Intramolecular Charge Transfer Photoemission of a Silicon-Based Copolymer Containing Carbazole and Divinylbenzene Chromophores. Electron Transfer Across Silicon Bridges

Intramolecular Charge Transfer Photoemission of a Silicon-BasedCopolymer Containing Carbazole and DivinylbenzeneChromophores. Electron Transfer Across Silicon BridgesMalgorzata 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-isopropylcar-bazole/dimethylsilylene bridge/divinylbenzene units wassynthesized and characterized. Dual fluorescence was observedin this copolymer in polar solvents. The absence of the secondband at the lower transition energy of the two emissionmaxima in nonpolar solvents and the quantitative correlationof the lower-energy emission band maxima with solventpolarity indicate that the lower-energy emission band arisesfrom an intramolecular charge transfer (ICT) state. A series ofmodel compounds was synthesized to investigate the source ofthe charge transfer. It was found that the Si-bridged dyad witha single N-isopropylcarbazole and a single divinylbenzene wasthe minimum structure necessary to observe dual lumines-cence. The lack of dual luminescence in low-temperature glasses indicates that the ICT requires a conformation change in thecopolymer. Analogous behavior in the Si-bridged dyad suggests that the ICT in the copolymer is across the silicon bridge. Resultsfrom time-resolved luminescence measurements with picosecond and subnanosecond excitation were used to support the thesisthat twisted charge-transfer states are the likely source of the observed dual luminescence.

■ INTRODUCTION

Copolymers with donor and acceptor groups separated bysilicon atom spacers display a wide array of photophysical andphotochemical phenomena.1,2 Synthesis of these polymers hasbeen motivated by the realization that electroluminescencefrom them is of interest in producing polymer-based, light-emitting diodes (LED).3 The silicon spacers were inserted tobreak the electronic conjugation1 in the archetypical conductingpolymer, polyphenylene-vinylene. If the conjugation in thispolymer were not broken, polyphenylene-vinylene would emitat longer wavelengths only. However, even with the siliconspacers breaking the electronic conjugation,4 electrolumines-cence can still be seen in a variety of homopolymers andcopolymers.5 This indicates that the transport of holes andelectrons is still possible even though electronic conjugation hasbeen broken.Silicon-bridged donor/acceptor copolymers with carbazoles

as the donors show both large fluorescence yields6 and largehole mobilities.7 This makes it possible to use photophysicaltechniques to understand the nature of the excited states8 thatcould be precursors of electroluminescence and chargetransport. To date, most of the photoluminescence work onthese polymers has been steady-state luminescence spectra thatare routinely taken along with new syntheses.1,9−12 On the basis

of these fluorescence studies, charge-transfer character has beenattributed to the excited states responsible for the red-shiftedspectra in some of the donor−acceptor copolymers.11 Thesespectral assignments are consistent with earlier work on silicon-bridged donor−acceptor dyads.13 In addition, there has beensome indication that dual luminescence was involved with thesebroad emission spectra (i.e., in copolymers and dyads).11,13 Sofar there has been no discussion of whether either these charge-transfer excited states or their dual luminescences were due totwisted intramolecular charge transfer (TICT) states.14

However, systematic studies have not been done that areaimed at the time-dependent excited-state processes involvedwith this class of copolymers. Understanding these processeshas the potential for gaining insight into two physical featuresthat could effect charge transport in such polymers and couldalso have wider theoretical implications. One issue is whetherthe charge can be transported through or around the siliconbridges (through space versus through bridge). Anotherquestion is whether significant conformational changes arenecessary to populate charge-transfer excited states (implica-tions for the TICT literature).14 In order to address both of

Received: May 12, 2014Published: June 5, 2014

Article

pubs.acs.org/JPCA

© 2014 American Chemical Society 4750 dx.doi.org/10.1021/jp504649p | J. Phys. Chem. A 2014, 118, 4750−4758

pubs.acs.org/JPCA

these two general topics via photophysical techniques, acopolymer was synthesized containing an electron donor (N-isopropylcarbazole) connected to an electron acceptor(divinylbenzene) by a silicon bridge. In addition, supplemen-tary syntheses of small model compounds were used to addressthese same two issues by providing contrasting structuralconstraints relative to the copolymer.

■ EXPERIMENTAL METHODSGeneral Considerations. 1H NMR (400 MHz), 13C NMR

(100 MHz), 29Si NMR (80 MHz), and DEPT spectra wererecorded on a Bruker Avance II 400 MHz spectrometer inCDCl3 solution. Chemical shifts are reported below in δ (ppm)with reference to the residue solvent (CDCl3) peak for

