Vectorial Charge Separation and Selective Triplet-State Formation

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Vectorial Charge Separation, and Selective Triplet-State Formation during Charge Recombination in a Pyrrolyl Bridged BODIPY-Fullerene Dyad Venugopal Bandi, Habtom B. Gobeze, Vellanki Lakshmi, Mangalampalli Ravikanth, and Francis D'Souza J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02712 • Publication Date (Web): 24 Mar 2015 Downloaded from http://pubs.acs.org on March 30, 2015

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The Journal of Physical Chemistry

Vectorial Charge Separation, and Selective Triplet-State Formation during Charge Recombination in a Pyrrolyl Bridged BODIPY-Fullerene Dyad Venugopal Bandi,a Habtom B. Gobeze,a Vellanki Lakshmi, b Mangalampalli Ravikanth,b,* and Francis D’Souzaa,* a

Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton,

TX 76203-5017, USA b

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, 400076,

India

Received *** ; Revised*** ; Accepted*** DOI:

1 ACS Paragon Plus Environment


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ABSTRACT: A donor-acceptor dyad comprised of BF2-chelated dipyrromethene (BDP or BODIPY) and fullerene connected with a pyrrole ring spacer, 1 has been newly synthesized and characterized. Due to carbon substitution and extended conjugation offered by the pyrrole ring, the optical absorbance and emission spectra of BDP macrocycle was found to be red-shifted significantly.

Electrochemical studies

provided information on the redox potentials while computational studies performed at the B3LYP/6-31G* level yielded an optimized geometry of the dyad that was close to that reported earlier for a BDP-C60 dyad covalently connected through the central boron atom, 2. The HOMO of the dyad was found to be on the BDP macrocycle, extended over the pyrrole bridging group, a property that is expected to facilitate electronic communication between the BDP and fullerene entities. The established energy level diagram using spectral, redox and optimized structural results predicted possibility of photoinduced electron transfer in both benzonitrile and toluene, representing polar and nonpolar solvents. However, such energy diagram suggested different routes for the charge recombination processes, that is, direct relaxation of the radical ion-pair in polar solvent while populating the triplet level of the sensitizer ( 3BDP* or 3C60*) in nonpolar solvent. Proof for charge separation and solvent dependent charge recombination processes were established from studies involving femto- and nanosecond pump-probe spectroscopy.

The

measured rate of charge separation, kCS for 1 was higher in both solvents compared to the earlier reported values for 2 due to electronically well-communicating pyrrole spacer. The charge recombination in toluene populated 3BDP* as an intermediate step while in benzonitrile it yielded directly ground state of the dyad. The present findings reveal the significance of pyrrole spacer between the donor and acceptor to facilitate charge separation and solvent polarity dependent charge recombination processes.


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INTRODUCTION The ability of natural photosynthetic pigments to transform solar light into chemical energy1-6 have steered research in artificial photosynthesis7-32 leading to the development of molecularly engineered electron donor-acceptor systems, and devices built using these systems for light-toelectricity and light-to-fuel production.33-34 The key steps in artificial photosynthesis involve broadband light capture and funneling, charge separation and migration, combined with slow charge recombination.7-32

In building donor-acceptor conjugates, electron donors such as

porphyrin,35 phthalocyanine,36-37 and other structurally relevant sensitizers38-41 linked to electron acceptors such as quinone, methyl viologen and fullerene have been used. Among the different acceptors, the delocalization of charges within the spherical structure of the rigid aromatic π-sphere of fullerenes is known for generating long-lived charge-separated states.42-45 Taking advantage of this property, novel electron donor-acceptor systems capable of performing fast charge separation and relatively slow charge recombination, have been designed and studied.7-32 Using such welldesigned systems, effect of donor-acceptor distance and orientation, role of solvent and temperature, and significance of spacer connecting the donor and acceptor in the process of charge separation and charge recombination have been systematically investigated and wealth of information has been acquired.7-32 Recently, BF2-chelated dipyrromethenes (BODIPY® or BDP), which are derived from 4,4difluoro-1,3,5,7-tetramethyl-4-bora-3a,4adiaaza-s-indacene,46-47 and their structural analogs, BF2chelated tetraarylazadipyrromethanes (azaBODIPY or ADP)48 have attracted much attention as energy funneling antenna, and electron donors and electron acceptors.49-50 Relatively simple to complex multi-modular systems, performing antenna and reaction center functionalities of natural photosynthesis, have been elegantly designed and studied. In combining BDP and ADP to fullerene, one of the elegant approach developed in our laboratory involve utilization of the central boron, viz., replacing fluorines with a catechol group to which fullerene is attached, BDP-C60 (see compound 2 in Figure 1).51-56 In an effort to rationalize the effect of boron-connected fullerene over peripheral carbon-connected fullerene, in the present study we have synthesized a new dyad using 3-pyrrolyl functionalized BDP, BDPPy-C60 (compound 1 in Figure 1). Systematic spectral and photochemical studies have been performed in benzonitrile and toluene on 1, and compared to the results reported earlier on compound 2.



