Improved light absorbance does not lead to better DSC performance : studies on a ruthenium porphyrin – terpyridine conjugate

Complete List of Authors: Housecroft, Catherine; University of Basel, Department of Chemistry Constable, Edwin; University of Basel, Departement Chemie Prescimone, Alessandro; University of Basel, Chemistry Wenger, Oliver; University of Basel, Department of Chemistry Bündt, Laura; University of Basel, Chemistry Schmidt, Hauke; University of Basel, Chemistry Lanzilotto, Angelo; University of Basel, Chemistry


Introduction
Ever since the rst development of Grätzel n-type dye-sensitized solar cells (DSCs) employing sintered semiconductor nanoparticles functionalized with ruthenium(II)-based dyes, [1][2][3] efforts have been concentrated on the improvement of the photoconversion efficiency.][6][7][8][9][10][11][12][13][14][15][16] Nature's reliance on porphyrins in photosystem II has lead to signicant interest in bioinspired devices utilizing porphyrin or metalloporphyrin-based sensitizers in DSCs, 4,[17][18][19] and a power conversion efficiency of 13% has been reported by Grätzel and coworkers for a porphyrin dye incorporating a donor-p-bridge-acceptor domain combined with a cobalt(II)/(III)redox shuttle. 141][22] Our approach to ruthenium(II) dyes containing light-harvesting porphyrin domains is predicated upon heteroleptic {Ru II (tpy) 2 } domains in which the two tpy ligands bear anchoring and porphyrin substituents, respectively.The majority of oligopyridine-porphyrin conjugates are characterized by the attachment of the metal-binding domains directly or with a spacer to the phenyl-substituents of 5,10,15,20-tetraphenyl-21H,23H-porphyrins. 23e have now developed new approaches for the functionalization of porphyrins in which oligopyridines and their metal complexes are attached directly to a pyrrole ring of the porphyrin core.The selective monobromination of H 2 TPP in the 7-position developed by Zhang and coworkers, 24 provided an attractive opening for our synthetic investigations into monofunctionalization of a porphyrin core with a 2,2 0 :6 0 ,2 00 -terpyridine (tpy) domain.

General
Microwave reactions were carried out in a Biotage Initiator 8 reactor. 1 H, 13 C and 31 P NMR spectra were recorded at room temperature using a Bruker Avance III-500 NMR spectrometer. 1 H and 13 C NMR chemical shis were referenced to residual solvent peaks with respect to d(TMS) ¼ 0 ppm and 31 P NMR chemical shis with respect to d(85% aqueous H 3 PO 4 ) ¼ 0 ppm.Solution absorption and emission spectra were measured using an Agilent 8453 spectrophotometer and a Shimadzu RF-5301PC spectrouorometer, respectively.Spectroelectrochemical and solid-state absorption spectroscopic measurements used a Varian-Cary 5000 spectrophotometer.Electrospray (ESI) mass spectra and high resolution ESI-MS were measured on Bruker Esquire 3000 plus and Bruker maXis 4G instruments, respectively.Nanosecond transient absorption spectra were measured on an LP-920KS spectrometer from Edinburgh Instruments using a frequency-doubled Quantel Brilliant B laser as a pump source.Transient absorption measurements with picosecond time resolution were performed with the TRASS instrument from Hamamatsu, equipped with a C7701-01 streak camera.Excitation occurred with a picosecond mode-locked Nd:YVO 4 / YAG laser (PL2251B-20-SH/TH/FH) with PRETRIG option from Ekspla.
Electrochemical measurements were carried out using a CH Instruments 900B potentiostat with [ n Bu 4 N][PF 6 ] (0.1 M) as supporting electrolyte and at a scan rate of 0.1 V s À1 .The working electrode was glassy carbon, pseudo-reference electrode silver wire and counter-electrode platinum wire; potentials were referenced with respect to the Fc/Fc + couple.Spectroelectrochemical measurements were performed using a CH 2 Cl 2 solution of 3 (1 mM) and an MeCN solution of [Ru(3)( 4

