Charge Stabilization in High-Potential Zinc Porphyrin-Fullerene via

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Charge Stabilization in High-Potential Zinc PorphyrinFullerene via Axial Ligation of Tetrathiafulvalene Christopher O. Obondi, Gary N. Lim, Youngwoo Jang, Prajay Patel, Angela K Wilson, Prashanth K. Poddutoori, and Francis D'Souza J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00028 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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The Journal of Physical Chemistry

Charge Stabilization in High-Potential Zinc Porphyrin-Fullerene via Axial Ligation of Tetrathiafulvalene

Christopher O. Obondi,a Gary N. Lim,a Youngwoo Jang,a Prajay Patel,b Angela K. Wilson,b,* Prashanth K. Poddutoori,c,* Francis D’Souzaa,*

a

Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX 76203-

5017, USA. b

Department of Chemistry, Michigan State University, 578 S. Shaw Lane, East Lansing, MI 48824-1322,

USA. c

Department of Chemistry, University of Prince Edward Island, 550 University Avenue, Charlottetown,

PE, C1A 4P3, Canada.

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ABSTRACT: Extending the lifetime of the charge separated states generated during photoinduced electron transfer in a covalently linked high-potential zinc porphyrin-fullerene dyad, (F15P)Zn-C60, was accomplished by metal-ligand axial coordination of pyridine functionalized tetrathiafulvalene (TTF) via a dual electron transfer/hole migration mechanism. The meso-aryl positions of the zinc porphyrin carried three penta-fluorophenyl substituents that made the zinc porphyrin ring harder to oxidize by 0.43 V compared to zinc porphyrin with meso-phenyl substituents. Two TTF derivatives, a first with a pyridine directly linked to TTF (Py-TTF), and a second with a phenyl spacer between the pyridine and TTF (PyphTTF) were employed to vary the distance between the primary photosensitizer/electron donor, zinc porphyrin, and the secondary electron donor, TTF. Both Py-TTF and Py-phTTF coordinated via the pyridine entity to the Zn center with 1:1 molecular stoichiometry and moderate binding constants. The supramolecular triads were characterized by optical absorption and emission, electrochemistry and computational studies. An energy level diagram was established to realize the different photochemical events in the triads. Using femtosecond transient absorption spectroscopy, it was possible to show that the coordinated TTF participated in electron transfer from the 1(F15P)Zn* in the case of the C60(F15P)Zn:TTF triads to produce C60-(F15P)Zn•-:TTF•+ charge separated state competitively with the electron transfer from the 1(F15P)Zn* to covalently linked C60 to produce C60•--(F15P)Zn•+:TTF charge separated state.

The two charge separated states, C60-(F15P)Zn•-:TTF•+ and C60•--(F15P)Zn•+:TTF, were further

involved in electron migration in the former case and hole transfer in the latter case to produce the C60•-(F15P)Zn:TTF•+ charge separated state as the ultimate electron transfer product.

Due to distal

separation of the positive and negative radical ions, long-lived charge separated states persistant for about 0.2 microseconds was possible to accomplish, as shown by nanosecond transient absorption spectral studies.


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INTRODUCTION Sustainable production of electricity and fuel using abundant solar photons is one of the highly researched topics in modern science.1-12 Often, the design of light energy harvesting materials follows the concepts developed by Mother Nature13-15 in bacterial and green plant photosynthetic systems. The primary photochemical events in natural photosynthesis involves capturing and funneling of sunlight by a group of well-organized chromophores called ‘antenna’ systems and promoting electron transfer using the funneled light into the ‘reaction center’ where a cascade of electron transfer events occurs leading to the generation of long-lived charge separated states. Over the last two-to-three decades, researchers have mainly focused on mimicking the early photo-events of natural photosynthesis by building donoracceptor systems to visualize energy and electron transfer or a combination of these two events.16-45 Recent efforts on this research topic include designing multi-modular systems that are capable of capturing most of the useful sunlight and generating charge separated states of prolonged lifetimes with adequate amounts of energy. Higher amounts of stored energy and longer lifetimes of the charge separated states are essential to drive catalytic reactions related to solar fuel or synthesizing valueadded products. However, only a handful of systems have fulfilled these expectations.46-49 The strategies used in building donor-acceptor systems that are capable of producing high-energy charge separated states include choosing donors that are difficult to oxidize and acceptors that are difficult to reduce; under these conditions, the stored energy in the charge separated state is equivalent to the potential difference between the oxidation and reduction potentials of the donor and acceptor, respectively. However, challenges exist to accomplish this goal where the excited state energy from either the donor or the acceptor may not be sufficient to drive the electron transfer process in an energetically feasible fashion.

