High-Generation Dendrimers with Excimer-like Photoluminescence for

Article pubs.acs.org/JPCC

High-Generation Dendrimers with Excimer-like Photoluminescence for the Detection of Explosives Paul E. Shaw,* Simon S. Y. Chen, Xin Wang, Paul L. Burn,* and Paul Meredith* Centre for Organic Photonics & Electronics, The University of Queensland, Brisbane, Queensland 4072, Australia S Supporting Information *

ABSTRACT: We report three generations of dendrimers incorporating either a fluorene or spirobifluorene core with carbazole dendrons and fluorene surface groups that are effective sensing materials for the detection of nitrated explosives by fluorescence quenching. The photophysical properties of the dendrimers were investigated with a combination of steady-state absorption and photoluminescence and time-resolved photoluminescence. We show that the first-generation dendrimers behave as single chromophores while the higher-generation dendrimers contain multiple chromophores that interact to give excimer-like emissive states. Stern−Volmer measurements with nitrated analytes show that the quenching efficiency decreases with generation for the planar fluorene-cored dendrimers and increases with generation for the more three-dimensional spirobifluorene-cored dendrimers. These contrasting trends are shown to be caused primarily by changes in the quenching efficiency of static interactions with the nitrated analytes, which is a consequence of the choice of core. Our results highlight the potential for exploiting such excimer-like states for chemical sensing, particularly in the case of nitrated explosives. (DNT), 7−9 a byproduct found in 2,4,6-trinitrotoluene (TNT).10 It was shown that, in contrast to conjugated polymers, which tend to form long-lived bound dark states with nitroaromatic analytes,11,12 the dendrimers could also exhibit a significant proportion of quenching through short-lived collisional interactions. Furthermore, dendrimers containing multiple chromophores were demonstrated to be quenched by a single analyte thus resulting in an amplified response.8 In this article, we present two series of dendrimers, each containing three generations (see Figure 1). The first series comprises a 9,9-di-n-propylfluorene core and surface groups with differing generations of carbazole dendrons. These compounds will be referred to as Fl(Gx)2 where x = 1, 2, or 3 denotes the generation. The second series are based on a 9,9′-spirobifluorene core with the same carbazole dendrons and 9,9-di-n-propylfluorene surface groups. The spirobifluorene compounds are named SBF(Gx)4 where again x denotes the generation. However, the choice of the 9,9′-spirobifluorene core allows four sets of dendrons to be attached versus the two that can be bound to the 9,9-di-n-propylfluorene core. In addition to the two dendrimer series, the individual dendrons were also studied with the G2 and G3 dendrons being N-protected by benzylation. It is important to note that the 9,9′-spirobifluorene core provides a pseudospherical shape to the dendrimers. The use of fluorene and carbazole moieties also means that each dendrimer can in principle contain multiple chromophores, potentially resulting in an amplified sensing response.8,13 These compounds therefore

1. INTRODUCTION The ability to rapidly and reliably detect traces of explosive compounds is a critical aspect of providing security in both civilian and military environments. The detection of explosives by photoluminescence (PL) quenching shows great potential as a technology that can be incorporated into compact portable sensors,1,2 as has been demonstrated with the “Fido” detector from FLIR Systems, Inc.3 The key component in any such fluorescence-based sensing device is the emissive chromophore, which is typically responsible for both the interaction with the target analyte and giving rise to the measurable response. In the case of oxidative quenching, the transfer of an electron from the excited chromophore to the target analyte results in efficient nonradiative relaxation to the ground state and thus a loss of fluorescence.4 It is therefore critical that the electron in the first excited state of the sensing chromophore has an energy that is greater than the electron affinity of the compound it has to detect by more than the exciton binding energy. Furthermore, the chromophore and the target analyte must have a degree of structural compatibility to promote interaction and potentially electrostatic binding between them.5,6 Ideally, the interaction between the sensing material and the target should be selective to minimize the potential of other compounds generating a “false” response.7 Meeting the requirements outlined above requires fine control of the structure of the sensing compound, which should also possess high PL efficiency. Conjugated dendrimers are able to address these requirements by virtue of their monodisperse tunable structure and high PL quantum yields (PLQYs). We have previously shown how dendrimers can be used to sense nitroaromatic compounds such as 2,4-dinitrotoluene © 2013 American Chemical Society

Received: January 9, 2013 Revised: January 29, 2013 Published: March 4, 2013 5328

dx.doi.org/10.1021/jp4002884 | J. Phys. Chem. C 2013, 117, 5328−5337

The Journal of Physical Chemistry C

Article

Figure 1. Structures of the dendrimers as defined by the core (SBF and Fl) and generation (G1, G2, and G3).

