Photoinduced Electron and Energy Transfer in a Molecular Triad

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Photoinduced Electron and Energy Transfer in a Molecular Triad Featuring a Fullerene Redox Mediator Antaeres Antoniuk-Pablant, Gerdenis Kodis, Ana L. Moore, Thomas A. Moore, and Devens Gust J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b03470 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 9, 2016

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The Journal of Physical Chemistry

Photoinduced Electron and Energy Transfer in a Molecular Triad Featuring a Fullerene Redox Mediator Antaeres Antoniuk‐Pablant, Gerdenis Kodis, Ana L. Moore,* Thomas A. Moore,* and Devens Gust* School of Molecular Sciences, Arizona State University, Tempe, AZ 85287 ABSTRACT: In order to investigate the possibility of a fullerene acting as an electron and/or singlet energy relay between a donor chromophore and an acceptor, a triad consisting of a fullerene (C60) covalently linked to both a porphyrin energy and electron donor (P) and a ‐tetracyanoporphyrin energy and electron acceptor (CyP) was synthesized. Steady state and time resolved spectroscopic investigations show that the porphyrin first excited singlet state donates singlet excita‐ tion and an electron to the fullerene, and also donates singlet excitation to the CyP. All three processes differ in rate con‐ stant by factors of ≤ 1.3, and all are much faster than decay of 1P‐C60‐CyP by unichromophoric processes. The fullerene excited state accepts an electron from P and donates singlet excitation energy to CyP. The P•+‐C60•‐‐CyP charge separated state transfers an electron to CyP to produce a final P•+‐C60‐CyP•‐ state. The same state is formed from P‐C60‐1CyP. Overall, the final charge separated state is formed with a quantum yield of 85% in benzonitrile, and has a lifetime of 350 ps. Rate constants for formation and quantum yields of all intermediate states were estimated from results for the triad and sever‐ al model compounds. Interestingly, the intermediate P•+‐C60•‐‐CyP charge‐separated state has a lifetime of 660 ps. It is longer lived than the final state in spite of stronger coupling of the radical ions. This is ascribed to the fact that recombi‐ nation lies far into the inverted region of the Marcus rate constant vs. thermodynamic driving force relationship.

1. INTRODUCTION Molecular donor‐acceptor systems capable of photoin‐ duced electron transfer have long been investigated as components of artificial photosynthetic systems because they convert light energy into electrochemical potential energy that can then be used to make fuels, electricity, etc. Since our original report of a porphyrin‐fullerene dyad that undergoes photoinduced electron transfer,1 this com‐ bination of chromophores has been employed numerous times in artificial photosynthetic reaction centers of vari‐ ous designs.2-9 Fullerenes are particularly suitable as ter‐ minal electron acceptors in such systems because they can reversibly accept up to 6 electrons and have reduction potentials that permit photoinduced electron transfer but retain a significant amount of the photon energy in the charge separated state. They also feature small reorganiza‐ tion energies for electron transfer. At modest driving forc‐ es for photoinduced electron transfer that conserve much of the excited state energy as charge separation, low reor‐ ganization energy favors rapid charge separation and slow charge recombination.10-12 Fullerene radical anions are relatively insensitive to solvent stabilization, which allows photoinduced electron transfer to occur at low tempera‐ tures in glasses and in nonpolar media.12

In principle, fullerenes could also act as electron relays, accepting an electron from an excited‐state donor and transferring it to another, non‐fullerene electron acceptor. This behavior would permit the use of fullerenes as inter‐ mediate electron donor‐acceptor relays in complex, multi‐ component molecules for artificial photosynthesis, molecu‐ lar logic devices, molecular electronics and similar studies. Using a fullerene in this way requires both a terminal elec‐ tron acceptor more easily reduced than C60 and a method for chemically linking both a donor and an acceptor to a fullerene in ways that facilitate electron transfer. Because fullerenes are visible light absorbing chromophores, they could also function as excitation energy relays between two other chromophores of suitable energies. Here, we report the preparation and spectroscopic study of molecu‐ lar triad 1 Chart 1 , which consists of a fullerene C60 covalently bonded to both a free base porphyrin excited state electron donor P and a ‐tetracyanoporphyrin elec‐ tron acceptor CyP . The triad features a pyrrolidine linker that couples both porphyrins to the fullerene via relatively short covalent linkages that severely restrict the distances between chromophores. Linking both porphyrins in this way also avoids the multiple isomers that arise when dou‐ bly substituting the fullerene, and leaves the derivatized fullerene with a reduction potential suitable for the pro‐ ject.

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As might be expected, the photochemical behavior of a molecule containing three chromophores that in principle can undergo both energy transfer among the chromo‐ phores and photoinduced electron transfer is complex. In fact, almost all thermodynamically allowed interchromo‐ phore processes are observed. Identifying the various pho‐ tochemical processes and their associated rate constants and quantum yields required steady state and transient spectroscopic investigations of 1, model P‐C60 dyad 2, model CyP‐C60 dyad 3, model porphyrin 4 and model CyP 5.

2. RESULTS AND DISCUSSION Synthesis The preparation of the triad required development of syn‐ thetic strategies for preparation of both amino acid por‐ phyrin 4 and CyP 5. Below, we will briefly outline the syn‐ thesis of each of these molecules, and then show how the final triad was prepared. The synthetic details and charac‐ terization of all new compounds are given in the Support‐ ing Information.

Amino acid porphyrin Scheme 1 shows the synthetic sequence for preparation of 4. Benzilic alcohol 8 was prepared using well‐known reac‐ tions and converted to the aldehyde 9 using a Dess‐Martin oxidation,13,14 which left the amino acid protecting groups intact. Aldehyde 9 was then allowed to react with dipyr‐ romethane 10 and mesityl aldehyde 11 to yield an amino acid porphyrin 12 in which the amine and carboxylate groups are protected as the amide and ester, respectively. Compound 12 was stable, and could be deprotected using acid to yield 4. Amino acid porphyrin 4 was stable under acidic conditions, but decomposed rapidly at neutral or basic pH. After deprotection, it was used immediately to prepare the triad.

-Tetracyanoporphyrin 5 ‐Tetracyanoporphyrins are excellent electron acceptors, with first reduction potentials generally more positive than benzoquinone. However, until recently, synthetic methods for their preparation from the corresponding ‐ tetrabromoporphyrins were of low yield, and required removal of the metal from copper porphyrins under harsh conditions that were unsuitable for making many substi‐ tuted porphyrins. We therefore developed a new, mild method using zinc cyanide and a palladium catalyst which gave the materials in improved yield as the zinc deriva‐ tives.15 The zinc can be readily removed from the porphy‐ rin when necessary. The preparation of CyP aldehyde 5 using this method is shown in Scheme 2. After condensation of 4‐ methoxycarbonylbenzaldehyde, 4‐t‐butylbenzaldehyde and dipyrromethane 13 to give porphyrin 14 the ester was reduced with lithium aluminum hydride to produce alco‐ hol 15. Oxidation of 15 with manganese dioxide yielded aldehyde 16, which was brominated with N‐ bromosuccinimide to yield 17. After introduction of zinc step e cyanation of 18 was carried out as described in

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detail in the Supporting Information to give 19. Removal of the zinc with trifluoroacetic acid resulted in the desired aldehyde 5.