Additional problems include a fast charge recombination process

prompted by high Coulombic forces and low-lying triplet excited states of the donor and acceptor entities.50 Recently, we reported on a donor-acceptor dyad, (F15P)Zn-C60, capable of generating charge separated state carrying an energy of 1.70 eV (see Figure 1 for structure of the dyad).51 In this study, the electron donor zinc porphyrin was functionalized with meso-pentafluorophenyl substituents that made the zinc porphyrin difficult to oxidize by 0.43 eV compared to simple zinc porphyrins. The singlet excited energy of 1(F15P)Zn* (= 2.21 eV) was sufficient to drive the electron transfer process. The lifetime of the charge separated state was persistent for about 50-60 ns. In the present study, to prolong the lifetime of the charge separated state, we have developed supramolecular triads using a hole transporting tetrathiafulvalene, TTF, linked via the well-known metal-ligand axial coordination approach.52 Here, the TTF was functionalized with either pyridine or phenylpyridine coordinating ligands. Supramolecular 3 ACS Paragon Plus Environment


triad formation including binding constants of the complexes were determined by spectroscopic methods. Using electrochemical methods, the redox states of different entities were identified and further used in establishing an energy level diagram to understand the mechanistic details of light induced electron/hole transfer processes.

Finally, transient absorption spectroscopic techniques

covering the wide femto-to-millisecond time scale were employed to characterize the electron transfer products and calculate the lifetime of the charge separated states.

Figure 1. Structures of the C60-(F15P)Zn:PyTTF (n=0) and C60-(F15P)Zn:Py-phTTF (n=1) supramolecular triads developed and investigated in the present study (: represents coordinate bond between Zn and pyridine).

EXPERIMENTAL SECTION Chemicals. Tetra-n-butyl ammonium perchlorate, (TBA)ClO4, was obtained from Fluka Chemicals. All the solvents in sure seal bottles were procured from Fisher Scientific and were used as received. Synthesis of (F15P)Zn-C6051 and Py-TTF and Py-ph-TTF52 are given elsewhere. Spectral measurements. The UV-visible and near-IR spectral measurements were carried out with a Shimadzu 2550 UV-Vis spectrophotometer or Jasco V-670 spectrophotometer. The steady-state fluorescence and phosphorescence spectral were recorded a Horiba Jobin Yvon Nanolog spectrofluorimeter equipped with PMT (for UV-visible) and InGaAs (for near-IR) detectors. A right angle detection method was used for fluorescence measurements at room temperature and phosphorescence at liquid nitrogen temperature. The fluorescence lifetimes were measured with the Time Correlated Single Photon Counting (TCSPC) lifetime option with nano-LED excitation sources on a Horiba Jobin Yvon Nanolog. All the solutions were purged prior to spectral measurements using argon gas. 4 ACS Paragon Plus Environment

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Spectroelectrochemical studies were performed by using a cell assembly (SEC-C) supplied by ALS Co., Ltd (Tokyo, Japan). This assembly comprised of a Pt counter electrode, a 6 mm Pt gauze working electrode, and an Ag/AgCl reference electrode in a 1.0mmpath length quartz cell. The optical transmission was limited to 6 mm covering the Pt gauze working electrode. Electrochemistry. Cyclic and differential pulse voltammetry was recorded on a Princeton Applied Research potentiostat/galvanostat Model 263A using a three electrode system. A platinum button electrode was used as the working electrode, while a platinum wire served as the counter electrode and an Ag/AgCl electrode was used as the reference electrode. Ferrocene/ferrocenium redox couple was used as an internal standard. All of the solutions were purged prior to electrochemical measurements with nitrogen gas. Femtosecond Laser Flash Photolysis: Femtosecond transient absorption spectroscopy experiments were performed using an Ultrafast Femtosecond Laser Source (Libra) by Coherent integrating a diodepumped, mode locked Ti:Sapphire laser (Vitesse) and diode-pumped intra cavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of 1.45 W. For optical detection, a Helios transient absorption spectrometer coupled with femtosecond harmonics generator both provided by Ultrafast Systems LLC (FL) 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 which produces second and third harmonics of 400 and 267 nm besides the fundamental 800 nm for excitation, while the rest of the output was used for generation of white light continuum. In the present study, the second harmonic 400 nm excitation pump was used in all the experiments. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. Data analysis was performed using Surface Xplorer software supplied by Ultrafast Systems. All measurements were conducted in degassed with argon solutions at 298 K. Nanosecond Laser Flash Photolysis: The studied compounds were excited by a Opolette HE 355 LD pumped by a high energy Nd:YAG laser with second and third harmonics OPO (tuning range 410-2200 nm, pulse repetition rate 20 Hz, pulse length 7 ns) with the powers of 1.0 to 3 mJ per pulse. The transient absorption measurements were performed using a Proteus UV-Vis-NIR flash photolysis spectrometer (Ultrafast Systems, Sarasota, FL) with a fibre optic delivered white probe light and either a fast rise Si photodiode detector covering the 200-1000 nm range or a InGaAs photodiode detector covering 900-1600 nm range. The output from the photodiodes and a photomultiplier tube was 5 ACS Paragon Plus Environment