1H and13C and to TMS for 29Si. Mass spectra of the organic andorganometallic molecular products were obtained by GCMSanalysis (Varian Saturn 2100T, equipped with a BD-5 capillarycolumn (30 m), and an ion trap detector). Gel permeationchromatography (GPC) analyses were performed using anAgilent HPLC system equipped with a UV absorbance detectorand 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), weight-average 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 fromAvantor, carbazole from Alfa Aesar, styrene, 1,2-dibromo-ethane, ethanol, methanol, isopropyl bromide, tetrabutylammo-nium bromide, N-bromosuccinimide (NBS), magnesiumsulfate, calcium hydride, sodium hydride, potassium hydroxide,and CDCl3 from Aldrich, and chlorodimethylvinylsilane fromGelest. The solvents used for photophysical measurements(chloroform, cyclohexane, dichloromethane, diethyl ether, anddimethyl sulfoxide) were of spectroscopic grade from Merckand were used as received. Acetonitrile (gradient grade forliquid chromatography, Merck) and sulfuric acid (POCH) wereused as received. THF (inhibitor free, for HPLC, Sigma-Aldrich) was distilled over sodium, under argon. 1,4-Divinylbenzene15 as well as chlorodimethylvinylsilane werepurified 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 argonatmosphere. The ruthenium complex [RuHCl(CO)(PCy3)2]was prepared according to a literature procedure.16

Syntheses of Organosilicon Compounds. 3-(Dimethyl-vinylsilyl)-N-isopropylcarbazole (1a). Compound 1a wasobtained by Grignard’s reaction of chlorodimethylvinylsilaneand 3-bromo-N-isopropylcarbazole. A solution of 3-bromo-N-isopropylcarbazole (1.58 g, 5.48 mmol) in 5 mL of THF wasadded slowly dropwise to a suspension of Mg (0.2 g, 8.23mmol; surface activated by 1,2-dibromoethane, 0.1 mL) andchlorodimethylvinylsilane (0.9 mL, 6.57 mmol) in 5 mL ofTHF. After the addition was completed, the reaction mixturewas refluxed, and the process of the reaction was controlled byGC analysis. The mixture was cooled to room temperature, thesolvent was evaporated, and the product was extracted withhexane/water solvents. The organic layer was dried undermagnesium 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-isopropyl-carbazole was obtained as a colorless oil (yield 81%).

The spectroscopic characteristics of synthesized compound1a are described as follows:

1H 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 = 8.2Hz, 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

following a general procedure for silylative coupling reactions.17

The silylative coupling syntheses were carried out in 5 mL glassreactors equipped with a magnetic stirring bar under an argonatmosphere. The reaction mixtures contained toluene (0.5 M),the vinylsilane derivative, olefin (DVB or styrene), and aruthenium−hydride complex [RuHCl(CO)(PCy3)2] (1−2 mol%). The molar ratios of the reagents were stoichiometric. The

Chart 1. Structures of Compounds 1−4

The Journal of Physical Chemistry A Article

dx.doi.org/10.1021/jp504649p | J. Phys. Chem. A 2014, 118, 4750−47584751

system was heated in an oil bath at 100 °C for 24 h (molecularcompounds) or 72 h (copolycondensation). After the reactionswere complete, the solvents were evaporated under vacuum.The molecular products were separated with a silica gel column(hexane/CH2Cl2 = 1:1). The macromolecular product wasdissolved in THF, purified by repeated precipitation frommethanol, filtered, and dried under vacuum.

The spectroscopic characteristics of the compoundssynthesized 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″).

29SiNMR (80 MHz, CDCl3): δ −6.23. MS (EI): m/z (relativeintensity %): 274•+ (60), 259 (80), 201 (37), 184 (25), 171(37), 144 (15), 73 (100), 59 (30), 45 (20). The compound wasisolated 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−

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).

13CNMR (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 (Cifrom 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 wasisolated 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,

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)-di-methylsilylene-(E)-vinylene-(1,4-phenylene)-(E)-vinylene] (4).

1H 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).13C 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 NCC−), 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 wasisolated as a white powder, yield 58%.

Photophysical Measurements. UV−vis spectra wererecorded at room temperature using a Cary 5000 UV−vis−NIR. Fluorescence spectra, at room temperature and at lowtemperature (77 K), were measured on a PerkinElmer LS 50Band were corrected for instrumental response.18 Bothabsorption and emission spectra at RT were recorded using 1cm × 1 cm rectangular cells.Low-temperature measurements were carried out in NMR

tubes (5 mm o.d.) immersed in liquid nitrogen in a quartz



Dewar. A neutral-density filter was employed to decrease thevery high fluorescence intensity of compounds 3 and 4 at 77 K.Fluorescence lifetime measurements were carried out using atime-correlated single-photon-counting technique (Fluores-cence Lifetime Spectrofluorimeter FluoTime 300 fromPicoQuant, equipped with a 300 ± 5 nm diode as its excitationsource whose time profile, measured by the detection systemfollowing scattering through Ludox, was approximately 750 psfull width at half-height). Time-resolved fluorescence experi-ments were also performed in the Centre of Ultrafast LaserSpectroscopy, Adam Mickiewicz University. The Centre’sTi:sapphire Tsunami laser, tunable in the 720−1000 nmrange, was pumped with a 10 W (532 nm) Millennia X Prolaser. The output of the Tsunami laser consisted of 2 ps pulsesat a repetition rate of about 82 MHz and a mean power of over1 W. A pulse selector reduced the repetition rate from 82 to 4MHz. A harmonic generator was used to generate 285 nm asthe excitation wavelength by tripling the fundamental beamfrequency (855 nm).19