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Figure 1. Structure of BDPPy-C60, 1 and BDP-C60, 2 and the control compounds, BDPPy-CHO and BDP investigated in the present study. BDP = BF2-chelated dipyrromethene and C60 = fullerene.

RESULTS AND DISCUSSION Synthesis of BDPPy-C60, 1 Dyad. The synthetic details of BDPPy-CHO is given elsewhere.57 Briefly, the meso-phenyl pyrrolyl boron dipyrromethene57 was subjected to Vilsmeier-Haack conditions to afford BDPPy-CHO in decent yield.58 Subsequently, BDPPy-CHO was reacted with fullerene and sarcosine in toluene, according to Prato’s method of fulleropyrrolidine synthesis59 followed by chromatographic separation on silica gel column to obtain BDPPy-C60, 1. The purity of the dyad was ascertained by thin-layer chromatography. The newly synthesized compound was characterized by 1H and

13

C NMR, MALDI-TOF-mass (see supporting information for details),

absorption and emission, and electrochemical methods. The compounds were stored in dark prior doing spectral and photochemical studies. Spectral Characterization. Figure 2a shows the absorption spectra, normalized to the intense visible band, of BDPPy-C60, and control compounds, BDP and BDPPy-CHO in benzonitrile. The intense band of BDPPy-CHO located at 586 nm was red-shifted by about 85 nm accompanied by significant spectral broadness compared to that of pristine BDP as a result of carbon-substitution of pyrrole ring. Covalent attachment of C60 to the spacer pyrrole ring, caused another 11 nm redshift implying electronic interactions between the BDP and C60 macrocycles in the BDPPy-C60. 4 ACS Paragon Plus Environment

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The characteristic fulleropyrrolidine peak at 432 nm (shown by asterisk) was also observed. The C60 peak in 325 nm range was overlapped with BDP bands in this wavelength region. Better spectral coverage of BDPPy (300-650 nm) compared to BDP (300-525) nm was evident from this study. The fluorescence spectrum of BDPPy-CHO revealed a band at 610 nm with a shoulder band at 654 nm (Figure 2b). The peak was red-shifted over 90 nm compared to that of BDP (em = 520 nm). Interestingly, no emission corresponding to either BDPPy or C 60 in the BDPPy-C60

Figure 2. (a) Normalized optical absorbance and (b) steady-state fluorescence spectrum of the indicated compounds in Ar-saturated benzonitrile. The samples were excited at the most intense visible band maxima. (c) Fluorescence decay curve for BDPPy in benzonitrile using 561 nm nanoLED as the excitation source (pink line = light scatter). The emission was collected at 610 nm. (d) Phosphorescence spectrum of BDPPy (ex = 590 nm) at liquid nitrogen temperature in benzonitrile and toluene.



