ARTICLE pubs.acs.org/JPCA
Highly Twisted Triarylamines for Photoinduced Intramolecular Charge Transfer J. Matthew Chudomel,† Boqian Yang,†,‡ Michael D. Barnes,†,‡ Marc Achermann,‡,|| Joel T. Mague,§ and Paul M. Lahti*,† †
Department of Chemistry and ‡Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003, United States Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States
§
bS Supporting Information ABSTRACT: 9-(N,N-Dianisylamino)anthracene (9DAAA), 9-(N,N-dianisylamino)dinaphth([1,2-a:20 -10 -j]-anthracene (9DAAH), and 9,10-bis(N,N-dianisylamino)anthracene (910BAA) were synthesized as highly twisted triarylamines with potential for photoexcited internal charge transfer. Crystallography of 9DAAA shows its dianisylamino group to be twisted nearly perpendicular to its anthracene unit, similar to a report for 910BAA. The solution fluorescence spectra show strong bathochromic shifts for each of the three molecular systems with strongly decreased quantum efficiency in higher polarity solvents. Solutionphase (ensemble) time-resolved photoluminescence measurements show up to 4-fold decreases in fluorescence lifetime in acetonitrile compared to hexane. The combined results are consistent with photoinduced, transient intramolecular charge-transfer from the bis-anisylamine unit to the polycyclic aromatic unit. Computational modeling is in accord with intramolecular transfer of electron density from the bis-anisylamino unit to the anthracene, based on in comparisons of HOMO and LUMO.
’ INTRODUCTION Triarylamines are important electron donor and fluorophore compounds, with potential applications to advanced organic photovoltaic systems. They have seen much use in the fabrication of various types of electronic devices based primarily or partly on organic electroactive components.1 Varying the structure of attached aryl units allows tuning of the electronic properties of triarylamines. As part of such tuning, we aimed to take advantage of the tendency of twisted geometry chromophores to generate charge-separated states,2 a potentially very useful property for many organic optoelectronic applications including light-harvesting and photoconductivity technologies. In this article, we report photophysical characterizations of 9-(N,N-dianisylamino)anthracene (9DAAA), 9,10-bis(N,N-dianisylamino)anthracene (910BAA), and 9-(N,N-dianisylamino)dinaphth([1,2-a:20 -10 -j]-anthracene (9DAAH) that have a sterically, enforced highly twisted geometry between a dianisylamine unit and an attached, larger aromatic moiety. 9DAAA and 9DAAH are new compounds, whose syntheses and characterization are reported herein; 910BAA was previously reported by Lambert et al.3 in studies of aminium radical cations, but without detailed photophysical characterization. All show electronic effects of torsional deconjugation and exhibit strong fluorescence spectral position and fluorescence lifetime solvatochromism that are consistent with the formation of charge-separated states in polar environments. r 2011 American Chemical Society
’ EXPERIMENTAL SECTION General Methods. All reagent chemicals were purchased commercially and used as received, unless otherwise described. Melting points are uncorrected. 1H NMR samples were analyzed at 400 MHz in deuterated solvents, and are reported in parts per million relative to tetramethylsilane standard. UVvis and emission spectral maximum positions are reported in nanometers. Relative quantum yields were determined ratiometrically by the method of Magde et al.4 using fluorescein in 0.1 M aqueous NaOH as standard (ΦFL = 0.924). Cyclic voltametry (CV) measurements were made in degassed acetonitrile solutions, using a 10 10 0.5 mm platinum foil suspended by a 0.5 mm diameter Pt wire (Goodfellow Corp.) as the working electrode, a CH Instruments CHI115 Pt wire counter electrode, and a CHI112P Ag/AgCl reference electrode immersed in 0.1 M NBu4PF6 as the supporting electrolyte. Voltages are reported versus the ferrocene/ferrocenium oxidation (Fc/Fc+) half-wave potential in acetonitrile as reference. 9-(N,N-Dianisylamino)anthracene (9DAAA). Reddish cubes with mp 177180 °C. HRMS (EI, m/z): calcd for C28H23NO2 405.1729; found 405.1733. 1H NMR (400 MHz, CDCl3, δ): 8.48 (s, 1 H), 8.14 (d, 2 H, J = 8.3 Hz), 8.05 (d, 2 H, J = 8.3 Hz), 7.44 Received: April 16, 2011 Revised: June 24, 2011 Published: July 11, 2011 8361
dx.doi.org/10.1021/jp203563y | J. Phys. Chem. A 2011, 115, 8361–8368
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Figure 1. Synthetic schemes for 9DAAA, 910BAA, and 9DAAH.