Electrochemical Measurements. The cyclic voltammetryexperiments were carried out with a potentiostat VersaSTAT3-400 from Princeton Applied Research. The measurements weremade with a standard three-component cell that contained aPTE platinum working electrode (6 mm o.d. × 1.6 mm i.d.), aplatinum-wire counter electrode and an SCE (saturated calomelelectrode) as the reference electrode. To determine theoxidation potential, Eox, of compound 1a (0.01 M) and thereduction potential, Ered, of compound 1b (0.01 M),voltametric measurements were performed on deoxygenatedACN solutions (purged with argon) with the addition of 0.1 Mtert-butylammonium perchlorate as the supporting electrolyte.

■ RESULTS AND DISCUSSION

From Chart 1, it can be seen that copolymer 4 is made up ofunits of model compound 3, but alternately copolymer 4 can beseen as being made of overlapping units of model compounds1a and 1b. This latter parsing of the copolymer into units isconvenient for analyzing the chromophoric content of thecopolymer because the silicon atoms can shift the spectra of thearomatic moieties. With this in mind, the absorption spectra ofthese two chromophores 1a and 1b are shown along with theabsorption spectrum of the copolymer 4 in Figure 1.

The additivity of the absorption spectra of the twochromophores to give the copolymer’s spectrum is taken toshow that there is little communication between the donor(carbazole, modeled by compound 1a) and acceptor(divinylbenzene, modeled by 1b) moieties across the siliconatom bridge in the ground state of the copolymer 4. Figure 1shows much less interaction between the monomers 3 and thecopolymer 4 with the small shift and broadening probably dueto exciton interactions.8 Furthermore, the absorption spectra ofthe 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, theemission spectrum (Figure 2) of the copolymer 4 was

reminiscent of the fluorescence spectrum of carbazole20 and,in particular, of compound 1a (Figure 3). However, the

copolymer 4 emission spectrum in acetonitrile (Figures 2 and3) showed that a second band appeared at long-wavelengths(low energies) in addition to the 370 nm band system in itsemission spectrum in cyclohexane (Figure 2). The emissionspectrum of 1a in acetonitrile (Figure 3) showed no such bandin the green part of its emission spectrum. The emissionspectrum of 4 in cyclohexane was similar to the emission

Figure 1. Absorption spectrum of 4 is displayed as a least-squares fit tothe sum of spectra 1a and 1b, and the absorption spectrum of 3 isdisplayed as a least-squares fit to the absorption spectrum of 4. Thesolvent was ACN in all spectra.

Figure 2. Fluorescence spectra of compound 4 in various solvents, λexc= 310 nm.

Figure 3. Normalized fluorescence spectra of compounds 1a, 1b, 3,and 4 in ACN.



spectra of 1a in both cyclohexane and acetonitrile (see Figures2 and 3).In contrast to the absorption spectrum (Figure 1) of

compound 1b, which appeared prominently as a component inthe absorption spectrum of copolymer 4, there was no evidencethat the fluorescence (Figure 3) of 1b makes any appearance inthe fluorescence spectra of copolymer 4. This could be due tothe very low fluorescence quantum yield of 1b relative to 1a(see Table 1), which could be masking the fluorescence of thefragment of 1b in compound 4’s luminescence.However, it can also be seen from Figure 3 (and also Table

1) that the first excited singlet state of 1b is higher in energythan the first excited singlet state of 1a. This could mean thatradiationless transitions23 or Forster singlet energy transfer24 israpid from the divinylbenzene component 1b to the carbazolefragments 1a in the copolymer. For example, Forster singletenergy transfer goes as the reciprocal of the sixth power of thedistance between energy donor−acceptor centers and couldaccount for the lack of a contribution to the luminescence of 4from its 1b components. The silicon spacers disrupt theconjugation, but they do not disrupt the communicationbetween energy (or electron) donors and acceptors, see alsobelow.The dual luminescence of copolymer 4 in acetonitrile was

seen in other solvents, particularly in polar solvents. Figure 2shows the emission spectra of 4 in the different solvents. Fromthis figure it can be seen that the carbazole-like emission bandsof copolymer 4 did not shift significantly but that the low-energy transition shifted significantly with solvent polarity.These shifts to lower energy with solvent polarity are

reminiscent of the Lippert−Mataga theory describing the shiftin luminescence with solvent polarity as arising from largerdipole moments in the excited states of the moleculescompared to the dipole moments in their ground states.24−26

The Lippert−Mataga equation is given by

ν νμρ = − Δ

hcf(0)

2CT CT

e2

3 (1)

where

εε

Δ =−+

− −+

fnn

12 1

14 2

S

S

2

2(2)

νCT is the maximum of the emission band in wavenumbers,νCT(0) is the hypothetical maximum of emission in vacuum, μeis the dipole moment of the excited state, h is Planck’s constant,c is the speed of light, ρ is Onsager’s solute cavity radius withinthe solvent, εS is the dielectric constant of the solvent, and n isthe solvent’s refractive index.