recorded with a digitizing Tektronix oscilloscope. Data analysis was performed using Surface Xplorer software supplied by Ultrafast Systems. The estimated error in the kinetic values of both femtosecond and nanosecond transient data is +10%. RESULTS AND DISCUSSION In the present study, we have employed relatively nonpolar, non-coordinating toluene and odichlorobenzene (DCB) solvents. The spectral and transient absorption studies were performed in toluene which proved to be a stable solvent for laser irradiation experiments, and DCB which was useful for electrochemical studies since the supporting electrolyte is soluble only in DCB and not in toluene.

Figure 2. (a) Normalized to the Soret band absorption spectra of (i) (F15P)Zn-C60 dyad and (ii) (F15P)Zn control, (iii) 2-phenyl fulleropyrrolidine and (iv) Py-TTF in toluene. The figure inset shows the phosphorescence spectrum of (F15P)Zn control in methyl cyclohexane containing 5% CH3I at 77K. (b) Steady-state fluorescence spectra of (i) (F15P)Zn-C60 dyad, (ii) (F15P)Zn control (ex = 546 nm), and (iii) 2phenyl fulleropyrrolidine (ex = 432 nm) in toluene. 6 ACS Paragon Plus Environment

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Figure 2a shows the normalized Soret band absorption spectra of the investigated dyad and the control compounds in toluene. Peak maxima for (F15P)Zn-C60 were located at 420, 546 and 576 nm that were similar to that of the control (F15P)Zn indicating the absence of ground state interactions between the (F15P)Zn and C60 entities. The C60 peak in the 320 nm region overlapped with the UV band of (F15P)Zn located at 310 nm. The TTF derivatives revealed two peaks at 326 and 440 nm. Fluorescence quenching in the (F15P)Zn-C60 dyad compared to that in the control compound was observed, as shown in Figure 2b. The two emission peaks of the (F15P)Zn control were located at 592 and 644 nm and were quenched by about 40% in the dyad suggesting an occurence of excited state events from the 1(F15P)Zn* state. Fluorescence lifetimes were also measured for the control and the dyad. The decay profiles could be fitted satisfactorily to a monoexponential decay function (2 < 1.2) in both cases. The lifetimes of the (F15P)Zn control and (F15P)Zn-C60 dyad were found to be 1.83 ns and 0.82 ns in toluene, respectively. The fluoresence spectrum of 2-phenyl fulleropyrrolidine was also measured. A weak peak in the 715 nm range was observed (see Figure 2b). To assist in establishing the energy levels, the phosphorescence spectrum of (F15P)Zn was recorded in methyl cyclohexane containing 5% methyl iodide as solvent at liquid nitrogen temperature. As shown in the Figure 2a inset, a peak at 758 nm was observed. Supramolecular C60-(F15P)Zn:PyTTF formation- Spectral and computational studies It is well-known that zinc porphyrins bind to nitrogenous ligands with 1:1 molecular stoichiometry with moderate binding stability.20 This was also found to be the case for the binding of pyridine functionalized TTFs to the (F15P)Zn-C60 dyad. Figure 3a shows spectral changes associated during PyphTTF binding to (F15P)Zn-C60 in DCB. Binding was accompanined by red-shfited spectra with isosbestic points at 523, 554, 590 and 615 nm indicating the existence of only one equilibrium process. The binding constant, K, was evaluted from the Benesi-Hildebrand plot53 for C60-(F15P)Zn:Py-phTTF and was found to be 6.4  103 M-1 (Figure 3b). Similar spectral changes were also observed when Py-TTF was binding to the dyad, and when Py-TTF and Py-phTTF was binding to the control (F15P)Zn. That is, red shifted spectra with isosbestic points were observed. The calculated binding constants were found to be 3.9  103 M-1 for C60-(F15P)Zn:Py-TTF, 3.2  103 M-1 for (F15P)Zn:Py-TTF, and 4.8  103 M-1 for (F15P)Zn:PyphTTF, respectively. The K values were slightly higher for the dyads compared to the (F15P)Zn control compound.