Crystallography
Data were collected on a Bruker Kappa Apex2 diffractometer with data reduction, solution and renement using APEX 28 and CRYSTALS. 29The program Mercury v. 3.7 30,31 was used for structural analysis.The working and counter-electrodes were joined using thermoplast hot-melt sealing foil (Solaronix Test Cell Gaskets, 60 mm) by heating while pressing together.The electrolyte (LiI (0.1 M), I 2 (0.05 M), 1-methylbenzimidazole (0.5 M), 1-butyl-3methylimidazolinium iodide (0.6 M) in 3-methoxypropionitrile) was inserted between the electrodes by vacuum backlling through a hole in the counter electrode; this was sealed (Solaronix Test Cell Sealings) and capped (Solaronix Test Cell Caps).All DSCs were fully masked for measurements. 32,33ectrodes for solid-state absorption spectroscopy Dye-functionalized electrodes were assembled as above but using Solaronix Test Cell Titania Electrodes Transparent.

DSC measurements
Masks for the DSCs were made from a black-coloured copper sheet with an aperture of average area 0.06012 cm 2 (1% standard deviation) placed over the active area of the DSC.The area of the mask hole was less than the surface area of TiO 2 (0.36 cm 2 ).Black tape was used to complete the masking of the cell.Performance measurements were made by irradiating the DSC from behind with a LOT Quantum Design LS0811 instrument (100 mW cm À2 ¼ 1 sun), and the simulated light power was calibrated with a silicon reference cell.

Synthesis and characterization of compound 3
The synthetic route to the porphyrin-functionalized 2,2 0 :6 0 ,2 00terpyridine 3 is summarized in Scheme 1.The selective bromination of H 2 TPP in the 7-position was carried out using NBS as described by Zhang and coworkers, 24 and an excess of zinc(II) acetate 25 was added to yield zinc(II) complex 1.When the metallation of H 2 TPP with zinc(II) was carried out prior to reaction with NBS, selective halogenation was no longer observed and a mixture of brominated derivatives was obtained.The reaction of 1 with boronic acid 2 26 under Suzuki-Miyaura cross-coupling conditions led to 3 in 69% yield aer workup.Metallation of the porphyrin core before the coupling reaction is essential.Although 7-bromo-5,10,15,20-tetraphenyl-21H,23H-porphyrin undergoes coupling with 2 to give the zinc-free analogue of 3, subsequent reaction with Zn(OAc) 2 $4H 2 O leads to competition between the porphyrin and tpy metal-binding domains for coordination to zinc(II).Thus, the sequence of steps presented in Scheme 1 is the optimal route to 3.
The highest mass peak envelope in the electrospray mass spectrum of 3 came at m/z 984.7 and exhibited a characteristic isotope pattern for zinc.The 1 H NMR spectrum of 3 is shown in Fig. 1a and was assigned using COSY and NOESY methods.The spectrum is consistent with the desymmetrization of the [Zn(TPP)] domain.This most noticeably affects the orthoprotons (H D2 , H E2 and H G2 ) of the phenyl rings and the remaining protons in phenyl ring D (Fig. 1a).The shi to lower frequency of the signals for H D3 and H D4 compared to the metaand para-protons in rings E and G is attributed to the proximity of H D3 and H D4 to the arene ring C. The 13 C NMR spectrum of 3 was assigned using HMQC and HMBC methods (Fig. S1 and  S2 †).
Single crystals of 3$Me 2 CO were grown by slow evaporation of solvent from an acetone solution of 3. The acetone adduct of 3 (Fig. 2) crystallizes in the monoclinic space group P2 1 /c.The {Zn(TPP)} unit is structurally as expected with the atom Zn1 lying only 0.14 A out of the mean plane of the porphyrin N 4 -donor set; Zn-N bond distances and N-Zn-N bond angles are given in the caption to Fig. 2. The acetone molecule is axially coordinated and the Zn-O bond distance of 2.345(4) A is within the range of observed for axial ketones in porphyrinato zinc(II) complexes. 34The twist angles between the planes of the phenyl rings with C3, C14, C25 and C36 and the porphyrin core are in the range 59.8 and 67.4 ; for the phenyl ring containing C45, the corresponding angle is 61.9 , and the pyridine ring with N6 is then twisted through 32.5 with respect to the plane of the phenyl spacer.The similar twist angles for the arene rings containing C36 and C45 permit the rings to engage in a p-stacking interaction although the 16.2 angle between their planes is not ideal; the centroid/centroid distance is 3.51 A. The tpy unit is virtually planar (angles between planes of adjacent pyridine rings are 2.7 and 6.2 ).The planarity is associated with a face-to-face p-interaction between centrosymmetric pairs of tpy units (Fig. 3a).The centrosymmetric pairing of the {Zn(TPP)} units (Fig. 3b) is typical and has been extensively discussed in the literature.7][38] The aromatic region of the [Ru(3)( 4)][PF 6 ] 2 is shown in Fig. 1b and its signature is consistent with the presence of two different tpy domains.COSY, NOESY, HMQC and HMBC methods were used to assign the signals in the 1 H and 13 C NMR spectra, although not all quaternary signals could be unambiguously ascribed (see Experimental section).A starting point for distinguishing between the two tpy ligands was assignment of the ipso-C atom of the arene ring attached to the phosphonate group; the resonance for C C4 00 (Scheme 2) was a doublet (J PC ¼ 189 Hz) at d 131.8 ppm.A comparison of Fig. 1a with Fig. 1b shows that formation of the {Ru(tpy) 2 } 2+ domain leads to the characteristic shi of the H A6 signal to lower frequency (d 8.76 ppm in 3 to d 7.47 ppm in [Ru(3)(4)][PF 6 ] 2 ); the signal for H A6 00 (d 7.53 ppm) appears close to that for H A6 , consistent with these protons lying over the ring current of the adjacent tpy ligand in the octahedral {Ru(tpy) 2 } 2+ unit.