marker. The tetrahydrofuran was pumped at a rate of 1 mL/min at 40 °C. Mass spectra were recorded on an Applied Biosystems Voyager-DE STR instrument with a matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS; positive-ion, reflectron) setup, with (E)-2[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) as the matrix. Loading of the sample on the analysis stage was done by first spotting a solution of matrix in dichloromethane onto the plate, and when dried, a solution of dendrimer in a dichloromethane/light petroleum ether mixture (1:1) was spotted. Finally, a further aliquot of the matrix solution was deposited on top. Mass units (m/z) are presented in daltons, and intensities are quoted in percentages of the largest peak. TGA and DSC were performed on a Mettler Toledo TGA/ DSC/STARe and a Mettler Toledo DSC/STARe, respectively, with the former analyzed under a nitrogen flow (20 mL/min) from 25 to 600 °C at 10 °C/min in aluminum crucibles. Melting points were obtained in air using a Gallenkamp melting point apparatus and are reported uncorrected. Elemental analyses were carried out using a NA 1500 Carlo Erba NCHS analyzer. 2.1.1. 2,7-Bis[3,6-bis(9,9-{di-n-propyl}fluoren-2-yl)carbazolyl]-9,9-di-n-propylfluorene’. A mixture of 2,7-diiodo9,9-di-n-propylfluorene (152 mg, 0.30 mmol),15 3,6-bis[9,9-di-npropylfluoren-2-yl)carbazole16 (477 mg, 0.72 mmol), and tritert-butylphosphonium tetrafluoroborate (135 mg, 0.47 mmol) was deoxygenated in a Schlenk tube by placement under vacuum and backfilling with argon three times. Sodium tert-butoxide (108 mg, 1.12 mmol) and tris(dibenzylideneacetone)dipalladium(0) (53.4 mg, 51.7 μmol) were added, and the mixture was deoxygenated by placement under vacuum and backfilling with argon a further three times. Anhydrous xylenes (2 mL) were added, and the mixture was deoxygenated by placement under vacuum and backfilling with argon three times. The mixture was stirred vigorously for 48 h under argon in an oil bath held at 130 °C. After cooling, water (10 mL) was added, and the two layers were separated. The organic layer was washed with water (2 × 10 mL), and the combined aqueous fractions were extracted with dichloromethane (2 × 10 mL). The combined organic fractions were dried over magnesium sulfate and filtered, and the solvent

provide a platform for investigating the effects of dendrimer generation, shape, and number of chromophores on the photophysics and thus the sensing performance.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis and Characterization. The synthesis of the two families of dendrimers was achieved by a convergent method. That is, the complete dendrons were made before coupling to either 2,2′,7,7′-tetrabromo-9,9′-spirobifluorene (SBF) or 2,7-diiodo- or 2,7-dibromo-9,9-di-n-propylfluorene (Fl) under Buchwald−Hartwig amination conditions to furnish the corresponding dendrimer. The first-, second-, and thirdgeneration (denoted G1, G2, and G3, respectively) dendrimers with an Fl core were isolated in yields of 58%, 52%, and 82%, respectively. The synthesis of the SBF-cored dendrimers have been previously reported.14 Gel permeation chromatography against poly(styrene) standards showed that the dendrimers formed this way were all monodisperse; that is, they were discrete macromolecules. Thermal gravimetric analysis (TGA) showed that the dendrimers were stable to temperatures greater than 300 °C, and differential scanning calorimetry (DSC) showed that the dendrimers did not have a glass transition temperature within the experimental limits (0−280 °C). 1 H NMR spectra were recorded on Bruker Avance spectrometers (300, 400, or 500 MHz). Chemical shifts (δ) are reported in parts per million (ppm) relative to the residual solvent peak. Multiplicities are reported as singlet (s), doublet (d), triplet (t), and multiplet (m), and coupling constants (J) are quoted to the nearest 0.5 Hz. Assignments of peaks are as follows: carb H = carbazole H with G1, G2, or G3 denoting the generation number; surf Fl H = surface fluorenyl H; core Fl H = core fluorene H; where a fluorene proton cannot be definitively assigned as surface or core, they are denoted as Fl H. Gel permeation chromatography was carried out on a Polymer Laboratories PL-GPC 50 using PLgel mixed-A columns (600 mm + 300 mm lengths, 7.5 mm diameter) from Polymer Laboratories calibrated with poly(styrene) narrow standards (Mp = 162−6.03 × 106) in tetrahydrofuran with toluene as the flow 5329