In Figure 4 the wavenumbers of the maxima of the low-energy transitions are plotted against the solvent parameter Δf.

Figure 4 shows a roughly linear plot as predicted by theLippert−Mataga equation for emission from an excited statewith a large dipole moment. In donor−acceptor compoundswith the moieties separated by silicon bridges, van Walee etal.13 saw similar linear Lippert−Mataga plots with slopes of−3.651 × 104 cm−1 for NH2−C6H4−Si(CH3)2−C6H4CNcompared to −3.2 × 104 cm−1 for the copolymer 4 in Figure4. They interpreted the behavior of their compounds as arisingfrom intramolecular charge transfer from the donor part of theirmolecules to the acceptor part of their molecules. Given thesimilarity of the slopes of their Lippert−Mataga plots and thatin Figure 4, this is suggestive that the solvent shifts in the low-energy transition in copolymer 4 are also due to emission froma charge-transfer excited state.The most likely charge transfer would be the full transfer of a

charge from the carbazole moiety 1a to the divinylbenzenemoiety 1b. The energy of this excited state can be estimatedfrom the Rehm−Weller equation:27

= − +E E E C(carbazole) (divinylbenzene)ICT ox red (3)

The Eox(1a) = 1.18 V vs SCE in acetonitrile and the Ered(1b) =−2.32 V vs SCE in acetonitrile. Taking C, the Coulombicenergy for an oppositely charged ion pair in a polar solvent as−0.056 eV as is commonly used for exciplex calculations inacetonitrile,27 the energy of the ICT state relative to theequilibrium ground state is 27 780 cm−1. This is below the localexcited state (LE) of the carbazole moiety 1a, 28 650 cm−1 (λ =349 nm, Table 1). The energy of the transition from this ICTstate would be expected to shift with solvent because of thelarge dipole moment of this state. The Coulombic energy willalso change with dielectric constant of the solvent. Thiscalculated energy is consistent with the scenario of the LE state

Table 1. Absorption and Emission Properties of Compounds 1a, 1b, 3, and 4 in ACN

λf,max (nm)

compd λabs,max (nm) LE ICT λS0−0 (nm) E(S1)(kJ mol−1) Φf

a τf (ns)

1a 346 353, 369 − 349 343 0.37b 15.31b 297 344 − 322 372 0.003b <0.5c

3 346 353, 370 488 349 343 0. 17d 20.4e

4 347 355, 370 494 350 342 0.17d 22.0e (15)f

aCalculated as total fluorescence quantum yields. bCarbazole in cyclohexane used as a reference compound to determine fluorescence quantumyields (0.38).20 cFor a similar system (E,E)-{1,4-bis(2-dimethylphenylsilyl)ethenyl}benzene, its fluorescence lifetime was determined to be 13.2 ±2.0 ps in acetonitrile.21 dQuinine sulfate in 1 N H2SO4 used as a reference compound to determine fluorescence quantum yields (0.54).22eFluorescence lifetimes determined for ICT band as single-exponential decays. fLongest fluorescence lifetime determined for ICT band in THF.

Figure 4. Fluorescence solvatochromism of compound 4.



being able to populate the closely lying ICT state as the solventbecomes more polar.There are several possible means by which the ICT could

occur. Intermolecular transfer seems unlikely because of thelow concentration of the copolymer and lack of evidence foraggregation in the absorption spectrum in Figure 1. However,being a copolymer, the formation of the ICT excited state couldbe formed by an intramolecular electron transfer from thecarbazole moiety to the divinylbenzene in two distinct ways.First, there could be transfer in the excited state from thecarbazole to the first adjacent divinylbenzene moieties on eitherside of the carbazole, or, second, this excited-state electrontransfer could happen between nonadjacent carbazole anddivinylbenzene units. Electron transfer would occur through theoverlap between occupied orbitals of the donor and unoccupiedorbitals of the acceptor. Since this overlap decreasesexponentially with distance,28 at first glance, one might expectthat electron transfer between adjacent donor and acceptorunits would not be particularly favorable. It might seem thatnonadjacent transfer of an electron between two intrachainmoieties that have diffused together might afford a betteroverlap of donor and acceptor orbitals than the overlapbetween adjacent donor and acceptor orbital across a siliconbridge. (For details see eqs 4 and 5 below.)A synthetic strategy was initiated to put some limits on the

answer to these questions. First of all, the absence of dualluminescence in compound 1a indicates that just having adimethylsilylvinylene group as the acceptor in the ICT is notsufficient to form an excited intramolecular CT state. Kim et al.interpreted their data as being due to ICT to a negative ionicsite on the silicon atom that, in their case, was stabilized byphenyl groups in their diphenylsilylene bridges.9 In the currentwork, the bridges are dimethylsilylene groups, and these bridgesdo not apparently provide adequate stabilization to form astable negative ionic site for an ICT state. Even with theterminal vinyl group for stabilization, it was not enough. Thenext degree of complexity was to synthesize 2, i.e., a carbazolelinked to a styrene through the dimethylsilylene bridge. Thiscompound also showed only carbazole-like fluorescence.However, when divinylbenzene was linked to the carbazolemoiety through the dimethylsilylene bridge in compound 3,dual luminescence was observed; see Figure 3.This is consistent with the van Walree et al. observations

