On–off switch of charge-separated states of pyridine-vinylene-linked porphyrin–C60 conjugates detected by EPR

The on–off switch of charge separated states in a new series of pyridine-vinylene linked porphyrin–C60 conjugates was detected by EPR at 77 K.


Introduction
In Nature, photosynthesis is by far the best method to convert solar energy into chemical energy. It involves complex processes based on intramolecular electron/energy transfer reactions between molecular components within photoactive membranes. [1][2][3][4][5] In recent decades, photosynthesis has served as an inspiration to design and synthesize new articial photosynthetic arrays that mimic the function of plants. 6 In this context, organic chemists have prepared many articial photosynthetic systems that have enabled the study of the fundamental chemistry and the reaction mechanisms involved in the biological processes that are responsible for solar energy conversion in nature. [7][8][9][10] The synthesis of molecular architectures consisting of electron donors and acceptors, covalently linked by p-conjugated molecular spacers (D-p-A) is one of the strategies for probing photoinduced electron transfer processes on a molecular level. 8,11 The electronic properties of these molecules make them potentially useful in molecular photonics, optoelectronics, nanoscale applications, and in solar energy conversion. [12][13][14][15][16][17][18] Porphyrins represent an important class of molecular building blocks, which in biological architectures are responsible for oxygen-electron transport, light-to-energy conversion, etc. 19 They have been used frequently as electron donors in Dp-A conjugates, mainly because of their ease of synthesis, versatile electrochemical and photochemical properties, and their presence in the naturally occurring chlorophyll. [20][21][22] As a complement, C 60 is an excellent electron acceptor because it features low-energy triply degenerate LUMOs and is able to accept up to six electrons. 2,23-27 C 60 exhibits low reorganization energies upon electron transfer processes, which is essential to obtain ultrafast charge separation and slow charge recombination. [28][29][30][31] Thus, a wide variety of porphyrin arrays -H 2 P or ZnP have been covalently linked to C 60 in many different ways and using various wire-like molecular spacers. [32][33][34][35][36] It has been observed that the linker in D-p-A conjugates has a profound effect on the rates of the intramolecular photoinduced charge-transfer 37 and on the mechanism by which the charge transfer occurs, 38,39 which can be via "superexchange"mediated coherent tunnelling between the electronic states of the D/A pair or via a "hopping" mechanism through localized electronic states on the linker. 40 Both charge separation and recombination occur and are dened by the electron-transfer rate constant k ET as Ae ÀbR D-A , where A is the Arrhenius constant, b the damping factor, and R D-A the distance between the electron donors and acceptors. A decreased damping factor therefore means an increase of the distance over which charges can be efficiently transported. [41][42][43][44] b depends primarily on the length of the wire-like molecular spacer, conformational rigidity, and the electronic properties of the electron donors and acceptors. [45][46][47][48] Several groups have studied p-conjugated oligomers as wire-like molecular spacers connecting the photoactive termini in H 2 P/C 60 and ZnP/C 60 conjugates. p-extended spacers such as para-phenylene vinylene (oPPV), 49 [2,20]paracyclophaneoligophenylene-vinylene (pCp-oPPV), 50-52 oligothiophene (nOT), 53 and oligothienylenevinylene (nTV) 54,55 are ideal connectors for effective charge transfer from the electron donors to the acceptors with maximum rates and relatively small damping factors. [49][50][51][52][53][54][55] p-decient molecular linkers, such as pyridine-vinylene groups have not been investigated so far, so we now report the synthesis and electronic/photophysical properties of a new homologous series of H 2 P/C 60 and ZnP/C 60 electron donoracceptor conjugates bridged covalently by various pyridinevinylene linkers 15-20a (H 2 P-n-mC 60 , ZnP-n-mC 60 , n ¼ 1-3) and 15-20b (H 2 P-n-pC 60 , ZnP-n-pC 60 , n ¼ 1-3) in addition to their precursors 9a-14a (H 2 P-n-mCHO, ZnP-n-mCHO, n ¼ 1-3) and 9b-14b (H 2 P-n-pCHO, ZnP-n-pCHO, n ¼ 1-3). We have used pyridine-vinylene spacers of different lengths and substitution patterns to conduct a systematic evaluation of the inuence of both the length and nature of the linker on intramolecular charge-transfer processes that yield long-and short-lived chargeseparated states in solvents of different polarities (Chart 1).