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was removed in vacuo. The crude residue was purified by column chromatography over silica using a dichloromethane/light petroleum mixture (3:7) as eluent. The main band was collected, and the solvent was removed. The residue was precipitated by adding a saturated dendrimer solution in dichloromethane dropwise to methanol at room temperature. The mixture was stirred for 30 min, and the precipitate was collected at the filter to give Fl(G1)2 as a cream solid (276 mg, 58%). Mp 296−299 °C; Found: C, 90.4; H, 7.15; N, 1.8. C119H116N2 requires: C, 90.8; H, 7.4; N, 1.8; δH (500 MHz, CDCl3) 8.55 (4 H, d, J = 1.5, G1-carb H), 8.08 (2 H, d, J = 8, core Fl H), 7.83 (8 H, m, surf Fl H), 7.77− 7.68 (16 H, m, G1-carb H and Fl H), 7.59 (4 H, d, J = 8.5, G1carb H), 7.41−7.30 (12H, m, surf Fl H), 2.14−1.97 (20 H, m, surf Fl CH2), 1.04−0.68 (46 H, m, surf Fl CH2 and CH3); λmax(THF)/nm 359sh (log ε/dm3 mol−1 cm−1 5.14), 336 (5.30), 316sh (5.23), 302sh (5.14), 270 (5.01), 226sh (5.36); m/z [MALDI-TOF, DCTB] Anal. Calcd for C119H116N2: 1572.9 (74%), 1573.9 (100%), 1574.9 (67%), 1575.9 (29%), 1576.9 (10%), 1577.9 (2%). Found: 1572.8 (74%), 1573.7 (100%), 1574.7 (73%), 1575.8 (32%), 1576.8 (11%), 1577.8 (3%) [M+]. M̅ n = 1.8 × 103, M̅ v = 1845, M̅ w = 1859, p.d. = 1.05. 2.1.2. 2,7-Bis[3,6-bis(3,6-bis{9,9-di-n-propylfluoren-2-yl}carbazolyl)carbazolyl]-9,9-di-n-propylfluorene [Fl(G2)2]. A mixture of 2,7-diiodo-9,9-di-n-propylfluorene (82.2 mg, 0.16 mmol), 3,6-bis[3,6-bis(9,9-di-n-propylfluoren-2-yl)carbazolyl}carbazole16 (578 mg, 0.39 mmol), and tri-tert-butylphosphonium tetrafluoroborate (73 mg, 0.25 mmol) was deoxygenated in a Schlenk tube by placement under vacuum and backfilling with argon three times. Sodium tert-butoxide (66.7 mg, 0.69 mmol) and tris(dibenzylideneacetone)dipalladium(0) (32 mg, 31 μmol) were added, and the mixture was deoxygenated by placement under vacuum and backfilling with argon a further three times. Anhydrous xylenes (1.5 mL) were then added, and the mixture was deoxygenated by placement under vacuum and backfilling with argon a further three times before being sealed and heated in an oil bath held at 130 °C for 48 h with vigorous stirring. After cooling the mixture, water (10 mL) and diethyl ether (10 mL) were added, and the organic and aqueous layers were separated. The organic layer was extracted with water (10 mL), and the combined aqueous layers were extracted with dichloromethane (2 × 10 mL). The combined organic layers were dried over magnesium sulfate and filtered, and the solvent was removed in vacuo. The residue was filtered through a plug of silica using a dichloromethane/light petroleum mixture (2:3) as eluent. The filtrate was collected, and the solvent was removed. The crude residue was then dissolved in dichloromethane (10 mL), washed with saturated aqueous sodium thiosulfate (2 × 10 mL), dried over magnesium sulfate, filtered, and the solvent removed. The residue was then purified by column chromatography over silica using dichloromethane/light petroleum mixtures (3:7 to 2:3) as eluent to give Fl(G2)2 as a cream solid (275 mg, 52%). Mp 296−299 °C; Found: C, 90.5; H, 7.4; N, 2.6. C243H224N6 requires: C, 90.4; H, 7.0; N, 2.6; δH (400 MHz, CDCl3) 8.56 (8 H, d, J = 1.5, G2-carb H), 8.48 (4 H, d, J = 1.5, G1-carb H), 8.23 (2 H, d, J = 8, core Fl H), 7.93−7.88 (4 H, m, Fl or carb H), 7.86−7.72 (48 H, m, Fl H and/or G1-carb H and G2carb H), 7.60 (8 H, d, J = 8.5, G2-carb H), 7.41−7.28 (24 H, m, surf Fl H), 2.34−2.26 (4 H, bm, core Fl CH2), 2.13−1.96 (32 H, bm, surf Fl CH2), 1.20−1.07 (4 H, bm, core Fl CH2), 0.94 (6 H, t, J = 7.5, core Fl CH2), 0.84−0.63 (80 H, m, surf Fl CH2 and CH3); λmax(THF)/nm 338sh (log ε/dm3 mol−1 cm−1 5.53), 322 (5.58), 305sh (5.49), 269 (5.34), 229sh (5.67); m/z [MALDI-TOF, DCTB] Anal. Calcd for C243H224N6: 3225.8 (27%), 3226.8

