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Ultrafast Photoinduced Electron Transfer between Carbon Nanoparticles and Cyclometalated Rhodium and Iridium Complexes Somen Mondal, Sourav Kanti Seth, Parna Gupta, and Pradipta Purkayastha* Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER), Kolkata, Mohanpur 741246, WB, India S Supporting Information *

ABSTRACT: Light-harvesting features of cyclometalated complexes of Ir(III) and Rh(III) contribute toward photoinduced electron and energy transfer for solar energy conversion and photocatalysis. Here we report four cyclometalated complexes of Rh and Ir among which one is heterometallic. These complexes, on interaction with fluorescent carbon nanoparticles (CNPs) in acetone medium, form molecular composites through hydrophobic interaction in the ground state followed by photoinduced electron transfer (PET). Quenching of CNP fluorescence and electrochemical measurements indicate occurrence of electron injection from the complexes to the CNPs.

1. INTRODUCTION Cyclometalated complexes of Ir(III) and Rh(III) have been of much scientific interest from long back because of their outstanding photophysical and electrochemical properties that make them promising candidates for photochemical energy conversion studies.1−4 Photoinduced energy and electron transfer processes are observed in supramolecular Ir(III) and Rh(III) cyclometalated species.5,6 Such polymetallic complexes possess enhanced extinction coefficients compared to their mononuclear analogues.7,8 The light-harvesting features of these coordination complexes stimulate the areas of photoinduced electron and energy transfer for solar energy conversion and in photocatalysis.9,10 The presence of additional metal centers may facilitate spin−orbit coupling and enhance radiative rate constants that, in turn, may increase the efficiency of phosphorescence. This phenomenon is useful for organic light emitting diodes.11,12 The process of organized energy transfer can be envisaged to transfer the excitation energy from many chromophoric components to a common acceptor.13−15 Working with a series of binuclear Ru(II)−Rh(III) complexes, Indelli and coworkers reported that energy transfer from excited Rh(III) to Ru(II) is efficient for all the complexes at both 77 and 150 K.16 In another work on a series of rod-like Ru(II)−Rh(III) polypyridine dyads, they showed that rigid oligophenylene bridges are efficient mediators for long distance photoinduced electron transfer (PET) between transition metal polypyridine units.17 Barcina and co-workers worked on a dyad consisting of Ru(II) and Ir(III) species separated by a homoconjugated bridge and inferred that such a bridge can efficiently mediate PET from the Ir(III) to the Ru(II) center through a Dextertype mechanism.18 Ultrafast PET from iridium-based water© 2015 American Chemical Society

oxidation catalyst to perylene diimide derivatives shows strong irreversible oxidation current similar to that for model catalysts.19 In a recent report, Sykora and co-workers showed that photoinduced charge transfer (PCT) can occur between CdSe quantum dots (QDs) and Ru-polypyridine complexes.20 They attributed the quenching of emission of the QDs by the Rucomplexes to gradual adsorption of the complexes onto the QD surface. A simultaneous effect on quenching of the emission from both QDs and the Ru-complexes suggests electronic interaction due to the attachment. However, they have excluded the possibility of energy transfer because of insignificant spectral overlap between the absorption spectrum of the complexes and the QD emission. Recently, Verma et al. synthesized Os(II)-polypyridyl complexes with pendant catechol functionality and showed that they form CT complex with TiO2 and ZrO2 nanoparticles where photoinduced electron injection takes place from the Os(II) complexes to the nanoparticles following excitation of the respective CT complexes.21 Later they also showed that polynuclear− polypyridyl complexes exhibit directional energy-transfer property that can improve their photosensitization activity.22 The authors immobilized the complexes on the nanoparticles by the catechol functional group and observed the electron injection. Herein, we demonstrate ultrafast PET between Rh(III) and Ir(III) with cyclometalated Ir and Rh units and fluorescent carbon nanoparticles (CNPs). CNPs are biocompatible and Received: September 4, 2015 Revised: October 19, 2015 Published: October 19, 2015 25122