Synthesis
New photoactive conjugates were prepared using different reactions: cross-coupling reactions, such as the Stille and Heck, condensation reactions, such as the Knoevenagel and Wadsworth-Horner-Emmons, and the 1,3-dipolar cycloaddition reaction. All reactions were conducted under argon and Schlenk conditions. The solvents were rst dried by standard procedures such as sodium/benzophenone or CaH 2 and freshly distilled before use. Pyridine-vinylene spacers of different lengths 2-6a and 2-6b were synthesized by successive Stille and Heck crosscoupling reactions starting with 6-bromo-2-pyridine-carboxaldehyde 1a, 6-bromo-3-pyridinecarboxaldehyde 1b and 2,6dibromopyridine 3 as the main building blocks. The synthetic procedures are shown in Scheme 1. Monomers 2a and 2b were prepared using Stille reactions starting from the corresponding bromo-pyridinecarboxaldehydes 1a or 1b and tributyl(vinyl)tin using Pd(PPh 3 ) 4 as catalyst and anhydrous toluene as solvent. These monomers were treated with 2,6-dibromopyridine 3 under palladium-catalyzed Heck coupling conditions to give intermediates 4a and 4b, which were subsequently converted to the desired dimers 5a and 5b via Stille cross-coupling reactions in anhydrous toluene using Pd(PPh 3 ) 4 as catalyst. Finally, 6a and 6b were synthesized starting from the previously obtained monomers 2a,b by a Pd(OAc) 2 catalyzed double Heck reaction with 2,6-dibromopyridine 3 in dimethylformamide (DMF). For these reactions it was necessary to use an excess of the monomers (2.5 eq.).

Ground-state interactions
Electrochemistry. The redox properties of the new porphyrin-fullerene conjugates were studied to probe the electronic coupling between the electron donor and the acceptor in the ground state through each molecular pyridinevinylene linker. The electrochemistry of D-p-A conjugates 15-20a (H 2 P-n-mC 60 , ZnP-n-mC 60 ) and 15-20b (H 2 P-n-pC 60 , ZnP-n-pC 60 ) was studied by cyclic and differential pulse voltammetry ( Fig. 1 and S1 †) at room temperature in dry dichloromethane (DCM) solutions containing tetra-n-butylammonium hexa-uorophosphate (TBAPF 6 0.1 M) as supporting electrolyte. A glassy carbon electrode was used as the working electrode, an Ag-wire as reference and a Pt-wire as the counter-electrode. The redox potentials are referenced to the internal ferrocene/ferrocenium couple (Fc/Fc + ). The corresponding redox potentials are summarized in Table S1 † along with those for C 60 used as reference for comparison. All the systems show oxidation waves between +0.271 and +0.692 V corresponding to the oxidation processes of the H 2 P and ZnP, and reversible waves between À1.125 and À2.259 V due to reduction processes of both the fullerene and porphyrin fragments (Fig. 1). Fig. 1 shows the cyclic voltammograms of the porphyrinfullerene conjugates 15-20a (H 2 P-n-mC 60 , ZnP-n-mC 60 , n ¼ 1-3) and 15-20b (H 2 P-n-pC 60 , ZnP-n-pC 60 , n ¼ 1-3). In the reductive scan, each conjugate shows the reduction prole of four or ve one-electron reversible reduction waves, respectively, corresponding to C 60 and porphyrin centred processes. The rst, second, and fourth reductions can be assigned to the fulleropyrrolidine-centred processes by comparison with C 60 . The third and h reduction waves are centred on the porphyrin subunit with Zn 18-20a (ZnP-n-mC 60 , n ¼ 1-3), 18-20b (ZnP-n-pC 60 , n ¼ 1-3) and without Zn 15-17a (H 2 P-n-mC 60 , n ¼ 1-3), 15-17b (H 2 P-n-pC 60 , n ¼ 1-3). The reduction potentials for the new electroactive conjugates are cathodically shied relative to the values for pristine C 60 , as expected for a monofunctionalized C 60 . 28,62 The oxidative scans show the rst and second one-electron reversible oxidation waves corresponding to H 2 P and ZnP centred processes. H 2 P derivatives 15-17a (H 2 P-n-mC 60 , n ¼ 1-3), 15-17b (H 2 P-n-pC 60 , n ¼ 1-3) exhibit one oxidation wave, while the ZnP derivatives 18-20a (ZnP-n-mC 60 , n ¼ 1-3) and 18-20b (ZnP-n-pC 60 , n ¼ 1-3) feature two oxidation waves at potentials similar to those observed for the reference tetraphenylporphyrin (TPP). 50,52 The electron donor ability of H 2 P and ZnP within electroactive systems was conrmed by the remarkably low value of their rst oxidation potential, between +0.27 and +0.48 V, similar to those found for other compounds. For the reduction processes (shown in Table S1 †) the rst reduction potential is shied cathodically by 110-148 mV compared to the rst reduction potential of C 60 , which shows the strong push-pull nature of the electroactive species, allowing the prediction of the formation of a charge-separated state by photoexcitation. The reduction potentials of each series show that the shis for the different molecular linker lengths are not signicant. However, the isomeric 1,3 or 1,4-disubstituted systems (meta or para), do exhibit differences of $20 to 27 mV in the cathodic shis. The 1,4-disubstituted systems 15-20b (H 2 P-n-pC 60 , ZnPn-pC 60 ) display rst reduction potentials that are less negative than their 1,3-disubstituted counterparts 15-20a (H 2 P-n-mC 60 , ZnP-n-mC 60 ).
