Steps To Demarcate the Effects of Chromophore Aggregation and Planarization in Poly(phenyleneethynylene)s. 2. The Photophysics of 1,4-Diethynyl-2-fluorobenzene in Solution and in Crystals Marcia Levitus, Gerardo Zepeda, Hung Dang, Carlos Godinez, Tinh-Alfredo V. Khuong, Kelli Schmieder, and Miguel A. Garcia-Garibay* Department of Chemistry and Biochemistry, The University of California, Los Angeles, California 90095-1569 mgg@chem.ucla.edu Received February 22, 2001 Crystals of 1,4-bis(2-hydroxy-2-methyl-3-butynyl)-2-fluorobenzene 4 have a rich packing structure with four distinct molecules in the unit cell. A complex hydrogen bonding network results in the formation of cofacial trimers, cofacial dimers, and monomers within the same unit cell. Given a remarkable opportunity to investigate the effect of aggregation on the photophysics of 1,4- diethynylbenzenes, we analyzed the absorption, diffuse reflectance, and emission spectra of compound 4 in solutions and in crystals. Diffuse reflectance and fluorescence excitation revealed a red-shifted absorption that is absent in dilute solution but becomes observable at high concentrations and low temperatures. The fluorescence emission in the solid state is dual with components assigned to monomers and aggregates. The excitation and emission assigned to the monomer are nearly identical in crystals and dilute solutions. The absorption and emission bands assigned to aggregates are broad and red-shifted by 60-80 nm. As expected for a sample with absorbers and emitters with different energies and incomplete equilibration, efficient monomer- to-aggregate energy transfer was observed by a proper selection of excitation wavelengths. The fluorescence quantum yield of 4 in solution is relatively low (Φ F ) 0.15) and the singlet lifetime short (τ F ) 3.8 ns). A lower limit for the triplet yield of Φ T ) 0.64 was determined indirectly in solution by 1 O 2 sensitization, and a relatively strong and long-lived phosphorescence was observed in low-temperature glasses and in crystals at 77 K. Introduction Several poly(phenyleneethynylene)s (1) 1,2 and other arylethynyl fluorophores (e.g., 2-5, Scheme 1), 3 have become very attractive for a wide variety of photonics and sensing applications due to their electron-transport abili- ties and intense fluorescence emission. Among their many interesting properties, red shifts in the absorption and emission spectra are frequently observed upon concentration and thin film formation 4 and by addition of nonsolvents. 5 Although these spectral changes and the observation of self-quenching have been primarily ex- plained in terms of aggregation effects, possible contribu- tions from coplanarization and twisting of the aryl groups have been recently suggested by Bunz et al. (Scheme 2). 5,6 While it is reasonable that both planarization and aggregation may have an effect on ground and excited state properties of arylethynyl-containing compounds, the separation of these effects has been difficult to factorize and remains somewhat controversial. 4a Ambiguities may arise because spectroscopic manifestations of chro- (1) (a) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605-1644. (b) Levitsky, I. A.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 1999, 121, 1466-1472. (c) Sato, T.; Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1999, 121, 10658-10659. (d) Samori, P.; Francke, V.; Mu ¨ llen, K.; Rabe, J. P. Chem. Eur, J. 1999, 5, 2312-2317. (2) (a) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537-2574, and references therein. (3) (a) Hanhela, P. J.; B. D. P. Aust. J. Chem. 1984, 37, 553. (b) Gisser, D. J.; Johnson, B. S.; Ediger, M. D.; Von Meerwall, E. D. Macromolecules 1993, 26, 512. (c) Lindasy, J. S.; Prahapan, S.; Johnson, T. E.; Wagner, R. W. Tetrahedron 1994, 50, 8941-8968. (4) (a) McQuade, D. T.; Kim, J.; Swager, T. M. J. Am. Chem. Soc., 2000, 122, 5885-5886. (b) Walters, K. A.; Ley, K. D.; Schanze, K. S. Langmuir 1999, 15, 5676-5680. (c) Weder, C.; Wrighton, M. S. Macromolecules 1996, 29, 5157-5165. (5) (a) Miteva, T.; Palmer, L.; Kloppenburg, L.; Neher, D.; Bunz, U. H. F. Macromolecules 2000, 33, 652-654. (b) Halkyard, C. E.; Rampey, M. E.; Kloppenburg, L.; Studer-Martinez, S. L.; Bunz, U. H. F. Macromolecules 1998, 31, 8655. (6) Planarization effects on the electronic properties of conducting polymers had been previously reported by Wudl and co-workers: Rughooputh, S. D. D. V.; Hotta, S.; Heeger, A. J.; Wudl, F. J. Polym. Sci. B 1987, 25, 1071. Scheme 1 3188 J. Org. Chem. 2001, 66, 3188-3195 10.1021/jo015589e CCC: $20.00 © 2001 American Chemical Society Published on Web 04/04/2001