where they saw intramolecular CT states forming acrossdimethylsilylene bridges.13 Through their electrochemicalmeasurements they got redox potentials for the substitutedaromatic donors (typically p-aminophenyl-) and acceptors(typically p-cyanophenyl-) that showed that the silicon atomwas involved in more favorable redox potentials for the electrontransfer. We saw no difference between the Eox of N-isopropylcarbazole and 1a. Furthermore, in the current work,the overlap between the donor and acceptor orbitals is not asfavorable due to the more extended nature of the donor andacceptor moieties. So it is likely that the silicon atom plays akinetic role in the electron-transfer process such as through asuperexchange mechanism.28,29

Thus, the ICT state can be formed on adjacent moieties inthe copolymer 4. Whether or not there are more complicatedkinetics was investigated by performing time-resolved single-photon-counting experiments. The luminescent decay tracesare shown in Figure 5 for copolymer 4 in THF.The 370 nm decay gave a very poor quality fit unless three

exponentials were used. In contrast, the ICT decay trace at 500

nm could be fit very well with a double exponential. The 370nm fluorescence trace is characterized initially, at least, by veryshort fluorescence lifetime, <500 ps, followed by longerlifetimes (3.8 and 14 ns). The ICT emission shows an apparentvery rapid growth following closely the pulse shape. This rapidgrowth is then followed by slow decays of 4.4 and 15 ns.The source of the multiple decays of the 370 and 500 nm

traces is most likely related to multiple conformations of thepolymer, but there are several possibilities for the nature of thisdecay. If the small perturbations on the absorption spectrum of3 compared to 4 are due to excitons, then exciton dynamicscould be responsible for the complex decay. Such variations inthe exciton dynamics would then also be related to the differingpolymer conformations. These conformations would havediffering resonance interactions between excited carbazolesites because of varying distance between these sites in thevarious polymer strands. If this is the source of the multiplelifetimes, the divinylbenzene units would have to be involvedsince only a single time constant was seen in the homopolymersinvolving silicon-bridged N-isopropyl carbazoles.12

Intramolecular electron transfer is often associated withconformal changes. For instance, there is a large body ofliterature on twisted intramolecular charge transfer (TICT)states.14,23 The issues of whether or how conformationalchanges affect the ICT state and its formation were pursued byperforming experiments in low-temperature glasses. Both BUNand DCM:MeOH (1:1) were used to form 77 K glassescontaining 4 in solution. Fluorescence spectra were measuredin these systems. The fluorescence spectra of 4 in BUN andDCM:MeOH showed similar behavior. See Figure 6 forDCM:MeOH. Both of these solvents are polar enough suchthat red-shifted fluorescences (500 nm) were seen at RT alongwith the carbazole-like fluorescence (370 nm). However, thesignificant feature of Figure 6 is that there was no dualluminescence of 4 in DCM:MeOH at 77 K. The same was seenfor 4 in BUN at 77 K.The contrasting fluorescence behavior of 4 in DCM:MeOH

and BUN at the two temperatures is suggestive that there arecritical conformational changes that are responsible for the low-energy fluorescence band at RT. There are three likelycandidates for this behavior. The first is that the ICT is dueto a twisted intramolecular charge transfer (TICT) state. In theTICT literature14 there are many examples of TICT statesresulting from conformational changes. If such a rotation isnecessary for the ICT and if the low-temperature glass hindersthe rotation, this would account for the difference in the

Figure 5. Fluorescence decays of compound 4 in THF, λmonit = 370and 500 nm (λexc = 300 nm).



fluorescence spectra at RT (room temperature) and LT (lowtemperature, 77 K) as seen in Figure 6.The second possible explanation for this difference in

behavior is that there are conformations having nonadjacentcarbazole and divinylbenzene in close contact that are notfavored in low-temperature glasses. This was mentioned aboveas a possibility that could account for the low-energyfluorescence band that would involve ICT without having touse the silicon bridges and hence have better overlap betweeninitial and final electron orbitals.Orbital overlap plays a critical role because electron transfer

involves the electronic exchange interaction matrix element

= Ψ Ψ | − |Ψ ΨV r r r r r r( ) ( ) 1/( ) ( ) ( )ET D 1 A 2 1 2 D 2 A 1 (4)

where ΨD and ΨA are one-electron molecular orbitals on thedonor and acceptor, respectively. Because the two molecularorbitals involve overlaps with themselves at the two differentsites, r1 and r2, the square of the electronic exchange interactionvaries approximately exponentially with the distance (r1 − r2)between donor and acceptor.28