Absorption spectroscopy. The optical UV-Vis absorption spectra of the electron donor-acceptor conjugates exhibited the contributions and features of their components, pyridinevinylene linkers, C 60 , H 2 P, and ZnP, as shown in Fig. S2 and S3. † The absorption spectra of the H 2 P-based conjugates 15-17a and 15-17b exhibit strong maxima at around 421 nm in addition to four weaker absorption bands in the range from 500 to 700 nm, corresponding to the H 2 P Soret-and Q-band absorptions, respectively. Compared to the H 2 TPP reference, the Soret bands are red shied by about 5 nm, while the Q bands exhibit redshis between 1 and 4 nm. In contrast, ZnP-containing conjugates 18-20a and 18-20b exhibit absorptions at 427 nm compared to 423 nm for ZnTPP and only two Q bands at 557 and 597 nm, 2-3 nm red shied compared to ZnTPP. Furthermore, a rather broad, though weak, absorption evolves between 300 and 400 nm, which is assigned to C 60 . The characteristic absorption of the mono-functionalized C 60 that usually appears at 430 nm overlaps with the stronger absorptions of the porphyrin fragments. These shis and the fact that the extinction coefficients are comparable but not identical to those observed for the references, suggests signicant electronic interactions between  the individual components in the ground state. In this context, we focused on the low-energy region of the absorption spectra, as shown in Fig. 2. In this instance, features whose origins are neither linked to C 60 nor to pyridine-vinylene or H 2 P/ZnP are discernable. A weak maximum is seen at around 860 nm that is assigned to a charge-transfer transition. [63][64][65][66] Within the ZnPcontaining conjugates, 20b (ZnP-3-pC 60 ) exhibits the strongest charge-transfer band and 18b (ZnP-1-pC 60 ) and 19b (ZnP-2-pC 60 ) show weaker interactions, with electronic coupling elements of 400 and $100 cm À1 , respectively. Weaker and energetically shied interactions for H 2 P-containing conjugates with electronic coupling elements of 20-40 cm À1 preclude a meaningful analysis.
At rst glance, comparing the absorptions of the meta and para linked pyridine-C 60 conjugates revealed no particular differences. The more pyridine-vinylene groups are present in the linker, the higher the extinction coefficient between 300 and 400 nm becomes. Thus, the increased absorption in this wavelength range is attributed to the linker.

Excited-state interactions
To gain further insights into the excited-state interactions, both steady state and time-resolved emission spectroscopy (time correlated single photon counting, TCSPC) and time-resolved absorption spectroscopy (transient absorption) were employed.