(73%), 3227.8 (100%), 3228.8 (91%), 3229.8 (62%), 3230.8 (33%), 3231.8 (15%), 3232.8 (6%), 3233.8 (2%). Found: 3226.0 (38%), 3227.0 (79%), 3228.0 (100%), 3229.0 (92%), 3230.0 (78%), 3231.1 (52%) [M+]. M̅ n = 3351, M̅ v = 3481, M̅ w = 3505, p.d. = 1.05. 2.1.3. 2,7-Bis[3,6-bis(3,6-bis{3,6-bis[9,9-di-n-propylfluoren2-yl]carbazolyl}cabazolyl)carbazolyl]-9,9-di-n-propylfluorene [Fl(G3)2]. A mixture of 2,7-dibromo-9,9-di-n-propylfluorene (4.3 mg, 10.5 μmol),15 3,6-bis[3,6-bis(3,6-bis{9,9-di-n-propylfluoren2-yl}carbazolyl)cabazolyl]carbazole (238 mg, 23.3 μmol),16 tritert-butylphosphonium tetrafluoroborate (5.8 mg, 20 μmol), and sodium tert-butoxide (6.0 mg, 62 μmol) in a Schlenk tube was deoxygenated by placement under vacuum and backfilling with argon three times. Tris(dibenzylideneacetone)dipalladium(0) (2.7 mg, 2.6 μmol) and anhydrous xylenes (0.7 mL) were added, and the mixture was deoxygenated by placement under vacuum and backfilling with argon a further three times. The reaction mixture was then stirred and heated in an oil bath held at 130 °C for 48 h. After cooling, the mixture was filtered through a plug of silica using dichloromethane as eluent (70 mL). The filtrate was collected, and the solvent was removed. The crude solid was then purified by column chromatography over silica using a dichloromethane/light petroleum (3:7 to 2:3) as eluent to give Fl(G3)2 as a cream solid (53 mg, 82%). Mp 316−320 °C; Found: C, 90.0; H, 7.0; N, 3.0. C491H440N14 requires: C, 90.2; H, 6.8; N, 3.0; δH (400 MHz, CDCl3) 8.73 (4 H, bs, G1-carb H), 8.53 (16 H, d, J = 1.2, G3-carb H), 8.47 (8 H, d, J = 1.5, G2-carb H), 8.31 (2 H, bd, J = 7.8, core Fl H), 8.01−7.96 (12 H, bm, carb H and/or Fl H), 7.84−7.70 (96 H, m, Fl H and carb H), 7.57 (16 H, d, J = 8.5, G3-carb H), 7.37−7.27 (48 H, m, surf Fl H), 2.34 (4 H, bs, CH2), 2.10−1.92 (64 H, bm, CH2), 1.18 (4 H, bs, CH2), 0.97 (6 H, bt, J = 6.5, CH3), 0.84−0.59 (160 H, bm, CH3); λmax(THF)/ nm 338sh (log ε/dm3 mol−1 cm−1 5.77), 320 (5.85), 300sh (5.71), 271 (5.54), 231 (5.73); m/z [MALDI-TOF, DCTB] Anal. Calcd for C491H440N14: 6536.7 (100%). Found: 6539.1 (53%), 6551.7 (100%), 6565.7 (90%), 6579.9 (73%), 6595.8 (41%) [M+]; M̅ n = 5455, M̅ v = 5801, M̅ w = 5864, p.d. 1.08. 2.2. Spectroscopy. All solutions of the dendrimers were prepared in spectrophotometric grade tetrahydrofuran with peak absorbances of approximately 0.1. Absorbance spectra were measured with a Varian Cary 5000 spectrophotometer and PL spectra were measured with a Fluorolog Tau-3 fluorometer. Solution PLQYs were measured relative to quinine sulfate in 0.5 M sulfuric acid.17 Fluorescence lifetimes were measured with a Fluorolog 3 with time-correlated single photon counting (TCSPC) capability. The excitation source was an LED emitting at 372 nm pulsed at 1 MHz with a pulse width of 1.2 ns. The instrument response function (IRF) was determined with a dilute Ludox solution, and all fits to the data were performed with the application DAS6 (supplied by Jobin−Yvon) following convolution with the IRF. 2.3. Fluorescence Quenching. For the Stern−Volmer measurements, a stock solution of the dendrimer with a peak absorbance of ∼0.1−0.2 was prepared in spectrophotometric grade tetrahydrofuran. Analyte solutions were prepared by dissolving measured quantities of each analyte into known volumes of the dendrimer solution. The absorbance and fluorescence of 2.5 mL of the dendrimer solution in a cuvette was measured before and after a series of 25 μL additions of the analyte solution. Since the analyte solution is prepared using the same dendrimer solution, the dendrimer concentration within the cuvette mixture remains constant with each analyte addition. The excitation wavelength for collecting the PL was in the range 5330



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330−355 nm, and the emission was measured over the range 360−600 nm. The PL data were corrected for both selfabsorption and absorption of the UV excitation by the analyte.18 For the time-resolved measurements, the PL decay of the dendrimer solution was measured at the emission peak before and after a series of 50 μL analyte additions.

3. RESULTS AND DISCUSSION 3.1. Photophysics. In order to understand the effects of dendrimer structure, generation, and shape on the photophysics, a combination of UV−visible absorbance, PL excitation, PL emission, and time-resolved PL decay measurements were performed. The absorbance and PL emission for the dendrimers and dendrons are displayed by generation in Figure 2 with the PL

Figure 3. Time-resolved PL decays of the dendrimers and dendrons in solution, which have been offset (by factors of 10) for clarity. The black lines are fits to the data with eq 1 following convolution with the IRF.

the emission of the former is not due to aggregates or excimers (if this was the case then there would be a longer lifetime component). Combined with the PLQY values of 0.57 for Fl(G1)2 and 0.65 for SBF(G1)4 (given in Table 1), it is clear that Table 1. PLQY Values of the Dendrimers and the Lifetime Parameters from eq 1 Used to Fit the Data in Figure 3

Figure 2. Absorbance and PL spectra of the dendrimers and dendrons in solution.

decays in Figure 3. In the case of the first-generation compounds, the absorption spectra for the two dendrimers are similar in shape, and both are broader and red-shifted relative to the G1 dendron, which has a maximum at 312 nm. This indicates that the chromophore in the dendrimers includes the core. For the G1 dendron and Fl(G1)2, the PL emission occurs at similar wavelengths indicating that the emissive chromophores within the dendron and the dendrimers have similar energies. In contrast, the PL emission of SBF(G1)4 is slightly red-shifted relative to the G1 dendron and Fl(G1)2. This result is a little unexpected as the two halves of the spirobifluorene core are normally assumed to be non-interacting because of their relative 90° orientation and the rigidity of the structure.19 While there is no formal delocalization through the central spiro carbon, it is clear that the chromophores within the two halves of the dendrimer are interacting, which is at least consistent with molecular orbital calculations showing that the highest occupied molecular orbital (HOMO) of SBF(G1)4 is delocalized over the two halves of the dendrimer.14 The time-resolved measurements of the PL decay show that the emission from the G1 dendron, which has a biexponential decay, is much longer lived than that from the dendrimers. Of the first-generation dendrimers, Fl(G1)2 has the longest lifetime of 2.4 ns while SBF(G1)4 has a lifetime of 1.6 ns. The fact that the lifetime of SBF(G1)4 is shorter than Fl(G1)2 also provides evidence that the red-shift in