β| | = | | − −V V r rexp{ ( )}ET2

ET,02

1 2 (5)

where the time dependence has been factored out of VET and βis the reciprocal of the distance between donor and acceptorthat the interaction goes to 1/e. Because of the flexibility of thecopolymer chain nonadjacent carbazole and divinylbenzenesites could involve smaller intersite distances than adjacent siteswhich include the extra distance of the silicon linker.One final low-temperature experiment was performed that

helped to distinguish between these two possible explanations.The fluorescence spectra of compound 3 was also measured inDCM:MeOH at RT and 77 K. The preliminary spectra wereperfectly analogous to the fluorescence spectra of 4 in Figure 6.In particular, a fluorescence spectrum of 3 in DCM:MeOH atRT showed both low- and high-energy bands, but at 77 K onlythe high-energy band was present. In compound 3, there is onlyadjacent carbazole and divinylbenzene, so the ICT has to beacross the silicon bridge. Furthermore, there is likely aconformational change that is being inhibited by the LTglass. So even though this does not totally eliminate thepossibility of an ICT between nonadjacent carbazole anddivinylbenzene moieties in 4, the existence of dualluminescence in 3 at RT and the lack of it at LT makes itplausible that the low-energy band of the dual luminescence in

4 is due to electron transfer across the silicon bridge betweenadjacent carbazole and divinylbenzene moieties.The third possibility for dual luminescence at RT and none

at LT is that separate conformations are present at RT in whichICT is possible in one such conformation, but that thisconformation is not present at LT. The absence of the ICTenabled conformation at LT could be that the conformation isinhibited by the structure of the LT glasses. But the moreplausible scenario is that the ICT-enabled conformation issomewhat more unstable than 3 or 4 in their LE-enabledconformation. The low temperatures would then favor themore stable conformation leading to the lack of ICTfluorescence. Such ground-state conformers of varying stabilityhave proven to be critical in understanding photophysical andphotochemical phenomena, for instance, s-cis and s-trans inpolyenes. Work on vitamin D precursors is a recent example.30

The difference between these two competing hypotheses canbe summarized by the TICT phenomena being a conforma-tional change in the excited-state manifold whereas the two-conformer model relates to ground-state conformations. Thereis an experimental way to distinguish between these possiblescenarios by looking at the formation kinetics of thefluorescence following a short excitation pulse. In the two-ground-state-conformer scenario, one would expect a minimumof difference in the rise times of the two fluorescence traces.Any shifts in the rising part of the trace would be related tonormal shifts, dependent on the decay time. In any event, therewould be little correlation between the two traces. In the caseof the TICT scenario, it would be expected that the LE statewould show a rapid decay that would match a rise time of theICT state’s fluorescence trace. Then the two traces might decaywith the same lifetime if they are in thermal equilibrium.The luminescence traces in Figure 5 do show the hint of a

fast decay of the 370 nm LE fluorescence trace, but the growthof the two traces (370 and 500 nm) appeared to convoluteadequately with the immediate response function (IRF) of thedetection system. However, the full width at half-height of thisdetection system and exciting diode is about 750 ps. In order toput more stringent limits on the rise times, solutions of 4 wereexcited with a picosecond excitation pulse.Room temperature fluorescence traces of 4 in THF at 370

and 500 nm following excitation by a 2 ps laser pulse at 285 nmare shown in Figure 7. Also shown is a fluorescence trace ofxanthione (XT) in CY at 460 nm. A fluorescence trace ofxanthione was used to reconstruct the shape of the IRF to beused in deconvoluting the 500 nm fluorescence trace of 4.31

Figure 6. Fluorescence spectra of compound 4 in DCM:MeOH (1:1),λexc = 310 nm.

Figure 7. Fluorescence traces of compound 4 in THF, λmonit = 370 and500 nm, in the picosecond range (λexc = 285 nm).



The decay of the LE fluorescence trace at 370 nm and thegrowth of the ICT fluorescence trace at 500 nm weredetermined taking into account deconvolution with the IRFswith respect to the respective wavelengths.Following deconvolutions, the decay of the 370 nm LE

fluorescence trace matches the rise of the 500 nm ICTfluorescence trace. Both time constants are approximately 25ps. This agreement between these two time constants isconsistent with the notion that the ICT state is being formedfrom the LE state which, in turn, is consistent with the TICThypothesis. Furthermore, the long decay times of the LE stateat 370 nm and the ICT state at 500 nm, shown in Figure 5, areconsistent with the two states being in thermal equilibrium.Combining the results in Figures 5 and 7, the time constant ofthe initial decay of LE and the ICT growth is associated withtheir approach to thermal equilibrium, and, after achievingequilibrium, they decay together.Finally, it was also necessary to show that the low-energy

emission was a fluorescence from a singlet ICT state and not aphosphorescence from a triplet state of the copolymer. To thelatter issue, it is noted that the emission lifetimes were of thesame order of magnitude as the fluorescence of carbazole, andthe triplet yield of a related silicon-bridged compound had avery low triplet yield.21 Lack of the lower-energy luminescenceat 77 K also can be counted as evidence against this emissionbeing phosphorescence. These observations suggest that theemission is fluorescence and not phosphorescence.