Steady-state uorescence. Upon excitation of H 2 TTP at 420 nm, characteristic uorescence maxima at 650 and 720 nm are discernable, while ZnTTP exhibits maxima at 605 and 655 nm, as shown in Fig. 3 and S4 in the ESI. † The values for the uorescence maxima of the electron donor-acceptor conjugates are listed in Table 1. Notably, the uorescence maxima of the different electron donor-acceptor conjugates and their references that lack C 60 evolve at the same wavelength. Compared to the references H 2 TPP and ZnTPP, they are shied 3-4 nm to the red. Furthermore, red shis were found when emission spectra of electron donor-acceptor conjugates were obtained in solvents with higher polarity, such as benzonitrile. These red shis of the bands have been assigned for other D-p-A systems to an intramolecular electron transfer process in the excited state between the porphyrin and C 60 . 67 References 9-14a,b exhibit higher uorescence quantum yields than H 2 TPP and ZnTPP -0.15 versus 0.10 and 0.065 versus 0.040. 68,69 Thus, it can be assumed that at 420 nm the pyridine-vinylene linker is also excited and transfers energy to the porphyrin. More importantly, for the electron donor-acceptor conjugates, the uorescence intensity decreases as the linker length decreases. This observation is quantied by the uorescence quantum yields (F F ) shown in Table 1. Consequently, 18a (ZnP-1-mC 60 ) and 18b (ZnP-1-pC 60 ) show the lowest uorescence quantum yields. With values of 4.6 Â 10 À4 and 3.7 Â 10 À4 in THF, their ZnP uorescence is almost completely quenched. 15a (H 2 P-1-mC 60 ) and 15b (H 2 P-1-pC 60 ) exhibit the lowest uorescence quantum yields among the H 2 P-containing systems with values of 0.003 and 0.007 in THF. In contrast, 17b (H 2 P-3-pC 60 ) and 20b (ZnP-3-pC 60 ) feature values of 0.10 and 0.04, which are similar to those seen for H 2 TPP and ZnTPP. Compared to 11b, (H 2 P-3-pCHO) and 14b (ZnP-3-pCHO), their uorescence is quenched by about 30%. Additionally, F F depends on the solvent polarity. In the more polar solvents, the uorescence is more efficiently quenched than in the less polar solvents. When comparing the meta-substituted conjugates to the para substituted ones, the latter exhibit slightly higher quantum yields that the former. Since uorescence quenching is only observed for the electron donor-acceptor conjugates and not for the references that lack C 60 , the quenching is attributed to an energy and/or electron transfer involving the porphyrin and C 60 .
References 9-14a,b exhibited lifetimes of 10 and 2 ns, respectively, comparable to the values for H 2 TPP and ZnTPP. 70,71 These results conrm the conclusions drawn from the steady-state uorescence experiments indicating that an energy and/or electron transfer takes place between H 2 P/ZnP and C 60 .
Transient absorption spectroscopy. Transient absorption measurements were conducted in solvents of different polarity (toluene, THF, and PhCN) using two different setups in order to investigate the formation and deactivation processes of excited states upon photoexcitation of the conjugates and the corresponding reference compounds. To investigate processes in the ps/ns time scale (up to 7.5 ns), the sample was excited with a 150 fs laser pulse at either 387 nm (200 nJ; c ¼ 10 À5 M) or 420 nm (200 nJ; c ¼ 10 À6 M) using the Helios spectrometer. To follow processes on the ns/ms/ms time scale, the EOS spectrometer was employed, exciting at 387 nm (1 mJ, c ¼ 10 À5 M) with time scales up to 400 ms. Transient absorptions were also investigated by exciting at either 355 nm (6 ns laser pulse, 10 mJ, c ¼ 10 À5 M) or 420 nm (3 ns laser pulse, 5 mJ, c ¼ 10 À6 M) using time scales up to 1 ms.
Upon excitation at 387 and 420 nm, the differential absorption spectra of 9a-11a (H 2 P-n-mCHO, n ¼ 1-3) and 9b-11b (H 2 Pn-pCHO, n ¼ 1-3) are dominated by features of the H 2 P singlet excited state, 71,72 as shown in Fig. 4, le. This state is formed immediately upon excitation and exhibits maxima at 450, 540, 575, 630, and 670 nm in addition to a broad transient absorption between 1000 and 1150 nm. Additionally observed groundstate bleaching at 420, 520, 550, 590, and 650 nm correspond to the Soret-and Q-band absorptions of H 2 P, respectively. The porphyrin's singlet excited state is stable during the time scale of our fs transient-absorption setup (7.5 ns, see Fig. 4). The differential absorption spectra of all H 2 P based references look nearly identical and also resemble the transient absorption spectra of H 2 TTP closely. Thus, it can be assumed that the pyridine-vinylene linker does not show transients upon photoirradiation. The ZnP-pyridine-vinylene reference compounds give similar results, as shown in Fig. S5. † As for ZnTPP, upon 420 nm excitation the 1 *ZnP 70,72 state dominates the visible region of the differential absorption spectrum with maxima at 460, 580, and 630 nm and minima at 560 and 600 nm (groundstate bleaching). The singlet excited porphyrin then undergoes  intersystem crossing within 2 ns to give the 3 *ZnP, which is stable within the 7.5 ns time scale of the system and shows maxima at 480 and 840 nm. When exciting the H 2 P-C 60 electron donor-acceptor conjugates with a fs-laser pulse at 387 and 420 nm, respectively, the most prominent features of the transient absorption spectra are in the visible region, as seen for the references, those belonging to the H 2 P singlet excited state, (maxima at $450, 540, 575, 630, and 670 nm) and ground-state bleaching at $420, 520, 550, 590, and 650 nm; a representative example is shown Fig. 5 and S6 in the ESI. † The features of the 1 *H 2 P, with a broad maximum between 1000 and 1150 nm, can be observed in the NIR region. However, in contrast to the references, the singlet excited state of the porphyrin is shorter lived in the presence of C 60 . 15a (H 2 P-1-mC 60 ) and 15b (H 2 P-1-pC 60 ) exhibit the shortest singlet state lifetimes (hundreds of picoseconds in THF). With increasing length of the linker, the decay of the singlet excited state of the porphyrin becomes slower. In 17b (H 2 P-3-pC 60 ) the 1 *H 2 P lifetime even exceeds the time scale of our fs setup, as observed for the reference systems that lack C 60 (compare Fig. S6 †).
These results also conrm those from the steady-state and time-resolved emission experiments. Furthermore, additional features corresponding to the C 60 singlet excited state are observed upon 387 nm excitation, as a 920 nm absorption maximum. The transient characteristics, which correlate with the triplet excited state of C 60 at 690 nm are, however, masked by the more intense H 2 P transients and thus barely visible. Finally, a distinct peak arises in the NIR region of the differential absorption spectra of 15a (H 2 P-1-mC 60 ) and 15b (H 2 P-1-pC 60 ) (Fig. S6 †) with a maximum at $1010 nm. This feature is assigned to the singly reduced fullerene's ngerprint, which is well known from the literature. 24,73 Although this signal coincides with the singlet features of H 2 P, it can be clearly distinguished, since they exhibit comparably short-lived singlets. For 16a (H 2 P-2-mC 60 ) (Fig. 5), 17a (H 2 P-3-mC 60 ) and 16b (H 2 P-2-pC 60 ) (Fig. S6 †) the fullerene anion ngerprint cannot be clearly identied in the differential absorption spectra because of the increased signal of the 1 *H 2 P. However, upon closer examination of the NIR region ( Fig. 5, above, and Fig. S6 †) an individual peak can be discerned at $1010 nm. In contrast, for 17b (H 2 P-3-pC 60 ), the singlyreduced C 60 cannot be identied unambiguously, since it is masked by the rather long lived 1 *H 2 P. Therefore, we cannot rule out the formation of a CSS for the latter.
The presence of the C 60 anion signature in the transient absorption spectra proves that charge transfer takes place in the conjugates. The associated radical cation transient absorption again overlaps with the porphyrin signatures. However, it can be probed by analyzing the decay kinetics. Even though the triads show the same transients initially, the transient absorption spectra show considerably different lifetimes of their excited states. As discussed above, the singlet excited-state lifetimes increase with the length of the linker. Furthermore, the latter also decrease with solvent polarity, as shown in Table 1. Not only the lifetimes of the singlet excited state vary with the length of the linker and with the solvent