J. Phys. Chem. C 2009, 113, 17927–17935
Photophysical Properties of Tolan Wavelength Shifters in Solution and Embedded in Polymeric Organic Thin Films Claudia Aurisicchio,† Barbara Ventura,‡ Davide Bonifazi,*,† and Andrea Barbieri*,‡ Department of Chemistry, UniVersity of Namur (FUNDP), Rue de Bruxelles 61, 5000 Namur, Belgium, and Istituto per la Sintesi Organica e la FotoreattiVita` (ISOF), Consiglio Nazionale delle Ricerche (CNR), Via Gobetti 101, 40129 Bologna, Italy ReceiVed: June 9, 2009; ReVised Manuscript ReceiVed: August 24, 2009
Linear organic conjugated molecules, peripherally equipped with electron-donating and electron-accepting moieties, are recognized as one of the most promising classes of nonlinear optical materials for potential application in energy conversion devices, organic electronics, optical communication, information storage, and nuclear medicine techniques. In this work, we have synthesized and photophysically characterized a series of organic molecules constituted by a 1,2-diphenyl acetylene core (tolan) bearing electronically active groups directly linked to the π-conjugated backbone. Tuning of the absorption and emission energies has been achieved via the push-pull effect. All investigated compounds displayed very high luminescence in condensed media from intramolecular charge transfer excited states with large Stokes shifts. These features revealed to be of particular interest for the engineering of new wavelength shifters for spectral conversion of deep ultraviolet to visible light. Introduction The so-called wavelength shifters, molecules that absorb ultraviolet (UV) light and emit at longer wavelengths, are of great interest in the research fields based on the detection of ionizing radiation, as in the study of cosmic rays1,2 or in nuclear medicine techniques, such as positron emission tomography (PET).3,4 Among all, PET is one of the most appealing and emerging medical and research tool for metabolic 3D imaging of tissue function in vivo. In PET scanners, scintillator crystals detect coincident pairs of γ-rays indirectly generated by a positron-emitting radionucleotide introduced into the body by means of a labeled biologically active molecule, usually a sugar (e.g., [18F]flourodeoxyglucose, 18F-FDG, a glucose analog with replacement of the oxygen in C-2 position with a 18-fluorine atom). The performance of PET scanners, in terms of spatial resolution and signal-to-noise ratio, can be improved by using faster scintillators that emit UV light, combined with wavelength shifter materials that absorb in the region of the scintillator luminescence and emit in the visible (vis) region, where photomultiplier tubes have the highest sensitivity. To optimize the overall detection sensitivity, the organic molecule acting as wavelength converter must combine the desired spectroscopic features with an high emission quantum yield and a short lifetime. Linear conjugated organic molecules, peripherally functionalized with electron-donating (D) and electron-accepting (A) species at the extremities, are good candidates for the role of wavelength shifters.5 Due to the push-pull effect of the donor and acceptor groups, in fact, upon photoexcitation a large change in the charge distribution is induced in the excited state, resulting in an intramolecular charge transfer (ICT) from donor to acceptor side.6-8 The large change in the molecular dipole * To whom correspondence should be addressed. E-mail: [email protected]
fundp.ac.be (D.B.), [email protected]
(A.B.). † University of Namur (FUNDP). ‡ Consiglio Nazionale delle Ricerche (CNR).
moment in going from the ground state to the low-lying chargetransfer excited state makes this class of chromophores also widely studied in terms of their nonlinear optical (NLO) properties with potential application in energy conversion devices, organic electronics, optical communication, and information storage.9-11 In view of designing wavelength shifters for spectral conversion from deep UV (DUV) to vis light, the interest in organic molecules with low-lying ICT states comes from their large Stoke’s shifted emission. Since the lifetime of the emitting state is inversely proportional to its energy and to the area of the absorption spectrum calculated over the whole absorption region,12 a way to combine a large Stoke’s shifted emission with a reasonable short lifetime is to design a molecule with a strong absorption in the UV region, a characteristic that can be achieved when the bridge connecting the donor and the acceptor groups is largely conjugated. Previous studies on D-π-A polyenic push-pull systems in solution evidenced large Stoke’s shifted emission due to intramolecular charge transfer and the dependence of their photophysical behavior on the electronic character of the donor-acceptor couple.13-17 For these reasons, we have considered linear conjugated molecules, in particular, diphenyl acetylene derivatives doubly substituted in 4,4′ positions with donor and acceptor groups. Here we report on the synthesis and photophysical characterization of a series of asymmetric diphenyl acetylene derivatives (2-5), para-substituted with different electronically active moieties (Chart 1). Two symmetric derivatives (6 and 7) and the precursor tolan (1) were also prepared for comparison purposes. All componds were synthesized by exploiting Sonogashira-type cross-coupling reactions. Thin polymethylmetacrylate (PMMA) films doped with fluorophores 1-7 ([email protected]
, n ) 1-7) have been prepared by spin-coating techniques.18 A comprehensive photophysical investigation has been performed by means of steady-state and time-resolved techniques in solvents of different polarity and in the plastic thin films at room temperature (rt) and in rigid matrices at 77 K. All fluorophores