Triad 1 Final assembly of the triad was accomplished by the Prato reaction11,12,16,17 of amino acid porphyrin 4, porphyrin al‐ dehyde 5 and C60. This reaction involves 1,3‐dipolar cy‐ cloaddition of an azomethine ylide generated in situ with C60, and produced 1 in 19% yield. The product could in principle be a mixture of diastereomers due to the pres‐ ence of two stereocenters on the pyrrolidine,18-20 but NMR and thin layer chromatography indicated that only one isomer was present as a pair of enantiomers . This is con‐ sistent with a concerted reaction of the azomethine ylide with C60.20

Electrochemistry The redox potentials of the triad and various model com‐ pounds were determined by cyclic voltammetry in order to estimate the energies of charge separated states formed following photoinduced electron transfer. Measurements were performed under an argon atmosphere in distilled dichloromethane containing 0.1 M tetra‐n‐ butylammonium hexafluorophosphate as the supporting electrolyte. The working and counter electrodes were Pt, and an Ag /Ag quasi‐reference electrode was employed. Potentials were determined using a ferrocene internal ref‐ erence: the results were converted to SCE by referencing peaks to the first oxidation wave of ferrocene, which is at 0.46 V vs SCE in this solvent.21 Typical voltammograms for triad 1 are shown in Figure 1, and relevant oxidation and reduction potentials for 1 and model compounds are given in Table 1. The first oxidation of 1 is partially reversible, and attributed to oxidation of the porphyrin moiety. The reduction voltammograms, which are reversible, are superpositions of the waves of the various moieties. The first reduction, at ‐0.32 V vs SCE, is due to the CyP, and is similar to the reduction potential of model CyP 7,8,17,18‐tetracyano‐5‐ 4‐carboxyphenyl ‐ 10,15,20‐tris‐ 4‐t‐butylphenyl porphyrin 20 . The second reduction, at ‐0.60 V vs SCE, is ascribed to the fullerene moiety, and is similar to that of a fulleropyrrolidine model compound.22 The redox potentials permit estimation of the energies of the relevant charge separated states of the triad as 1.52 eV for P• ‐C60•‐‐CyP and 1.24 eV for P• ‐C60‐CyP•‐. Table 1. Relevant redox potentials (E1/2, V vs. SCE) cmpd

red2

red1

ox1

4

‐1.68

‐1.33

1.00

20a

‐0.61

‐0.31

1.32

model C60

‐1.04b

‐0.57a

‐‐‐

3

‐0.65

‐0.38

1.34

1

‐0.60

‐0.32

0.92


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b7,8,17,18‐Tetracyano‐5‐

4‐carboxyphenyl ‐10,15,20‐tris‐ 4‐t‐butylphenyl ‐porphyrin.15 bTaken from the literature.22

Spectroscopy Absorption spectra Figure 2 shows the absorption spectra in benzonitrile of triad 1, P‐C60 dyad 2 and cyanoporphyrin 5. The porphyrin moiety of dyad 2 features a typical Soret band at 420 nm and Q‐band maxima at 515, 549, 593 and 648 nm. Fuller‐ ene absorption is weak, and is not visible in Figure 2 ex‐ cept for some broad absorbance between 300 and 400 nm. There is a very weak long‐wavelength maximum at ca. 705 nm. The spectrum of CyP 5 shows two Soret peaks at 445 and 458 nm and Q‐bands at 520, 557, 601, 676 and 740 nm. In terms of the Gouterman four orbital theory,23 the Soret B band is due to electronic transitions from the b2 orbital HOMO to the c1 and c2 LUMO orbitals. These transitions involve two transition dipole moments in directions along a set of axes which pass through the two sets of opposing pyrrole rings of the porphyrin macrocycle. In porphyrins with only hydrogen at the ‐positions, these two dipoles are nearly degenerate, and the transitions are often dis‐ cussed in terms of effective transition dipoles passing through the meso‐carbon atoms.24 In the cyanoporphyrins, however, because of the electronic effects of the cyano groups these two transitions are no longer degenerate, and two absorption bands are observed. Mixing of the Q and B states may also contribute to the difference in energy of the two transitions. The electron withdrawing effects of the cyano groups are also responsible for the changes in the Q‐band region relative to most porphyrins, including the nearly 100 nm shift of the longest‐wavelength absorp‐ tion. The spectrum of the triad is nearly a linear combination of the spectra of the model compounds, with Soret maxima at 422, 444 and 459 nm and Q‐band absorbance at 516, 556, 601, 648, 675 and 740 nm. In addition, there is a small shoulder at around 430 nm that reflects weak excitonic interaction between the two porphyrins. The spectra do not suggest any strong ground‐state electronic interactions among the chromophores.

Fluorescence spectra Figure 3 shows the fluorescence emission spectra of triad 1, dyad 2 and CyP 5 in benzonitrile. The triad and dyad were excited into the Soret absorption at 424 and 419 nm, respectively, whereas the CyP excitation was at 457 nm, into the Soret of that compound. The emission spectra were normalized at their maxima. The dyad has emission maxima at 649, ~709 sh , 719 and 785 nm, which are typical of porphyrins of this type, and the CyP features a single broad maximum at 795 nm. The emission Stokes shift for the dyad 2 is 1 nm; many porphyrins have little or no Stokes shift. However, CyP 5 exhibits a Stokes shift of 55 nm. This large value is consistent with some charge trans‐ fer character in the excited state. Turning to the triad,

