, E. J., Ed.; Plenum: New York, 1984; Vol. 5, pp 321-350. (c) Bredas, J. L.; Street, G. B. Ace. Chem. Res. 1985, 18, 309-315. (d) MacDiarmid, A. G.; Mammone, R. J.; Kaner, R. B.; Porter, S. J. Philos. Trans. R. SOC.London, B 1985, 314, 3-15. (e) Frommer, J. E. Acc. Chem. Res. 1986, 19, 2-9. (2) (a) Fincher, C. R., Jr.; Ozaki, M.; Heeger, A. J. MacDiarmid, A. G. Phys. Rev. E Condens. Matter 1979,19,4140-4148. (b) Baughman, R. H.; Moss, G. J . Chem. Phys. 1982, 77, 6321-6336. (c) Baughman, R. H.; Murthy, N. S.; Miller, G. G. J . Chem. Phys. 1983, 79, 515-520. (d) Moraes, F.; Chen, J.; Chung, T.-C.; Heeger, A. J. Synth. Met. 1985, 11, 271-292. (e) Jeyadev, S.; Conwell, E. M. Phys. Rev. B: Condens. Matter 1986, 33, 2530-2539. ( f ) Chien, J. C . W.; Schen, M. A. Macromolecules 1986, 19, 1042-1049.
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anilix~e,~ and p~lythiophene;~ these all have in common the existence of extended ?r systems that can be readily oxidized or (3) (a) Bredas, J. L.;Scott, J. C.; Yakushi, K.; Street, G. B. Phys. Reu. B: Condens. Matter 1984, 30, 1023-1025. (b) Pfluger, P.; Gubler, U. M.; Street, G. B. Solid Stare Commun.1984, 49, 911-915. (c) Bredas, J. L.; Themans, B.; Fripiat, J. G.; Andre, J. M.; Chance, R. R. Phys. Reu. B: Condens. Matter 1984, 29, 6761-6773. (4) (a) MacDiarmid, A. G.; Chiang, J.-C.; Halpern, M.; Huang, W.4.; Mu, S.-L.; Somasiri, N. L.D.; Wu, W.; Yaniger, S. I. Mol. Cryst. Liq. Cryst. 1985, 121, 173-180. (b) Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J . Phys. Chem. 1985,89, 1441-1447. (c) Hjertberg,T.; Salaneck, W. R.; Lundstrom, I.; Somasiri, N. L.D.; MacDiarmid, A. G. J. Polym. Sci., Polym. Lett. Ed. 1985,23, 503-508. (d) McManus, P.; Yang, S.C.; Cushman, R. J. J. Chem. Soc., Chem. Commun. 1985, 1556-1557. (e) Brahma, S. K. Solid State Commun. 1986, 57, 673-675. ( f ) Chiang, J.-C.; MacDiarmid, A. G. Synth. Met. 1986, 13, 193-205. ( 5 ) (a) Tourillon, G.; Garnier, F. J . Phys. Chem. 1983,87,2289-2292. (b) Pfluger, P.; Street, G. B. J . Chem. Phys. 1984.80, 544-553. (c) Tourillon, G.; Gourier, D.; Garnier, P.; Vivien, D. J. Phys. Chem. 1984, 88, 1049-1051. (d) Chung, T.-C.; Kaufman, J. H.; Heeger, A. J.; Wudl, F. Phys. Reu. B: Condens. Matter 1984, 30, 702-710. (e) Davidov, D.; Moraes, F.; Heeger, A. J.; Wudl, F.; Kim, H.; Dalton, L. R. Solid State Commun. 1985, 53, 497-500. (f) Chen, J.; Heeger, A. J.; Wudl, F. Solid State Commun.1986, 58, 251-257.
0 1987 American Chemical Society