Figure 3. (a) Spectral changes observed during Py-phTTF binding to (F15P)Zn-C60. (b) Benesi-Hildebrand plot for evaluating the binding constant. (c) Fluorescence spectra of (F15P)Zn control on increasing addition of Py-phTTF (ex = 555 nm). (d) Fluorescence spectra of (F15P)Zn-C60 control on increasing addition of Py-phTTF (ex = 555 nm). All spectra were measured in DCB. The fluorescence behavior of (F15P)Zn and (F15P)Zn-C60 in the presence of coordinating TTF derivatives was subsequently investigated. From the spectra shown in Figure 2b it was clear that appended C60 quenches the fluorescence of (F15P)Zn in the dyad significantly. In general, due to the presence of electron withdrawing fluoro-substituents, the (F15P)Zn ring is electron deficient making the ring oxidation harder while at the same time making ring reduction easier (see electrochemical section for details). Thus, there is a possibility that the coordinated electron rich TTF might be involved in the excited state electron transfer.

Under such circumstances, quenching of fluorescence of pristine

(F15P)Zn could be expected.54 This was indeed to be the case as shown in Figure 3c where increasing addition of Py-phTTF to a solution of pristine (F15P)Zn revealed red-shifted peaks due to axial coordination and the queching was due to the occurrence of excited state events. Isosbestic points at 603, 616 and 658 nm were also observed. Similar results were also observed when Py-TTF was allowed 8 ACS Paragon Plus Environment

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to interact with (F15P)Zn (See Figure S1 in SI). Interestingly, when Py functionalized TTF was allowed to interact with the (F15P)Zn-C60 dyad, fluoresence spectral features similar to TTF binding to (F15P)Zn was observed. That is, red-shifted fluorescence peaks due to axial coordination and quenching due to additional excited state events were observed (Figure 3d). Isosbestic points at 598, 620 and 650 nm were also observed. These results suggest that the 1(F15P)Zn* formed upon excitation in the C60(F15P)Zn:Py-TTF and C60-(F15P)Zn:Py-phTTF triads have at least two relaxation pathways involving electron rich TTF and electron deficient C60. This is in addition to the usual intersystem crossing of 1(F15P)Zn* to populate the 3(F15P)Zn* state. The geometry and electronic structures of the (F15P)Zn-C60 dyad, and C60-(F15P)Zn:Py-TTF and C60(F15P)Zn:Py-phTTF triads were probed using the hybrid-meta Minnesota functional M06-2X with 54% exact exchange and the 6-31G* basis set.55-58 Figure 4 depicts molecular electrostatic potential (MEP)

Figure 4. The M06-2X/6-31G* molecular electrostatic potential maps, and the frontier HOMO and LUMO of the optimized structures of (a) (F15P)Zn-C60 dyad and (b) C60-(F15P)Zn:Py-phTTF triad. The isovalue used for the MO depictions was 0.02 while the density value used was 0.0004.



maps and frontier HOMO and LUMO for the optimized structures. In the case of the dyads, the frontier HOMO was on the (F15P)Zn and LUMO on the C60 making them the donor and acceptor sites,respectively. Interestingly, for the triad, HOMO was shifted to the TTF site without altering the location of LUMO. This is understandable due to easier oxidation of TTF over (F15P)Zn, discussed in the next section. HOMO-1 occupied the (F15P)Zn for the triads. It may be pointed out here that the density of the dyad was not affected by the addition of TTF ligand except at the porphyrin center where the potential was neutral, which indicates that the central metal is fully coordinated and not likely to bind an additional ligand. The estimated center-to-center distances between Zn and C60 in the dyad and triads were ~ 17.5 Å while these distances between Zn and TTF were ~18.0 and ~17.7 Å, respectively, in the case of C60-(F15P)Zn:Py-TTF and C60-(F15P)Zn:Py-phTTF triads. Electrochemistry, spectroelectrochemistry and energy level diagram Cyclic (CVs) and differential pulse voltammograms (DPVs) of the control (F15P)Zn and (F15P)Zn-C60 dyad in DCB containing 0.1 M (TBA)ClO4 are shown in Figure 5a. The first two oxidation and first reduction of (F15P)Zn were located at 0.60, 0.78 and -1.82 V vs. Fc/Fc+, respectively (Figure 5a(i)). Upon appending the C60, there was a slight anodic shift of potentials, that is, oxidations at 0.66, 0.92 and a reduction at -1.92 V vs. Fc/Fc+ were observed. In addition, C60 reductions at -1.13, -1.53 and -2.05 V were also observed (Figure 5a(ii)). Cyclic voltammetric studies confirmed these processes to be oneelectron reversible processes (Figure 5a(iii)), as determined from anodic-to-cathodic peak current ratio and scan rate studies.59 The different pulse voltammograms (DPVs) during the formation of C60(F15P)Zn:Py-phTTF triad are shown in Figure 5b; similar results were obtained during the formation of the C60-(F15P)Zn:Py-TTF triad (see Figure S2 in SI). A slight cathodic shift of the first oxidation and first reduction corresponding to (F15P)Zn (~ 20-30 mV) were observed upon Py-phTTF coordination, however, without altering the potential corresponding to the first reduction of C60. In addition, two new anodic processes at -0.08 and 0.28 V were observed corresponding to TTF oxidation. While the first oxidaton process of TTF was reversible, the second oxidation process was quasireversible and produced a shoulder peak at 0.41 V. From these studies, facile oxidation of TTF over (F15P)Zn, and facile reduction of C60 over (F15P)Zn were borne out.