DOI: 10.1021/acs.jpcc.5b08633 J. Phys. Chem. C 2015, 119, 25122−25128

Article

The Journal of Physical Chemistry C

2.2.3. Synthesis of 3. Measured amount of [Rh(ppy)2Cl]2 with H2bpib in EtOH/dichloromethane mixture (2:1 v/v) was refluxed at 90 °C for 4 h. The mixture was heated until the volume was reduced to 8 mL, and subsequently, water (10 mL) was added to it. The compound was precipitated by adding an excess of aqueous NH4PF6 to the solution. This was filtered, and a yellow colored compound was obtained. 1 H NMR [DMSO-d6]: δ 14.47 (s, broad, 2H), 9.27 (d, 4H), 8.60 (s, 4H), 8.30 (d, 4H), 8.23 (d, 4H), 8.12 (t, 4H), 8.03 (d, 4H), 7.97 (t, 4H), 7.49 (d, 4H), 7.15 (t, 4H), 7.07 (d, 4H), 7.03 (d, 4H), 6.32 (d, 4H). υ̅max/cm−1: 3430s(br), 1606s, 1578s, 1480s, 1451m, 846vs, 757s, 733m. 2.2.4. Synthesis of 4. A measured mixture of [Ir(ppy)2Cl]2, H2bpib, and acetonitrile/dichloromethane (1:1 v/v) was refluxed for 4 h at 90 °C to obtain an orange colored mixture. It was cooled to room temperature, and the solvent was evaporated using a rotavapor. The residue was purified by column chromatography (5% methanol in dichloromethane was used as eluent). A brownish orange compound was obtained. 1 H NMR [CDCl3]: δ 15.42 (s, 2H). υ̅max/cm−1: 3412s (br), 2924vs, 1605s, 1581s, 1477s, 1451s, 1377s, 758s, 729s. 2.3. Chemicals and Methods. All the chemicals and solvents were procured either from from Sigma-Aldrich Chemicals Pvt. Ltd., Bangalore, India or Merck, India and used without further purification. The 1H NMR studies were obtained on a Bruker Avance III-500 NMR spectrometer using TMS as the internal standard. The hydrodynamic diameter of the nanoparticles was determined to be approximately 5 nm by dynamic light scattering method (DLS) (Malvern Zetasizer Nanoseries). Absorption spectra were taken on a Varian Cary 300 Biospectrophotometer, and steady-state fluorescence measurements were done on a PTI QuantaMaster 40 spectrofluorometer. The FTIR spectrum was recorded using a Perkin−Elmer Spectrum RX1 spectrophotometer. The TEM images were taken on an FEI (Czech Republic) type FP5018/ 40 Technai G2 Spirit BioTWIN transmission electron microscope. A drop of the corresponding solution was taken on a strong carbon film on 300 mesh Cu grid (GSCu300C, ProSciTech, Thuringowa, Australia) and dried in air to make the sample for the TEM experiment. The fluorescence lifetimes were measured by the method of time-correlated single-photon counting (TCSPC) using a picoseconds spectrofluorimeter from Horiba Jobin Yvon IBH equipped with a FluoroHub single photon counting controller and FC-MCP-50SC MCPPMT detection unit. A 402 nm laser head (fwhm = 70 ps) was used as the excitation source. The average lifetime, ⟨τ⟩, is determined as ⟨τ⟩ = Σ(aiτi2)/Σ(aiτi), where the parameters ai are the relative amplitudes of each lifetime component and τis are the different decay times of the excited-states. In the femtosecond upconversion setup (FOG 100, CDP), the sample was excited at 405 nm using the second harmonic of a modelocked Ti-sapphire laser (Tsunami, Spectra Physics), pumped by a 5 W Millennia (Spectra Physics). A nonlinear crystal (1 mm BBO, θ = 25°, φ = 90°) was used in order to generate second harmonic. The fluorescence emitted from the sample was up-converted in a nonlinear crystal (0.5 mm BBO, θ = 38°, φ = 90°) using the fundamental beam as a gate pulse. The upconverted light is dispersed in a monochromator and detected using photon counting electronics. A cross-correlation function obtained using the Raman scattering from ethanol displayed a full width at half-maximum (fwhm) of 350 fs. The femtosecond fluorescence decays were fitted using a Gaussian shape for the

chemically inert having advantages over fluorescent semiconductor QDs.23−28 Photoexcited CNPs can function as electron acceptor as well as donor depending on the associated systems.29,30 CNPs show broad optical absorptions due primarily to π-plasmon and photoluminescence emissions associated with structural and surface defects.31 Carbon dots with effective surface passivation are brightly fluorescent.31 Photoexcitation of CNP core results in efficient charge separation, with the electrons and holes trapped at various surface sites of the nanoparticles, with the radiative recombinations of the electrons and holes being responsible for the observed fluorescence emissions.31,32 Thus, PET from cyclometalated Rh−Ir to CNPs is a new and improvising system for further applications.32 CNPs are capable of harvesting photons over the solar spectrum, with their photoexcited states and the associated transient species responsible for bright fluorescence emissions. Their optical and photoinduced redox properties derived from defects in the CNP core are enhanced dramatically by effective passivation of the defects. Due to this process, CNPs may play a dominating role among carbon-based nanostructures.