10.1021/jp9053988 CCC: $40.75 2009 American Chemical Society Published on Web 09/18/2009
J. Phys. Chem. C, Vol. 113, No. 41, 2009
CHART 1: Schematic Structure of the Investigated Compounds
showed an extraordinary improvement of the luminescent properties compared to the solution state when embedded in the polymer matrix by reduction of the nonradiative processes. Materials and Methods General Details. Compounds 1,19 6,20 7,21 4-(cyanophenylethynyl)benzene (8),22 and 4-(nitrophenylethynyl)benzene (9),23 were prepared according to literature procedures. Solvent and reagents were purchased as reagent-grade and used without further purification. All new compounds were characterized by 1 H-, 13C-, and 19F-NMR; IR; and HR-MS. 1H NMR spectra were recorded at 25 °C in CDCl3 on a 400 and 270 MHz JEOL instruments. Chemical shifts are reported in ppm downfield from Me4Si using the residual solvent signals as an internal reference. Coupling constants (J) are given in hertz. The IR spectra were collected with a Perkin-Elmer spectrometer RX I FT-IR system, and the selected absorption bands are reported by wavenumber (cm-1). EI-HRMS and ESI-HRMS measurements were performed respectively on a H2Os AutoSpec 6 F mass spectrometer and H2Os QToF2 mass spectrometer both operating in positive mode. Microwave reactions were performed with a Biotage Initiator 2.0. Synthetic Procedures. 1,2-Diphenylethyne (1). To a dried Schlenk tube were added iodobenzene (0.5 g, 2.45 mmol), dried (i-Pr)2NH (42 mL), phenylacetylene (0.25 g, 2.45 mmol), [PdCl2(PPh3)2] (91 mg, 0.13 mmol), and CuI (40 mg, 0.21 mmol). The solution was degassed by three “freeze-pumpthaw” cycles and stirred for 18 h at rt. Upon completion, as monitored by TLC (eluent: hexane), the crude mixture was filtered through Celite and washed with hexane, and the solvent was evaporated in vacuo. The residue was purified by FCC (SiO2; hexane) affording desired compound as a white crystalline solid (0.4 g, 90% yield): δH (CDCl3, 400 MHz) 7.36-7.39 (m, 6H, Ph); 7.58-7.61 (m, 4H, Ph); δC (CDCl3, 100 MHz) 131.56, 128.30, 128.21, 123.22, 89.35. Characterization was as reported in the literature.19 4-Cyanophenylethynyl-4-N,N-dihexylaniline (2). To a dried Schlenk tube were added 4-(N,N-dihexylamino)iodobenzene (10) (40 mg, 0.17 mmol), dried (i-Pr)2NH (5 mL), 8 (0.05 g, 0.4 mmol), [PdCl2(PPh3)2] (6 mg, 0.008 mmol), and CuI (3 mg, 0.017 mmol). The solution was degassed by three “freezepump-thaw” cycles and stirred overnight at rt. Upon completion, as monitored by TLC (eluent: hexane/AcOEt 9:1), the crude
Aurisicchio et al. mixture was filtered through Celite and cleaned with repeated hexane washing, and the solvent was evaporated in vacuo. The residue was purified by FCC (SiO2; hexane/AcOEt 9:1), affording the desired compound as a yellow oil (40 mg, 61% yield): νmax (film)/cm-1 2928.36, 2206.07, 1596.09, 1523.7, 1132.86; δH (CDCl3, 400 MHz) 7.58-7.50 (m, 4H, Ph); 7.34 (d, 2H, J 12, Ph), 6.55 (d, 2H, J 12, Ph), 3.26 (m, 4H, 2 NCH2), 1.56-1.53 (m, 4H, 2 NCH2CH2), 1.30-1.23 (m, 12H, 6 CH2), 0.90-0.85 (m, 6H, 2 CH3); δC (CDCl3, 100 MHz) 148.6, 133.23, 132.0, 131.5, 129.43, 118.92, 111.16, 110.03, 107.3, 96.13, 86.03, 50.96, 31.82, 27.12, 26.75, 22.77, 14.01; EI-MS (m/z) 386 [M]+, 315 [M - CH3(CH2)4]+, 245 [M - (CH3(CH2)4)2]+. 4-(N,N-Dihexylaminophenyl)ethynyl-4-nitrobenzene (3). To a dried Schlenk tube were added 10 (50 mg, 0.2 mmol), dried (i-Pr)2NH (6 mL), 9 (66 mg, 0.45 mmol), [PdCl2(PPh3)2] (7 mg, 0.01 mmol), and CuI (3.8 mg, 0.02 mmol). The solution was degassed by three “freeze-pump-thaw” cycles and stirred overnight at rt. Upon completion, as monitored by TLC (eluent: hexane/AcOEt 3:7), the crude mixture was filtered through Celite and cleaned with repeated hexane washing, and the solvent was evaporated in vacuo. The residue was purified by FCC (SiO2; hexane/AcOEt 3:7), affording the desired compound as a red oil (44 mg, 54% yield): νmax (film)/cm-1 2927.22, 2207.34, 1588.3, 1509.17, 1337.77, 687.5; δH (CDCl3, 400 MHz) 8.18 (d, 2H, J 8.7, Ph), 7.58 (d, 2H, J 8.7, Ph), 7.38 (d, 2H, J 9, Ph), 6.58 (d, 2H, J 9, Ph), 3.28 (t, 4H, J 7.6, 2 NCH2), 1.58 (m, 4H, 2 NCH2CH2), 1.31-1.25 (m, 12H, 6 CH2), 0.9-0.86 (m, 6H, 2 CH3); δC (CDCl3, 100 MHz) 148.7, 146.37, 133.46, 131.6, 130.7, 123.7, 111.28, 107.32, 97.47, 86.42, 51.06, 31.75, 27.21, 26.85, 22.74, 14.11; EI-HRMS calcd 406.2620, m/z 406.2627 ([M]+, C26H34N2O2+). 4-(4-Trifluoromethylphenyl)ethynyl-4-N,N-dihexylaniline (4). To a dried Schlenk tube were added 4-iodobenzotrifluoride (0.1 g, 0.36 mmol), dried (i-Pr)2NH (10 mL), 12 (0.24 g, 0.83 mmol), [PdCl2(PPh3)2] (13 mg, 0.018 mmol), and CuI (7 mg, 0.036 mmol). The solution was degassed by three “freeze-pumpthaw” cycles and stirred overnight at rt. Upon completion, as monitored by TLC (eluent: hexane/CH2Cl2 85:15), the crude mixture was filtered through Celite and cleaned with repeated hexane washing, and the solvent was evaporated in vacuo. The residue was purified by FCC (SiO2; hexane/CH2Cl2 85:15), affording desired compound as yellow oil (90 mg, 58% yield): νmax (film)/cm-1 2957.91, 2930.14, 2858.37, 2210.23, 1602.49, 1520.13, 1404.23, 1369.31, 1322.36, 1197.56, 1161.03, 1106.77, 1066.50, 840.64, 813.33; δH (CDCl3, 400 MHz) 7.56 (s, 4H, Ph); 7.37 (d, 2H, J 8.3, Ph), 6.57 (d, 2H, J 8.3, Ph), 3.28 (m, 4H, 2 NCH2), 1.57 (m, 4H, 2 NCH2CH2), 1.32 (m, 12H, 6 CH2), 0.89 (m, 6H, 2 CH3); δC (CDCl3, 100 MHz) 148.3, 133.1, 131.25, 128.22, 125.1 (d, CF3), 122.84, 111.15, 107.74, 93.71, 86.1, 50.95, 31.7, 27.15, 26.77, 22.7, 14.02; δF(CDCl3, 376 MHz) -62.47. ESI-MS 452.2 ([M + Na+], C27H34NF3Na+). 4-(Pentafluorophenyl)ethynyl)-4-(N,N-dihexyl)aniline (5). To a dried Schlenk tube were added pentafluoroiodobenzene (0.22 g, 0.76 mmol), dried (i-Pr)2NH (15 mL), 12 (0.1 g, 0.34 mmol), [PdCl2(PPh3)2] (12 mg, 0.017 mmol), and CuI (6 mg, 0.034 mmol). The solution was degassed by three “freeze-pumpthaw” cycles and stirred overnight at rt. Upon completion, as monitored by TLC (eluent: hexane/CH2Cl2 9:1), the crude mixture was filtered through Celite and cleaned with repeated hexane washing, and the solvent was evaporated in vacuo. The residue was purified by FCC (SiO2; hexane/CH2Cl2 9:1), affording the desired compound as a yellow oil (90 mg, 60% yield): νmax (film)/cm-1 2958.39, 2931.37, 2207.52, 1605.63, 1526.62, 1499.28, 989.46, 966.48, 813.87; δH (CDCl3, 400