emissions from the porphyrin are observed at 652 and ca. 722 nm, and emission from the CyP is found at 788 nm. Thus, in the triad, P emission has essentially no Stokes shift while the Stokes shift for CyP is 48 nm. The difference from the Stokes shift for the triad is likely due to differences in solvation of the excited state. Even though the triad emission spectrum was obtained by excitation at 424 nm where almost all of the light is ab‐ sorbed by the porphyrin moiety P, emission from that chromophore is weak whereas that from the CyP is strong. This suggests that some amount of singlet‐singlet energy transfer from the porphyrin to the cyanoporphyrin has occurred. The emission spectra of CyP model 5 and CyP‐C60 dyad 3 were measured in toluene solution with excitation at 320 nm. The solutions had equal absorbance at 320 nm, and at this wavelength, the fullerene absorbs 33 % of the light and the CyP absorbs 67%. The emission spectra Figure S1, Supporting Information show significant emission only from CyP, indicating singlet‐singlet energy transfer from the fullerene to the CyP. Photoinduced electron transfer does not occur in this solvent; see next section. Integra‐ tion of the emission spectra indicates that the energy transfer efficiency is ~79 %. The fluorescence quantum yields of 5 and 1CyP in dyad 3 are identical. The fluorescence data coupled with the absorption data allow estimation of the energies of the first excited singlet states of the various chromophores. This was done by tak‐ ing the wavenumber average of the longest‐wavelength absorption band maximum and the shortest‐wavelength fluorescence band maximum. The energies of 1P, 1C60, and 1CyP in benzonitrile were determined to be 1.91, 1.7622 and 1.64 eV above the ground state. These energies and the energies of the charge‐separated states estimated above allow us to construct Figure 4, which shows the transient states and their relevant interconversion path‐ ways.

Time resolved spectroscopy experiments Time resolved absorption measurements were performed in order to learn more about the interconversion pathways among the various high energy states of the triad. Fluorescence decay measurements were made using the time‐correlated single photon counting method. These experiments were done in two solvents; benzonitrile and toluene. Due to the time resolution of the instrument used to obtain these data, time constants of 30 ps have great‐ er uncertainty than longer time constants. Model porphy‐ rin 12 was studied in toluene solution with excitation at 515 nm and emission monitoring at 650 nm. A single ex‐ ponential decay with a time constant of 9.5 ns was ob‐ served, corresponding to decay of 1P by the usual photo‐ physical pathways of internal conversion, intersystem crossing and fluorescence. In benzonitrile, the correspond‐ ing lifetime was 10.1 ns.



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The time‐resolved fluorescence experiments for the CyP model 5 were obtained with excitation at 515 nm in both toluene and benzonitrile. In toluene, exponential compo‐ nents of 21 ps and 2.1 ns were found. The 21 ps compo‐ nent whose measured lifetime is inaccurate because it is shorter than the time resolution of the instrument is as‐ cribed to relaxation and solvation of the excited state, whereas the 2.1 ns lifetime represents the decay of the relaxed first excited singlet state by fluorescence, intersys‐ tem crossing and internal conversion. In benzonitrile, the corresponding lifetimes are 45 ps and 1.9 ns. The relaxa‐ tion/solvation process is slower in benzonitrile due to an increase of solvent polarity and viscosity going from tolu‐ ene to benzonitrile. A toluene solution P‐C60 dyad 2 was excited at 515 nm one of the porphyrin Q‐band maxima and emission was de‐ tected at 650 nm and 720 nm. Lifetimes of 76 ps and 1.3 ns were observed. The first time constant is associated with decay of the porphyrin first excited singlet state. It is much shorter than the excited state lifetime of porphyrin 12 due to a new decay pathway, which is attributed to singlet‐ singlet energy transfer to the fullerene step 1 in Figure 4 . Photoinduced electron transfer does not occur in this sol‐ vent vide infra . The 1.3 ns decay is due to decay of the fullerene first excited singlet state by the usual photophys‐ ical pathways step 6 . In benzonitrile, a single lifetime of 28 ps is observed, using the same excitation and emission wavelengths. Thus, new pathways for decay of both 1P‐C60 and P‐1C60 are present in this polar solvent. They are as‐ cribed to photoinduced electron transfer to yield P• ‐C60•‐ steps 2 and 5 in Figure 4 . Accurate rate constants for these processes cannot be determined from these experi‐ ments because 28 ps is within the time resolution of the spectrometer. When a benzonitrile solution of a previously‐reported P‐ C60 dyad related to 2 but lacking the tolyl group was excit‐ ed at 705 nm, where only the fullerene absorbs, fullerene emission with a lifetime of 200 ps was observed at 740 nm. As the lifetime of the fullerene excited singlet state in the absence of interchromophoric decay pathways is 1.3 ns, 1C60 is substantially quenched in the dyad in benzonitrile. This quenching is ascribed to photoinduced electron trans‐ fer to form P• ‐C60•‐ step 5 in Figure 4 . Experiments on the CyP‐C60 dyad 3 were performed with excitation at 515 nm and emission at 5 wavelengths in the 740‐820 nm region. The resulting decay associated spectra DAS are shown in Figure 5a. In toluene, lifetimes of 50 ps and 2.0 ns were found. The 50 ps DAS clearly shows the shift of CyP fluorescence to longer wavelengths with time positive amplitude at the short wavelength side of the fluorescence band and negative amplitude at the long‐ wavelength side , and represents excited state relaxation and solvation. The 2.0 ns DAS has the shape of CyP steady‐ state fluorescence see Figure 3 and is associated with the decay of the relaxed excited singlet state analogous to step 8 in Figure 4 . Excitation of a toluene solution at 500 nm, where a significant fraction of the light is absorbed by the fullerene, yielded an emission decay with lifetimes of 70 ps and 2.0 ns. This decay showed no contribution that

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could be clearly attributed to fullerene emission, but both the fullerene and CyP have emission in essentially the same spectral region, and fullerene emission is very weak compared to that of the porphyrin. Thus, it is not possible to say whether the 70 ps component includes any contri‐ bution from energy transfer from the fullerene to the CyP step 4 in Figure 4 . However, we know that such transfer occurs, based on the steady state fluorescence experiments discussed above. Figure 5b shows the decay associated spectra from a simi‐ lar experiment on triad 1 in toluene. The 28 ps DAS shows decay positive amplitude of 1P‐C60‐CyP around 660 nm and a corresponding rise negative amplitude of CyP emission in the 760‐780 nm region, and represents singlet‐ singlet energy transfer to CyP and C60 by steps 1 and 3 in Figure 4. The 140 ps DAS, which shows a fluorescence band shift similar to that observed in 3, is due to relaxa‐ tion/solvation of P‐C60‐1CyP, and the 2.2 ns spectrum with the shape of CyP emission shows the decay of P‐C60‐1CyP step 8 . The time resolved emission experiments yield valuable data for the decays of the excited states of the chromo‐ phores, but do not inform us concerning the fate of any charge separated states. In addition, the time scales of some of the processes of interest are shorter than the time resolution of our fluorescence instrumentation. Thus, we turn to transient absorption experiments with ca. 100 fs excitation pulses for additional information. In general, a transient absorption spectrum intensity A is a sum of a positive signal associated with excited state absorption induced absorption and two kinds of negative signals, associated with ground state absorption ground state bleaching and stimulated emission. Therefore, in the DAS a negative amplitude shows decay of the ground state bleaching/stimulated emission and concurrent formation of induced absorption while a positive amplitude corre‐ sponds to the decay of induced absorption and concurrent formation of ground state bleaching/stimulated emission. Transient absorption decays were obtained for CyP model 5 in deoxygenated toluene solution with excitation at 725 nm. The global analysis of the data gave 4 DAS with time constants of 1.5 ps, 23 ps, 2.1 ns, and component that did not decay within the time window of the instrument ~5 ns see Figure S2, Supporting Information . The 1.5 ps and 23 ps components are ascribed to relaxation and solv‐ ation of the CyP singlet excited state. The 2.1 ns DAS repre‐ sents the decay of the relaxed singlet excited state, and its lifetime is similar to the value found by fluorescence decay measurements. The final DAS shows Q‐band bleaching and induced absorption around 550 nm, but no stimulated emission, and is assigned to the CyP triplet state. In benzo‐ nitrile solution, four DAS are also observed see Figure S2, Supporting Information . The shorter components 2.9 ps and 50 ps are due to solvation and relaxation of the excit‐ ed singlet state, and a component of 1.9 ns represents de‐ cay of the relaxed 1CyP. The nondecaying on this time scale component is due to 3CyP. These results for CyP are