dendrimer

PLQY

A1

τ1

A2

τ2

Fl(G1)2 Fl(G2)2 Fl(G3)2 SBF(G1)4 SBF(G2)4 SBF(G3)4

0.57 ± 0.01 0.20 ± 0.01 0.19 ± 0.01 0.65 ± 0.01 0.29 ± 0.01 0.16 ± 0.01

1.00 0.33 0.45 1.00 0.33 0.36

2.40 ns 1.31 ns 2.50 ns 1.64 ns 1.94 ns 2.88 ns

− 0.67 0.55 − 0.67 0.64

− 4.37 ns 5.04 ns − 4.84 ns 6.58 ns

the spirobifluorene core is responsible for an increase in the radiative rate, which could be caused by increased rigidity. The PL spectra of the two dendrimers were found to be independent of the excitation wavelength, and the PL excitation spectra closely resembled the absorbance spectra for emission wavelengths of 390−460 nm (see the Supporting Information). Coupled with the monoexponential PL lifetime, this strongly suggests that the emission occurs from a single chromophore containing the fluorene or spirobifluorene core. Increasing the dendrimer generation has a significant impact on the photophysics. The absorbance spectrum of both Fl(G2)2 and SBF(G2)4 are very similar and are less red-shifted from the G2 dendron absorbance peak than their first-generation counterparts. This is consistent with the spectrum being more strongly weighted toward the dendron absorbance. The emission from both second-generation dendrimers is broader than that of the first-generation dendrimers and lacks clear vibronic structure. While the emission from Fl(G2)2 is similar to that of the G2 dendron albeit slightly broader, the emission from SBF(G2)4 is significantly broader and red-shifted. The values of the PLQY (see Table 1) of 0.2 for Fl(G2)2 and 0.29 for SBF(G2)4 are less 5331



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Figure 4. Steady-state Stern−Volmer plots for the two dendrimer series with (a) DNB, (b) DNT, (c) pNT, and (d) DMNB. Data points for the firstgeneration are shown as red circles, the second-generation as blue squares, and the third-generation as green triangles. Fits to the data with eq 2 are represented with black lines.

than half of the value obtained for the corresponding firstgeneration dendrimers and suggests a significant difference in the emissive chromophore. Full mapping of the PL emission and excitation spectra showed no dependence of the emission on the excitation wavelength and good agreement between the excitation spectra and the absorbance (see the Supporting Information) demonstrated that the emissive chromophores are efficiently populated. In contrast to the first-generation dendrimers, the PL decays of the second-generation dendrimers and dendron are all very similar, suggesting that the emissive states are located on the dendrons. The PL decays required fitting with a biexponential function, indicating that there are multiple emissive species present, and given by I(t ) = A1e−t /τ1 + A 2 e−t /τ2

states.20,21 Since there is no clear broadening of the absorbance spectrum to suggest a ground-state interaction, it is likely that excimer-like states rather than aggregates are the cause of the photophysical changes. Such states have been previously observed in multichromophore dendrimer systems where adjacent chromophores were strongly interacting.22,23 The greater effect of interdendron chromophore interactions can be readily understood from the energy minimized structure of SBF(G2)4,14 which shows that the carbazole and or fluorene units are interacting due to steric constraints. As will be discussed later, this will have important ramifications for the sensing ability of the dendrimers. As with the second-generation compounds, the absorbance spectra for the third-generation dendrimers are very similar and dominated by the contribution from the dendron although the onset and peaks of the absorption of the third generation materials are slightly blue-shifted relative to the secondgeneration materials. Measurements of the PL excitation spectra for both third-generation dendrimers (see the Supporting Information) matched their respective absorption spectra indicating that the emissive sites are all efficiently populated. The PL spectra of the third-generation materials are similar. The PL decay kinetics show that the emission from Fl(G3)2 is slightly longer lived than that from the G3 dendron while the PL from SBF(G3)4 has a significantly longer lifetime than either of the

(1)

where τ1 and τ2 are lifetimes and A1 and A2 are their respective amplitudes. All fits were convolved with the IRF. The lifetimes and amplitudes used for the fits are summarized in Table 1. It can be deduced from the decrease in PLQY and the general increase in the PL lifetime that the radiative decay rate has decreased in the second-generation dendrimers relative to the first. Taking into account the broad featureless PL, which is also weighted to the red relative to the first-generation dendrimers, the changes are consistent with the presence of either aggregate or excimer 5332