Properties of Tolan Wavelength Shifters MHz) 7.4 (d, 2H, J 8.6, Ph); 6.57 (d, 2H, J 8.6, Ph), 3.29 (m, 4H, 2 NCH2), 1.59 (m, 4H, 2 NCH2CH2), 1.32 (m, 12H, 6 CH2), 0.92-0.89 (m, 6H, 2 CH3); δC (CDCl3, 100 MHz) 148.78, 147.9 (m, CF), 145.5 (m, CF), 142.9 (m, CF), 138.97 (m, CF), 136.45 (m, CF), 133.3, 111.07, 106.64, 103.9, 101.5 (m, CF), 71.06, 50.95, 31.67, 27.10, 26.75, 22.65, 14.0; δF (CDCl3, 376 MHz) -137.15 (d, 2F, ArF), -155.26 (m, 1F, ArF), -162.60 (m, 2F, ArF); ESI-MS 452.2 ([M + H]+, C26H31NF5+). 4-(N,N-Dihexylamino)iodobenzene (10). To a 100 mL roundbottom two-necked flask were added 1-bromo-n-hexane (10.5 mL, 75.3 mmol), dried and degassed DMF (50 mL), and KI (12.5 g, 75.3 mmol) under Ar. The mixture was heated at 95 °C under vigorous stirring for 30 min. Subsequently, Na2CO3 (4.35 g, 41 mmol) and 4-iodoaniline (5.0 g, 22.8 mmol) were added. The resulting solution was allowed to stir for a further 20 h at 95 °C. After cooling to rt, the mixture was diluted with AcOEt and poured in H2O. The organic layer was washed with brine (1 × 30 mL) and H2O (5 × 30 mL), dried on MgSO4, and evaporated in vacuo. The residue was then purified by FCC (SiO2; cyclohexane), affording the desired compound as a pale yellow oil (5.74 g, 65% yield): δH (CDCl3, 400 MHz) 7.40 (d, 2H, J 9, Ph), 6.39 (d, 2H, J 9, Ph), 3.20 (m, 4H, 2 NCH2), 1.55 (m, 4H, 2 NCH2CH2), 1.29 (m, 12H, 6 CH2), 0.89 (m, 6H, 2CH3); δC(CDCl3, 100 MHz) 147.69, 137.72, 114.07, 76.40, 51.11, 31.80, 27.09, 26.9, 27.77, 14.14. Characterization was as reported in the literature.24 4-[(N,N-Dihexylamino)-2-(trimethylsilyl)ethyn-1-yl]benzene (11). To a dried Schlenk tube were added compound 10 (0.5 g, 1.2 mmol), dried (i-Pr)2NH (18 mL), (trimethylsilyl)acetylene (TMSA) (0.28 g, 2.9 mmol), [PdCl2(PPh3)2] (42 mg, 0.06 mmol), and CuI (22 mg, 0.12 mmol). The solution was degassed by three “freeze-pump-thaw” cycles and stirred overnight at rt. Upon completion, as monitored by TLC (eluent: hexane/AcOEt 9:1), the crude mixture was filtered through Celite and cleaned with repeated hexane washing, and the solvent was evaporated in vacuo. The residue was then purified by FCC (SiO2; hexane/AcOEt 9:1), affording the desired compound as yellow oil (0.26 g, 61% yield): δH (CDCl3, 400 MHz) 7.27 (d, 2H, J 13, Ph), 6.48 (d, 2H, J 13, Ph), 3.22 (m, 4H, 2 NCH2), 1.56-1.53 (m, 4H, 2 NCH2CH2), 1.33-1.29 (m, 12H, 6 CH2), 0.98-0.85 (m, 6H, 2 CH3), 0.21 (s, 9H, 3 SiCH3); δC (CDCl3, 100 MHz) 148.12, 133.35, 111.1, 108.6, 106.92, 90.85, 51.0, 31.78, 27.22, 26.86, 22.76, 14.13, 0.34. Characterization was as reported in the literature.24 4-(N,N-Dihexylamino)ethynylbenzene (12). To a degassed solution of THF/MeOH (50 mL, 1/1) were added compound 11 (2.28 g, 6.4 mmol) and K2CO3 (1.77 g, 12.8 mmol) under Ar. The mixture was stirred for 1 h at rt. Upon completion, as monitored by TLC (eluent: CH2Cl2), the crude mixture was filtered through Celite and the solvent evaporated in vacuo. The residue was purified by FCC (SiO2; CH2Cl2), affording the desired compound as a colorless oil (1.82 g, 100% yield): δH (CDCl3, 270 MHz) 7.31 (d, 2H, J 8.9, Ph), 6.51 (d, 2H, J 8.9, Ph), 3.24 (m, 4H, 2 NCH2), 2.94 (s, 1H, CCH), 1.54 (m, 4H, 2 NCH2CH2), 1.29 (m, 12H, 6 CH2), 0.89 (m, 6H, 2 CH3); δC (CDCl3, 100 MHz) 148.28, 133.45, 111.09, 107.42, 85.19, 74.48, 51.03, 31.79, 27.22, 26.88, 22.77, 14.13. Characterization was as reported in the literature.24 Preparation of Plastic Thin Film. Thin films of polymethylmethacrylate (PMMA) doped with the organic fluors were prepared from 200 g · L-1 PMMA (MW ) 35 000) solutions in chloroform containing 1% w/w organic dyes (2 g · L-1), spin coated on 1” × 1” borosilicate glass substrates that were cut from microscope slides. Before spin-coating, the substrates were
J. Phys. Chem. C, Vol. 113, No. 41, 2009 17929 cleaned by first sonicating them for 10 min in water with detergent (Carlo Erba AUSILAB 104, 2% deionized water solution at pH ) 12.5), then rinsing them with a slightly acidic water solution, and finally washing them with acetone. Spincoating was performed by spreading from 50 to 100 µL of the starting solution onto the spinning substrate. The spin-coating conditions (4000 rpm for 60 s) were adjusted to give substrates fully covered with a photoactive layer of ca. 2.5 µm thickness. Determination of Film Thickness. When a beam of light passes through two partially reflecting surfaces (like in a thin transparent film) it generates an interference pattern that can be used to calculate the thickness of the film. Provided the film thickness is on the order of magnitude of the wavelength of the incident light, if a vis/NIR optical absorption spectrum is recorded for such a film, the spectrum appears in the form of maxima and minima. For regions where individual cycles can be clearly resolved, the film thickness can be easily determined by using the following equations25
n(λ2) n(λ1) λ1 λ2 V) Ncyc d)