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in agreement with data we have reported earlier for an‐ other CyP derivative.25 Transient absorption experiments were also performed on P‐C60 dyad 2 with excitation at 590 nm, where most of the light is absorbed by the porphyrin. The resulting DAS from toluene solution are shown in Figure 6a. Components of 75 ps, 1.9 ns, and a lifetime too long to measure with the ap‐ paratus were found. The 75 ps DAS shows decay of por‐ phyrin Q‐band bleaching and stimulated emission at 650 and 720 nm, as well as a broad negative amplitude in the 680 – 780 nm region that represents formation of induced absorption in that spectral region. This component is asso‐ ciated with singlet‐singlet energy transfer from the por‐ phyrin to the fullerene. The 1.9 ns DAS is due to the fuller‐ ene first excited singlet state, which decays with a typical lifetime for this state to form 3C60. The final DAS, with strong fullerene triplet absorption in the 700 nm region, has a lifetime that is too long to measure with the appa‐ ratus. No evidence for formation of a P• ‐C60•‐ charge sepa‐ rated state was observed in toluene. In benzonitrile, the DAS shown in Figure 6b were ob‐ served. The associated lifetimes of the three DAS are 33 ps, 660 ps, and too long to measure. The 33 ps DAS shows de‐ cay of porphyrin stimulated emission at 650 and 720 nm and concurrent rise of broad induced absorption around 680 nm, which is characteristic of the porphyrin radical cation. Thus, this DAS reflects decay of 1P‐C60 by both elec‐ tron and singlet energy transfer to the fullerene analo‐ gous to steps 1 and 2 in Figure 4 . The 660 ps DAS is char‐ acteristic of the porphyrin radical cation, and gives the lifetime of P• ‐C60•‐. The fullerene radical anion has absorp‐ tion in the ca. 1000 nm region. This region was not exam‐ ined for 2, but the fullerene radical anion absorption ac‐ companying the porphyrin radical cation absorption has been reported in many closely related dyads.26-29 The long‐ lived DAS is very weak, and represents a small amount of fullerene and porphyrin triplet states formed via intersys‐ tem crossing. The transient absorption spectra for CyP‐C60 dyad 3 were also measured in toluene and benzonitrile with excitation at 595 nm Figure 7 . In toluene, four DAS with time con‐ stants of 10 ps, 68 ps, 2.2 ns, and 5 ns were observed Figure 7a . The 10 ps and 68 ns DAS are ascribed to relax‐ ation and solvation of the initially‐formed CyP excited state. The 2.2 ns DAS represents decay of the relaxed CyP excited singlet state, and the longest component is associ‐ ated with the CyP triplet state. The 2.2 ns lifetime of 1CyP‐ C60 is virtually identical to that of the first excited singlet state of CyP model 5, indicating that there is no significant electron transfer from the fullerene to CyP. Figure 7b shows the results of a similar experiment in de‐ aerated benzonitrile solution. Time constants of 8.9 ps, 130 ps, 1.9 ns and a time to long to measure were observed. The 8.9 ps and 130 ps DAS are due to relaxation and solva‐ tion of the initially formed CyP excited state. The 1.9 ns time constant represents decay of the relaxed 1CyP‐C60, and is identical to the lifetime of 1CyP in model porphyrin 5 in this solvent. Thus, there is no photoinduced electron

transfer from the fullerene to the cyanoporphyrin. No transient absorption from the fullerene was observed in dyad 3. This is likely due to the fact that the extinction co‐ efficient of the fullerene is very small at 595 nm, and the transient extinction coefficient is also small compared to that of the CyP. Turning now to the triad 1, Figure 8a shows the DAS in toluene with excitation at 510 nm, where nearly all of the absorption is due to the porphyrin moiety. Four DAS with time constants of 29 ps, 143 ps, 2.2 ns, and too long to measure were found. The 29 ps DAS, with Q‐band bleach‐ ing and stimulated emission at 660 and 720 nm, shows decay of 1P‐C60‐CyP. This DAS is consistent with singlet‐ singlet energy transfer to the fullerene to give P‐1C60‐CyP step 1 in Figure 4 and singlet‐singlet energy transfer to the CyP to give P‐C60‐1CyP step 3 in Figure 4 . The time constant is smaller than the 75 ps time constant observed for decay of 1P‐C60 in 2 because of the influence of step 3. The 143 ps DAS is associated with relaxation and solvation of P‐C60‐1CyP as was seen in the other molecules containing CyP. The relaxed P‐C60‐1CyP decays in 2.2 ns, which is simi‐ lar to the 2.1 ns lifetime of the relaxed 1CyP in model 5 in toluene. The decay of P‐C60‐1CyP yields P‐C60‐3CyP, whose lifetime is too long to measure with the apparatus. No transient for decay of P‐1C60‐CyP is observed, suggesting a low transient concentration of this species. No evidence for photoinduced electron transfer was observed in toluene. Figures 8b and 8c show DAS from excitation of triad 1 in benzonitrile at 510 nm in two spectral regions. The DAS of 22 ps represents decay of 1P‐C60‐CyP by steps 0 – 3 in Fig‐ ure 4 and formation of the P• ‐C60‐CyP•‐ charge‐separated state. The strong transient absorption at ca. 945 nm is characteristic of the cyanoporphyrin radical anion CyP•‐ , and the absorption around 660 nm is due mainly to the porphyrin radical cation. The 350 ps DAS is ascribed to decay of P• ‐C60‐CyP•‐ by charge recombination step 11 in Figure 4. The long‐lived transient is assigned to P‐C60‐3CyP. Although the experiments on 1 and dyad 2 discussed above indicate that excitation of the porphyrin will lead to for‐ mation of significant amounts of P• ‐C60•‐‐CyP, the for‐ mation and decay of this species are not observed. This suggests that conversion of P• ‐C60•‐‐CyP to P• ‐C60‐CyP•‐ by charge shift reaction step 9 is comparable to or faster than formation of P• ‐C60•‐‐CyP by steps 2 and 5 in Figure 4. Figure 9 shows the DAS from excitation of a benzonitrile solution of triad 1 at 725 nm, where all absorption is due to the cyanoporphyrin. Four time constants of 14 ps, 350 ps, 500 ps and a time too long to measure were found. The 14 ps DAS is due to relaxation and solvation of the CyP first excited singlet state. The 500 ps DAS shows Q‐band bleach‐ ing of P‐C60‐1CyP and induced absorption around 660 nm that is characteristic of the porphyrin radical cation. There is no stimulated emission. On the other hand, the 350 ps DAS shows negative amplitude in the 660 nm region for‐ mation of the porphyrin radical cation and at around 780 nm decay of stimulated emission of CyP . The experi‐ ments with excitation at 510 nm Figure 8 have shown that the P• ‐C60‐CyP•‐ charge‐separated state has a lifetime