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dyad was observed indicating occurrence of photochemical events.60 Similar spectral observations were also made for the dyad in toluene (see Figure S1 in the supporting information). Next, fluorescence lifetime of BDPPy-CHO was measured using time correlated single photon counting method (TCSPC) using 561 nm nanoLED as the excitation source. The monoexponential decay fit (χ2 = 1.2) gave a lifetime,  of 4.5 ns (Figure 2c). This lifetime is slightly higher than that reported for BDP being 3.2 ns in benzonitrile. Finally, the phosphorescence spectrum of BDPPy-CHO was also recorded at liquid nitrogen temperature (Figure 2d). In both benzonitrile and toluene, phosphorescence emission at 828 nm was observed. Electrochemical Studies. Using differential pulse voltammetry (DPV) technique, redox states of the donor and acceptor components of the dyad were established. Figure 3a show the DPVs of the dyad along with the control compounds, BDP and BDPPy-CHO in benzonitrile containing 0.1 M (n-Bu4N)ClO4. The first one-electron oxidation and reduction processes of BDP were located at 0.73 and -1.62 V vs. Fc/Fc+,51-52 while these processes for BDPPy-CHO were at 0.75 and -1.07 V vs. Fc/Fc+. Easier reduction of BDPPy-CHO compared to BDP with almost the same oxidation potential corroborated absorption studies where smaller HOMO-LUMO gap for BDPPy-CHO compared to BDP was observed. In the BDPPy-C60 dyad, the first oxidation of BDPPy entity was cathodically shifted compared to either BDP or BDPPy and appeared at 0.58 V while the reduction was also cathodically shifted and appeared at -1.32 V vs. Fc/Fc+. The first two one-electron reduction of the fulleropyrrolidine was located at -1.00 and -1.45 V, respectively. Easier reduction of electron acceptor, fullerene compared to BDPPy and easier oxidation of electron donor BDPPy was evident from this study. Further, spectral studies to characterize the one-electron oxidation product of the BDPPy-C60 dyad, to help interpret the transient absorption spectral data discussed in the next section, were performed. For this, nitrosonium tetrafluoroborate, NOBF4 was used as an oxidizing agent. As shown in Figure 3b, increased addition of NOBF4 to a solution BDPPy-C60 dyad in benzonitrile diminished the intensity of the 600 nm band with concomitant development of a new peak at 584 nm, corresponding to the formation of BDPPy•+. Appearance of a transient band at this wavelength would serve as a proof for BDPPy acting as an electron donor in light induced electron transfer reaction.


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Figure 3. (a) Differential pulse voltammograms (DPVs) of BDP, BDPPy-CHO and BDPPy-C60 dyad in benzonitrile containing 0.1 M (n-Bu4N)ClO4. Experimental conditions: scan rate = 5 mV/s, pulse width = 0.25 s, pulse height = 0.025 V. (b) UV-vis spectral changes during chemical oxidation of BDPPy-C60 dyad in benzonitrile. Nitrosonium tetrafluoroborate (0.2 eq. each addition) was used as an oxidizing agent. Computational Studies and Energy Level Diagram. The optimized structure and frontier orbitals of the BDPPy-C60 dyad at the B3LYP/6-31G* level61-62 is shown in Figure 4. The dyad was optimized to a stationary point on a Born-Oppenheimer potential energy surface. The structure of the optimized dyad resembled that of BDP-C60 dyad, 2 in terms of disposition of the fullerene and BDP entities with one another. That is, the fullerene entity was closer to the BF2 segment. The center-to-center distance between the boron atom and the center of fullerene was found to be ~6.8 Å, slightly smaller than that reported for 2. Importantly, the HOMO was found to be on the BDPPy segment with contributions extending to the pyrrolidine ring. This suggests that the spacer pyrrole ring extends the -conjugation all the way to the electron acceptor, 7 ACS Paragon Plus Environment


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fullerene. Such electronic structure would accelerate the electron transfer process.62 Predictably, the LUMO of the dyad was fully localized on the fullerene entity.