where n(λ1) and n(λ2) are the refractive indices of the film at wavelengths λ1 and λ2, Ncyc is the number of cycles in the interference fringes between λ1 and λ2, and d is the thickness of the film. In our experiments, vis/NIR absorption spectra of the films prepared on borosilicate glass substrates have been recorded between 300 and 1500 nm. Film thicknesses of 2.5 ( 0.2 and 2.8 ( 0.2 µm, for samples prepared from 50 and 100 µL of the starting solution, respectively, have been calculated using the refractive index nPMMA ) 1.4896, and assuming it to be wavelength-independent. Absolute Photoluminescence Quantum Yield Measurement. The procedures used to determine the photoluminescence quantum yield (PLQY) are generally based on two different experimental approaches: the comparative and the absolute methods. In contrast to measurements of PLQY in solution, where a comparative method can be used, measurements on thin solid films are not straightforward. The absolute method has been the normal approach for measurements on thin films over the past decade. Several methods for the measurement of the absolute photoluminescence efficiency of solid-state samples using an integrating sphere to collect the emitted light have been proposed.26 For the absolute PLQY determination of our sample, we used the method proposed by deMello and co-workers in 1997. Three measurements are necessary to evaluate the absolute quantum yield, η: (i) the excitation source spectrum with the empty sphere; (ii) the excitation source spectrum and the emission spectrum of a sample placed in the sphere, when the excitation beam hit the sample only after diffusion by the sphere wall; and (iii) the excitation source spectrum and the emission spectrum of a sample placed in the sphere, when the excitation beam hit the sample directly. The absorption coefficient, A, is defined as
J. Phys. Chem. C, Vol. 113, No. 41, 2009
where Lb and Lc are the spectral areas of the excitation source spectrum detected for the second and the third measurements, respectively. The number of absorbed photons is proportional to A times the spectral area, La, of the first measurement. Correcting for the emissions induced indirectly from the reflected excitation beam, the quantum yield can be expressed by
Pc - (1 - A)Pb ALa
where Pb and Pc are the spectral areas of the emission profiles measured in the second and third measurements, respectively. All measurements were repeated four times. In our experimental setup, a collimated 100 W Hg arc lamp was coupled to the integrating sphere input port. Inside the sphere, a baffle is placed in front of the sphere output port in order to protect the detector from direct illumination by nondiffuse light. The sample is placed at the center of the sphere in a home-build Teflon sample holder. The sphere output port is connected by means of a pure silica core (600 µm) solarization-resistant optical fiber to the emission monochromator of a SPEX Fluorolog II fluorimeter (see the next section). With this assembly, great care is taken in order to avoid perturbation by external light. The total response of the integrating sphere/detection system has been corrected by using a calibrated 45 W quartz-halogen tungsten coiled filament lamp standard of spectral irradiance by Optronics Laboratories (calibration range from 280 to 800 nm). General Photophysical Characterization. Solvents used for photophysical determinations were spectroscopic grade (C. Erba). Acidification of the solutions was obtained by adding one drop of hydrochloridric acid 37% (ACS For Analysis, C. Erba) to 3 mL of solution. The absorption spectra of dilute solutions and plastic thin films were obtained with a PerkinElmer Lambda 950 UV/vis/NIR spectrophotometer. Molar absorptivity values (ε) were calculated by applying the Lambert-Beer law to low absorbance spectra (A < 1) of compounds recorded at successive dilutions. Steady-state photoluminescence spectra were measured in right angle mode for solutions and in front face mode for solid samples, using a Spex Fluorolog II spectrofluorimeter, equipped with a Hamamatsu R928 phototube. The concentration of airequilibrated sample solutions was adjusted to obtain absorption values A < 0.1 at the excitation wavelength. While uncorrected luminescence band maxima are used throughout the text, corrected spectra were employed for the determination of the luminescence quantum yields (φ). The correction curve of the wavelength dependent phototube response between 280 and 900 nm has been obtained by using a calibrated halogen lamp source. Luminescence quantum efficiencies (φem) in solution were evaluated using the method of Demas and Crosby by comparing the wavelength integrated intensities (I) with reference to quinine sulfate as the standard (φr ) 0.546 in air-equilibrated 1 N H2SO4)27 and by using the following equation
( InA )( I n )φ Ar
where A and n are the absorbance values at the employed excitation wavelength and refractive index of the solvent, respectively. Band maxima and relative luminescence intensities
Aurisicchio et al. are obtained with uncertainty of 2 nm and 20%, respectively. One cm path length square optical Suprasil Quartz (QS) cuvettes were used for measurements at rt of dilute solutions, while capillary tubes immersed in liquid nitrogen in a coldfinger quartz Dewar were used for measurements of MeOH/EtOH (1:4) frozen glasses at 77 K. Fluorescence lifetimes were measured with an IBH 5000F time-correlated single-photon counting device, by using pulsed NanoLED excitation sources at 278, 331, and 373 nm. Analysis of the luminescence decay profiles against time was accomplished with the Decay Analysis Software DAS6 provided by the manufacturer. Experimental uncertainties in the lifetime determinations are estimated to be 10%. Phosphorescence spectra and decays were measured at 77 K in MeOH/EtOH (1:4) frozen dilute solutions on a Perkin-Elmer LS-50B spectrofluorimeter equipped with a pulsed Xe lamp and in gated detection mode. The phosphorescence decay analysis was performed with the PHOSDecay software provided by the manufacturer. Results and Discussion Synthesis of the Substituted Diphenyacetylene Derivatives. Molecules 1, 6, and 719-21 have been synthesized as reference compounds according to the protocols reported in the literature, while the synthesis of compounds 2-5 have been achieved by exploiting Pd-catalyzed cross-coupling reactions starting from 4-(N,N-dihexylamino)iodobenzene 10 as obtained in good yield by substitution reaction of 1-bromo-n-hexane with 4-iodoaniline (Scheme 1, path a). Compound 10 was then converted to its acetylenic TMS-protected derivative 11 thorough a Sonogashiratype cross-coupling reaction28 in the presence of [PdCl2(PPh3)2] and CuI in (i-Pr)2NH and subsequently deprotected under basic conditions to give 12 (Scheme 1, paths b and c). Two coupling reactions, respectively, with 4-iodobenzotrifluoride and pentafluoroiodobenzene, provided then the targeted products 4 and 5 (Scheme 1, paths d and e). Intermediate 10 was also used for the preparation of two other targeted molecules, 2 and 3. These were in fact obtained again by applying a Pd-catalyzed coupling reaction to acetylenic derivatives 822 and 9,23 prepared in quantitative yields according to protocols already reported in the literature (Scheme 1, paths f and g). Photophysics in Solution. Absorption, emission, and lifetime data have been recorded at rt in three solvents of different polarity, i.e., MeCN, MeOH, and CH2Cl2, with dielectric constant values εr ) 35.94, 32.66, and 8.93, respectively.27 Lowtemperature emission spectra and excited state lifetimes have been obtained in MeOH/EtOH (1:4) mixtures at 77 K. The electronic absorption and emission spectra (at room and low temperature) for the compounds under investigation are shown in Figures 1-3, and concerned data are collected in Table 1. Absorption. The unsubstituted diphenylethynylene 1 displays an envelope of intense absorption bands (ε ) 20-30 000 mol-1 · L · cm-1 in MeOH) in the UV region of the spectrum (Figure 1, top), between 220 and 300 nm, which are assigned to π,π* S0fS2 electronic transitions, since the direct excitation from the ground state S0 to the lowest singlet S1 is forbidden.29 The observed progression of peaks can be ascribed to both CtC stretching and ring-breathing modes.15 The 4-(N,N-dihexyl)amino derivatives 2, 3, 4, and 5 present, in addition to the same high energy π,π* transitions, now structureless and of lower intensity (λmax ) 267, 294, 245, and 241 nm with ε ) 16 800, 22 000, 13 100, and 15 200 mol-1 · L · cm-1, respectively, in MeOH), an additional intense absorption band at lower energy (λmax ) 378, 417, 356, and 357 nm with ε ) 32 700, 18 300,
Properties of Tolan Wavelength Shifters
J. Phys. Chem. C, Vol. 113, No. 41, 2009 17931
SCHEME 1: Synthesis of the Donor/Acceptor-Substituted Diphenyl Acetylene Derivatives 2-5a
Reagents and conditions: (a) KI, Na2CO3, DMF, 95°C, 20 h, 65%; (b) [PdCl2(PPh3)2], CuI, (i-Pr)2NH, rt, 12 h, 61%; (c) K2CO3, MeOH/THF, rt, 1 h, 100%; (d) [PdCl2(PPh3)2], CuI, (i-Pr)2NH, rt, 12 h, 58%; (e) [PdCl2(PPh3)2], CuI, (i-Pr)2NH, rt, 12 h, 60%; (f) [PdCl2(PPh3)2], CuI, (iPr)2NH, rt, 12 h, 61%; (g) [PdCl2(PPh3)2], CuI, (i-Pr)2NH, rt, 12 h, 54%.
Figure 2. Emission spectra of the four asymmetric diphenyl acetylene derivatives and the model 1 in air-equilibrated CH2Cl2 solution, scaled according to the photoluminescence quantum yield.
Figure 1. Absorption spectra of the symmetric (top) and asymmetric (bottom) diphenyl acetylene derivatives in MeOH solutions.
34 400, and 42 800 mol-1 · L · cm-1, respectively, in MeOH), the position of which is solvent sensitive (see Table 1). The polarizability of the solvent, which follows the order CH2Cl2 . MeCN > MeOH, can account for the observed shifts. According to semiempirical calculations (performed using the PM3 method as implemented in HYPERCHEM 8.02 for Windows), the transitions in the region between 230 and 330 nm for molecule 2 relate to a state with a calculated energy of 35 920 cm-1, identified as the S2 π,π* state in precursor 1. The band peaking at 378 nm results from the overlap of two bands, one at 30 740 cm-1, which can be attributed to the S0fS1 transition, red-shifted by 4790 cm-1 compared to the same
forbidden transition for diphenyl acetylene 1, and one at 28 600 cm-1, a band peculiar of the disubstituted phenyl acetylene that can be assigned to an ICT state (see Supporting Information). It is possible to assume the same distribution of states in the description of the absorption features of the other 4-dihexylamino derivatives. A clear trend is observed in the position of the lowest energy absorption band along the series of the four studied asymmetric derivatives: a bathochromic shift occurs in passing from molecule 4 and 5, the bands of which appear at the same energy, to 2 and 3, the latter presenting the most redshifted absorption (Figure 1, bottom). The trend follows the withdrawing ability of the acceptor group present in the dye: the “pull” effect of a fully fluorinated phenyl ring is moderate and similar to that of a CF3 para-substituted ring; the CtN and, in particular, the p-nitro substitution has a much higher effect, as expected on the basis of the relative values of the Hammett constants of the substituents in the para position (σp ) 0.54, 0.66, and 0.78 for -CF3, -CtN, and -NO2, respectively).27 These observations confirm the effectiveness of the desired push-pull effect and the formation of an ICT state in the D-π-A dyes under examination; the stabilization of such an ICT transition, in fact, should follow the withdrawing strength of the accepting group. Another confirmation of the right assignation of the absorption transitions comes from the
J. Phys. Chem. C, Vol. 113, No. 41, 2009
Figure 3. Emission spectra of the symmetric (top) and asymmetric (bottom) diphenyl acetylene derivatives in MeOH/EtOH 1:4 frozen glass at 77 K.