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other third-generation compounds and for both dendrimers the PL lifetime is on average longer than for the second-generation materials. A longer lifetime would normally correspond to a greater proportion of excimer-like emission, but this appears to be at odds with the PL emission, which are not broader or more red-shifted compared to the second-generation dendrimers. However, this apparent dichotomy can be easily explained by recognizing that there are two competing issues: first, steric hindrance causes twisting of the chromophores, and this would normally lead to a blue-shift in the emission, and second, the steric hindrance also increases the interchromophore interactions that gives rise to red-shifted excimer emission. Hence, the PL emission of the third-generation dendrimers is similar to the second as these two effects are essentially counterbalanced. The PLQY values of 0.19 and 0.16 for Fl(G3)2 and SBF(G3)4, respectively, are much lower than the corresponding values for the first-generation dendrimers and are consistent with the radiative rate decreasing with generation. Thus, as was discussed for the second-generation dendrimers, the emission is consistent with that of excimer-like states formed between strongly interacting moieties in the dendrons. 3.2. Steady-State Stern−Volmer Measurements. Stern− Volmer measurements to determine the effects of dendrimer generation and shape, and/or changes in the emissive chromophore, on the sensing properties of the carbazole dendrimers were performed.24 The compounds DNT, 1,4dinitrobenzene (DNB), and 4-nitrotoluene (pNT) were chosen as model analytes due to their chemical similarity to TNT. The compound 2,3-dimethyl-2,3-dinitrobutane (DMNB) was also included because of its role as a legally required taggant in commercial plastic explosives to enable their detection by canines. For fluorescence quenching to occur, electron transfer from the lowest excited state of the dendrimer to the analyte must be energetically possible. In other words, the ionization potential of the first excited state of the dendrimer must be less negative than the analyte electron affinity (taking the vacuum level as zero). Often the ionization potential of the first excited state is estimated by measuring the reduction potential or by adding the energy of the optical gap in electronvolts to the oxidation potential, which is measured in volts. However, although these methods can give a rough estimate of the relevant energy levels in this work neither could be applied in practice. Chemically reversible reduction processes could not be measured, and for the “optical gap/oxidation potential” method to have any validity, there must be symmetry between the ground state and first excited state,24 which means this calculation could not be performed on the second- and third-generation dendrimers because of the excimer-like emissive states. For this reason, the best approach for determining whether the fluorescence of the dendrimers was quenched by the target analytes was simply to test them experimentally. The results of the Stern−Volmer measurements for the two series of dendrimers with each of the analytes are shown in Figure 4. Also included in the plots are fits to the data with F0 = 1 + KSV[Q ] F

Figure 5. Stern−Volmer constants for the dendrimers with each analyte.

there is a decrease in the quenching efficiency with increasing generation with all the analytes except DMNB, which shows a small increase. In contrast, the SBF(Gx)4 dendrimers show a substantial increase in the Stern−Volmer constant for all the analytes between the first- and second-generation with little difference between the second- and third-generation. The differences in the Stern−Volmer constants between analytes with any one of the dendrimers follows the trend of the electron affinities of the analytes, that is, the smaller the electron affinity the smaller the response,5 confirming that to first order the electron transfer rate dominates the quenching process. The magnitudes of the Stern−Volmer constants for the dendrimers with all the nitroaromatic analytes and DNB and DNT in particular are higher (KSV ∼ 200−230 M−1) than what has been reported for conjugated polymers.12,25,26 Also of note are the quenching constants with DMNB, which at 35 M−1 for Fl(G3)2, are at the high end of reported values for what is normally a difficult compound to detect via fluorescence quenching.9,27−29 From the steady-state data alone, it is not clear why the trends between the two series of dendrimers are remarkably different especially when considering the similarities between the compounds. The most obvious way in which the two series differ is through the choice of core and thus the number of dendrons and electroactive moieties. To probe how these factors influence the quenching efficiency, we undertook time-resolved Stern−Volmer measurements to provide a deeper insight into the mechanism of the quenching interaction. 3.3. Time-Resolved Stern−Volmer Measurements. We have previously shown how the combination of steady-state and time-resolved measurements can provide important insight into the nature of the interaction between the sensing compound and the analyte by determining the relative contribution of collisional and static quenching.7,8 Time-resolved Stern−Volmer measurements are performed using the same method as for the steadystate, with the sole difference being that the PL lifetime rather than the emitted intensity is measured. Since only collisional interactions between the analytes and the dendrimers in the excited state will result in a change in the rate of PL decay, a direct measure of the collisional quenching component can be obtained. The time-resolved equivalent of eq 2 is given by τ0 = 1 + K C[Q ] (3) τ where τ0 is the PL lifetime in the absence of any analyte, τ is the PL lifetime in the presence of analyte with concentration Q, and KC is the collisional Stern−Volmer constant. One problem with

(2)

where F0 is the fluorescence intensity from the dendrimer solution prior to the addition of any analyte, F is the fluorescence intensity with a concentration Q of analyte added, and KSV is the Stern−Volmer constant. The Stern−Volmer constants obtained from the fits are summarized in Figure 5, and there are clear but differing trends for each dendrimer series. For the Fl(Gx)2 series, 5333



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for the first-generation dendrimers in Figure 7. The linearity of the data confirms that the quenching rate depends only on the

the use of eq 3 is that it is derived for monoexponential lifetimes, which is not a concern for the first-generation dendrimers but is a problem for the higher generations. So while the data for the firstgeneration dendrimers was analyzed in terms of the change in PL lifetime, the following approach was taken for the higher generations that had biexponential decays. The PL decay of the higher-generation dendrimers in solution can be described in terms of the biexponential decay given in eq 1. In the presence of the quenching analyte, there will be an additional decay rate, and the PL will then be described by IQ (t ) = (A1e−t /τ1 + A 2 e−t /τ2)e−kQ t

(4)

where kQ is the quenching rate in the presence of analyte concentration Q. If the PL decay measured in the presence of the analyte is divided by the PL decay measured in the absence, we get

IQ (t ) I (t )

= e −k Q t

(5)

thus isolating the component of the decay attributed to the quenching. This greatly simplifies the fitting of the data as it allows quenching rates to be determined from biexponential PL decay data without resorting to complex fitting functions.30 A single rate is expected as the rate of collisions between the dendrimers and the analytes will be time-independent for a solution in thermal equilibrium. The ratio of the PL decays for SBF(G3)4 with each of the analytes is shown in Figure 6 and is