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of 350 ps. Thus, the results with 725 nm excitation repre‐ sent a case of “inverted kinetics” where the 500 ps DAS represents formation of P• ‐C60‐CyP•‐ by photoinduced electron transfer from P to the cyanoporphyrin first excit‐ ed singlet state step 7 in Figure 4 , and the 350 ps DAS is associated with the decay of P• ‐C60‐CyP•‐. Finally, the long‐ lived DAS is due to the cyanoporphyrin triplet state.

Kinetic analysis From the spectroscopic data discussed above, we can ex‐ tract estimates for the rates of the various decay processes shown in Figure 4 for triad 1 in benzonitrile solution. These are summarized in Table 2. The rate constants for the decays of the two porphyrin excited singlet states in the absence of interchromophore processes, k0 and k8, may be estimated from the excited state lifetimes of model por‐ phyrins 12 and 5. The corresponding lifetime of the fuller‐ ene excited singlet state, 1.3 ns for 2 in benzonitrile, yields a value for k6. Table 2. Rate constants for triad 1 in benzonitrile (s‐1) ko

9.9 × 107

k6

7.7 × 108

k1

1.3 × 1010

k7

1.5 × 109

k2

1.7 × 1010

k8

5.3 × 108

k3

1.5 × 1010

k9

>> k2

k4

2.9 × 109 9

k10

1.5 × 109

k11

2.9 × 109

k5

4.2 × 10

The transient results for dyad 2 in toluene yield a lifetime of 75 ps for 1P‐C60. The rate constant for singlet energy transfer from the porphyrin to the fullerene, k1, may be calculated from eq. 1, where k0 is the reciprocal of the 9.5 ns lifetime of porphyrin 12 in toluene and τa 75 ps equals 1.3 1010 s‐1. Because energy transfer rate constants are relatively independent of solvent, we assume that k1 for triad 1 in benzonitrile has a similar value.

1

A value for the rate constant for photoinduced electron transfer from the porphyrin first excited singlet state to the fullerene step 2 may be derived from the transient absorption results for model dyad 2 in benzonitrile with excitation of the porphyrin moiety. Rate constant k2 is giv‐ en by eq. 2, where τb is 33 ps, and it equals 1.7 1010 s‐1. The DAS for dyad 2 in benzonitrile also allow an estimate for the rate constant for charge recombination of the P• ‐ C60•‐‐CyP charge separated state k10 as the reciprocal of 660 ps, or 1.5 109 s‐1.

2

Because we now have estimates for k0, k1 and k2, we may estimate k3 as 1.5 1010 s‐1 using eq. 3, where τc equals 22 ps from the transient absorption results for the triad in benzonitrile. The rate constant for decay of the P• ‐C60‐ CyP•‐ charge separated state, k11, is 2.9 109 s‐1.

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3

The value of k8 estimated above and the 500 ps lifetime for the CyP first excited singlet state measured for triad 1 in benzonitrile allow estimation of k7 as 1.5 109 s‐1. Photoinduced electron transfer from P‐1C60 to yield P• ‐ C60•‐ was found in dyad 2 in benzonitrile. From the 200 ps lifetime of P‐1C60 and the value of k6 in Table 2, a value for the photoinduced electron transfer rate constant analo‐ gous to k5 of 4.2 109 was calculated using an equation similar to eq. 1. It is assumed that k5 in the triad is similar. With regard to step 4, singlet‐singlet energy transfer from P‐1C60‐CyP to yield P‐C60‐1CyP, it was mentioned above that we were not able to obtain a rate constant for this process from the time‐resolved data. However, the steady‐state emission data did reveal a ~79% energy transfer efficien‐ cy. Given that the lifetime of 1C60 in the absence of inter‐ chromophore processes is 1.3 ns, we can calculate an esti‐ mated rate constant for energy transfer analogous to step 4 in Figure 4 of ~2.9 109 s‐1. Having obtained estimates of the rate constants for all the steps shown in Figure 4, we may now calculate quantum yields for the various decay processes. In general, the quantum yield Φn of a given intermediate n is given by eq. 4, where kn is the rate constant for formation of intermedi‐ ate n and τ is the reciprocal of the sum of all of the rate constants that depopulate the excited state giving rise to the intermediate the lifetime of the excited state .

Φ

τ

4

Applying this equation to the decay products of the initial 1P‐C60‐CyP in benzonitrile, we find that the quantum yield of P‐1C60‐CyP by step 1, Φ1 0.29, the quantum yield of P• ‐ C60•‐‐CyP directly from 1P‐C60‐CyP, Φ2 0.37 and the quan‐ tum yield of P‐C60‐1CyP by step 3, Φ3 0.33. The total yield of P• ‐C60•‐‐CyP by all pathways, ΦCSi, is given by eq. 5.

Φ

Φ

Φ

5

The value of Φ5 equals Φ1 times the yield of step 5, which can be calculated from the rate constants in Table 2, and equals 0.15. Thus, ΦCSi 0.52. The quantum yield of P‐C60‐1CyP by all pathways, Φ1CyP, is given by eq. 6, and equals 0.44.

Φ

Φ

Φ

6

Turning now to the final P• ‐C60‐CyP•‐ charge‐separated state, the quantum yield for its formation by step 9, equals k2 and k10. The yield of step 7 from P‐ ΦCSi 0.52 , as k9 C60‐1CyP, calculated using an equation similar to eq. 1 with k7 and k8 as given in Table 2, is 0.76. Thus, the total quan‐ tum yield of P• ‐C60‐CyP•‐ via step 7 based on light absorbed by the porphyrin P equals 0.33. The overall yield of the


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final P• ‐C60‐CyP•‐ state by all pathways, ΦCSf, equals 0.33 0.52, or 0.85.