observation that the acidification of methanol and acetonitrile solutions of 2 determines a relevant change in the absorption spectra: the low-energy band disappears quite completely and the transitions below 300 nm closely resemble those of the unsubstituted 1 (see Supporting Information). This can be explained by considering that the protonation of the 4-dihexylamino substituent causes the loss of the electron-donating ability of the “push” group and only the π,π* transitions of the diphenylethynylene core are allowed. The two symmetric derivatives 6 and 7 display an envelope of intense absorption bands between 300 and 400 nm (λmax ) 354 and 329 nm with ε ) 55 200 and 23 400 mol-1 · L · cm-1, respectively, in MeOH; Table 1 and Figure 1, top). These bands can derive from π(n),π* transitions, but their attribution is not straightforward. The values of the extinction coefficients, very high in molecule 6 and almost double with respect to molecule 7, are ascribable to the presence of two strong electron-donating groups in the former and two strong electron-withdrawing groups in the latter attached to the aromatic core. The position of these bands slightly blue-shifts in passing from CH2Cl2 to MeOH (Table 1), as observed for the asymmetric derivatives. Emission. For the tolan reference molecule (1), a weak emission at rt has been registered in both CH2Cl2 and MeOH (emission quantum yield, φ ∼ 10-3; see Table 1 and Figure 2). An intense and well-resolved emission is recorded, instead, in frozen glass at 77 K (Figure 3, top). The strong dependence of the emission quantum yield of this molecule from the temperature is well-documented in the literature and is attributed to the close vicinity of the S2 and S1 excited states, the latter being a nonemissive “dark” state with a distorted geometry.15,29 Quantum mechanical calculations confirmed the vicinity of the two states: energy levels of 36 000 cm-1 and of 35 500 cm-1 were calculated for S2 and S1, respectively (details of the calculation are given in Supporting Information). The activation energy barrier renders the S2fS1 decay thermally activated and
Aurisicchio et al. thus operative only at rt: the fast depopulation of S2 causes its very weak and short living emission. The rt emission is not solvatochromic (Table 1), and its Stokes shift is very low (ca. 1000 cm-1) as expected for emission coming from π,π* and not distorted states. The 77 K emission presents a Stoke’s shift close to zero and a vibrational progression mirroring that observed in the absorption spectrum. Low emission quantum yields (from 1 × 10-3 to 1 × 10-2; Table 1) have been observed at rt for the four asymmetric derivatives in MeOH. In CH2Cl2, all these compounds, but molecule 3, show a higher quantum yield that reaches the significant values of 0.24 and 0.37 for 5 and 2, respectively. At variance with the behavior of precursor 1, the emission of the D-π-A dyes presents a large Stoke’s shift. Its value in MeOH solutions, ca. 6400 cm-1 for dyes 4 and 5, increases to 7000 and 8100 cm-1 for 2 and 3, respectively (Table 1). These observations indicate that the energy level of the emitting state, which is not the same emitting state as in the tolan reference 1, is lowered accordingly to the withdrawing ability of the “pull” substituent. The position of the emission maximum of each asymmetric derivative also depends on the polarity of the solvent: a bathochromic shift of the maximum is observed as the polarity of the solvent increases for all compounds (Table 1), except for molecule 5, for which the apparent emission maximum is red-shifted in CH2Cl2 with respect to MeOH. Peak fitting of the emission profile of 5 in CH2Cl2, however, reveals that the E00 transition peaks at 452 nm (a shoulder in the spectrum of Figure 2; see Supporting Information) and is thus slightly blue-shifted with respect to MeOH, similarly to the other compounds. The fluorescence quenching observed in MeOH, with a decrease in quantum yield larger than 1 order of magnitude with respect to CH2Cl2, could in part be accounted for by the polarity change of the solvent. In addition, in linear D-π-A push-pull systems, where a large change in the dipole moment takes place upon excitation, a significant change in the energy of the hydrogen bonds formed with the protic solvent molecules may be expected.30 Accordingly, in the direct neighborhood of the donor group (D) and/or acceptor group (A), the excited state relaxation by intermolecular hydrogen bonding could contribute substantially to the radiationless deactivation of the excited molecule.31 In fact, the formation, or the change in the strength, of the intermolecular hydrogen bonds following excitation may yield new vibronic dissipative modes that couple excited and ground states.32,33 Moreover, excited state hydrogen-bond interactions might also be accompanied by various degrees of electron-proton transfers, leading to reversible or irreversible proton displacements or H-atom transfer with formation of nonradiative species.34,35 However, the study of these fundamental relaxation processes falls outside the scope of the present work. For molecule 2 the emission properties were studied in three different solvents, and a linear correlation was established between the transition energy and the solvent polarity expressed by its dielectric constant (see Supporting Information). This solvatochromic behavior has been already observed for parasubstituted push-pull tolans and has been explained by considering an emitting ICT state with an increased dipole moment compared to that of the ground state.13,14,36,37 In MeOH acid solutions the rt emission of 2 has a very low Stoke’s shift (see Supporting Information) and the yield is quite high (0.29) with a lifetime below the experimental resolution (<0.5 ns), indicating a very high radiative constant. On the other hand, the emission at 77 K is well-resolved (see Supporting Information) with a lifetime of 0.6 ns. The similarity of the spectroscopic
Properties of Tolan Wavelength Shifters
J. Phys. Chem. C, Vol. 113, No. 41, 2009 17933
TABLE 1: Photophysical Parameters of the Investigated Compounds in Solution emission absorption (rt)
εmax (M-1 cm-1)
278 295 281 299 267 378 268 385 266 379 294 417 299 431 245 282 356 246 285 362 241 357 243 363 354 380 360 387 329 335
30 800 26 200 -
16 800 32 700 -
22 000 18 300 -
13 100 9 700 34 400 13 800 9 300 36 200 15 200 42 800 -
55 200 49 300 56 700 51 100 23 400 25 400
nd nd nd
MeOH DCM MeCN
3.1 × 106
2.4 1.8 × 106
In MeOH, CH2Cl2, and MeCN solutions. b In MeOH/EtOH (1:4) mixture. c Apparent maximum; see discussion in the text.
