Article pubs.acs.org/JPCA
Intramolecular Charge Transfer Photoemission of a Silicon-Based Copolymer Containing Carbazole and Divinylbenzene Chromophores. Electron Transfer Across Silicon Bridges Malgorzata Bayda,*,†,‡ Monika Ludwiczak,‡ Gordon L. Hug,‡,§ Mariusz Majchrzak,‡ Bogdan Marciniec,†,‡ and Bronislaw Marciniak‡ †
Center of Advanced Technologies, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89 b, 61-614 Poznan, Poland § Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡
ABSTRACT: A new copolymer consisting of N-isopropylcarbazole/dimethylsilylene bridge/divinylbenzene units was synthesized and characterized. Dual fluorescence was observed in this copolymer in polar solvents. The absence of the second band at the lower transition energy of the two emission maxima in nonpolar solvents and the quantitative correlation of the lower-energy emission band maxima with solvent polarity indicate that the lower-energy emission band arises from an intramolecular charge transfer (ICT) state. A series of model compounds was synthesized to investigate the source of the charge transfer. It was found that the Si-bridged dyad with a single N-isopropylcarbazole and a single divinylbenzene was the minimum structure necessary to observe dual luminescence. The lack of dual luminescence in low-temperature glasses indicates that the ICT requires a conformation change in the copolymer. Analogous behavior in the Si-bridged dyad suggests that the ICT in the copolymer is across the silicon bridge. Results from time-resolved luminescence measurements with picosecond and subnanosecond excitation were used to support the thesis that twisted charge-transfer states are the likely source of the observed dual luminescence.
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of these fluorescence studies, charge-transfer character has been attributed to the excited states responsible for the red-shifted spectra in some of the donor−acceptor copolymers.11 These spectral assignments are consistent with earlier work on siliconbridged donor−acceptor dyads.13 In addition, there has been some indication that dual luminescence was involved with these broad emission spectra (i.e., in copolymers and dyads).11,13 So far there has been no discussion of whether either these chargetransfer excited states or their dual luminescences were due to twisted intramolecular charge transfer (TICT) states.14 However, systematic studies have not been done that are aimed at the time-dependent excited-state processes involved with this class of copolymers. Understanding these processes has the potential for gaining insight into two physical features that could effect charge transport in such polymers and could also have wider theoretical implications. One issue is whether the charge can be transported through or around the silicon bridges (through space versus through bridge). Another question is whether significant conformational changes are necessary to populate charge-transfer excited states (implications for the TICT literature).14 In order to address both of
INTRODUCTION Copolymers with donor and acceptor groups separated by silicon atom spacers display a wide array of photophysical and photochemical phenomena.1,2 Synthesis of these polymers has been motivated by the realization that electroluminescence from them is of interest in producing polymer-based, lightemitting diodes (LED).3 The silicon spacers were inserted to break the electronic conjugation1 in the archetypical conducting polymer, polyphenylene-vinylene. If the conjugation in this polymer were not broken, polyphenylene-vinylene would emit at longer wavelengths only. However, even with the silicon spacers breaking the electronic conjugation,4 electroluminescence can still be seen in a variety of homopolymers and copolymers.5 This indicates that the transport of holes and electrons is still possible even though electronic conjugation has been broken. Silicon-bridged donor/acceptor copolymers with carbazoles as the donors show both large fluorescence yields6 and large hole mobilities.7 This makes it possible to use photophysical techniques to understand the nature of the excited states8 that could be precursors of electroluminescence and charge transport. To date, most of the photoluminescence work on these polymers has been steady-state luminescence spectra that are routinely taken along with new syntheses.1,9−12 On the basis © 2014 American Chemical Society
Received: May 12, 2014 Published: June 5, 2014 4750
dx.doi.org/10.1021/jp504649p | J. Phys. Chem. A 2014, 118, 4750−4758
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The spectroscopic characteristics of synthesized compound 1a are described as follows:
these two general topics via photophysical techniques, a copolymer was synthesized containing an electron donor (Nisopropylcarbazole) connected to an electron acceptor (divinylbenzene) by a silicon bridge. In addition, supplementary syntheses of small model compounds were used to address these same two issues by providing contrasting structural constraints relative to the copolymer.