Absorption spectra and spectroelectrochemistry
Upon formation of the ruthenium(II) complex [Ru(3)( 4)] [PF 6 ] 2 , the high-energy bands (below 350 nm) approximately doubled in intensity with respect to the absorptions in 3 (blue trace in Fig. 5), consistent with the presence of two tpy domains.The Soret band again decreases in intensity (3 ¼ 370 000 dm 3 mol À1 cm À1 ) but is little shied from the free ligand 3 (425 versus 427 nm), providing evidence for electronic communication between the porphyrin and tpy domains.Fig. 5 shows that there is also little difference in the Q bands comparing 3 with [Ru(3)( 4)][PF 6 ] 2 , (in the complex, 599 and 559 nm, 3 ¼ 28 000, 10 000 dm 3 mol À1 cm À1 , respectively).Conrmation of the presence of the {Ru(tpy) 2 } 2+ chromophore comes from the appearance of the broad band at 492 nm arising from the 1 MLCT absorption of this chromophore. 42e commence the spectroelectrochemical discussion by presenting the results obtained from a study of a CH 2 Cl 2 solution of compound 3. Fig. 6a shows a superimposition of the Fig. 4 Oxidative processes in the cyclic voltammograms of (a) 3 and (b) [Ru(3)( 4)][PF 6 ] 2 .For conditions, see Table 1.7 and 8.At the end of the oxidative cycle, the regeneration of the absorptions associated with the {Ru(tpy) 2 } 2+ domain (the MLCT band at 492 nm, and the bands at 284 and 310 arising from the phenyltpy p* ) p transitions) conrms the reversibility of these processes.In contrast, the oxidation of the [Zn(TPP)] moiety within the complex is irreversible, the processes mimicking those of compound 3 with the exception that the band at 650 nm is now transient.This may be due to over-oxidation of the porphyrin core.During the reductive cycle (Fig. 8), the absorptions arising from both the porphyrin and {Ru(tpy) 2 } 2+ domains are irreversibly transformed.The irreversible changes to the Soret and Q bands are consistent with those observed for 3 and [Zn(TPP)] 43 while irreversible reduction processes centred on the phenyltpy units are responsible for the loss of the bands associated with the p* ) p and MLCT transitions.Fig. 9b provides evidence for a transient band between 800 and 900 nm, which can be assigned to the [3]c À radical anion. 44u(3)( 4