features of 2 in acidified solvent with those of the precursor 1 indicates that the ICT state is destabilized in the acidified solvent, as expected considering that the protonation of the aminic nitrogen preclude the charge migration. The fact that the emission quantum yield of 2-H+ is 50-100 times higher than that of tolan seems to indicate that the introduction of the substituents renders the S1 state less effective in producing the nonradiative deactivation of S2. For all the asymmetric derivatives the CT nature of the emission is confirmed by the observation of an important hypsochromic shift in the maximum of the low-temperature spectrum compared to the rt one (Table 1 and Figures 2 and 3). It is worthwhile to note that, in rigid matrix at 77 K, all examined push-pull dyes showed brighter emission with respect to the rt case, due to the decreased operativity of nonradiative deactivating pathways. This effect is important also in view of the preparation of highly performing films: all studied asymmetric compounds, in fact, present a high emission quantum yield when embedded in plastic thin films (see below). The two symmetric derivatives display a very weak (6) or almost absent (7) rt emission both in MeOH and in CH2Cl2 (Table 1), whereas at 77 K a weak structured fluorescence was observed (Figure 3 top and Table 1). In contrast to the asymmetric derivatives, a phosphorescence emission, peaking around 510 nm, was observed in frozen glass at low temperature (77 K) for the symmetrically substituted compounds 6 and 7. The normalized phosphorescence spectra are shown in Figure 4, and concerned photophysical parameters are collected in Table 1. In the case of the dinitro derivative 7 a vibronic structure in the emission spectrum is observed, while in the case of the
Figure 4. Phosphorescence spectra of the symmetric diphenyl acetylene derivatives 6 and 7 in MeOH/EtOH 1:4 frozen glass at 77 K.
amino compound 6 the bands are broadened and the vibrational peaks appear only as shoulders. The separation of ca. 1200 cm-1 is consistent with phenyl ring symmetric vibrational modes. The observed phosphorescence can be ascribed to Coulomb exchange and spin-orbit coupling between orbitally distinct states, enhanced by the internal heavy atoms (O or N), that would lead to a “concerted” charge recombination and intersystem crossing process, as previously stated by Khundkar and co-workers for similar symmetric ethynyl-bridged aromatic compounds.38 Photophysics in PMMA Thin Film. All the studied fluorophores, when embedded in the solid PMMA matrix ([email protected]
, n ) 1-7), show absorption spectra similar to those recorded in solution, with only a slight blue-shift of the band maxima (Table 2). The optical density of the prepared films
J. Phys. Chem. C, Vol. 113, No. 41, 2009
Aurisicchio et al.
TABLE 2: Photophysical Parameters of the Investigated Compounds in PMMA Thin Films emission
absorption: λmax (nm)
1 2 3 4 5 6 7
299 384 429 361 361 359 333
nd 434 545 416 422 422 nd
nd 0.92 ( 0.01 0.52 ( 0.05 0.75 ( 0.02 1.00 ( 0.05 <0.02 nd
nd 1.9 2.8 1.7 2.4 <0.5 nd
(thickness ca. 2.5 µm) ranges between 0.1 and 0.4 absorbance units, corresponding to 80% and 40% of transmitted light, respectively. The emission from the [email protected]
systems is blue-shifted compared to the emission of the corresponding asymmetric 2-5 derivatives in solution (Figure 5, Tables 1 and 2), according to the ICT characteristic. All organic dyes show now a monoexponential luminescence decay with excited state lifetimes very similar to those recorded from the emission in solution or in frozen glass matrix at 77 K (Tables 1 and 2). In general terms, most of the luminescent material studied shows a neat quantum yield increase with respect to solution (see Tables 1 and 2) when the luminophore is embedded in the polymer matrix. In the case of dyes 2 and 4, a 3 time increase in quantum yield is observed in passing from the dichloromethane solution to the polymer film. This effect is attributed to the increase of the medium rigidity that inhibits the nonradiative relaxation processes between various vibronic levels of the dye molecules, thus resulting in a lowering of the nonradiative constant knr and hence an increase of φ ) kr/(kr + knr). In the case of the nitro (3) and pentafluoro (5) derivatives, the matrix effect on the quantum yield is more important (40 and 350 time increase for [email protected]
and [email protected]
, respectively) and the contribution of other processes has to be taken into account, as discussed below. Organic dyes in thin solid films are able to form ordered aggregates, classified as H- and J-type on the basis of the observed spectral shift of the absorption maximum relative to the monomer absorption band (blue-shifted for the face-to-face H-type dimers and red-shifted for the side-by-side J-type dimers). J-aggregates usually exhibit enhanced fluorescence intensity with respect to the monomer,39 while in contrast H-aggregates are characterized by poor photoluminescence efficiency, although a few exceptions to this general rule have
Figure 5. Emission spectra of the asymmetric diphenyl acetylene derivatives 1% (w/w) in PMMA thin film (thickness ca. 2.5 µm), scaled according to the photoluminescence quantum yield.