3. Conclusions The spectroscopic data show that after excitation of the porphyrin in benzonitrile, the molecule evolves via a series of electron and energy transfer steps into a final charge separated state P• ‐C60‐CyP•‐ that is formed with a quantum yield of 85%. The fullerene moiety interposed between the porphyrin donor and cyanoporphyrin acceptor acts as a relay for both electron transfer and singlet energy transfer from the porphyrin to the cyanoporphyrin. A total of 61 % of the charge separation arises from electron transfer via the fullerene radical anion relay, and 39 % comes from direct electron transfer from the porphyrin to the CyP ex‐ cited singlet state. This P‐C60‐1CyP state is formed by ener‐ gy transfer either directly from 1P‐C60‐CyP or from 1P‐C60‐ CyP via an excitation relay from P‐1C60‐CyP.

porphyrin moieties is faster than energy transfer from 1P to C60, even though the distance between the porphyrins is substantially larger than that between P and C60. This effect is ascribed to the much better spectral overlap between the two porphyrins, as the C60 absorption is extremely weak in the region of P emission whereas the absorption of CyP is much stronger. Likewise, the weak emission of 1C60 relative to that of 1P leads to much slower energy transfer from 1C60 to CyP than from 1P to CyP. It is clear from this work that fullerenes can function as both energy and electron transfer relays between donors and acceptors, assuming that the acceptor species has a lower lying excited singlet state than that of the fullerene for energy transfer or is more easily reduced than the fullerene for electron transfer . The ‐ tetracyanoporphyirn used in this study meets both re‐ quirements.

4. Experimental The rate constants for the three interchromophore pro‐ cesses that depopulate the porphyrin first excited singlet state are nearly identical, differing by a factor of less than 1.3. This congruence is partially responsible for the rich photochemistry of the molecule, and is due to a fortuitous balance of the factors that control electron transfer such as energetics, reorganization energies and electronic cou‐ pling and singlet energy transfer such as spectral over‐ lap, distances and angles between transition moments . Interestingly, the time constant for charge recombination of the intermediate P• ‐C60•‐‐CyP charge‐separated state 660 ps, step 10 in Figure 4 is about twice that for recom‐ bination of the final P• ‐C60‐CyP•‐ state 350 ps, step 11 . Molecular modeling MM2 indicates that the center‐to‐ center distance between the porphyrins in 1 is 19 Å, whereas that between the porphyrin P and the fullerene is 13 Å. Both distances are too large to permit very rapid through‐space electron transfer, and the transfer is ex‐ pected to occur through the linkage bonds, as is the case with most covalently linked donor‐acceptor systems. The electronic coupling for electron transfer step 10 is almost certainly larger than that for step 11. The fullerene is joined to the phenyl ring of P via two saturated bonds, whereas there are four such bonds between the porphy‐ rins. The most likely explanation for the rate difference lies in the driving forces for electron transfer. Both charge re‐ combination steps probably lie in the inverted region of the Marcus electron transfer rate vs. free energy change relationship.30-33 The driving force for step 10 is signifi‐ cantly larger than that for step 11, and thus step 10 will lie farther into the inverted region, and this will lead to slow‐ er reaction. Evidently, this effect outweighs electronic cou‐ pling differences. Any differences in reorganization ener‐ gies between the two charge‐separated states will also play a role in determining the rate constants, as will differ‐ ences in solvent stabilization of the radical anions. Turning now to singlet‐singlet energy transfer, this process undoubtedly occurs by the Förster mechanism, which does not depend on electronic coupling and orbital overlap. It is clear from Table 1 that energy transfer between the two

Spectroscopic measurements Steady‐state fluorescence spectra were measured using a Photon Technology International MP‐1 spectrometer and corrected for detection system response. Excitation was provided by a 75 W xenon‐arc lamp and single‐grating monochromator. Fluorescence was detected at 90° to the excitation beam via a single‐grating monochromator and an R928 photomultiplier tube having S‐20 spectral re‐ sponse and operating in the single photon counting mode. Fluorescence decay measurements were performed on optically dilute ca. 1  10‐5 M samples in solution under an air atmosphere by the time‐correlated single‐photon‐ counting method. Two different excitation systems were employed. The excitation source for the first system was a mode‐locked Ti:Sapphire laser Spectra Physics, Millennia‐ pumped Tsunami with a 130‐fs pulse duration operating at 80 MHz. The laser output was sent through a frequency doubler and pulse selector Spectra Physics Model 3980 to obtain 370‐450 nm pulses at 4 MHz. The excitation source for the second system was a fiber supercontinuum laser based on a passive mode locked fiber laser and a high‐nonlinearity photonic crystal fiber supercontinuum generator Fianium SC450 . The laser provides 6‐ps pulses at a repetition rate variable between 0.1 – 40 MHz. The laser output was sent through an Acousto‐Optical Tunable Filter Fianium AOTF and the relevant 10‐nm interfer‐ ence filter to obtain excitation pulses at a desired wave‐ length. Fluorescence emission was detected at the magic angle using a double grating monochromator Jobin Yvon Gemini‐180 and a microchannel plate photomultiplier tube Hamamatsu R3809U‐50 . The instrument response function was 35‐55 ps. The spectrometer was controlled by software based on the LabView programming language and data acquisition was done using a single photon count‐ ing card Becker‐Hickl, SPC‐830 . Transient absorption experiments were carried out on samples in solution under an atmosphere of air. The femtosecond transient absorption apparatus consisted of a



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kilohertz pulsed laser source and a pump‐probe optical setup. Laser pulses of 100 fs at 800 nm were generated from an amplified, mode locked titanium sapphire laser system Millennia/Tsunami/Spitfire, Spectra Physics . Part of the laser pulse was sent through an optical delay line and focused on to a 3 mm sapphire plate to generate a white light continuum for the probe beam. The remainder of the pulse energy was used to pump an optical paramet‐ ric amplifier Spectra Physics to generate excitation puls‐ es, which were selected using a mechanical chopper. The white light generated was then compressed by prism pairs CVI before passing through the sample. The polarization of pump beam was set to the magic angle 54.7° relative to the probe beam and its intensity was adjusted using a continuously variable neutral density filter. The white light probe was dispersed by a spectrograph 300 line grating onto a charge‐coupled device CCD camera DU420, An‐ dor Tech. . The final spectral resolution was about 2.3 nm for over a nearly 300 nm spectral region. The instrument response function IRF was ca. 150 fs. Data analysis was carried out using locally written soft‐ ware ASUFIT developed under a MATLAB environment Mathworks Inc. . Decay‐associated spectra DAS were obtained by fitting the transient absorption or fluores‐ cence decay curves over a selected wavelength region sim‐ ultaneously as described by eq. 7 parallel kinetic model ,

∆

,

∑

 exp

7

where A ,t is the observed absorption or fluores‐ cence change at a given wavelength at time delay t and n is the number of kinetic components used in the fitting. A plot of Ai  versus wavelength is called a decay‐associated spectrum, and represents the amplitude spectrum of the ith kinetic component, which has a lifetime of i. The global analysis procedures described here have been extensively reviewed in literature.34 Random errors associated with the reported lifetimes obtained from fluorescence and transient absorption measurements were typically  5%.