■ CONCLUSIONS

Dual emission was observed upon photoexcitation of the newlysynthesized copolymer 4. The dependence of the lower energyemission on solvent polarity showed that this excited state hada large dipole moment. Lack of this low-energy emission at lowtemperature indicated that both emissions were from singletexcited states. Using a synthetic strategy for systematicallybuilding up the copolymer until dual luminescence occurred, itwas shown that dyad 3 is the minimum structure for observingICT in the copolymer 4.Additional detail about the nature of the ICT state was

obtained in the low-temperature (77 K) experiments on 3 and4 that showed no ICT fluorescence at LT in polar glasses. Thisobservation was interpreted as being due to the LT glassesrestricting conformational changes necessary for ICT or to thetemperature changing the equilibrium populations of con-formers associated with LE and ICT states. Both possibilitieswould involve electron transfer through the dimethylsilylenebridge. These conformational changes were shown to beassociated with excited-state processes (likely involving a TICTstate) using time-resolved fluorescence with picosecond andsubnanosecond excitations.Experiments and calculations are underway on dyad 3 to

determine what is the precise nature of the time-dependent,excited-state processes.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given the approval to the final versionof the manuscript.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSFinancial support from The National Science Centre Grant(Poland) (Grant MAESTRO 2011/02/A/ST5/00472) isgratefully acknowledged. This is document no. NDRL-4988from the Notre Dame Radiation Laboratory which is supportedby the Office of Basic Energy Sciences of the US Department ofEnergy. The authors thank Dr. Jerzy Karolczak from the Centreof Ultrafast Laser Spectroscopy Adam Mickiewicz Universityfor performing and analyzing the picosecond experiments.

■ DEDICATIONThis paper is dedicated to the memory of Professor Klaus-Dieter Asmus.

■ REFERENCES(1) Wang, F.; Kaafarani, B. R.; Neckers, D. C. Synthesis of Silicon-Containing Unsaturated Polymers by Hydrosilylation Reactions.Photophysical Studies. Macromolecules 2003, 36, 8225−8230.(2) Luh, T.-Y.; Cheng, Y.-J. Alternating Divinylarene-SilyleneCopolymers. Chem. Commun. 2006, 4669−4678.(3) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.;Holmes, A. B. Synthesis of Light-Emitting Conjugated Polymers forApplications in Electroluminescent Devices. Chem. Rev. 2009, 109,897−1091.(4) Luh, T.-Y.; Hu, Z. Thorpe−Ingold Effect in OrganosiliconChemistry. Dalton Trans. 2010, 39, 9185−9192.(5) Jung, S.-H.; Kim, H. K.; Kim, S.-H.; Kim, Y. H.; Jeoung, S. C.;Kim, D. Palladium-Catalyzed Direct Synthesis, Photophysical Proper-ties, and Tunable Electroluminescence of Novel Silicon-BasedAlternating Copolymers. Macromolecules 2000, 33, 9277−9288.(6) Sasano, T.; Sogawa, H.; Tamura, K.; Shiotsuki, M.; Masuda, T.;Sanda, F. Synthesis and Properties of Conjugated PolymersContaining 3,9-Carbazolylene and Silylenevinylene Moieties in theMain Chain. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 1815−1821.(7) Zou, Y.; Gendron, D.; Badrou-Aïch, R.; Najari, A.; Tao, Y.;Leclerc, M. A High-Mobility Low-Bandgap Poly(2,7-Carbazole)Derivative for Photovoltaic Applications. Macromolecules 2009, 42,2891−2894.(8) Barford, W. Excitons in Conjugated Polymers: A Tale of TwoParticles. J. Phys. Chem. A 2013, 117, 2665−2671.(9) Paik, K. L.; Baek, N. S.; Kim, H. K.; Lee, J.-H.; Lee, Y. WhiteLight-Emitting Diodes from Novel Silicon-Based CopolymersContaining Both Electron-Transport Oxadiazole and Hole-TransportCarbazole Moieties in the Main Chain. Macromolecules 2002, 35,6782−6791.(10) Liu, X.-M.; Xu, J.; Lu, X.; He, C. A Comparative Study onLuminescent Copolymers of Fluorene and Carbazole with Conjugatedor δ-Si Interrupted Structures: Steric Effects. Macromolecules 2006, 39,1397−1402.(11) Wang, H.-W.; Cheng, Y.-J.; Chen, C.-H.; Lim, T.-S.; Fann, W.;Lin, C.-L.; Chang, Y.-P.; Lin, K.-C.; Luh, T.-Y. Photoinduced ElectronTransfer in Silylene-Spaced Copolymers Having Alternating Donor-Acceptor Chromophores. Macromolecules 2007, 40, 2666−2671.(12) Ludwiczak, M.; Majchrzak, M.; Bayda, M.; Marciniak, B.;Kubicki, M.; Marciniec, B. Stereoselective Synthesis and LuminescenceProperties of Novel Trans-Regular N-Alkylcarbazolylene-Silylene-Vinylene Polymers. J. Organomet. Chem. 2014, 750, 150−161.(13) van Walree, C. A.; Roest, M. R.; Schuddeboom, W.; Jenneskens,L. W.; Verhoeven, J. W.; Warman, J. M.; Kooijman, H.; Spek, A. L.Comparison between SiMe2 and CMe2 Spacers and σ-Bridges forPhotoinduced Charge-Transfer. J. Am. Chem. Soc. 1996, 118, 8395−8407.(14) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Structural ChangesAccompanying Intramolecular Electron Transfer: Focus on Twisted



mailto:[email protected]

