Photophysical Properties of Tolan Wavelength Shifters in Solution and

Sep 18, 2009 - Photophysical Properties of Tolan Wavelength Shifters in Solution and Embedded ... shifter materials that absorb in the region of the s...
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J. Phys. Chem. C 2009, 113, 17927–17935

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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: davide.bonifazi@ 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 (n@PMMA, 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

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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)

)

1 2V

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

(

A) 1-

Lc Lb

)

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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

η)

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

φem )

( InA )( I n )φ Ar

r

r r

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

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SCHEME 1: Synthesis of the Donor/Acceptor-Substituted Diphenyl Acetylene Derivatives 2-5a

a

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

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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 (