Figure 7. Stern−Volmer plots derived from the lifetime data for the (a) Fl(Gx)2 and (b) SBF(Gx)4 dendrimers with DNB (circles), DNT (squares), pNT (triangles), and DMNB (diamonds). The data is expressed in terms of the lifetime ratio for the first-generation dendrimers and the quenching rate for the second- and third-generation dendrimers.

concentration of the quencher present, a key assumption for Stern−Volmer analysis, and that the biexponential decay of the higher-generation dendrimers does not cause this to change. For the first-generation dendrimers, the collisional Stern−Volmer constant KC was obtained by fitting the data with eq 3, while for the higher-generation dendrimers, the collisional quenching rate constant KQ was similarly determined with a line fit. The collisional quenching rate constant is related to the collisional Stern−Volmer constant by K C = KQ τ (6)

Figure 6. PL decay ratios for SBF(G3)4 with analyte concentrations in the range 0.002−0.010 M for DNB, 0.004−0.019 M for DNT, 0.004− 0.018 M for pNT, and 0.010−0.046 M for DMNB. The black lines are monoexponential fits to the data.

representative of the data for the other higher-generation dendrimers. Each decay corresponds to a different analyte concentration, and it can be easily seen that solutions containing higher analyte concentrations decay faster. The quenching rate for each analyte concentration was determined by fitting to the data with a monoexponential decay. Plots of the quenching rate versus analyte concentration for the higher-generation dendrimers are displayed alongside the lifetime Stern−Volmer data

The time-averaged collisional Stern−Volmer constant for the higher-generation dendrimers can then be obtained by multiplying KQ by the average lifetime. The benefit of this approach is that the average lifetime is only invoked in the calculation at the last stage, and thus, all the previously calculated values are not 5334



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influenced by this simplification. The collisional quenching rate constants are summarized in Table 2 with the values for the firstTable 2. Collisional Quenching Rate Constants (in Units of ×1010 s−1) for the Dendrimers with Each Analyte dendrimer

DNB

DNT

pNT

DMNB

Fl(G1)2 Fl(G2)2 Fl(G3)2 SBF(G1)4 SBF(G2)4 SBF(G3)4

2.93 ± 0.08 1.71 ± 0.01 1.52 ± 0.01 2.55 ± 0.01 1.51 ± 0.01 1.18 ± 0.01

2.76 ± 0.03 1.73 ± 0.01 1.57 ± 0.02 2.52 ± 0.04 1.37 ± 0.01 1.28 ± 0.02

2.53 ± 0.01 1.50 ± 0.01 1.47 ± 0.02 2.14 ± 0.01 1.30 ± 0.01 1.10 ± 0.01

1.02 ± 0.01 0.65 ± 0.01 0.57 ± 0.01 0.63 ± 0.01 0.45 ± 0.01 0.37 ± 0.01

generation dendrimers calculated by dividing the collisional Stern−Volmer constant by the PL lifetime as per eq 6. The highest quenching rate constants were observed between the first-generation Fl(G1)2 and DNB and DNT, which are (2.93±0.08) × 1010 s−1 and (2.76±0.03) × 1010 s−1, respectively. These rates are on the order of the diffusion limit,24 indicating that electron transfer between the dendrimers and the higher electron affinity analytes is very efficient. In contrast, the rates for DMNB are about 2.5−3 times lower. The lower quenching rate for DMNB can be attributed to its lower electron affinity as well as it being able to adopt a conformation that is not conducive to electron transfer. This decrease in the quenching efficiency is the reason why DMNB is typically difficult to detect through oxidative quenching of fluorescence. Comparing the collisional quenching rate constants between the two first-generation dendrimers, the values for SBF(G1)4 are consistently lower than those for Fl(G1)2, which is attributed to a combination of slower diffusion due to its larger size and increased shielding of the core chromophore by the additional dendrons (four rather than two). Looking at the two dendrimer series, there is a clear decrease of the collisional quenching rate constant with generation, which can be largely explained by the increase in the size of the dendrimers, and the resulting decrease in diffusion coefficient and collision rate. As the dendrimers exhibit a combination of static and collisional quenching, eq 3 should be rewritten as

Figure 8. Stern−Volmer constants for (a) Fl(Gx)2 and (b) SBF(Gx)4, with the collisional and static components represented by the filled and striped areas, respectively.

with increasing dendrimer generation. The decrease in static quenching is clearly not due to a difference in the ability of the photoexcited state of the dendrimer to be oxidized by the analyte, as both static and collisional quenching require the same energy offset. The difference therefore arises from the position of the bound analyte relative to an emissive chromophore. Given that static quenching arises from a ground-state complexation, we can consider the position of analyte binding to the dendrimer based on the HOMO orbital distribution. For both series the firstgeneration dendrimers have the HOMO on the core and the carbazole units of the dendrons whereas for the secondgeneration dendrimers there is less on the fluorenyl units that make up the core, and this trend follows for the third-generation dendrimers.14 Given that the PL studies have shown that emission occurs from the chromophore containing the core for the first-generation dendrimers and excimer-like states within the dendrons of the higher-generation dendrimers of both series then the position of binding of the analyte becomes important for static quenching. Clearly, as the number of carbazole units increases, there is an increasing probability that a binding event will occur to a site that is either not excited or emissive and hence does not lead to PL. Therefore, the decrease in static quenching for the Fl(Gx)2 series can be conceptually understood as an analyte binding to a non-emissive site on one dendron with the excitation formed on the second dendron rapidly migrating to an excimer-like state and being trapped. As a consequence, the