ASSOCIATED CONTENT Supporting Information. Synthetic details for preparation of all new compounds, steady‐state emission spectra of 3 and 5 and transient spectra of , CyP 5, dyad 2, dyad 3, and triad 1. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E‐mail: [email protected]; TEL: 1‐480‐965‐4547

Author Contributions The manuscript was written through contributions of all authors.

ACKNOWLEDGMENTS This work was supported by the Office of Basic Energy Sci‐ ences, Division of Chemical Sciences, Geosciences, and Ener‐ gy Biosciences, Department of Energy under contract DE‐

Page 8 of 22

FG02‐03ER15393 and the Center for Bio‐Inspired Solar Fuel Production, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE‐SC0001016. The authors acknowledge the mass spectrometry core of the research technology support facility at Michigan State Uni‐ versity for the analysis of several compounds synthesized in this work.

REFERENCES (1) Liddell, P. A.; Sumida, J. P.; Macpherson, A. N.; Noss, L.; Seely, G. R.; Clark, K. N.; Moore, A. L.; Moore, T. A.; Gust, D. Preparation and Photophysical Studies of Por‐ phyrin‐C60 Dyads. Photochem. Photobiol. 1994, 60, 537‐ 541. (2) Gust, D.; Moore, T. A. In The Porphyrin Handbook, Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000; pp 153‐190. (3) Imahori, H. Porphyrin‐Fullerene Linked Systems As Artificial Photosynthetic Mimics. Org. Biomol. Chem. 2004, 2, 1425‐1433. (4) Guldi, D. M. Fullerene‐Porphyrin Architectures; Photosynthetic Antenna and Reaction Center Models. Chem. Soc. Rev. 2002, 31, 22‐36. (5) Boyd, P. D. W.; Reed, C. A. Fullerene‐Porphyrin Constructs. Acc. Chem. Res. 2005, 38, 235‐242. (6) Schuster, D. I. Synthesis and Photophysics of New Types of Fullerene‐Porphyrin Dyads. Carbon 2000, 38, 1607‐1614. (7) Wrobel, D.; Graja, A. Photoinduced Electron Transfer Processes in Fullerene‐Organic Chromophore Systems. Coord. Chem. Rev. 2011, 255, 2555‐2577. (8) El‐Khouly, M. E.; Ito, O.; Smith, P. M.; D'Souza, F. Intermolecular and Supramolecular Photoinduced Electron Transfer Processes of Fullerene‐ Porphyrin/Phthalocyanine Systems. J. Photochem. Photobiol., C 2004, 5, 79‐104. (9) Lemmetyinen, H.; Tkachenko, N. V.; Efimov, A.; Niemi, M. Transient States in Photoinduced Electron Transfer Reactions of Porphyrin‐ and Phthalocyanine‐ Fullerene Dyads. J. Porphyrins Phthalocyanines 2009, 13, 1090‐1097. (10) Guldi, D. M.; Asmus, K.‐D. Electron Transfer From C76 (C2v') and C78 (D2) to Radical Cations of Various Arenes: Evidence for the Marcus Inverted Region. J. Am. Chem. Soc. 1997, 119, 5744‐5745. (11) Imahori, H.; Hagiwara, K.; Akiyama, T.; Aoki, M.; Taniguchi, S.; Okada, T.; Shirakawa, M.; Sakata, Y. The Small Reorganization Energy of C60 in Electron Transfer. Chem. Phys. Lett. 1996, 263, 545‐550. (12) Liddell, P. A.; Kuciauskas, D.; Sumida, J. P.; Nash, B.; Nguyen, D.; Moore, A. L.; Moore, T. A.; Gust, D. Photoinduced Charge Separation and Charge Recombination to a Triplet State in a Carotene‐Porphyrin‐ Fullerene Triad. J. Am. Chem. Soc. 1997, 119, 1400‐1405. (13) Fernandez, M. I.; Gabriel, T. Oxidation of Alcohols to Aldehydes and Ketones; Springer: New York, 2006.


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(14) Dess, D. B.; Martin, J. C. Readily Accessible 12‐I‐5 Oxidant for the Conversion of Primary and Secondary Alcohols to Aldehydes and Ketones. J. Org. Chem. 1983, 48, 4155‐4156. (15) Antoniuk‐Pablant, A.; Terazono, Y.; Brennan, B. J.; Sherman, B. D.; Megiatto, J. D.; Brudvig, G. W.; Moore, A. L.; Moore, T. A.; Gust, D. A New Method for the Synthesis of B‐Cyano Substituted Porphyrins and Their Use As Sensitizers in Photoelectrochemical Devices. J. Mater. Chem. A. 2016, 2976‐2985. (16) Maggini, M.; Scorrano, G.; Prato, M. Addition of Azomethine Ylides to C60: Synthesis, Characterization, and Functionalization of Fullerene Pyrrolidines. J. Am. Chem. Soc. 1993, 115, 9798‐9799. (17) Prato, M.; Maggini, M.; Giacometti, C.; Scorrano, G.; Sandona, G.; Farnia, G. Synthesis and Electrochemical Properties of Substituted Fulleropyrrolidines. Tetrahedron 1996, 52, 5221‐5234. (18) Leupold, S.; Shokati, T.; Eberle, C.; Borrmann, T.; Montforts, F. P. Synthesis of Porphyrin‐Fullerene Dyads and Their Spectroscopic Properties. Eur. J. Org. Chem. 2008, 15, 2621‐2627. (19) Prato, M.; Maggini, M. Fulleropyrrolidines: A Family of Full‐Fledged Fullerene Derivatives. Acc. Chem. Res. 1998, 31, 519‐526. (20) Novello, F.; Prato, M.; Daros, T.; DeAmici, M.; Bianco, A.; Toniolo, C.; Maggini, M. Stereoselective Additions to [60]Fullerene. Chem. Commun. 1996, 903‐ 904. (21) Noviandri, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T.; Lay, P. A.; Masters, A. F.; Phillips, L. The Decamethylferrocenium/Decamethylferrocene Redox Couple: A Superior Redox Standard to the Ferrocenium/Ferrocene Redox Couple for Studying Solvent Effects on the Thermodynamics of Electron Transfer. J. Phys. Chem. B 1999, 103, 6713‐6722. (22) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. Photoinduced Intramolecular Electron Transfer in a Bridged C60 (Acceptor)‐Aniline (Donor) System. Photophysical Properties of the First "Active" Fullerene Diad. J. Am. Chem. Soc. 1995, 117, 4093‐4099. (23) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138‐163. (24) Matile, S.; Berova, N.; Nakanishi, K.; Fleischhauer, J.; Woody, R. W. Structural Studies by Exciton Coupled Circular Dichroism Over a Large Distance: Porphyrin Derivatives of Steroids, Dimeric Steroids, and Brevetoxin B. J. Am. Chem. Soc. 1996, 118, 5198‐5206. (25) Megiatto, J. D.; Antoniuk‐Pablant, A.; Sherman, B. D.; Kodis, G.; Gervaldo, M.; Moore, T. A.; Moore, A. L.; Gust, D. Mimicking the Electron Transfer Chain in Photosystem II With a Molecular Triad Thermodynamically Capable of Water Oxidation. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15578‐15583. (26) Bahr, J. L.; Kuciauskas, D.; Liddell, P. A.; Moore, A. L.; Moore, T. A.; Gust, D. Driving Force and Electronic Coupling Effects on Photoinduced Electron Transfer in a Fullerene‐Based Molecular Triad. Photochem. Photobiol. 2000, 72, 598‐611.