Intramolecular Charge-Transfer States and Structures. Chem. Rev.2003, 103, 3899−4031.(15) Ludwiczak, M.; Majchrzak, M.; Marciniec, B.; Kubicki, M.Highly Stereoselective Synthesis of Novel Trans-Thiophenylene-Silylene-Vinylene-Arylene Molecular and Macromolecular Com-pounds. J. Organomet. Chem. 2011, 696, 1456−1464.(16) Yi, C. S.; Lee, D. W.; Chen, Y. Hydrovinylation and [2 + 2]Cycloaddition Reactions of Alkynes and Alkenes Catalyzed by a Well-Defined Cationic Ruthenium−Alkylidene Complex. Organometallics1999, 18, 2043−2045.(17) Marciniec, B. Catalysis by Transition Metal Complexes ofAlkene Silylation - Recent Progress and Mechanistic Implications.Coord. Chem. Rev. 2005, 249, 2374−2390.(18) Gardecki, J. A.; Maroncelli, M. Set of Secondary EmissionStandards for Calibration of the Spectral Responsitivity in EmissionSpectroscopy. Appl. Spectrosc. 1998, 52, 1179−1189.(19) Karolczak, J.; Komar, D.; Kubicki, J.; Wrozowa, T.; Dobek, K.;Ciesielska, B.; Maciejewski, A. The Measurements of PicosecondFluorescence Lifetimes with High Accuracy and SubpicosecondPrecision. Chem. Phys. Lett. 2001, 344, 154−164.(20) Berlman, I. B. Handbook of Fluorescence Spectra of AromaticMolecules, 2nd ed.; Academic Press: New York, 1971.(21) Burdzinski, G.; Bayda, M.; Hug, G. L.; Majchrzak, M.;Marciniec, B.; Marciniak, B. Time-Resolved Studies on the Photo-isomerization of a Phenylene-Silylene-Vinylene Type Compound in ItsFirst Singlet Excited State. J. Lumin. 2011, 131, 577−580.(22) Meech, S. R.; Phillips, D. Photophysics of Some CommonFluorescence Standards. J. Photochem. 1983, 23, 193−217.(23) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern MolecularPhotochemistry of Organic Molecules; University Science Books:Sausalito, CA, 2010.(24) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; SpringerScience+Business Media: New York, 2010.(25) Lippert, E. Habilitationsschrift Zur Erlangung Der Lehrber-ichtigung (Venia Legendi) Fur Das Fach Physikalische Chemie AnderTechischen-Hochschule-Stuttgart - Spektroskopische Bestimmung DesDipolmomentes Aromatischer Verbindungen Im Ersten AngeregtenSingulettzustand. Z. Elektrochem. 1957, 61, 962−975.(26) Mataga, N.; Kaifu, Y.; Koizumi, M. Solvent Effects UponFluorescence Spectra and the Dipolemoments of Excited Molecules.Bull. Chem. Soc. Jpn. 1956, 29, 465−470.(27) Rehm, D.; Weller, A. Kinetics of Fluorescence Quenching byElectron and H-Atom Transfer. Isr. J. Chem. 1970, 8, 259−271.(28) Newton, M. D. Quantum Probes of Electron-Transfer Kinetics:The Nature of Donor-Acceptor Interactions. Chem. Rev. 1991, 91,767−792.(29) McConnell, H. M. Intramolecular Charge Transfer in AromaticFree Radicals. J. Chem. Phys. 1961, 35, 508−515.(30) Redwood, C.; Bayda, M.; Saltiel, J. Photoisomerization of Pre-and Provitamin D3 in EPA at 77 K: One-Bond-Twist, Not Hula-Twist.J. Phys. Chem. Lett. 2013, 4, 716−721.(31) Wro zowa, T.; Ciesielska, B.; Komar, D.; Karolczak, J.;Maciejewski, A. Measurements of Picosecond Lifetimes by TimeCorrelated Single Photon Counting Method: The Effects of theRefraction Index of the Solvent on the Instrument Response Function.Rev. Sci. Instrum. 2004, 75, 3107−3121.

■ NOTE ADDED IN PROOFWe have been made aware that an alternate explanation is thatthe conformation that takes place in the excited state ofcompound 4 might be the conformational relaxation of thesolvent. Further experiments are planned.



Documents

Intramolecular Charge Transfer Photoemission of a Silicon-Based Copolymer Containing Carbazole and Divinylbenzene Chromophores. Electron Transfer Across Silicon Bridges