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Figure 5. (a) Cyclic and differential pulse voltammograms of (F15P)Zn (i), and (F15P)Zn-C60 (ii and (iii)), and (b) DPVs of (F15P)Zn-C60 on increasing additions of Py-phTTF to form the C60-(F15P)Zn:Py-phTTF triad. The voltammograms were recorded in DCB containing 0.1 M (TBA)ClO4. Spectral characterization of the one-electron oxidized and one-electron reduced species of (F15P)Zn, and the one-electron oxidized species of pyridine derivatized TTF, needed to characterize the transient spectral species during photoinduced electron transfer process, were performed. As shown in Figure 6a, during the course of the first oxidation of (F15P)Zn, peaks corresponding to the neutral species revealed decreased intensity with the appearance of new peaks at 790 and 905 nm, corresponding to the formation of (F15P)Zn•+. Similarly, during the first reduction of (F15P)Zn, new peaks at 438, 571 and 616 nm corresponding to the formation of (F15P)Zn•- were observed (see Figure 6b). The one-electron oxidized pyridine derivatized TTF revealed new peaks at 450 and 610 nm corresponding to TTF•+ (see Figure 6c). These processes were found to be fully-reversible on the spectroelectrochemical time scale, that is, most of the initial spectrum could be recovered by reversing the potential.



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Figure 6. Spectral changes observed during controlled potential (a) first oxidation of (F15P)Zn, (b) first reduction of (F15P)Zn, and (c) first oxidation of Py-phTTF. The applied potentials were 100 mV past the peak potential of the given redox process, dicussed in Figure 5. The free energy change for charge recombination (ΔGCR) and charge separation (ΔGCS) from the singlet excited state 1(F15P)Zn* within the triad was calculated using spectroscopic, computational and electrochemical data following equations 1-3, according to the Rehm and Weller approach.60 -GCR = Eox – Ered + GS

(1)

-GCS = GCR

(2)

where and GS correspond to the energy of the excited singlet state of (F15P)Zn (=2.21 eV) and electrostatic energy (= 0.06 eV), respectively. The Eox and Ered represent the oxidation potential of the electron donor, (F15P)Zn or TTF and the reduction potential of the electron acceptors, C60 or (F15P)Zn, respectively. The term Gs was calculated by using the “dielectric continuum model” according to equation (3):

GS  e

2

 1  1   1   1   40  2 R 2 R     R  R   CC R   12 ACS Paragon Plus Environment

(3)

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The symbols 0, and R represent vacuum permittivity and dielectric constant of the solvents used for photochemical and electrochemical studies, respectively. RCC is the center-to-center distance between donor and acceptor entities. The symbols R+ and R- refer to radii of the cation and anion species, respectively. Figure 7 shows the energy level diagram constructed to visualize the different photochemical events occurring in the C60-(F15P)Zn:TTF triads. In the present investigation, the samples were excited at 400 nm (100 fs pulses) that predominantly excited the (F15P)Zn entity of the triad to produce the C601

(F15P)Zn*:TTF excited state. The 1(F15P)Zn* thus formed in the triad could undergo either energy

transfer to produce (F15P)Zn-1C60* (minor process due to lack of spectral overlap), or intersystem crossing to produce 3(F15P)Zn* or sequential electron transfer to yield different charge separated states. The steady-state fluorescence spectrum of the (F15P)Zn revealed quenching of about 40% upon appending C60 (Figure 2b); this was also the case when Py-TTF or Py-phTTF was coordinated to (F15P)Zn where quenching of about 40% of the initial fluorescence emission was observed (Figure 3c). These results indicate that the initially formed 1(F15P)Zn* is only partially involved in the electron transfer processes either with the appended C60 or TTF entities. The remainder of the 1(F15P)Zn* could undergo intersystem crossing to populate the 3(F15P)Zn* state or relax to the ground state in a non-radiative path. The C601