Figure 4. (a) B3LYP/6-31G* optimized structure, (b) highest occupied molecular orbital, HOMO and (c) lowest unoccupied molecular orbital, LUMO of the BDPPy-C60 dyad. Further, using the redox, optimized structural and excited singlet energy data, the free-energies of charge-separation (GCS) and charge recombination (radical-ion pair formation) (GCR) were calculated using Rehm-Weller’s approach according to equations 1 and 2.63 −GCS = E0-0 − GCR

(1)

−GCR = (Eox − Ered) + GS

(2)

where Eox is the first oxidation potential of the donor, Ered is the first reduction potential of acceptor, E0-0 is the energy of the lowest excited state of the sensitizer, BDPPy (2.07 eV) or 1C60* (1.75 eV).64 GS refers to the solvation energy, calculated by using the ‘Dielectric Continuum Model’ according to equation 3, -Gs = e2/4o[(1/2R+ + 1/2R- - 1/RCC)(1/s)

(3)

where R+ and R- refer to radii of the radical cation and radical anion species, respectively. RCC is center-to-center distance between the donor and acceptor entities from the optimized structures. 8 ACS Paragon Plus Environment

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Symbols 0 and R refer to vacuum permittivity and dielectric constant of the solvents, respectively. Such calculations revealed GCS = -0.60 eV and GCR = -1.47 eV in benzonitrile. In toluene, the value of GCS would be higher by another 150 mV. 65 The negative GCS values suggest electron transfer to be thermodynamically possible in both solvents. Figure 5 shows an energy level diagram of the BDPPy-C60 dyad established using excited state energies of the donor and acceptor entities, and the free-energy associated for the radical ion-pair in polar benzonitrile and nonpolar toluene.

The

3

BDPPy* was calculated from the

phosphorescence spectra shown in Figure 2d being 1.50 eV and was similar in magnitude to that of 3C60*. It is interesting to note that the free-energy of the radical ion pair, GRIP (= GCR) is

Figure 5. Energy level diagram showing the possible photochemical events in the investigated BDPPy-C60 dyad. Solid arrow – most likely process; dashed arrow – less likely process. Abbreviations: EnT = energy transfer, CS = charge separation, CR = charge recombination, ISC = intersystem crossing, and T = triplet relaxation. slightly lower than that of either 3BDPPy* or 3C60* in benzonitrile while it is slightly higher in toluene. This suggest that the electron transfer product, BDPPy•+-C60•- radical ion-pair, would relax to the ground state directly in benzonitrile (shown in blue arrow) while in toluene, it might 9 ACS Paragon Plus Environment


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populate one of the triplet states (shown in green arrows) prior returning to the ground state. 66 That is, different photochemical routes depending upon the polarity of the solvent is envisioned from this energy level diagram. In order to unravel these possibilities, further studies involving femtosecond and nanosecond transient absorption were systematically performed, as discussed in the next section. Femto- and Nanosecond Transient Pump-Probe Studies. First, femtosecond transient absorption spectra of BDPPy-CHO was investigated in toluene and benzonitrile, as shown in Figure 6. In toluene, the instantaneously formed singlet excited BDPPy-CHO revealed

Figure 6. Femtosecond transient absorption spectra of the BDPPy-CHO in Ar-saturated (a) toluene and (b) benzonitrile at the indicated delay times. Figure inset displays time profiles of the 480 nm (red line) and 590 nm (blue line) peaks. positive peaks at 480, 524(sh), 561, 636, 718, and 1412 nm while a strong negative band at 590 nm, opposite mimic of ground state absorption implying ground state depletion with contributions 10 ACS Paragon Plus Environment