polarity; those of the radical ion pair state also do. From a multi-wavelength analysis of the decays of the C 60 radical anion and of the H 2 P radical cation, a lifetime of 1.4 ns was determined for 15a (H 2 P-1-mC 60 ) in THF. The lifetime of the radical ion pair state changed on varying the solvent. In toluene, for example, a lifetime of 2.0 ns was found for 15a (H 2 P-1-mC 60 ), while in PhCN the lifetime was only 662 ps, as shown in Table S2 of the ESI. † Even more distinct differences in the radical ion pair lifetime are observed, when different linker lengths are considered, as shown in Fig. 5 (below) for kinetic measurements. Ongoing from one pyridine-vinylene group in 15a (H 2 P-1-mC 60 ) to two in 16a (H 2 P-2-mC 60 ), the lifetime increases in THF. Because the 1010 nm decay is not complete for 16a (H 2 P-2-mC 60 ) within the timescale of 7.5 ns, we turned to EOS fs-measurements and nanosecond transient absorption spectroscopy. Upon excitation of 16a (H 2 P-2-mC 60 ) with ns laser pulses at either 355 or 420 nm under different conditions, the features of the H 2 P radical anion are discernable, but are superimposed with those of the C 60 triplet excited state. The ngerprint of the C 60 radical anion is clearly visible in the near-infrared region (Fig. 6, below). Its decay at 1010 nm, for example, (Fig. 7) ts by a single exponential function to afford a radical ion pair state lifetime of 35 ns for 16a (H 2 P-2-mC 60 ) in THF. Interestingly, the radical ion pair lifetime in 17a (H 2 P-3-mC 60 ) is 2.2 ns in THF, appreciably shorter than for 16a (H 2 P-2-mC 60 ) ( Table 2). This behaviour will be discussed further later. 74 When probing 16a (H 2 P-2-mC 60 ) in THF with the EOS fssetup (Fig. 8, above), the visible region is again dominated by the porphyrin's triplet excited-state features, while the C 60 radical anion 1010 nm ngerprint is discernible in the near infrared region. The 1010 nm decay (Fig. 8, below) is best t by a biexponential function, affording a short-lived component of 4.3 ns attributable to an H 2 P-centered singlet excited state and a longer lived one of 50 ns assigned to a C 60c À centered charge separation. By comparison with the ns transient absorption measurements, where the laser power is four orders of magnitude higher (1 mJ compared to 10 mJ), we conclude that charge recombination is independent of the applied laser power. The same trend is observed for the radical ion pair state lifetimes for the para substituted conjugates, as shown in Table  2. While for 15b (H 2 P-1-pC 60 ) the radical ion pair state decays with a lifetime of 1.3 ns in THF, that of 16b (H 2 P-2-pC 60 ) does not decay within the time scale of our femtosecond setup. A lifetime of 65 AE 21 ns was determined for 16b (H 2 P-2-pC 60 ) in THF in complementary nanosecond experiments, as shown in Fig. 7. It was reassuring that EOS fs-measurements yielded a lifetime of 58 ns (Table 2). 75 The formation of the charge-separated states was investigated in order to obtain further insight into the charge-transfer dynamics of the electron donor-    Table 2). Clear trends can be observed for 15a (H 2 P-1-mC 60 ) and 15b (H 2 P-1-pC 60 ). The more polar the solvent, the faster the charge separation process occurs. Furthermore, the chargeseparated state is formed more rapidly in 15a (H 2 P-1-mC 60 ) than for 15b (H 2 P-1-pC 60 ). For the systems with longer linkers, the electron is transferred to the fullerene in less than 1 ps, so that no further conclusions can be drawn from these results. The C 60 radical anion absorption at 1010 nm can be identied even more clearly for the ZnP-C 60 D-A conjugates, since ZnP does not have transients in this region of the spectrum, as shown in Fig. 9 and S7 of the ESI. † Nevertheless, the visible region is again dominated by porphyrin features. To be more precise, maxima at 460, 580, and 620 nm and minima at 420, 560, and 600 nm evolve immediately aer the 387 nm laser pulse. These correspond to the ZnP singlet excited state and ground state bleaching, respectively. While for 18a (ZnP-1-mC 60 ) 1 *ZnP decays within 400 ps and only weak triplet signatures are discernible, the singlet lifetimes and the intensity of the 3 *ZnP peaks (850 nm) increase with increasing length of the linker up to $1 ns for 20b (ZnP-3-pC 60 ). Additionally, at 920 nm a transient arises that can be assigned to 1 *C 60 . The same trend as found for the H 2 P systems was observed for the charge-separated state lifetimes. The shortest CSS lifetime of the ZnP compounds was found for 18b (ZnP-1-pC 60 ) with $150 ps in PhCN, while 19a (ZnP-2-mC 60 ) and 19b (ZnP-2-pC 60 ) in THF and toluene do not decay within the 7.5 ns time scale of our fs-setup (Fig. 9, below).
Lifetimes of 98 ns for 19a (ZnP-2-mC 60 ) and 165 ns for 19b (ZnP-2-pC 60 ) in THF were determined in complementary ns experiments. In EOS experiments, however, slightly different lifetimes, 116 ns for 19a (ZnP-2-mC 60 ) and 79 ns for 19b (ZnP-2-pC 60 ), were determined. As described for the H 2 P-conjugates, the CSS lifetime decreases with increasing solvent polarity and the longest lifetimes were determined for the compounds with two pyridine-vinylene groups as linkers rather than of those with the longer linker. All CSS lifetimes determined from multiwavelength ts either from fs or ns transient absorption experiments are summarized in Tables 2 and S2. † Analysis of the charge-separation kinetics of the ZnP D-p-A conjugates did not yield a clear trend. For 18a (ZnP-1-mC 60 ) in  THF and 18b (ZnP-1-pC 60 ) in toluene, CS takes place within $10 ps, while the CSS is formed within 2 ps for both in PhCN. CS is too fast to be monitored with our setup in all other ZnP compounds.
Electron spin resonance. As a complement to the room temperature (298 K) measurements, charge separation was also probed at low temperature (77 K) by means of EPR measurements. Fig. 10 shows a typical example, where the two signals due to C 60 c À and H 2 Pc + are discernable upon photoirradiation, at g ¼ 2.0002 and 2.0026, respectively, related to the triplet charge-separated state. The rather sharp signal at g values smaller than that of the free spin value is diagnostic for the presence of pristine C 60 . 82 Interestingly, the ne structure of the triplet chargeseparated state is also observable at g ¼ 4. Its amplitude is, however, rather weak due to the forbidden nature of the "DM s ¼ 2" transitions (see Fig. S7 in the ESI †). Similar triplet EPR signals were observed for the charge-separated states of the remaining porphyrin-C 60 conjugates (Fig. S11 and S12 in the ESI †).
Repeated on-off switching of the charge-separated state formation was realized by turning on and off the irradiation source many times, see Fig. 11. The corresponding lifetimes at 77 K are long enough to be detected during the on-off cycling for both series of porphyrin-C 60 conjugates ( Fig. S13 and S14 in the ESI †). Table 3 lists the CS lifetimes determined from the EPR experiments at 77 K. Due to the time resolution of the available equipment, only lifetimes >200 ms could be detected.

Molecular modelling
We turned to molecular modelling to investigate the fact that the D-A conjugates with the longest linker do not exhibit the longest lived CSS. Conformational analysis of the free base molecules was performed with the Conformer program 76 to determine average electron donor-acceptor distances. 10 000 conformations were determined for each molecule via a Metropolis Monte-Carlo algorithm. Each conformation was optimized with the COMPASS force eld. 77 The D-A distance for the lowest-energy conformer and the mean distances (including standard deviations) for all conformers within 20 kcal mol À1 of the lowest-energy conformation are given in Table 4. For both the meta-and para-series, the longest molecules do not follow the trend of increased D-Adistance with increased linker length. This is because the longest linkers allow the formation of a porphyrin-C 60 van-der-Waals dimer (Fig. 12), which is the lowest energy conformer. Meta isomers, usually display shorter D-A distances and a higher standard deviation (i.e. higher conformational freedom) than their para-equivalents.   To assess whether these results also apply to the metalated system, we compared the dimerisation energy of unsubstituted H 2 P and Zn porphyrins with C 60 using dispersion-corrected density functional theory (DFT) (PBE+TS/DND). [78][79][80] This reveals that the dimer is stabilized by 2.2 kcal mol À1 through metalation. Since the dimer is already the most stable conformation for the free base molecules, the overall picture of the conformational analysis should not change for the metalated case.
Furthermore, the energies of frontier molecular orbitals were calculated for DFT-optimized structures obtained with the MO6 functional. 80,81 The frontier orbital energies for all computed structures are summarized in Table S3. † In general, the orbital energies are quite constant, with energy differences on the order of several meV. Energy differences of the HOMO orbitals are observed in the presence of the metal atom, which lead to 40 AE 1 meV stabilization. The larger variations observed for the LUMOs are the result of the linkage between the pyridine and the pyrrolidine: meta-conformations have LUMO energies around 70 AE 5 meV higher than the corresponding para-ones. Analyses of the orbital shapes showed clear electron donor-acceptor interactions. The LUMO is well distributed around the fullerene cage for all the cases (Fig. S9 †). The HOMO is mainly localized on the porphyrin and shows a good overlap with the p-orbitals of the benzyl group, which provides electronic coupling with the aromatic chain. The orbitals of the metal atoms showed a high contribution to the electronic distribution of the HOMO around the entire porphyrin, increasing the electronic distribution around it (see Fig. S9 in the ESI †). Table 2 summarizes the radical ion pair state lifetimes for all electron donor-acceptor conjugates in toluene, THF, and PhCN.