been reported.40 However, in our case no spectral variation, neither in shape nor in energy of the absorption band (Table 1 and 2), has been observed in passing from the CH2Cl2 solution to the solid PMMA thin film (see Supporting Information, Figure 7_SI). Furthermore, the crystalline structure in thin films of linear conjugated molecules, as oligothiophenes or oligophenylenes, is based on stacked layers in which the long molecular axes are parallel to each other with a herringbone configuration.41-43 This configuration naturally leads to nonemitting H-aggregates because the transition dipoles are oriented along the molecular axis,44-46 as it happens for our diphenylacetylene push-pull derivatives. Thus, although we cannot exclude in principle the presence of highly luminescent J-aggregates, literature data and our experimental results suggest an unlikely contribution of these species to the quantum yield enhancement observed in passing from the solution to the solid state. Another process that might account for the observed results is the presence of nonemissive charge separated (CS) excited states. In fact, CS states with a hole localized on the amine substituent and an electron localized on the acceptor might lie lower in energy than the corresponding ICT state and contribute to its deactivation. CS states are stabilized by polar solvents and, on the contrary, are destabilized in rigid environments (like in glassy solution at 77 K or in PMMA film at rt), where the rearrangement of the solvating molecules is prevented. It should be noted that ab initio quantum mechanical calculation performed on the asymmetric derivatives places the LUMO energy of 3 roughly 1.2 eV below that of 4, while 5 lays in between; i.e., ELUMO(4) > ELUMO(5) > ELUMO(3) (see Supporting Information for calculation details). This trend parallels that already observed for the photoluminescence quantum yields in CH2Cl2 solution, namely, φ(2, 4) > φ(5) > φ(3). Therefore, the neat quantum yield increase observed in 3,[email protected]
, together with the bright emission shown by all asymmetric derivatives at 77 K, might be related to the destabilization of low-energy CS states with consequent reactivation of the radiative decay from ICT states. Only a faint emission has been detected from [email protected]
, while the other symmetric derivative [email protected]
and the tolan precursor [email protected]
have shown no luminescence at all. In the case of [email protected]
, this confirms that the thermal activation and not the fluidity of the medium is responsible for the depopulation of the emissive S2 excited state. Although in a rigid environment, no phosphorescence was observed at room temperature from the symmetric [email protected]
and [email protected]
systems. Conclusions In this work, the synthesis of several organic wavelength shifters with either asymmetrical D-π-A or symmetrical D-π-D/ A-π-A structure is reported, where D and A are an electrondonor and -acceptor group, respectively, and π is a conjugated diphenyl-ethynyl bridging unit. The photophysical properties of all chromophores have been investigated in solvents of different polarity and in PMMA thin films by steady-state absorption and fluorescence measurements. The spectral results in solution show that upon increasing the electron-donating and -accepting strength of the substituents, as well as the solvent polarity, a small solvatochromic shift in the absorption maxima but a large bathochromic shift in the fluorescence maxima are observed, accounting for a larger dipole moment in the excited state with respect to the ground state. The lowest energy transition is attributed to an intramolecular charge transfer (ICT) state populated upon photoexcitation. Such photoinduced ICT induces a strong solvent reorganization, which is correlated to the large Stokes shifts observed in the fluorescence spectra.
Properties of Tolan Wavelength Shifters All studied fluorophores, once embedded in solid PMMA matrix ([email protected]
), show slightly blue-shifted absorption spectra, similar to those recorded in solution. All compounds were revealed to be highly luminescent in condensed media, i.e., in frozen solutions at 77 K and in PMMA films at rt, with a hypsochromic shift of the emission band maxima, according to the ICT nature of the emitting state. In general terms, most of the studied luminescent molecules show a neat increase of the quantum yield when embedded in a rigid environment with respect to solution, in some cases remarkably higher than 1 order of magnitude, and monoexponential luminescence decays with excited state lifetimes shorter than 3 ns. The luminescence enhancement observed in polymer matrix is attributed mainly to the inhibition of nonradiative relaxation processes between various vibronic levels of the dye molecules caused by the increased medium rigidity. Nonemitting CS states below the active ICT states might play a role in the deactivation of the ICT excited state in solution, involving the LUMO level of the acceptor moiety. In a rigid environment (like in frozen glass at 77 K or in polymer thin film), the CS states are destabilized and radiative decay from ICT states becomes active. In principle, the formation of highly luminescent dye aggregates in the solid state could also come into play, but literature data and our experimental results suggest an unlikely contribution from these species. In conclusion, these observations show how the fluorescence behavior as measured in solution is not a good criterion for the selection and the evaluation of the potential materials properties of organic luminescent dyes. Thereby, the preparation of solid matrices,18 hosting wavelength shifters with good conversion performances, is a promising methodology for the design of new generations of organic-based devices for medical imaging applications. Acknowledgment. This work is supported by grants from the Italian National Research Council (CNR project PM.P04.010 “MACOL”), the Belgian National Research Foundation (FRSFNRS, contracts nr. 2.4.625.08 and 2.4.550.09), the “Loterie Nationale”, Re´gion Wallonne (SOLWATT program, contract nr. 850551), the University of Namur, and the European Union through the projects “STRING” (STRP-032636, FP6), “PRAIRIES” (MRTN-CT-2006-035810, FP6), and “FINELUMEN” (PITN-GA-2008-215399, FP7). Our colleague Francesco Barigelletti is gratefully acknowledged for helpful discussions. Supporting Information Available: Details of (i) the quantum mechanical calculations, (ii) the photophysical results in acid solution and (iii) the peak fitting procedure. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ruchti, R.; Karmgard, D.; Albrecht, M.; Andert, K.; Anselmino, P.; Baumbaugh, B.; Bishop, J.; Clendenen, V.; Dauerty, H.; Dreher, D.; Hurlbut, C.; Jensen, M.; Kamat, N.; Marchant, B.; Marchant, J.; McKenna, M.; Slusher, A.; Sommese, R.; Sparks, T.; Vigneault, M. IEEE 2004, 1086– 1090. (2) Dai, X. X.; Rollin, E.; Bellerive, A.; Hargrove, C.; Sinclair, D.; Mifflin, C.; Zhang, F. Nucl. Instrum. Methods Phys. Res. Sect. A 2008, 589, 290–295. (3) Sharma, N.; Neumann, D.; Macklis, R. Radiat. Oncol. 2008, 3, Art. 25. (4) Lewellen, T. K. Phys. Med. Biol. 2008, 53, R287–R317. (5) For some recent examples about linear conjugated molecules please refer to: (a) Diederich, F. Pure Appl. Chem. 2005, 77, 1851–1863. (b) Tykwinski, R. R.; Schreiber, M.; Carlon, R. P.; Diederich, F.; Gramlich, V. HelV. Chim. Acta 1996, 79, 2249–2281. (c) Tykwinski, R. R.; Schreiber, M.; Gramlich, V.; Seiler, P.; Diederich, F. AdV. Mater. 1996, 8, 226–231. (d) Miki, Y.; Momotake, A.; Arai, T. Org. Biomol. Chem. 2003, 1, 2655– 2660. (e) Anderson, A. S.; Kerndrup, L.; Madsen, A.; Kilsa, K.; Nielsen, M. B.; La Porta, P. R.; Biaggio, I. J. Org. Chem. 2008, 74, 375–382. (f) Moonen, N. P.; Pomerantz, W. C.; Gist, R.; Boudon, C.; Gisselbrecht, J. P.;
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