(t, 2 H, J = 7.8 Hz), 7.38 (t, 2 H, J = 7.8 Hz), 6.96 (d, 4 H, J = 8.1 Hz), 6.72 (d, 4 H, J = 8.1 Hz), 3.72 (s, 6 H). UVvis (hexane, 3.42 μM, λmax [ε/M1 cm1]): 290 [31900], 338 [9100], 356 [8800], 376 [6700], 450 [3790] nm. Fluorescence (450 nm excitation): hexane λmax = 505 nm, CH2Cl2 λmax = 587 nm, MeCN λmax = 612 nm. Cyclic voltammetry (MeCN, vs Fc/Fc+): Eox 1/2 = 306 mV. 9,10-Bis(N,N-dianisylamino)anthracene (910BAA). Reddish microcrystalline solid with mp >280 °C (dec), (lit.3 mp >280 °C). MS (EI, m/z): calcd for C42H36N2O4 632.3, found 632.2 (M + [29%]). 1H NMR(CDCl3, 400 MHz, δ): 8.19 (dd, 4 H, J = 6.6 Hz, J0 = 2.6 Hz), 7.34 (dd, 4 H, J = 6.8 Hz, J0 = 3.3 Hz), 7.01 (AA0 BB0 d, 8 H, J = 9.1 Hz), 6.75 (AA0 BB0 d, 8 H, J = 9.1 Hz), 3.74 (s, 12 H). UVvis (hexane, 24.6 μM, λmax [ε/ M1cm1]): 356 [3940], 376 [3740], 399 [2680], 470 [4350] nm. Fluorescence (500 nm excitation): hexane λmax = 531 nm, CH2Cl2 λmax = 562 nm, MeCN λmax = 630 nm. Cyclic voltammetry (MeCN, vs Fc/Fc+): Eox 1/2 = 288 mV. 9-(N,N-Dianisylamino)dinaphth[1,2-a:20 -10 -j]anthracene (9DAAH). Yellow microcrystalline solid with mp 238240 °C. HRMS (EI, m/z): calcd for C44H31NO2 605.2355; found 605.2363. 1H NMR (400 MHz, CDCl3, δ): 11.08 (s, 1 H),
9.04 (d, 2 H J = 8.3 Hz), 8.28 (d, 2 H J = 8 Hz), 8.07 (d, 2 H J = 8.1 Hz), 7.99 (d, 2 H J = 8 Hz), 7.86 (d, 2 H J = 8 Hz), 7.79 (d, 2 H J = 8 Hz), 7.62 (t, 2 H J = 7.1 Hz), 7.53 (t, 2 H J = 6.8 Hz), 7.11 (d, 4 H J = 9.1 Hz), 6.77 (d, 4 H J = 9.1 Hz), 3.72 (s, 6 H). UVvis (hexane, 26.4 μM, λmax [ε/M1 cm1]): 258 [50800], 293 [42900], 332 [38000], 451 [3800] nm. Fluorescence (450 nm excitation): hexane λmax = 507 nm, CH2Cl2 λmax = 593 nm, MeCN λmax = 609 nm. Cyclic voltammetry (MeCN, vs Fc/Fc+): Eox 1/2 = 333 mV. Fluorescence Lifetime Measurements. Time-resolved photoluminescence (PL) measurements of solution samples were carried out using a time-correlated single-photon counting (TCSPC) approach. Solution samples were excited with PicoQuant pulsed diode lasers at wavelenghs of 407 (9DAAA) or 440 nm (910BAA, 9DAAH) and a typical excitation pulse width of 50 ps. The sample PL was spectrally dispersed with a monochromator, detected with a cooled multichannel plate photomultiplier tube (Hamamatsu R3809U-51) or an avalanche photodiode (Id Quantique 100-APD), and analyzed with a TCSPC system (Becker-Hickl SPC-630 or PicoQuant PicoHarp 300). The laser and detection systems provided a 70 ps time resolution in timeresolved PL measurements. 8362
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910BAA3 and 910BTA9 were previously shown to have crystal structures with similarly twisted diarylamino groups. Despite multiple efforts, we were unable to obtain diffraction grade crystals for 9DAAH. Semiempirical PM310 calculations found two nearly isoenergetic structures for 9DAAH with strong twisting of the dianisylamino group relative to the polycyclic aromatic moiety: the structures differ only in having butterflyshaped versus helical11 dinaphthanthracene units, with the butterfly lower by 2 kcal/mol (no solvent effects included). Whatever the actual structure of the polycyclic aromatic portion of 9DAAH in solution or in solid, its main conformer in solution will have the dianisylamine unit strongly twisted like those in 9DAAA and 910BAA, based on its properties described below.