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from stimulated emission. In benzonitrile, the positive peaks were located at 482, 560, 637, 708 and 1420 nm and a negative peak at 590 nm. The near-IR band in the 1420 nm range was attributed to singlet-singlet transition, similar to that reported by us for other BDP derivatives. 67 Figures 6a and 6b inset show the time profiles corresponding to the recovery of the 590 nm peak (blue line) and decay of the 480 nm peak (red line). Similar decay curves for the 1420 nm peak was also observed. The recovery and decay of the monitored signals persisted beyond the 3 ns monitoring window of our instrument, in agreement with earlier discussed longer fluorescence lifetime of BDPPy-CHO, being 4.5 ns. Femtosecond transient absorption spectra of the BDPPy-C60 dyad in toluene and benzonitrile are shown in Figure 7. As anticipated from the energy level diagram discussions, the spectral features in toluene and benzonitrile were significantly different although evidence for charge separation could easily be established. The instantaneously formed 1BDPPy* in toluene decayed with the appearance of a new transient band at 584 nm (in the area of depleted BDPPy signal) corresponding to the formation of BDPPy•+ and at 1020 nm to the formation of C60•-, confirming formation of BDPPy•+-C60•- charge separated state (Figure 7a). In the 900 nm range, broad absorbance was also observed at the early delay times corresponding to 1C60* which could be due to direct excitation of the dyad or ultrafast energy transfer from 1BDPPy* to C60 at the excitation wavelength of 400 nm. After reaching a maxima, the peaks started decaying with the appearance of new peaks at 475, 500(sh), 844 and 940 nm. These peaks were different from that of 3C60* where peaks at 700 and 825(sh) are expected.64 This suggests that the spectrum recorded at the end of 3 ns delay time could be due to the formation of 3BDPPy*. Figure 7a inset show the time profiles of the 584 nm peak corresponding to BDPPy•+ (navy blue line), 1020 nm peak corresponding to C60•- (red line), and 482 nm peak corresponding to 3BDPPy* (blue line). The decay time profile of BDPPy•+ and C60•- were slightly different that could be attributed to their different spectral environments. That is, BDPPy•+ in the area where BDPPy ground state depletion and recovery occurs, and for C60•-, overlap of the 3BDPPy* at the monitoring wavelength of 1020 nm. However, the decay time profile of both peaks persisted beyond 3 ns indicating charge stabilization in toluene. The time profile of the 482 nm peak corresponding to the formation of 3

BDPPy* during charge recombination revealed a steady increase and almost reached a maxima

by 3 ns. The kinetics of rate of charge separation, kCS and charge recombination, kCR, calculated from the growth and decay of the C60•- peak at 1020 nm, were found to be 8.1 x 1010 s-1 and 7.1 x 11 ACS Paragon Plus Environment


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108 s-1, respectively. The kCR determined by monitoring the decay profile of 584 nm corresponding to BDPPy•+ was found to be 8.2 x 108 s-1, slightly higher than that obtained from monitoring time profile of C60•-.

Figure 7. Transient absorption spectra BDPPy-C60 dyad in Ar-saturated (a) toluene and (b) benzonitrile at the indicated delay times. Figure 7a inset illustrates time profile of the 584 nm peak (navy blue line) corresponding to BDPPy•+, 1020 nm peak (red line) corresponding to C60•-, and 482 nm peak (blue line) corresponding to 3BDPPy*. Figure 7b inset shows the time profile of the 1020 nm peak of C60•-. Figure 7b shows the transient absorption spectra at different time intervals of the BDPPy-C60 dyad in benzonitrile. Similar to that observed in toluene, the instantaneously formed 1BDPPy* decayed with the simultaneous appearance of new bands at 584 nm corresponding to BDPPy•+ and at 1020 nm to the formation of C60•-, establishing formation of BDPPy•+-C60•- charge separated 12 ACS Paragon Plus Environment

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state. In addition, in the 900 nm range, broad spectral features of 1C60* were also observed. Interestingly, in contrast to toluene, the decay of the radical ion-pair peaks revealed no new spectral features corresponding to either 3BDPPy* or 3C60* suggesting that the charge recombination process directly leads to the ground state of the dyad. The determined kCS and kCR by monitoring the growth and decay of the C60•- peak at 1020 nm (see Figure 7b inset) were found to be 8.3 x 1010 s-1 and 2.4 x 1010 s-1, respectively. The determined kCS values in benzonitrile and toluene were almost the same indicating that the charge separation process to be a highly exothermic process (belonging to the top of the Marcus parabola),68 that is, originating from 1BDPPy* instead of 1C60* (see Figure 5 for the energy level diagram). Additionally, the kCS values of BDPPy-C60 dyad were slightly higher than that of BDPC60 (7.5 x 1010 s-1 in toluene and 4.9 x 1010 s-1 in benzonitrile) suggesting occurrence of facile charge separation in the pyrrole ring connected donor-acceptor dyad. It appears that the extended -conjugation (see Figure 4b) accelerates the charge separation process. Generally, kCR values are lower than that of kCS, a general trend often observed for fullerene based donor-acceptor dyads.725