)][PF 6 ] 2 as a dye in DSCs
Although phosphonic acid anchors 45 bind more strongly than phosphonate esters, 46 it is has been demonstrated that TiO 2 surfaces can be functionalized using phosphonate esters 47,48 with immobilization of the anchor taking place by hydrolysis of POR groups by surface-OH groups. 49We therefore investigated the use of [Ru(3)(4)] 2+ as a dye in DSCs.First, we conrmed that the dye bound to a TiO 2 surface.TiO 2 electrodes (without a scattering layer) were soaked in an MeCN solution of [Ru(3)( 4)] [PF 6 ] 2 for 3 days, and were then washed and dried.The electrode retained a red colour similar to that of reference electrodes with adsorbed N719.Compared to N719, the additional spectral response that the Soret band imparts to [Ru(3)(4)] 2+ is clear from the solid-state absorption spectra Fig. 9. Adsorbed  [Ru(3)(4)] 2+ exhibits l max at 432, 500, 564 and 641 nm.The Soret band at 432 nm is red-shied with respect to solution (425 nm) and absorptions at 500, 564 and 641 nm compare with bands in the solution spectrum (Fig. 5) at 492 nm (MLCT) and 560 and 600 nm (Q bands).
Photoanodes for n-type DSCs were made by immersion of FTO/TiO 2 electrodes in an MeCN solution of [Ru(3)( 4)][PF 6 ] 2 for 3 days, and reference electrodes were made similarly using an EtOH solution of N719.DSCs were fabricated using an I À /I 3 À electrolyte (see Experimental section) and were fully masked. 32,33he reproducibility of performance parameters (Table 2) was conrmed using duplicate DSCs for each dye.Despite the enhanced light absorption of [Ru(3)(4)] 2+ with respect to N719, the conversion efficiency (h) is poor; the main contributing factor is the extremely low short-circuit current density (J SC ) electron injection.The open-circuit voltage (V OC ) is about half that of N719.In order to understand the poor performance of [Ru(3)(4)] 2+ in DSCs, we have carried out a detailed investigation of the energy-transfer processes that follow excitation.

Emission properties
The solution emission behaviour of 3 and [Ru(3)( 4)][PF 6 ] 2 were investigated and compared to those of 1.As discussed earlier, in the absorption spectrum of the latter, bands arising from S 2 ) S 0 and S 1 ) S 0 transitions are observed.Normally, for an organic molecule, population of the S 2 excited state is followed by fast internal conversion to S 1 , 50 and the emission spectrum can be related to the radiative decay of the lowest excited state of same multiplicity.[Zn(TPP)] is emissive from both the S 2 and S 1 states, although the uorescence originating from the S 2 state has a much lower quantum yield and only a picosecond lifetime. 51Excitation of 1 at 400 nm (into the Soret shoulder) results in the emission spectrum shown in Fig. 10 with uorescence from both the S 2 (431 and 453 nm) and S 1 (607 and 659 nm) excited states.The assignments were conrmed from the excitation spectra.Note that l max of the Soret band (425 nm) is too  close to the l max em of 431 and 453 nm from the S 2 uorescence to observe these emissions using l exc ