J. Phys. Chem. 1983, 87, 4790-4792
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functions are not degenerate at a geometry where they should be of E symmetry, then the valley in the resulting Jahn-Teller surface may be distorted toward the wrong geometry or may overestimate the barrier to pseudo-ro-
tation. Examples of this phenomenon, discussed above, include trimethylenemethane, cyclopropenyl anion [and the isoelectronic (NH)32+],and cyclobutadiene radical cation.
ARTICLES Fluorescence of Polymerized Diacetylene Bilayer Films John Olmsted I I I
and Marglth Strand
Chemistry Department, Callfornia State Unlverslv, Fullerton, California 92634 (Received: November 15, 1982; In Flnal Form: March IO, 1983)
Monolayer films of 27-carbon-chain-length alkyldiacetylenecarboxylic acid, C14H29C=C-C=CCsHleCOOH, can be polymerized on the aqueous surface by UV irradiation. The resulting blue polydiacetylene is readily transferred onto glass slides to form multilayers. The blue form, which absorbs at 640 and 580 nm, is nonfluorescent, but treatment with heat or polar, nonaqueous solvent (e.g., pyridine) irreversibly transforms the polymer to a red form absorbing at 540 and 500 nm. This red form fluoresces with emission maxima at 570 and 640 nm. The fluorescence quantum yield, using cresyl violet and Rhodamine 6G embedded in nail polish films as standards, is measured to be (2.0 f 0.5) X
Introduction Films formed from surface-active diacetylene compounds containing long, straight-chain alkyl groups and a polar head group have been shown by several groups to be polymerizable when exposed to UV radiation in the absence of The resulting polymers display interesting spectral features, being blue upon initial formation and transforming subsequently to a stable red form.*+ Fluorescence of polydiacetylenes was first reported qualitatively by Baughman and Chance' and has since been reported for water-soluble urethane-substituted systems,8 a polymer containing chiral center^,^ and an amphiphilic systern.l0 For all cases studied thus far, the longer wavelength absorbing form of the polymer formed initially upon irradiation shows negligible fluorescence, significant emission occurring only after the polymer is converted to a form showing blue-shifted absorption. In none of the work reported to date were quantitative (1) Tieke, B.; Graf, H.-J.; Wegner, G.; Naegele, B.; Ringsdorf, H.; Banerjie, A.; Day, D.; Lando, J. B. Colloid Polym. Sci. 1977, 255, 521. (2) Day, D.; Ringsdorf, H. J. Polym. Sci., Polym. Lett. E d . 1978, 16, 205. (3) Day, D.; Lando, J. B. Macromolecules 1980,13, 1478. (4) Day, D.; Ringsdorf, H. Makrornol. Chem. 1979,180, 1059. ( 5 ) Day, D.; Hub, H. H.; Ringsdorf, H. Isr. J. Chern. 1979, 18, 325. (6) Tieke, B.; Lieser, G.; Wegner, G. J.Polym. Sci., Polyrn. Chern. E d . 1979,17, 1631. (7) Baughman, R. H.; Chance, R. R. J. Polyrn. Sci., Polym. Phys. E d . 1976,14, 2037. (8) Bhattacharjee, H. R.; Preziosi, A. F.;Patel, G. N. J. Chern. Phys. 1980, 73, 1478. (9) Wilson, R. B.; Duesler, E. N.; Curtin, D. Y.; Paul, I. C.; Baughman, R. H.; Preziosi, A. F.J . A m . Chem. SOC.1982, 104,509. (10) Bubeck, C.; Tieke, B.; Wegner, G. Ber. Bunsenges. Phys. Chem. 1982, 86, 495. 0022-3654/83/2087-4790$01.50/0
fluorescence data presented. Because bilayers formed from such polymerized diacetylene films display remarkable strength allowing them to span macroscopic holes: we have undertaken studies to determine their suitability as support structures for immobilized photosensitizers. As part of those studies, we have examined the luminescence of the polymer chromophore, which is the subject of this communication. Procedures and Results The diacetylene monomer of formula H3C(CH2),3-C= C-C=C-(CH2)&OOH was prepared from 10-undecynoic acid and 1-hexadecyne (both from Farchan Labs, Willoughby, OH). Hexadecyne was treated with mercuric acetate to yield the mercuric diacetylide, which was then treated with bromine in carbon tetrachloride to give 1bromohexadecyne." This was chromatographed on Pd-C and then reacted with the undecynoic acid in the presence of Cu+ to give the diacetylene via Chodkiewicz co~pling.'~J~ The resulting crystalline product, after recrystallization from boiling light petroleum ether (30-60 " C ) ,turned the characteristic deep blue color of polymerized diacetylenes upon irradiation with UV light. Oriented monolayer films of diacetylene monomer were produced by layering a drop of 1 mg/mL solution of the monomer in chloroform on the surface of doubly distilled water, allowing it to spread and the chloroform to evaporate, and then slowly compressing the resulting film until stress patterns began to appear on the surface. Compression was accomplished by drawing ferric stearate (11) Eglington, G.; McCrae, W. J. Chern. SOC.1963, 2295. (12) Chodkiewicz, W. Ann. Chim. (Paris) 1957, 2, 852. (13) Eglington, G.; McCrae, W. Adu. Org. Chem. 1963, 4, 228.
0 1983 American Chemical Society
Polymerized Diacetylene Bilayer Films
coated dental floss across the surface. Additional solution was layered on the aqueous surface behind the floss, and the layering-evaporation-compression sequence continued until addition of further chloroform solution led to lensing rather than spreading of the solution d r ~ p l e t Most . ~ films were doped with anthracene at about 1:20 weight ratio to monomer. Anthracene doping has been found by us to facilitate orientation of the monomer, leading to more consistent polymerization. The anthracene is initially incorporated into the polymer film but readily sublimes or is dissolved out. The monomer layers generated in this way were UV irradiated in a nitrogen atmosphere from a height of about 5 cm by using a Spectroline Model EF-16 UV lamp, irradiation times of about 10 min being sufficient to polymerize the films to the maximum possible extent. Polymerized films appeared as mosaics of bronze platelets when viewed in oblique sunlight. Judging from the fraction of the aqueous surface not covered by such polymer, polymerization was accompanied by shrinkage of the film area by about 20%. Films could be readily transferred to a ferric stearate coated glass slide by dipping the slide, held nearly horizontally, down through a polymer layer, turning the slide over under water, and bringing it back up through the layer, again nearly h~rizontally.~ Following drying of the slide, this process could be continued until multilayers of polymerized films of any desired number of layers were obtained. The resulting slides could be mounted in the cell compartment of a v i s i b l e w spectrophotometer (Cary 15) or spectrophotofluorimeter (Perkin-Elmer MPF 44B) for absorption and emission spectral measurements. When viewed in transmitted light, our polymerized films had a characteristic royal blue color and displayed broad absorption peaks a t 640 and 580 nm with a maximum optical density of around 0.01 per layer. This blue form of the polymer had no observable fluorescence (detection limit for fluorescence approximately The blue form of the polymer is stable indefinitely at room temperature in air and ambient light (no apparent change in color after 4 months), and it undergoes no spectral changes under our conditions when irradiated further with UV light, either while still layered on water or after deposition on glass slides. It can be readily and irreversibly converted to an equally characteristic red form by heating to 90 OC for 5-10 min, by soaking in acetone or ethanol for about 5 min, or instantaneously upon dipping in pyridine. Regardless of the mode of treatment, the red form has absorption peaks a t 540 and 500 nm with a maximum optical density per layer that is very similar to that of the blue form. This red form is also stable indefinitely. The red form of the polymerized diacetylene films has a fluorescence spectrum which is shown in Figure 1. Fluorescence maxima occur at 570 and 640 nm, the emission spectrum being reasonably mirror image to the absorption spectrum. The shape of the emission spectrum is independent of excitation wavelength, and the excitation spectrum for the fluorescence is identical with the absorption spectrum of the polymer. The fluorescence quantum yield of the polymer was determined by comparing the fluorescence intensity with that of a fluorescence standard that had been dissolved in clear nail polish and then painted onto a glass slide. Two different fluorescence standards were used: Rhodamine 6 G (4f= 0.88, A d = 573 nm in ethanol) and cresyl = 623 nm ethan01.l~ An absorption violet (4f= 0.50,, ,A (14)Olmsted, J., I11 J. Phys. Chern. 1979, 83,2581.
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
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Figure 1. Excitation spectrum (corrected, solid curve on right) and emission spectrum (uncorrected, solid curve on left) of polymerized . fluorescence spectra of films of C , , H 2 9 ~ - C ~ s H , B C 0 0 H The Rhodamine 6 0 (dotted curve) and cresyl violet (dashed curve) embedded in nail polish films are also shown. Abscissa is emission intensity in arbitrary units.
wavelength at which the fluorescence standard slide and polymer slide had identical optical densities was determined spectrophotometrically. Each slide was then excited a t that wavelength and the fluorescence spectrum recorded. Relative fluorescence intensities were found by determining the areas under these fluorescence curves, and the polymer fluorescence quantum yield was computed from the ratio of areas and the known yield of the standard substance. The measured quantum yield is independent of both the number of layers of polymer on the slide (measurements made on slides having from 8 to 24 layers) and, for a given standard, the excitation wavelength. Using cresyl violet as a fluorescence standard, we measured the polymer fluorescence quantum yield to be (2.9 f 1.0) X When Rhodamine 6G is used as a standard, the value is (1.2 f 0.1) x lo-'. We have also measured the fluorescence quantum yield of the polymer formed from the 29-carbon-chain-length monomer, which within the experimental error is the same as that of the 27-carbon system. Discussion The spectral characteristics which we find for polymerized 27-carbon diacetylenecarboxylic acid generally are comparable with results obtained by others, the emission and excitation spectra being very similar to those reported by Bubeck, Tieke, and Wegner for multilayer polymer films of the Cd salt of the C-25 diacetylene.1° Both the blue and red forms of the polymer are characteristic of the conjugated chromophore of the polydiacetylene, having been observed for monolayer films polymerizing on aqueous surface^,^^^ Cd salt multilayers polymerized on glass slides," and vesicles of polymerized diacetylene-containing phosph01ipids.l~ Details of the transition from blue to red form of the polymer as we observe it are somewhat different from those reported by others. Whereas Day and Ringsdorf found that UV irradiation of films on an aqueous surface generated blue polymer which upon further illumination was converted to the red form,4v5our irradiations generate only (15) O'Brien, D. F.; Whitesides, T. H.; Klingbiel, R. T. J. Polyrn. Sci., Polyrn. Lett. Ed. 1981, 19, 95.
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Olmsted and Strand
the blue form, which must be relaxed by heat or solvent treatment to generate the red form. In this regard our results parallel those of Tieke, Lieser, and Wegner on Cd salt multilayers on slides.6 Repeated efforts on our part to modify conditions (different doping levels including no doping, changes in film compression including no compression, repeated purification of the aqueous substrate, adjustment of substrate pH) have all failed to give red polymer upon irradiation. On the other hand, when blue films on slides are irradiated further with white light (medium-pressure Hg lamp) in air, the films are converted to the red form and subsequently slowly degrade. The exact nature of the transformation which converts the longer wavelength absorbing form of polymer to a shorter wavelength absorbing, fluorescent form is still uncertain. Similar absorption blue shifts have been observed in other polydiacetylene systems (though complicated by intramolecular hydrogen bonding) and attributed to R-group-induced distortion of the polymer backbone.16 Even larger shifts are observed in solvent-sensitive polydiacetylenes due to conjugation length d i s r u p t i ~ n . ~ J ' Bubeck, Tieke, and Wegner attribute the blue shift and fluorescence to a polymer degradation product formed in disordered areas.1° Both these proposals suggest a disruption of the extended conjugation of the a system, either by distortion or by photodegradaton. In our view, such disruption would, in the case of the polymer studied here, generate oligomers of variable chain length which would be expected to have different absorption and fluorescence spectra and hence would be likely to show wavelengthdependent emission and excitation spectra. It is possible, particularly if the oligomers are distributed in a characteristic way within each individual polymer chain, that such a distribution of absorbers would nevertheless give rise to wavelength-independent spectra. This would occur if nonradiative energy transfer occurred with unit efficiency among all chromophores, such that a photon absorbed by any chromophore generated fluorescence from the lowest energy chromophore with the same efficiency. While we cannot entirely rule out this possibility, two pieces of experimental evidence make it unlikely. First, we observe that partially converted films containing both red and blue form of the polymer show partial but not total quenching of red form fluorescence by the blue form. Second, the absorption and emission spectra of the red form are reasonably mirror image (see Figure l ) , a relationship that would be unlikely if the absorption spectrum were generated by many different-length chromophores while the emission originated from a single chromophore. Further, we observe that heat treatment, solvent immersion, or white-light irradiation all generate the identical fluorescence spectrum, mirror image to the absorption spectrum and with an excitation spectrum identical with the absorption spectrum. The features all suggest a discrete, invariant chemical species as their source, whose exact nature is yet to be elucidated. Because of the identical absorption spectra of red polymer generated from several different organized diacetylene film structures, we believe that the fluorescence spectrum and yield which we report here are characteristic of the red form of the polymerized diacetylene chromophore, whether in monolayers, multilayers, or vesicles. At the same time, since the fluorescence capability of monomeric diacetylenes is affected by the nature of substituent groups,18 one might expect that polydiacetylenes
having nonalkyl substituents close to the diacetylene chromophore may have different emission characteristics, an expectation that requires experimental verification. The reduced fluorescence yield of the blue form of the polydiacetylene polymer compared to the red form should be due at least in part to the smaller singlet-state energy gap for the blue form. It is well-known that internal conversion from SIto So is governed by an energy gap law of the form kIc 1013e~p(-cuAE).'~This expression can be used to predict a fluorescence quantum yield for the blue form of the polymer, given that blue and red forms have equal oscillator strengths and assuming negligible intersystem crossing and strict energy gap law dependence of kIc. From the red form fluorescence yield of 2 x and red and blue form energy gaps of 220 and 185 kJ/mol, respectively, the blue form fluorescence yield is predicted to be 6 X The actual yield is indicating that the energy gap difference by itself is insufficient to account for the nonfluorescence of the blue form. Although the fluorescence quantum yield value for red polymer obtained by using Rhodamine 6G as a standard is significantly lower than that obtained with (1.2 X cresyl violet (2.9 X we believe that the average of these two values, 2.0 X is accurate to about 25%. As shown in Figure 1, the fluorescence of Rhodamine 6G is blue shifted relative to the center of gravity of the polymer fluorescence while that of cresyl violet is red shifted relative to that center of gravity. Since these emission spectra are uncorrected for instrument response variations with wavelength (although the instrument has a corrected emission capability, it is not applicable beyond 600 nm), and since the instrument phototube response is diminished significantly as one scans to the red, the Rhodamine 6G result underestimates polymer intensity and quantum yield while the cresyl violet result overestimates it. Besides these spectral sensitivity effects, the quantum yield measurement may also be subject to errors arising from polarization phenomena, changes in the quantum yields of the standards when embedded in a polymer matrix, and refractive index differences between diaceylene polymer films and nail polish films. The reasonable agreement of the two sets of measurements and the fact that the two values disagree in the direction predicted by instrument sensitivity variations suggest that these factors, if contributing, are not substantial. The existence of measurable fluorescence from these polydiacetylene films suggests that they might be capable of efficiently transferring excitation energy to photochemical sensitizers embedded in them. We are currently pursuing experiments designed to explore this possibility.
(16) Chance, R. R.Macromolecules 1980, 13, 396. (17) Patel, G.N.;Chance, R. R.; Witt, D. J . Chem. Phys. 1979, 70, 4387.
(18) Beer, M. J. Chem. Phys. 1956, 25, 745. (19) Turro, N. J. 'Modern Molecular Photochemistry"; Benjamin Cummings: Menlo Park, CA, p 183.
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Acknowledgment. This research was supported by a grant from the U.S. Department of Energy, Grant No. DE-FG02-79ER-10546. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of DOE. The synthesis of CZ7and CZsdiacetylene monomers was carried out by Mr. Philip Miller, an undergraduate research assistant at California State University, Fullerton. The spectrofluorimeter was purchased with an NSF equipment grant. Registry NO.H3C(CH2)&4CS(CHz)&OOH, 67071-94-7; 10-undecyanoic acid, 2777-65-3; 1-hexadecyne, 629-74-3; 1bromo-1-hexadecyne, 13866-75-6; 10,12-heptacosadiynoic acid homopolymer, 67071-95-8.