(F15P)Zn*:TTF species could promote electron transfer in two different ways, first, involving the electron

acceptor, C60 to produce the C60•--(F15P)Zn•+:TTF charge separated state or involving electron donor, TTF to produce the C60-(F15P)Zn•-:TTF•+ charge separated state. From the earlier discussed fluorescence data, both photochemical paths seems to be possible where (F15P)Zn fluorescence quenching upon TTF coordination was observed (Figure 3d). In fact, energetically speaking, the ΔGCS value for the former process is -0.40 eV and for the latter process it is -0.80 eV; making both of them thermodynamically feasible, more so, for the latter process. These energy considerations also apply to simple dyads viz., C60-(F15P)Zn and (F15P)Zn:TTF. Furthermore, the C60•--(F15P)Zn•+:TTF and C60-(F15P)Zn•-:TTF•+ charge separated states could undergo a hole transfer (HS) in the former case and an electron migration (EM) in the latter case to produce the final C60•--(F15P)Zn:TTF•+ charge separated state. Energetically, both processes are thermodynamically feasible with GHS = -0.44 eV for the former process and GEM = -0.04 for the latter process. The C60•--(F15P)Zn:TTF•+ charge separated state, owing to the distal separation could slow the charge recombination process leading to the desired long-lived charge separated states. Inspired with these possibilities of charge stabilization, we have performed systematic studies using femto- and nanosecond transient spectroscopic techniques, the results of which are discussed in the next section. 13 ACS Paragon Plus Environment


Figure 7. Energy level diagram depicting the different photochemical events occuring in the C60(F15P)Zn:TTF dyad upon photoexcitation leading to the formation of initial C60-1(F15P)Zn*:TTF excited state that leads to long-lived charge separated state. Abbreviations: CS = charge separation, CR = charge recombination, ISC = intersystem crossing, HS = hole separation, EM = electron migration, T = triplet emission. The states are drawn according to their energy scales. An ET value for 3[(F15P)Zn]* = 1.64 eV from phosphorescence spectrum (Figure 2a inset) and ET value for 3C60* = 1.50 Ev61 are adopted. Femto- and nanosecond transient absorption spectral studies Femtosecond transient spectra of the control (F15P)Zn at the indicated delay times are shown in Figure S3 in SI. The transient spectra of (F15P)Zn in toluene revealed instanteneously formed 1(F15P)Zn* with positive peaks at 457, 571, 598, 702 and 1295 nm and negative peaks at 548 and 645 nm. The 1295 nm peak has been ascribed to the singlet-singlet transition originating from 1(F15P)Zn* due to its close similarity with other zinc porphyrins.62 The 548 nm peak was due to ground state depletion while the 645 nm peak was due to stimulated emission. Decay/recovery of the positive/negative peaks resulted in populating the 3(F15P)Zn* via intersystem crossing with characteristic peaks located at 463 and 848 nm. The decay of the 1295 nm peak (see Figure S1 inset) lasted beyond the 3 ns window, consistent with the earlier discussed fluorescence lifetime of 1.83 ns. The steady-state fluorescence results showed quenching of (F15P)Zn fluorescence upon axial coordination of TTF derivatives (Figure 3c), and the free-energy calculations indeed suggested that the


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formation of (F15P)Zn•-:TTF•+ from the 1(F15P)Zn* is energetically a feasible process. Figure 8a shows the transient spectrum of pristine (F15P)Zn and (F15P)Zn:Py-phTTF at a delay time of 10 ps (see Figure S4 in SI for complete spectra of (F15P)Zn:Py-TTF and (F15P)Zn:Py-phTTF)).

At this delay time, faster

decay/recovery of the transient peaks of 1(F15P)Zn* was clear. In addition, although weak, a positive peak as shoulder to the 550 depleted peak (570 nm range) corresponding to (F15P)Zn•- and in the 615 nm range having contributions from both (F15P)Zn•- and TTF•+ were observed providing evidence for the formation of (F15P)Zn•-:TTF•+ charge separated state. The decay profile of 1(F15P)Zn* at 1295 nm in the presence and absence of coordinated Py-phTTF is shown in Figure 8b.

Faster decay upon TTF

coordination was evident. By using this kenetic information, the rate constant for charge separation (kCS = 1/(F15P)Zn:TTF -1/(F15P)Zn)) was estimated. Such calculations revealed a kCS value of 3.02  109 s-1 for (F15P)Zn•-:Py-TTF•+ and 8.43  108 s-1 (F15P)Zn•-:Py-phTTF•+ formation.