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EXPERIMENTAL METHODS General Considerations. 1H NMR (400 MHz), 13C NMR (100 MHz), 29Si NMR (80 MHz), and DEPT spectra were recorded on a Bruker Avance II 400 MHz spectrometer in CDCl3 solution. Chemical shifts are reported below in δ (ppm) with reference to the residue solvent (CDCl3) peak for 1H and 13 C and to TMS for 29Si. Mass spectra of the organic and organometallic molecular products were obtained by GCMS analysis (Varian Saturn 2100T, equipped with a BD-5 capillary column (30 m), and an ion trap detector). Gel permeation chromatography (GPC) analyses were performed using an Agilent HPLC system equipped with a UV absorbance detector and a RI detector (analysis conditions: mobile phase, THF; flow rate 0.80 mL/min; temperature 20 °C; injection volume, 17 μL). The number-average molecular weight (Mn), weightaverage molecular weight (Mw), and polydispersity index (PDI) were determined by a polystyrene standard calibration curve. The chemical reagents were obtained from the following sources: hexane, toluene, diethyl ether, tetrahydrofuran (THF), dimethylformamide (DMF), and acetone were purchased from Avantor, carbazole from Alfa Aesar, styrene, 1,2-dibromoethane, ethanol, methanol, isopropyl bromide, tetrabutylammonium bromide, N-bromosuccinimide (NBS), magnesium sulfate, calcium hydride, sodium hydride, potassium hydroxide, and CDCl3 from Aldrich, and chlorodimethylvinylsilane from Gelest. The solvents used for photophysical measurements (chloroform, cyclohexane, dichloromethane, diethyl ether, and dimethyl sulfoxide) were of spectroscopic grade from Merck and were used as received. Acetonitrile (gradient grade for liquid chromatography, Merck) and sulfuric acid (POCH) were used as received. THF (inhibitor free, for HPLC, SigmaAldrich) was distilled over sodium, under argon. 1,4Divinylbenzene15 as well as chlorodimethylvinylsilane were purified by “bulb-to-bulb” distillation and stored under argon. All the syntheses of monomers, macromolecular compounds, and catalytic tests were carried out under an inert argon atmosphere. The ruthenium complex [RuHCl(CO)(PCy3)2] was prepared according to a literature procedure.16 Syntheses of Organosilicon Compounds. 3-(Dimethylvinylsilyl)-N-isopropylcarbazole (1a). Compound 1a was obtained by Grignard’s reaction of chlorodimethylvinylsilane and 3-bromo-N-isopropylcarbazole. A solution of 3-bromo-Nisopropylcarbazole (1.58 g, 5.48 mmol) in 5 mL of THF was added slowly dropwise to a suspension of Mg (0.2 g, 8.23 mmol; surface activated by 1,2-dibromoethane, 0.1 mL) and chlorodimethylvinylsilane (0.9 mL, 6.57 mmol) in 5 mL of THF. After the addition was completed, the reaction mixture was refluxed, and the process of the reaction was controlled by GC analysis. The mixture was cooled to room temperature, the solvent was evaporated, and the product was extracted with hexane/water solvents. The organic layer was dried under magnesium sulfate and filtered. The solvent was evaporated, and the residue was separated with a silica gel column (hexane/ CH2Cl2 = 1:1, Rf = 0.5). 3-(Dimethylvinylsilyl)-N-isopropylcarbazole was obtained as a colorless oil (yield 81%).