F0 = (1 + KS[Q ])(1 + K C[Q ]) F ≈ 1 + (KS + K C)[Q ]

(7)

where KS is the static Stern−Volmer constant. At low analyte concentrations, the quadratic component can be neglected and KSV ≈ (KS + KC). This approximation is consistent with the Stern−Volmer data shown in Figure 4, which appears linear. Therefore, taking the steady-state KSV constants and subtracting the collisional KC constants gives the static KS values. It is important to emphasize at this point that, in calculating the collisional constant, an average lifetime was used and in calculating the static constant a first-order approximation of the Stern−Volmer equation was employed. We must therefore emphasize that the numbers obtained are not absolute but show the important trends. Figure 8 displays both the collisional and static constants for each dendrimer with each of the analytes. The collisional Stern− Volmer constants for the Fl(Gx)2 series do not significantly change with generation as the decrease in the collisional quenching rate constant is counterbalanced by the increase in lifetime. In contrast, the static quenching component decreases 5335



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structural similarities of the two dendrimer series, increasing the generation results in opposite trends for the Stern−Volmer constants of the nitroaromatic analytes in solution: the more planar fluorene-cored Fl(Gx)2 dendrimers show a gradual decrease in sensing performance while the more three-dimensional SBF(Gx)4 dendrimers show an increase. Time-resolved measurements indicate that the differences in the Stern−Volmer constants are due to large changes in the level of static quenching, which is dependent on the location of the emissive chromophore relative to the binding site of the analyte. The differences in the static and collisional components of the quenching for the Fl(Gx)2 series with each of the analytes tested suggest that it may be possible to discriminate between them based on the relative ratio of the collisional and static components, a property that could potentially be harnessed to achieve selectivity.

proportion of static quenching decreases with increasing dendrimer generation. In contrast to the Fl(Gx)2 dendrimers, the results for the SBF(Gx)4 series in Figure 8 show an increase in the collisional constant and a larger increase in the static constant with generation. It is important to remember that, for the firstgeneration, the emission is essentially from a chromophore that comprises the spirobifluorene core but at higher generations emission is from excimer-like states within the dendrons. In the case of SBF(G1)4, the Stern−Volmer constant is approximately half that of Fl(G1) 2 , with the change resulting from corresponding decreases in both the static and collisional quenching constants. The change in the collisional constant can be explained in terms of a small reduction of the collisional rate due to the increased size of the dendrimer and a reduction of the excited state lifetime. The decrease in static quenching for SBF(G1)4 can be understood by the reduced probability of the analyte being able to bind to the chromophore containing the core due to steric hindrance. For the higher-generation SBF(Gx)4 dendrimers, the static component is larger than the first-generation SBF(G1)4 and the Fl(Gx)2 dendrimers. As previously stated, the PL emission is clearly dominated by the excimer-like states (long lifetime and lower PLQY) on the dendrons. As a consequence, the interactions of the analytes will be primarily with the dendrons and the excimer-like states within the dendrons. In the case of Fl(Gx)2 dendrimers, as the generation increases, the average distance between a bound analyte and a photoexcitation also increases. However, the more three-dimensional shape of the SBF(Gx)4 dendrimers creates a cleft for the analyte, and so, a single bound analyte can effectively interact with more than one dendron thus reducing the average distance between a bound analyte and an excited state for the equivalent generation. Clearly, there is a trade-off between steric hindrance of the analyte interacting with the chromophores within the dendrons and the number of chromophores as the second- and third-generation SBF(Gx)4 dendrimers have similar Stern−Volmer constants. While the nitroaromatic analytes exhibit quite different trends with each dendrimer series, the nitroaliphatic DMNB shows an increase in quenching efficiency with generation for both series. It is important to note that the interaction with DMNB is primarily collisional in nature and is therefore largely determined by the molecular diffusion rate, energy level offset, and the excited-state lifetime. The low static quenching contributions are consistent with weak binding between the DMNB and the dendrimers resulting from the absence of a π-conjugated system in the analyte. Not only does this reduce the binding constant but DMNB may not share the same preference for binding sites as the nitroaromatics. The trend for both dendrimers is thus consistent with weak binding of the DMNB to the dendrons.

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ASSOCIATED CONTENT

S Supporting Information *

Absorbance, PL excitation, and PL spectra of the dendrimers. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected] (P.E.S.); [email protected] (P.L.B.); [email protected] (P.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Australian Research Council through the Discovery Program (DP0986838). The Centre for Organic Photonics & Electronics is a strategic initiative of the University of Queensland. P.L.B. and P.M. are both supported by University of Queensland Vice Chancellor’s Strategic Research Fellowships.

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REFERENCES

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4. CONCLUSIONS We have developed two series of carbazole dendrimers for the detection of nitroaromatic and nitroaliphatic explosive analytes, which vary in terms of generation, core, shape, and the number of chromophores. Photophysical characterization of the two series indicates that the first-generation dendrimers behave as single chromophores with the excited state localized on a chromophore that includes the core. Increasing the generation increases the number of chromophores per dendrimer, but intramolecular interactions between the dendrons results in the formation of excimer-like excited states. Although such states reduce the PLQY, their long lifetimes are beneficial for sensing. Despite the 5336



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High-Generation Dendrimers with Excimer-like Photoluminescence for

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