Figure 8. (a) Femtosecond transient absorption spectrum of (F15P)Zn (green) and (F15P)Zn:Py-phTTF (magenta) at a delay time of 10 ps in toluene. (b) The time profile of the 1295 nm peak. Figure 9 shows the transient spectra of (F15P)Zn-C60 at the indicated delay time in toluene. The faster decay/recovery of the peaks of the instantaneously formed 1(F15P)Zn* started revealing new peaks in the 790 and 900 nm region corresponding to (F15P)Zn•+, and in the 1015 nm range corresponding to C60•- thus offering proof for charge separation (see spectrum at 25 ps). With time, the slow decay of (F15P)Zn•+ and C60•- started developing a new broad peak in the 700 nm region correponding to 3C60*. In agreement with our earlier studies, even at 3 ns, the decay of the radical ionpair peaks was not complete. This becomes clearer in the decay profiles shown in Figures 9b for the 1295 nm peak of the (F15P)Zn and (F15P)Zn-C60 revealing faster decay of 1(F15P)Zn* state due to the



occurence of competing charge separation involving C60, and Figure 9c for the 1015 nm peak of C60•-. The rate constant for charge separation, kCS, was estimated using the earlier described method and was found to be 1.7  108 s-1 in toluene.

Figure 9. (a) Femtosecond transient absorption spectrum of (F15P)Zn-C60 at the shown delay times in toluene. (b) The time profile of the 1295 nm peak of (F15P)Zn (green) and (F15P)Zn-C60 (red). (c) Time profile of the 1015 nm peak corresponding to C60•-. Figures 10a and 11a show the transient absorption spectra at the shown delay times of the C60(F15P)Zn:Py-TTF and C60-(F15P)Zn:Py-phTTF triads in toluene, respectively. Most notewothy observations include faster decay/recovery of the positive/negative transient peaks orginating from the initial 1

(F15P)Zn* state. In fact, such processes were faster than that observed for either (F15P)Zn:TTF dyad

(Figure 8) or (F15P)Zn-C60 dyad (Figure 9). This is understandable since the fluorescence spectrum of the (F15P)Zn-C60 dyad revealed additional quenching upon coordinating TTF as shown in Figure 3d. These observations point out to the formation of C60•--(F15P)Zn•+:TTF and C60-(F15P)Zn•-:TTF•+ charge separated states from the 1(F15P)Zn* state competitively. Transient peaks were observed (see spectrum at 5 ps


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Figure 10. Femtosecond transient absorption spectra at the shown delay times of the C60-(F15P)Zn:PyTTF triad in toluene. (b) Time profile of the 1295 nm peak of 1(F15P)Zn* before (red) and after coordinating Py-TTF (blue) to (F15P)Zn-C60. (c) Normalized decay profile of the C60•- monitored at 1015 nm of (F15P)Zn-C60 dyad (red) and C60-(F15P)Zn:Py-TTF triad (blue).

Figure 11. Femtosecond transient absorption spectra at the shown delay times of the C60-(F15P)Zn:PyphTTF triad in toluene. (b) Time profile of the 1295 nm peak of 1(F15P)Zn* before (red) and after coordinating Py-phTTF (blue) to (F15P)Zn-C60. (c) Normalized decay profile of the C60•- monitored at 1015 nm of (F15P)Zn-C60 dyad (red) and C60-(F15P)Zn:Py-phTTF triad (blue).