That is, the charge recombination process to belong to the inverted portion of the Marcus

parabola,68 although much faster charge recombination was observed in benzonitrile due to general solvent polarity effect. Finally, in order to characterize the 3BDPPy*-C60 formed during charge recombination process in toluene, nanosecond transient spectral studies were carried out. Figure 8a shows the transient spectra at different delay times. Peak maxima at 490, 652, 834(sh), 950, and 1010(sh) nm were observed that resembled the spectrum recorded at 3 ns delay time in Figure 7a. The 1010 nm shoulder band was close to that of C60•- at 1020 nm suggesting formation of long-lived radical ion-pair. However, lack of the 584 nm peak corresponding to BDPPy•+ suggests that it belongs to the 3BDPPy* species. The time profile of the 950 nm peak is shown in Figure 8b, a similar decay curve was obtained for the 490 nm peak. Sharp rise of the 950 nm peak (within 7 ns pulse width of the nanosecond laser) suggests that the spectral origin is via the singlet excited state of BDPPy and not from the triplet state. The measured decay rate constant for relaxation of 3BDPPy* was found to be 1.54 x 105 s-1 in toluene. It may be mentioned here that direct excitation of BDPPyCHO revealed no measurable transient signals, a problem often encountered in BDP and ADP



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based macrocycles.51-52 The triplet is often populated via a charge recombination process, similar to the one observed in the present investigation. 51-52

Figure 8. Nanosecond transient absorption spectra of the BDPPy-C60 dyad in Ar-saturated toluene at the indicated delay times (ex = 532 nm of 7 ns laser pulses). Figure b shows the kinetic trace of the 950 nm peak.

SUMMARY In summary, the role of a pyrrole ring spacer group connecting the donor and acceptor entities in promoting photoinduced electron transfer is investigated in the newly synthesized donoracceptor dyad comprised of BDP and fullerene entities. The presence of pyrrole ring on the BDP led to ring -extension as revealed by red-shifted absorption and emission spectra. This was also supported by computational studies wherein the HOMO was found to be fully delocalized over the BDP and pyrrole entities of the dyad. Consequently, accelerated charge separation was witnessed in the dyad in both polar and nonpolar solvent media from the transient absorption studies. Interestingly, the charge recombination was found to dependent on solvent media. In toluene, during charge recombination, population of intermediate 3BDPPy* was observed while in benzonitrile this process yielded directly ground state of the dyad. Currently, we are exploring the possibility of building solar cells using these dyads for direct light-electricity conversion.


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EXPERIMENTAL SECTION Chemicals and Materials. Reagents used in the present study were obtained from Aldrich Chemicals (Milwaukee, WI) while the bulk solvents used in the syntheses were from Fischer Chemicals (Plano, TX).

Tetra-n-butylammonium perchlorate, (n-Bu4N)ClO4 used in

electrochemical studies was from Fluka Chemicals (Ronkonkoma, NY). The synthesis of BDPPyCHO is given elsewhere.58 Syntheses of BDPPy-C60 Dyad. To a solution of C60 (65 mg, 0.09 mmol), in dry toluene (50 ml), sarcosine (14 mg, 0.157 mmol) and the BDPPy-CHO (11 mg, 0.03 mmol) were added. After refluxing the solution for 24 h, the solvent was removed under vacuum. The crude compound was purified by column chromatography on silica gel using 1:1 Hexane/toluene as eluent to give the product 1: Yield 24 mg (71%); 1H NMR (400MHz, CDCl3:CS2) δ = 11.25 (s, 1H), 7.72 (s, 1H), 7.52-7.45 (m, 6H), 7.02 (s, 1 H), 6.92 (s, 2H), 6.70 (s, 1H), 6.60-6.20 (d, 1H), 6.44-6.40 (d, 1H);13C NMR (400MHz, CDCl3:CS2) δ = 147-144 (m), 143-141(m), 140, 138, 136, 134, 132, 130, 128, 126, 121, 118, 116, 115, 40, 33, 31, 30, 22, 14; MALDI-TOF-MS (in DHB): m/z calcd for C82H19BF2N4 : 1108.86; found: 1108.1 and 1107.0. (See Figure S2-S3 for spectra). Spectral Measurements. The UV-visible and near-IR spectral measurements were carried out on a Jasco V-670 spectrophotometer. The steady-state fluorescence was recorded by using a Varian (Cary Eclipse) Fluorescence Spectrophotometer or a Horiba Jobin Yvon Nanolog spectrofluorimeter equipped with PMT (for UV-visible) and InGaAs (for near-IR) detectors. The fluorescence lifetimes were measured with the Time Correlated Single Photon Counting (TCSPC) option with nano-LED excitation sources on the Nanolog. All the solutions were purged prior to spectral measurements using argon gas. The 1H NMR studies were carried out on a Varian 400 MHz spectrometer with tetramethylsilane (TMS) as an internal standard. Computational geometry optimizations were performed at the B3LYP/6-31G* level using GAUSSIAN 03 software package.61 GaussView program of GAUSSIAN was used to generate frontier HOMO and LUMO orbitals. Electrochemistry. Redox potentials were measured using a three electrode system on a Princeton Applied Research potentiostat/galvanostat Model 263A instrument. A platinum bottom, a platinum wire and a Ag/AgCl served as working, counter and reference electrodes, respectively.