(27) Liddell, P. A.; Kodis, G.; de la Garza, L.; Bahr, J. L.; Moore, A. L.; Moore, T. A.; Gust, D. Photoinduced Electron Transfer in Tetrathiafulvalene‐Porphyrin‐ Fullerene Molecular Triads. Helv. Chim. Acta 2001, 84, 2765‐2783. (28) Liddell, P. A.; Kodis, G.; Moore, A. L.; Moore, T. A.; Gust, D. Photonic Switching of Photoinduced Electron Transfer in a Dithienylethene‐Porphyrin‐Fullerene Triad Molecule. J. Am. Chem. Soc. 2002, 124, 7668‐7669. (29) Kodis, G.; Liddell, P. A.; Moore, A. L.; Moore, T. A.; Gust, D. Synthesis and Photochemistry of a Carotene‐ Porphyrin‐Fullerene Model Photosynthetic Reaction Center. J. Phys. Org. Chem. 2004, 17, 724‐734. (30) Marcus, R. A.; Sutin, N. Electron Transfers in Chemistry and Biology. Biochim. Biophys. Acta 1985, 811, 265‐322. (31) Marcus, R. A. On the Theory of Chemiluminescent Electron‐Transfer Reactions. J. Chem. Phys. 1965, 43, 2654‐2657. (32) Marcus, R. A. Erratum: On the Theory of Chemiluminescent Electron‐Transfer Reactions. J. Chem. Phys. 1970, 52, 2803‐2804. (33) Marcus, R. A. On the Theory of Electrochemical and Chemical Electron Transfer Processes. Can. J. Chem. 1959, 37, 155‐163. (34) van Stokkum, I. H. M.; Larsen, D. S.; van Grondelle, R. Global and Target Analysis of Time‐Resolved Spectra. Biochem. Biophys. Acta 2004, 1657, 82‐104.



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TOC graphic


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Chart 1.



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Scheme 1. Synthesis of amino acid porphyrin 4: (a) methanesulfonic acid, (b) sodium bicarbonate, (c)diazomethane, (d) Dess‐Martin oxidation, (e) trifluoroacetic acid followed by 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone, (f) trifluoroacetic acid and aqueous HCl.


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Scheme 2. Synthesis of cyanoporphyrin 5: (a) trifluoroacetic acid followed by 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone, (b) lithium aluminum hydride, (c) manganese dioxide, (d) N‐bromosuccinimide, (e) zinc acetate, (f) cyanation, (g) tri‐ fluoroacetic acid.



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Figure 1. Cyclic voltammograms of triad 1 in dichloromethane containing 0.1 M tetra‐n‐butylammonium hexafluorophos‐ phate. (a) reduction, (b) oxidation.


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Figure 2. Absorption spectra of benzonitrile solutions of triad 1 (solid black), P‐C60 dyad 2 (dotted red) and CyP 5 (dashed blue). The insets are expansions of the Soret (top) and Q‐band (bottom) regions.



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Figure 3. Fluorescence emission spectra in benzonitrile of triad 1 (solid), P‐C60 dyad 2 (dashed) and cyanoporphyrin 5 (dotted). The samples were excited at 424, 419 and 457 nm, respectively, and their intensities have been normalized.


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Figure 4. Transient states and relevant interconversion pathways for triad 1.



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Figure 5. Fluorescence DAS in toluene with excitation at 510 nm. (a) CyP‐C60 dyad 3; associated time constants 50 ps (squares) and 2.02 ns (circles). (b) triad 1; time constants 28 ps (squares), 140 ps (circles) and 2.15 ns (triangles).


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Figure 6. Transient absorption DAS from P‐C60 dyad 2 with excitation at 590 nm (a) in toluene solution; lifetimes of 75 ps (blue squares), 1.9 ns (red circles) and too long to measure in the time window employed (black triangles) (b) in benzo‐ nitrile solution; lifetimes of 33 ps (blue squares), 660 ps (red circles), and too long to measure (black triangles).



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Figure 7. Transient absorption DAS of CyP‐C60 dyad 3 with 595 nm excitation in (a) toluene; lifetimes 10 ps (blue squares), 68 ps (red circles), 2.2 ns (green diamonds) and too long to measure (black triangles) and in (b) benzonitrile; lifetimes of 8.9 ps (blue squares), 130 ps (red circles), 1.9 ns (green diamonds) and too long to measure (black triangles)


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Figure 8. Transient absorption DAS for triad 1 with excitation at 510 nm. (a) in toluene solution; lifetimes 29 ps (blue squares), 143 ps (red circles), 2.2 ns (green diamonds) and too long to measure (black triangles) (b) in benzonitrile solu‐ tion (short wavelength region); lifetimes 22 ps (blue squares), 350 ps (red circles), too long to measure (black triangles) (c) in benzonitrile solution (long wavelength region); lifetimes 22 ps (blue squares), 351 ps (red circles), too long to measure (black triangles).



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Figure 9. Transient absorption DAS of triad 1 in benzonitrile with excitation into CyP at 725 nm; lifetimes 14 ps (squares), 350 ps (circles), 500 ps (diamonds) and too long to measure (triangles).


Photoinduced Electron and Energy Transfer in a Molecular Triad

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