delay time) at the expected wavelength regions providing evidence of charge separation. Figures 10b and 11b show the 1(F15P)Zn* peak at 1295 nm before (red) and after coordinating TTF (blue) to (F15P)ZnC60. Faster decay in the triad over the dyad were clear in both cases. Figures 10c and 11c show the decay profile of the C60•- monitored at 1015 nm for (F15P)Zn-C60 dyad (red) and C60-(F15P)Zn:TTF triads (blue). The initial signal growth (as determined by the peak intensity) of C60•- was found to be less in the triads as compared to the triad due to competing charge separation process of 1(F15P)Zn* to produce C60-(F15P)Zn•-:TTF•+ charge separated state. The energy level diagram shown in Figure 7 predicted a hole shift in the case of C60•--(F15P)Zn•+:TTF and an electron migration in the case of C60-(F15P)Zn•-:TTF•+ to produce C60•--(F15P)Zn•-:TTF•+ charge separated state as the ultimate product. Under such circumsances, due to distal separation of the positive and negative radical ions, long-lived charge separated state could be visualized. The decay profiles of the C60•- shown in Figures 10c and 11c indeed prove that to be the case. Slower decay of C60•- in the case of triads as compared to that in the dyad was observed providing proof for charge stabilization in the triads. Nanosecond transient absorption spectral studies on the dyad and triad were performed to evalute the final lifetime of the charge separated state, as the decay of C60•- in Figures 10c and 11c lasted over 3 ns, the time window of our instrumental setup. Figure 12 shows the nanosecond transient spectra at the indicated delay times of the dyad and triads while the time profile of the C60•- peak monitored at 1015 nm is shown in the right-hand panels. The transient spectra of 3(F15P)Zn* revealed peaks at 462, and 835 nm in toluene with a decay rate constant of kT = 1.3  104 s-1. In the spectra of the dyad and triads, peaks originated from 3(F15P)Zn* were apparant. This is concievable since the fluorescence quenching of (F15P)Zn in the donor-acceptor dyad was about 40% (Figure 2b) and the earlier discussed femtosecond transient spectra indeed showed competitive population of 3(F15P)Zn* both in the dyads and triads. In addition, the energy of the (F15P)Zn•+-C60•- and C60•--(F15P)Zn•+:TTF charge separated states are above that of 3(F15P)Zn* (1.64 eV) and 3C60* (1.55 eV) levels. Under these conditions, the process of charge recombination could populate either one of these states competitively. There was a broad peak in the 700 nm region corresponding to 3C60* suggesting that this process is also a possibility. Nevertheless, the nanosecond transient spectra had a weak spectral signature peak at 1015 nm suggesting the persistence of C60•-. The decay time profiles nicely demonstrate faster decay in the case of the dyad (Figure 11a right panel) and slower decay in the case of the triads (Figures 11b and c right panels) providing additional evidence of the formation of the C60•--(F15P)Zn:TTF•+ charge separated state from the earlier discussed dual electron and hole migration mechanism (see Figure 7 discussions). From these decay profiles the lifetime of the final charge separated states were estimated to be 0.05 s for 18 ACS Paragon Plus Environment

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(F15P)Zn•+-C60•-, 0.23 s for C60•--(F15P)Zn:Py-TTF•+, and 0.35 s for C60•--(F15P)Zn:Py-phTTF•+ charge separated states. As expected, the second triad with an additional phenyl spacer had a longer lifetime for the charge separated state.

Figure 12. Nanosecond transient spectra at the indicated delay times of (a) -(F15P)Zn-C60 (b) C60(F15P)Zn:Py-TTF and (c) C60-(F15P)Zn:Py-phTTF triad in toluene. The samples were excited at 425 nm corresponding to Soret band. The right hand panel shows the decay of C60•- monitored at 1015 nm, demonstrating long-lived charge separated state in the case of the triads. SUMMARY In conclusion, by coordinating pyridine derivatized TTF derivatives to a high potential zinc porphyrinfullerene dyad, (F15P)Zn-C60, distance dependent charge stabilization has been accomplished. The supramolecular triads, C60-(F15P)Zn:TTF were fully characterized by optical absorption and emission, electrochemical, spectroelectrochemical and computational studies. The measured binding constants were 3-5  103 M-1 revealing moderate stability. The established energy level diagram helped in 19 ACS Paragon Plus Environment


realizing the different photochemical events in the triads. Coordinated TTF was involved in photoinduced electron transfer in the (F15P)Zn:TTF dyad and C60-(F15P)Zn:TTF triads from the 1(F15P)Zn* as revealed by fluorescence and transient absorption spectral measurements. The measured kCS values for these processes were slightly smaller than that determined for the (F15P)Zn-C60 dyad, making these two processes kinetically competitive in the C60-(F15P)Zn:TTF triads. These two photochemical events are in addition to 1(F15P)Zn* undergoing intersystem crossing to populate the 3(F15P)Zn*. The two charge separated states, C60-(F15P)Zn•-:TTF•+ and C60•--(F15P)Zn•+:TTF were further involved in electron migration in the former case and hole tranfer in the latter case to produce C60•--(F15P)Zn:TTF•+ charge separated state as the ultimate product. Due to distal separation of the positive and negative radical ions, longlived charge separated states persistant for about 0.2 microseconds was possible to accomplish in the present study. ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website: DOI:” …. Additional spectral, electrochemical and femtosecond transient absorption spectral data. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] ORCID Francis D’Souza: 0000-0003-3815-8949 Angela K. Wilson: 0000-0001-9500-1628 Prashanth K. Poddutoori: 0000-0001-6007-8801 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the US-National Science Foundation (Grant No. 1401188 to FD). REFERENCES (1) Photochemical Conversion and Storage of Solar Energy; Connolly, R. S. Academic Press: New York, 1981. 20 ACS Paragon Plus Environment

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Charge Stabilization in High-Potential Zinc Porphyrin-Fullerene via

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