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The measured potentials were referenced against an internal ferrocene/ferrocenium redox couple. The solutions were degassed using argon gas prior electrochemical and spectral measurements. Transient Absorption Measurements.

Femtosecond pump-probe experiments were

performed using an Ultrafast Femtosecond Laser Source (Libra) by Coherent combining a diodepumped, mode locked Ti:Sapphire laser (Vitesse) and diode-pumped intra cavity doubled Nd:YLF laser (Evolution) to produce a compressed laser output of 1.45 W. For optical detection, a Helios transient absorption spectrometer coupled with a second and third harmonics generator, both provided by Ultrafast Systems was used. The source for the pump and probe pulses were derived from the fundamental output of Libra (Compressed output 1.45 W, pulse width 100 fs) at a repetition rate of 1 kHz. About 95% of the fundamental output of the laser was introduced into harmonic generator to produce the second and third harmonics of 400 and 267 nm besides the fundamental 800 nm for excitation. The rest of the output was used for generation of white light continuum. In the present study, the second harmonic output (400 nm) was used as excitation pump source. Kinetic profiles at appropriate wavelengths were assembled from the time-resolved spectral data. Data analysis was carried out using Surface Xplorer software from Ultrafast Systems. All spectral measurements were performed at room temperature in Ar-saturated solutions. Nanosecond Flash Photolysis. The instrumental setup comprised of a Opolette HE 355 LD pumped by a high energy Nd:YAG laser with second and third harmonics OPO (tuning range 4102200 nm, pulse repetition rate 20 Hz, pulse length 7 ns) with laser powers of 1.0 to 3 mJ per pulse. For spectral measurements, a Proteus UV-Vis-NIR flash photolysis spectrometer (Ultrafast Systems, Sarasota, FL) with a fibre optic delivered white light as probe, and either a fast rise Si photodiode detector (covering 200-1000 nm range) or a InGaAs photodiode detector (covering 900-1600 nm range) was used. The output from the photodiodes and a photomultiplier tube was recorded using a digitizing Tektronix oscilloscope. Data analysis was performed using Surface Xplorer software from Ultrafast Systems. ASSOCIATED CONTENT Supporting Information Absorbance and fluorescence spectra of BDPPy-C60 in toluene, Maldi-TOF-mass, 1H and 13C NMR spectra of the dyad, complete citation details of Ref. 61. See DOI: This material is available free of charge via the Internet at http://pubs.acs.org. 16 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (M. Ravikanth); [email protected] (F. D’Souza) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Authors are thankful to the National Science Foundation (Grant No. 1401188 to FD) and Department of Science and Technology, Govt. of India (File No. SR/S1/IC-12/2011 to MR) for support of this work.

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Table of contents Vectorial Charge Separation, and Selective Triplet-State Formation during Charge Recombination in a Pyrrolyl Bridged BODIPY-Fullerene Dyad V. Bandi, H. B. Gobeze, V. Lakshmi, M. Ravikanth, * and F. D’Souza*


Vectorial Charge Separation and Selective Triplet-State Formation

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