Discussion
In brief, several factors inuence the charge transfer of the porphyrin-fullerene D-A conjugates. On one hand, the polarity of the solvent affects the CSS lifetimes. In less polar solvents such as toluene, the longest lifetimes are observed. This leads to the assumption that the charge recombination occurs in the inverted region of the Marcus parabola. On the other hand, the length of the linker plays a key role in the electron transfer dynamics. The systems with just one pyridine-vinylene group exhibit the fastest charge recombination. 16a (H 2 P-2-mC 60 ) and 16b (H 2 P-2-pC 60 ) feature the longest lived radical ion pairs within the free base porphyrin series, while for the ZnP series 19a (ZnP-2-mC 60 ) and 19b (ZnP-2-pC 60 ) reveal the longest CSS lifetimes. Astonishingly, the triads with the longest linkers do not show the longest CSS lifetimes. Calculations suggest that the exible linkers lead to shorter through-space distances between the free base porphyrin and the fullerene. The reduced D-A distances and the shorter lifetimes observed for the radical ion pair for 17a (H 2 P-3-mC 60 ), 17b (H 2 P-3-pC 60 ), 20a (ZnP-3-mC 60 ) and 20b (ZnP-3-pC 60 ) lead to the conclusion that for this system electron transfer occurs through space rather than through the linker. The compounds with C 60 attached to the pyridine in a para-positions yield longer-lived CSS than those with C 60 in a meta position. Finally, it should be noted that the longest CS states are formed for 19b (ZnP-2-pC 60 ).

Conclusions
We have designed and synthesized a new series of H 2 P/C 60 and ZnP/C 60 electron donor-acceptor conjugates, in which the electron donating H 2 P/ZnP and the electron accepting C 60 are linked through a pyrrolidine ring covalently attached to pyridine-vinylene spacers of different lengths. Electrochemical experiments and molecular modelling at the DFT level revealed a strong push-pull nature between the electroactive constituents. Signicant interactions were observed in absorption measurements as 1-4 nm red shis of the H 2 P/ZnP absorptions. In addition, in the low-energy region of the spectra, charge transfer bands were identied that show considerably stronger interactions for the ZnP conjugates (100-400 cm À1 ) than for the H 2 P conjugates (20-40 cm À1 ). Among the ZnP conjugates, 20b (ZnP-3-pC 60 ) exhibits the strongest coupling between ZnP and C 60 . Fluorescence assays showed that the H 2 P/ZnP features depend on the length and the substitution pattern of the pyridine-vinylene spacers. H 2 P systems generally show stronger uorescence than ZnP ones. Conjugates with just one pyridinevinylene unit, that is, 15a, 15b, 18a, and 18b, display the weakest uorescence and shortest uorescence lifetimes, while uorescence quenching is barely detected for the conjugates with three pyridine-vinylene units, 17a, 17b, 20a, and 20b. Importantly, the uorescence is more intense in the less polar solvent environments, suggesting charge rather than energy transfer. To conrm this, charge transfer was veried using pump-probe experiments. Differential absorption spectra reveal features of oxidized H 2 P/ZnP and reduced C 60 in the visible and in the nearinfrared regions, respectively. Kinetic analyses yielded chargeseparated state lifetimes between 1.3 ns (15b) and 65 ns (16b) for the H 2 P conjugates and between 373 ps (18b) and 165 ns (19b) for the ZnP conjugates in THF. In toluene, the H 2 P/ZnP conjugates generally exhibit longer charge-separated state lifetimes than in THF and benzonitrile. This solvent dependence suggests that charge recombination occurs in the Marcus inverted region. Calculations revealed that conjugates with two pyridine-vinylene units and a para substitution exhibit the longest distances between D and A, $21Å. These ndings are perfectly compatible with the charge-separated state lifetimes for 16b and 19b, which were the longest within the series investigated. We infer from a detailed examination of the lowest-energy conformation of the conjugates with the longest spacers that the exible linkers enable electron donor and acceptor to approach through-space, thus decreasing the effective distance to $6-8Å.
Complementary EPR measurements in frozen PhCN and THF conrm the formation of charge-separated states. A sharp peak corresponding to reduced C 60 (g $ 2.000) was identied and a broader, less intense signal (g $ 2.003) was assigned to oxidized H 2 P/ZnP. Additionally, formation of the charge-separated states was switched on and off repeatedly by turning the irradiation on and off.