H NMR (400 MHz, CDCl3): δ 0.46 (s, 6H, Si−CH3), 1.73 (d, JHH = 7.0 Hz, 6H, H2′), 5.01 (septet, 1H, H1′), 5.83 (dd, JHH = 3.9, 20.4 Hz, 1H, H2″), 6.11 (dd, JHH = 3.9, 14.6 Hz, 1H, H2″), 6.41 (dd, JHH = 14.6, 20.4 Hz, 1H, H1″), 7.24 (t, 1H, H7), 7.45 (t, 1H, H6), 7.54 (m, 1H, H8 and 1H, H2), 7.60 (d, JHH = 8.2 Hz, 1H, H1), 8.15 (m, 1H, H5), 8.30 (s, 1H, H4). 13C NMR (100 MHz, CDCl3): δ −2.40 (Si−CH3), 20.81 (C2′), 46.65 (C1′), 109.70 (C1), 109.94 (C5), 118.5 (C8), 120.31 (C6), 123.28 (Ci−Si), 125.33 (C7), 126.16 (C4), 126.35 (Ci from N− CC−), 130.67 (C2), 132.48 (C2″), 138.86 (C1″), 139.41 (N− Ci), 140.13 (N−Ci). 29Si NMR (80 MHz, CDCl3): δ −10.60. MS (EI): m/z (relative intensity %): 294•+ (9), 293 (36), 278 (100), 252 (16), 85 (23), 59 (17). A General Procedure for Silylative Coupling (SC). Compounds 1b, 2, 3, and 4 (Chart 1) were synthesized 1
Chart 1. Structures of Compounds 1−4
following a general procedure for silylative coupling reactions.17 The silylative coupling syntheses were carried out in 5 mL glass reactors equipped with a magnetic stirring bar under an argon atmosphere. The reaction mixtures contained toluene (0.5 M), the vinylsilane derivative, olefin (DVB or styrene), and a ruthenium−hydride complex [RuHCl(CO)(PCy3)2] (1−2 mol %). The molar ratios of the reagents were stoichiometric. The 4751
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system was heated in an oil bath at 100 °C for 24 h (molecular compounds) or 72 h (copolycondensation). After the reactions were complete, the solvents were evaporated under vacuum. The molecular products were separated with a silica gel column (hexane/CH2Cl2 = 1:1). The macromolecular product was dissolved in THF, purified by repeated precipitation from methanol, filtered, and dried under vacuum. H1′), 5.25 (d, JHH = 10.8 Hz, 1H, −CHCH2), 5.76 (d, JHH =17.6 Hz, 1H, −CHCH2), 6.69 (d, JHH = 11.0 Hz, 1H, −CHCH2), 6.71 (d, JHH = 19.1 Hz, 1H, H2″), 6.98 (d, JHH = 19.1 Hz, 1H, H1″), 7.20−7.25 (t, 1H, H7), 7.37−7.40 (d, JHH = 8.3 Hz, 2H, H5″), 7.44 (d, JHH = 8.0 Hz, 2H, H4″), 7.45 (t, 1H, H6), 7.53 (d, JHH = 8.3 Hz, 1H, H8), 7.55 (d, JHH = 8.1 Hz, 1H, H2), 7.64 (d, JHH = 8.4 Hz, 1H, H1), 8.15 (d, 7.8, 1H, H5), 8.32 (s, 1H, H4). 13C NMR (100 MHz, CDCl3): δ −2.27 (Si−CH3), 20.82 (C2′), 46.68 (C1′), 109.75 (C1), 113.79 (−CHCH2), 118.5 (C8), 123.28 (Ci−Si), 125.36 (C7), 126.24 (C4), 126.38 (C5″), 126.71 (C4″), 126.45 (Ci from NCC−), 130.80 (C2), 136.45 (−CHCH2), 137.29 (>CiCHCH2), 137.88 (C3″), 139.42 (C2″), 140.17 (N−Ci), 144.50 (C1″). 29Si NMR (80 MHz, CDCl3): δ −10.02. MS (EI): m/z (relative intensity %): 395•+ (100), 380 (40), 338 (6), 276 (8), 171 (8), 59 (5). The compound was isolated as a white powder, yield 60%. Poly[dimethylsilylene-(3,6-N-isopropylcarbazolylene)-dimethylsilylene-(E)-vinylene-(1,4-phenylene)-(E)-vinylene] (4).
The spectroscopic characteristics of the compounds synthesized by SC are described as follows: [(E,E)-1,4-Bis(trimethylsilyl)ethenyl]benzene (1b). 1H NMR (400 MHz, CDCl3): δ 0.09 (s, 18H, Si−CH3), 6.41 (d, JHH =
19.1 Hz, 2H, H2″), 6.79 (d, JHH = 19.1 Hz, 2H, H1″), 7.33 (s, 4H, H4′). 13C NMR (100 MHz, CDCl3): δ −1.26 (Si−CH3), 126.53 (C4″), 129.52 (C2″), 137.94 (Ci), 143.16 (C1″). 29Si NMR (80 MHz, CDCl3): δ −6.23. MS (EI): m/z (relative intensity %): 274•+ (60), 259 (80), 201 (37), 184 (25), 171 (37), 144 (15), 73 (100), 59 (30), 45 (20). The compound was isolated as a yellow powder, yield 88%. {(E)-[(3-N-Isopropylcarbazolyl)dimethylsilyl]ethenyl}benzene (2). 1H NMR (400 MHz, CDCl3): δ 0.54 (s, 6H, Si−
H NMR (400 MHz, CDCl3): δ 0.52 (s, 12H, −CH3), 1.67 (d, 6H, H2′), 4.99 (m, 1H, H1′), 5.22 (trace H2CCH−), 5.73 (trace H2CCH−), 6.42 (trace H2CCH−), 6.67 (d, 1H, H2′), 6.95 (d, 1H, H1′), 7.41 (s, 4H, H4″, and H5″), 7.52 (d, 2H, H2), 7.61 (d, 2H, H1), 8.14 (H5), 8.27 (HIV), 8.33 (d, 2H, H4). 13 C NMR (100 MHz, CDCl3): δ −2.0 (Si−CH3), 20.8 (C2′), 46.7 (C1′), 109.7 (C1), 110.0 (Ci-Si), 118.7 (CVIII), 120.3 (CVI), 123.1 (CV), 125.3 (CVII), 126.1 (C4), 126.5 (Ci from NC C−), 126.7 (C4″ and C5″), 130.9 (C2), 138.0 (C2″), 140.15 (N− Ci), 144.5 (C1″). 29Si NMR (80 MHz, CDCl3): δ −10.02, −9.96. GPC analysis: Mw = 4663 [g·mol−1], Mn = 3768 [g· mol−1], PDI (Mw/Mn) = 1.24, n = 9. The compound was isolated as a white powder, yield 58%. Photophysical Measurements. UV−vis spectra were recorded at room temperature using a Cary 5000 UV−vis− NIR. Fluorescence spectra, at room temperature and at low temperature (77 K), were measured on a PerkinElmer LS 50B and were corrected for instrumental response. 18 Both absorption and emission spectra at RT were recorded using 1 cm × 1 cm rectangular cells. Low-temperature measurements were carried out in NMR tubes (5 mm o.d.) immersed in liquid nitrogen in a quartz 1
CH3), 1.72 (d, JHH = 7.0 Hz, 6H, H2′), 5.01 (septet, 1H, H1′), 6.70 (d, JHH = 18.9 Hz, 1H, H2″), 7.01 (d, JHH = 18.9 Hz, 1H, H1″), 7.20−7.28 (m, 2H, H7 and H6″), 7.34 (t, 2H, H5″), 7.44 (t, 1H, H6), 7.48 (d, JHH = 7.3 Hz, 2H, H4″), 7.53 (d, JHH = 8.2 Hz, 1H, H8), 7.56 (d, JHH = 8.2 Hz, 1H, H2), 7.64 (d, JHH = 8.2 Hz, 1H, H1), 8.15 (d, JHH = 7.6, 1H, H5), 8.33 (s, 1H, H4). 13C NMR (100 MHz, CDCl3): δ −1.98 (Si−CH3), 20.82 (C2′), 46.67 (C1′), 109.74 (C1), 109.93 (C5), 118.71 (C8), 120.35 (C6), 123.22 (Ci−Si), 125.35 (C7), 126.24 (C4), 126.52 (Ci from NCC−), 128.04 (C6″), 128.10 (C5″), 125.51 (C4″), 130.81 (C2), 138.33 (C2″), 139.42 (N−Ci), 140.17 (N−Ci), 144.98 (C1″). 29Si NMR (80 MHz, CDCl3): δ −10.01. MS (EI): m/z (relative intensity %): 369•+ (40), 354 (100), 312 (10), 252 (20), 194 (10), 145 (28). The compound was isolated as a white powder, yield 87%. {(E)-4-[(3-N-Isopropylcarbazolyl)dimethylsilyl]ethenyl}styrene (3). 1H NMR (400 MHz, CDCl3): δ 0.53 (s, 6H, Si− CH3), 1.72 (d, JHH = 7.09 Hz, 6H, H2′), 4.95−5.06 (septet, 1H, 4752
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Dewar. A neutral-density filter was employed to decrease the very high fluorescence intensity of compounds 3 and 4 at 77 K. Fluorescence lifetime measurements were carried out using a time-correlated single-photon-counting technique (Fluorescence Lifetime Spectrofluorimeter FluoTime 300 from PicoQuant, equipped with a 300 ± 5 nm diode as its excitation source whose time profile, measured by the detection system following scattering through Ludox, was approximately 750 ps full width at half-height). Time-resolved fluorescence experiments were also performed in the Centre of Ultrafast Laser Spectroscopy, Adam Mickiewicz University. The Centre’s Ti:sapphire Tsunami laser, tunable in the 720−1000 nm range, was pumped with a 10 W (532 nm) Millennia X Pro laser. The output of the Tsunami laser consisted of 2 ps pulses at a repetition rate of about 82 MHz and a mean power of over 1 W. A pulse selector reduced the repetition rate from 82 to 4 MHz. A harmonic generator was used to generate 285 nm as the excitation wavelength by tripling the fundamental beam frequency (855 nm).19 Electrochemical Measurements. The cyclic voltammetry experiments were carried out with a potentiostat VersaSTAT3400 from Princeton Applied Research. The measurements were made with a standard three-component cell that contained a PTE platinum working electrode (6 mm o.d. × 1.6 mm i.d.), a platinum-wire counter electrode and an SCE (saturated calomel electrode) as the reference electrode. To determine the oxidation potential, Eox, of compound 1a (0.01 M) and the reduction potential, E red, of compound 1b (0.01 M), voltametric measurements were performed on deoxygenated ACN solutions (purged with argon) with the addition of 0.1 M tert-butylammonium perchlorate as the supporting electrolyte.
The additivity of the absorption spectra of the two chromophores to give the copolymer’s spectrum is taken to show that there is little communication between the donor (carbazole, modeled by compound 1a) and acceptor (divinylbenzene, modeled by 1b) moieties across the silicon atom bridge in the ground state of the copolymer 4. Figure 1 shows much less interaction between the monomers 3 and the copolymer 4 with the small shift and broadening probably due to exciton interactions.8 Furthermore, the absorption spectra of the copolymer do not change significantly with solvent, i.e., acetonitrile (ACN), butyronitrile (BUN), chloroform (CHL), cyclohexane (CY), dichloromethane (DCM), diethyl ether (DEE), dimethyl sulfoxide (DMSO), or tetrahydrofuran (THF). In contrast, the emission spectra of the copolymer 4 were significantly dependent on the solvent. In cyclohexane, the emission spectrum (Figure 2) of the copolymer 4 was
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Figure 2. Fluorescence spectra of compound 4 in various solvents, λexc = 310 nm.
RESULTS AND DISCUSSION From Chart 1, it can be seen that copolymer 4 is made up of units of model compound 3, but alternately copolymer 4 can be seen as being made of overlapping units of model compounds 1a and 1b. This latter parsing of the copolymer into units is convenient for analyzing the chromophoric content of the copolymer because the silicon atoms can shift the spectra of the aromatic moieties. With this in mind, the absorption spectra of these two chromophores 1a and 1b are shown along with the absorption spectrum of the copolymer 4 in Figure 1.
reminiscent of the fluorescence spectrum of carbazole20 and, in particular, of compound 1a (Figure 3). However, the
Figure 3. Normalized fluorescence spectra of compounds 1a, 1b, 3, and 4 in ACN.
copolymer 4 emission spectrum in acetonitrile (Figures 2 and 3) showed that a second band appeared at long-wavelengths (low energies) in addition to the 370 nm band system in its emission spectrum in cyclohexane (Figure 2). The emission spectrum of 1a in acetonitrile (Figure 3) showed no such band in the green part of its emission spectrum. The emission spectrum of 4 in cyclohexane was similar to the emission
Figure 1. Absorption spectrum of 4 is displayed as a least-squares fit to the sum of spectra 1a and 1b, and the absorption spectrum of 3 is displayed as a least-squares fit to the absorption spectrum of 4. The solvent was ACN in all spectra. 4753
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Table 1. Absorption and Emission Properties of Compounds 1a, 1b, 3, and 4 in ACN λf,max (nm) compd
λabs,max (nm) 346 297 346 347
1a 1b 3 4
LE
ICT
353, 369 344 353, 370 355, 370
− − 488 494
λS0−0 (nm) 349 322 349 350
E(S1)(kJ mol−1) 343 372 343 342
Φfa b
0.37 0.003b 0. 17d 0.17d
τf (ns) 15.3