Figure 2. ORTEP diagram for one of the crystallographically distinct molecules of 9DAAA. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms generated by a riding model (see details in the Supporting Information).
Test TRPL lifetime decay curves were integrated against time, and the results were compared to the integrated steady-state fluorescence peak intensities that were recorded by an imaging spectrograph for the same solutions. A linear correlation was obtained, confirming the self-consistency of the TRPL methodology compared to the static spectral results.
’ RESULTS Figure 1 shows the syntheses of 9DAAA, 910BAA, and 9DAAH. N,N-Dianisylamine was made by literature5 methods, and coupled to 9-bromoanthracene with palladium catalysis6 to give 9DAAA as chunky reddish crystal aggregates that are best purified by chromatography followed by crystallization. Compound 910BAA was made by a similar procedure to give a red, microcrystalline product having properties and NMR spectra consistent with those reported3 by Lambert et al. Compound 9DAAH was synthesized by subjecting 9BrH to Pd-catalyzed coupling with dianisylamine to give the product as an orange, powdery solid. The synthetic intermediate 9BrH was made by the method of Wilcox et al.7 as shown in Figure 1. We previously studied 9BrH as a highly nonplanar polycyclic aromatic hydrocarbon (PAH) in the solid state,8 making it a convenient building block for the analogous 9DAAH, to see whether its behavior would be different from the other twisted triarylamines due to its much largeer solvent exclusion volume. Single crystal X-ray diffraction analysis of 9DAAA showed two crystallographically distinct molecules in its lattice, both with propeller-shaped structures (Figure 2) having strong twisting (73.1° and 77.7°) of the anthracenyl unit relative to the plane formed by the amino nitrogen and the phenyl carbons ipso to the nitrogen. The anisyl rings are twisted relative to one another by 47.7° and 47.2° in the two forms. The similar compounds
The strong twisting tendency of the dianisylamino groups in 9DAAA, 910BAA, and 9DAAH limits π-delocalization of their amino lone pair electrons into the polycyclic aromatic units, which would typically induce an upfield 1H NMR chemical shift of a proton para to the nitrogen. For example, the proton para to the amino nitrogen in N,N-dianisylaniline12 (circled position on structure DAAn) is found at δ 6.8, versus δ 7.3 for benzene and bromobenzene. By comparison, the protons para to the dianisylamino group in both 9DAAA and 9DAAH are shifted by