2. EXPERIMENTAL SECTION 2.1. Preparation of Carbon Nanoparticales (CNPs). Hydrophobic CNPs were synthesized by mixing 160 mg of glucose with 1.6 g of dodecylamine and heating the solution at 80 °C for 30 min. A brown colored solid was obtained on cooling, which was dissolved in hexane. The soluble CNPs were purified by solvent extraction using hexane and methanol. Hexane was evaporated out and the brown solid CNPs were solubilized in acetone. 2.2. Synthesis of Cyclometalated Complexes. The compounds [Rh(ppy)2Cl]2, [Ir(ppy)2Cl]2, 1,10-phenanthroline-5,6-dione,2-(4-formylphenyl)imidazo[4,5-f ][1,10]phenanthroline (fmp), and 2,2-p-phenylene (imidazo[4,5f ][1,10] phenanthroline (H2bpib) were obtained commercially and used without further purification. The detailed synthetic methods and characterization are reported in Seth et al.33 2.2.1. Synthesis of 1. A measured mixture of [Ir(ppy)2Cl]2, fmp, methanol, acetonitrile, and dichloromethane was refluxed for 5 h at 90 °C to produce a red colored solution. The product was purified by column chromatography with 5 to 10% (v/v) methanol in dichloromethane as eluent. The reddish orange compound was obtained after the eluent was dried using a rotavapor. 1 H NMR [CDCl3]: δ 10.06 (s, 1H), 9.59 (s, 1H), 8.79 (d, 2H), 8.13 (d, 2H), 8.02 (d, 2H), 7.94 (d, 2H), 7.73 (m, 7H), 7.38 (d, 2H), 7.10 (t, 2H), 6.99(t, 2H), 6.85 (t, 2H), 6.42 (d, 2H). υ̅max/cm−1: 3430m, 3043m, 2920m, 2850m, 1698s, 1607vs, 1582s, 1478s, 1383m, 757m, 730m. 2.2.2. Synthesis of 2. A measured mixture of [(ppy)2Ir(H2bpib)]Cl, [Rh(ppy)2Cl]2, and acetonitrile/dichloromethane/methanol (1:1:1, v/v) was refluxed at 90 °C for about 3 h. The yellowish orange colored solution obtained was cooled to room temperature. The compound was purified by thin layer chromatography with 10% methanol in dichloromethane mixture. The deep yellow colored compound was obtained after the solution was dried using a rotavapor. 1 H NMR [CDCl3]: 9.74 (s, 4H), 8.77 (s, 4H), 8.15 (d, 2H), 8.12 (d, 2H), 7.95 (d, 4H), 7.75 (m, 12H), 7.38 (t, 4H), 7.14 (t, 2H), 7.08 (t, 2H), 7.02 (t, 2H), 6.97 (t, 2H), 6.91 (t, 2H), −1 6.86 (t, 2H), 4.43 (d, 4H). υm ̅ ax/cm : 3433s(br), 3043m, 2922m, 1606s, 1581m, 1479s, 1449m, 758m, 731m. 25123

DOI: 10.1021/acs.jpcc.5b08633 J. Phys. Chem. C 2015, 119, 25122−25128

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The Journal of Physical Chemistry C Scheme 1. Representative Structures for the Metal−Ligand Complexes Used in the Studya

a

fmp = 1,10-phenanthroline-5,6-dione-2-(4-formylphenyl)imidazo[4,5-f ][1,10]phenanthroline; bpib = 2,2-p-phenylene(imidazo [4,5-f ][1,10] phenanthroline.

Figure 1. (A) Absorption and fluorescence spectra of the dodecylamine coated CNPs, (B) dynamic light scattering profile for the synthesized CNPs, (C) transmission electron micrograph (TEM) of the synthesized CNPs, and (D) FTIR spectrum of CNPs in hexane showing the amide stretch at 1640 cm−1.

glucose. The amide bond-linked dodecylamines imposed hydrophobicity to the CNPs.34 The Rh(III)/Ir(III) dyads in the present study (Scheme 1) were synthesized using standard protocols as explained elsewhere.33 The molecular dyads prefer relatively hydrophobic environment for residence. This has been proved by photophysical characterization of the complexes in mixtures of acetone and dichloromethane as shown in Figures S1 and S2 (Supporting Information). Clear hypsochromic shift of the charge transfer (CT) emission bands on reducing the polarity of the environment confirmed that the complexes prefer to move toward lesser polarity. These

exciting pulse. The ultrafast transient absorption measurements were performed by using a femtosecond pump−probe setup, which consisted of a mode-locked Ti-sapphire oscillator (Spitfire, Spectra Physics) that served as the seed laser for the amplifier, generating femtosecond pulses (fwhm = 70 fs, ∼4 W at 1 kHz with a tunable range of 400−750 nm).

3. RESULTS AND DISCUSSION 3.1. Attachment of the Molecular Dyads to the CNPs. The dodecylamine protected CNPs were produced from 25124

DOI: 10.1021/acs.jpcc.5b08633 J. Phys. Chem. C 2015, 119, 25122−25128

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The Journal of Physical Chemistry C

Figure 2. Absorption spectra of CNP and the CNP−complex adducts in acetone and ambient temperature.

Figure 5. Femtosecond transients (λex = 400 nm, λem = 560 nm) of CNP with and without the metal−ligand complexes. In the legend: M and D indicate monomer and dimer, respectively.

excitation wavelength is varied indicating close uniformity in size distribution for the CNPs. The diverse nature in structures and energetics of defect derived surface sites is likely responsible for a large population of emissive excited states accessible with a selection of different excitation energies. The small shift arises from the “surface states”, i.e., different functional groups, such as, hydroxyl, ketonic carbonyl, and ester carbonyl on the CNP surfaces. Dynamic light scattering (DLS) (Figure 1B) and TEM micrograph (Figure 1C) show that the average particle size of the CNPs is around 5 ± 1 nm (error equals standard deviation). Hydrophobicity has been imposed to the CNPs deliberately by coating them with dodecylamine where the amine groups react with the carboxyl functional groups to form amide bonds (Figure 1D). 3.3. Absorption Spectroscopy of the CNP−Complex Adducts. Absorption spectra of the CNPs and those for adducts with 1−4 are shown in Figure 2. In absence of CNPs, MLCT in the complex is negligibly small. In the presence of CNPs, broad shoulders are observed at ∼500 nm for the molecular adducts that can be attributed to t2g → π*ppy and t2g → π*bpib MLCT transitions. The bpib ligand has stronger π-acceptor characteristics compared to the ppy ligand. A distinct shift in the MLCT band of 2 (450−600 nm region) in comparison to that for the other complexes shows redox asymmetry of heterometallic Ir/Rh composite. The symmetric complexes (3 and 4) thus practically do not possess any redox asymmetry. Consequently, the CNP-induced MLCT in them is less. Structured bands, due to the absorption by the ligands, are also observed at ∼400 nm. Because of the ground state hydrophobic interaction between the dodecylamine coated CNPs and 1−4 and the electron acceptability of the CNPs, possibility of ET can be predicted.21,22 3.4. Emission Characteristics of the Molecular Adducts. Figure 3 shows emission spectra of the CNPs in the presence of various concentrations of 1−4. CNPs emit at 425 nm when excited at 350 nm (Figure 1A). The fluorescence band at 425 nm undergoes profuse quenching on progressive addition of 1−4 to the CNPs. The new band at ∼560 nm in Figure 3 is attributed to the emissions from the respective complexes in acetone. No CT band is observed for 1 in pure acetone. CT, however, originates when the solvent is predominated by a nonpolar gradient that supports 3 MLCT.35,36 This indicates that the complex gets trapped in the hydrophobic layer over the CNPs. Dipole moments of 2 and 4 are much less than the monomeric variant 1.33 Hence, complete disappearance of

Figure 3. Fluorescence spectra of CNPs in the presence of various concentrations of (A) 1, (B) 2, (C) 3, and (D) 4 in acetone. The samples were excited at 350 nm.

Figure 4. Relative quenching of CNP emission due to the addition of 1−4 in acetone. The data carry an error limit of ±5%.

observations indicate that the complexes, 1−4, intrude the hydrophobic layer of dodecylamine on the CNPs and get trapped. ET commences from the molecular dyads to the CNPs on shining light of a particular frequency. 3.2. Characterization of CNPs. The synthesized CNPs were characterized by their absorption and fluorescence spectra as shown in Figure 1A. The absorption spectrum exhibits a broad shoulder at ∼275 nm due to the π−π* transitions in the aromatic CC bonds and a weak band at ∼350 nm because of the n−π* transitions in the different functionalities in the CNPs.30 The emission maximum shows small shift as the 25125

DOI: 10.1021/acs.jpcc.5b08633 J. Phys. Chem. C 2015, 119, 25122−25128

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The Journal of Physical Chemistry C Table 1. Picosecond Decay Parameters of CNP Emission (at 460 nm) in the Presence of Complexes 1 to 4a system CNP 1−CNP 2−CNP 3−CNP 4−CNP

τ1 (ns)

a1

± ± ± ± ±

34 32 31 29 31

1.59 1.49 1.46 1.40 1.46

0.024 0.021 0.019 0.022 0.022

τ2 (ns)

a2

τ3 (ps)

a3

χ2

⟨τ⟩ (ns)

± ± ± ± ±

59 62 61 64 63

106 ± 2 93 ± 2 65 ± 2 65 ± 2 87 ± 2

7 6 8 7 6

1.30 1.28 1.21 1.24 1.22

4.09 3.74 3.78 3.81 3.80

6.00 5.27 5.45 5.31 5.30

0.018 0.016 0.018 0.017 0.017

The a values are the relative amplitudes of each lifetime component. The χ2 values determine the goodness of the fits. Standard deviation for amplitude analysis is ±5%. a

Table 2. Femtosecond Decay Parameters of CNP Emission (at 460 nm) in the Presence of Complexes 1 to 4a system CNP 1−CNP 2−CNP 3−CNP 4−CNP

τ1 (ps)

a1

± ± ± ± ±

61 64 90 62 69

1.12 0.35 0.70 0.80 0.85

0 0 0 0.03 0

τ2 (ps)

a2

± ± ± ± ±

24 21 5 20 17

17.47 20.00 15.12 18.50 17.50

1.04 1.14 1.64 0.95 0

τ3 (ps) 4090 3780 3810 3800 3740

± ± ± ± ±

0 0 0 0 0

a3

χ2

15 15 5 18 14

0.98 0.99 0.97 0.98 0.97

a The a values are the relative amplitudes of each lifetime component. The χ2 values determine the goodness of the fits. Standard deviation for amplitude analysis is ±5%.

3

(fs) time resolutions as shown in Figure 5. The data were collected at 460 nm exciting the samples at 405 nm. The lifetime decays of CNPs take multicomponent fits in the ps study suggesting that multiple radiative species are present in the samples. Hence, it is instructive to consider the average lifetimes (Table 1). There was practically no substantial difference in lifetimes in this time domain. The fs decay studies showed two ultrafast components indicating that one is for CT (τ1) and the other is for PET (τ2) (Table 2).20 This phenomenon takes place fastest for 2 leading to electron injection to CNP. Femtosecond transient absorption (FTA) spectroscopy helped us to confirm the PET phenomenon. Samples were subjected to pulsed excitation (λex = 400 nm) and probed by white light continuum (WLC) pulses, covering the 440−730 nm domain. Characteristic spectral features due to ground-state bleaching (GSB), excited-state absorption (ESA), and stimulated emission (SE) were observed. The positive signal at 510 nm in the ΔA spectrum is generally observed for CNPs due to ESA.38 The growth between 525 and 550 nm on addition of complexes 1−4 to the CNPs is due to formation of cation radical of the complex.22 A comparison of the transient absorption kinetics of 1−4 reveals a peripheral-to-core electron transfer. Figure S4 (Supporting Information) indicates that complex cation radical forms more effectively for 2 than the others. The results again corroborate our findings as explained above and show that the heterometallic cyclometalated Ir−Rh (2) complex is the most efficient in PET among the other variants.

MLCT may not take place due to lesser solvation effect. However, 3MLCT increases as the polarity of the solvent decreases. Unlike the Ir-containing complexes, 3 does not show any emission band at 560 nm. This is because, for Rh complexes, 3MLCT is replaced by 1MLCT.33 The band at 560 nm reappears for 4 because of the presence of Ir. The samples had been excited at 350 nm since the quenching of emission from CNPs in acetone due to 1−4 is the principal concern for the study. Concentration of the metal complexes is very less compared to that of the CNPs where no inner filter effect can persist. Fluorescence emission from CNPs may get quenched by two possible ways in this case: (i) due to distorted surface passivation because of hydrophobic interaction and (ii) through CT and ET mechanisms. Our studies show that the quenching efficiency of CNPs is highest due to interaction with the heterometallic complex 2, whereas, 1, 3, and 4 show lower and similar quenching effects (Figure 4). This difference is principally due to the redox asymmetry in 2. The extent of fluorescence quenching is measured by using Stern−Volmer equation given as I0/I = 1 + KSV[Q], where the I0 and I are the fluorescence intensities of CNPs in absence and presence of the complexes, respectively, KSV is the Stern−Volmer constant, and [Q] is the effective concentration of the quencher. All the traces show deviation from linearity indicating a dual quenching mechanism (static and dynamic) (Figure 4). Since the CNP− complex conjugate formation takes place in the ground state due to hydrophobic interaction, hence dynamic quenching could be due to ET in the excited state induced by light, i.e., PET. As a consequence of excitation of CNP, one electron gets transferred from its highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) that induces shifting of an electron from the donor inorganic complex to the HOMO of the CNP. Highest efficiency of PET is shown by 2; hence, possibility of interfacial remote electron injection from 2 to CNPs may well be considered.21,22,37 3.5. Time-Resolved Studies on the Dynamics of PET. To determine the time-resolved characteristics of PET in the systems, we performed emission decay studies in picoseconds (ps) (in Supporting Information, Figure S3) and femtoseconds

4. CONCLUSIONS Four cyclometalated complexes were synthesized using Rh and Ir among which 1 contains a single Ir metal, whereas the 2 and 3 have Ir−Rh, Rh−Rh, and Ir−Ir metal combinations. These complexes were allowed to interact with fluorescent CNPs in acetone medium through hydrophobic interaction in the ground state. Quenching of CNP fluorescence indicates the possibility of electron injection from the complexes to the CNPs in the excited state. The steady state and time-resolved fluorescence studies indicate that the heterometallic complex 2 is the most promising system for the PET phenomenon due to 25126

DOI: 10.1021/acs.jpcc.5b08633 J. Phys. Chem. C 2015, 119, 25122−25128

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The Journal of Physical Chemistry C

-Donating Substituents on the Photophysical and Electrochemical Properties. Inorg. Chem. 1995, 34, 2759−2767. (9) Kaveevivitchai, N.; Chitta, R.; Zong, R.; El Ojaimi, M.; Thummel, R. P. A Molecular Light-Driven Water Oxidation Catalyst. J. Am. Chem. Soc. 2012, 134, 10721−10724. (10) Deng, Z.; Tseng, H.-W.; Zong, R.; Wang, D.; Thummel, R. Preparation and Study of a Family of Dinuclear Ru(II) Complexes That Catalyze the Decomposition of Water. Inorg. Chem. 2008, 47, 1835−1848. (11) Yersin, H., Ed. Highly Efficient OLEDs with Phosphorescent Materials; Wiley-VCH: Weinheim, Germany, 2007. (12) Gildea, L. F.; Williams, J. A. G. In Organic Light-Emitting Diodes: Materials, Devices and Applications; Buckley, A., Ed.; Woodhead: Cambridge, U.K., 2013. (13) Anderson, S.; Anderson, H. L.; Sanders, J. K. M. Scavenger Templates: Synthesis and Electrospray Mass Spectrometry of a Linear Porphyrin Octamer. Angew. Chem., Int. Ed. Engl. 1992, 31, 907−910. (14) Denti, G.; Campagna, S.; Serroni, S.; Ciano, M.; Balzani, V. Decanuclear Homo- and Heterometallic Polypyridine Complexes: Syntheses, Absorption Spectra, Luminescence, Electrochemical Oxidation, and Intercomponent Energy Transfer. J. Am. Chem. Soc. 1992, 114, 2944−2950. (15) Serroni, S.; Denti, G.; Campagna, S.; Juris, A.; Ciano, M.; Balzani, V. Arborols Based on Luminescent and Redox-Active Transition Metal Complexes. Angew. Chem., Int. Ed. Engl. 1992, 31, 1493−1495. (16) Indelli, M. T.; Scandola, F.; Collin, J. P.; Sauvage, J. P.; Sour, A. Photoinduced Electron and Energy Transfer in Rigidly Bridged Ru(II)−Rh(III) Binuclear Complexes. Inorg. Chem. 1996, 35, 303− 312. (17) Indelli, M. T.; Chiorboli, C.; Flamigni, L.; De Cola, L.; Scandola, F. Photoinduced Electron Transfer across Oligo-p-phenylene Bridges. Distance and Conformational Effects in Ru(II)−Rh(III) Dyads. Inorg. Chem. 2007, 46, 5630−5641. (18) Osio Barcina, J.; Herrero-García, N.; Cucinotta, F.; De Cola, L.; Contreras-Carballada, P.; Williams, R. M.; Guerrero-Martínez, A. Efficient Photoinduced Energy Transfer Mediated by Aromatic Homoconjugated Bridges. Chem. - Eur. J. 2010, 16, 6033−6040. (19) Vagnini, M. T.; Smeigh, A. L.; Blakemore, J. D.; Eaton, S. W.; Schley, N. D.; D’Souza, F.; Crabtree, R. H.; Brudvig, G. W.; Co, D. T.; Wasielewski, M. R. Ultrafast Photodriven Intramolecular Electron Transfer from an Iridium-Based Water-Oxidation Catalyst to Perylene Diimide Derivatives. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15651− 15656. (20) Sykora, M.; Petruska, M. A.; Alstrum-Acevedo, J.; Bezel, I.; Meyer, T. J.; Klimov, V. I. Photoinduced Charge Transfer between CdSe Nanocrystal Quantum Dots and Ru−Polypyridine Complexes. J. Am. Chem. Soc. 2006, 128, 9984−9985. (21) Verma, S.; Kar, P.; Das, A.; Palit, D. K.; Ghosh, H. N. Interfacial Electron-Transfer Dynamics on TiO2 and ZrO2 Nanoparticle Surface Sensitized by New Catechol Derivatives of Os(II)-polypyridyl Complexes: Monitoring by Charge-Transfer Emission. J. Phys. Chem. C 2008, 112, 2918−2926. (22) Verma, S.; Kar, P.; Banerjee, T.; Das, A.; Ghosh, H. N. Sequential Energy and Electron Transfer in Polynuclear Complex Sensitized TiO2 Nanoparticles. J. Phys. Chem. Lett. 2012, 3, 1543− 1548. (23) Zyubin, A. S.; Mebel, A. M.; Hayashi, M.; Chang, H. C.; Lin, S. H. Quantum Chemical Modeling of Photoabsorption Properties of Two- and Three-Nitrogen Vacancy Point Defects in Diamond. J. Phys. Chem. C 2009, 113, 10432−10440. (24) Yu, S. J.; Kang, M. W.; Chang, H. C.; Chen, K. M.; Yu, Y. C. Bright Fluorescent Nanodiamonds: No Photobleaching and Low Cytotoxicity. J. Am. Chem. Soc. 2005, 127, 17604−17605. (25) Zhou, J.; Booker, C.; Li, R.; Zhou, X.; Sham, T. K.; Sun, X.; Ding, Z. An Electrochemical Avenue to Blue Luminescent Nanocrystals from Multiwalled Carbon Nanotubes (MWCNTs). J. Am. Chem. Soc. 2007, 129, 744−745.

redox asymmetry. Photoexcited CNP acts as a triggering device for the process due to its incredible dual property of electron donation and acceptance. In the present work, the CNPs acted as electron acceptors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08633. Absorption spectra of compounds 1−4 in acetone, emission spectra of 1−4 in acetone−dichloromethane mixture, picosecond transient data of CNP with and without 1−4, and transient absorption spectra recorded of CNP without and with 1−4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Department of Science and Technology (DST) (SR/S1/PC-35/2011) for funding. S.M. and S.K.S. thank University Grants Commission (UGC) and Council of Scientific and Industrial Research (CSIR) for their research scholarships. Help from Prof. K. Bhattacharyya of IACS, Kolkata and Dr. P. Sen from IIT Kanpur are deeply acknowledged for the ultrafast experiments.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b08633 J. Phys. Chem. C 2015, 119, 25122−25128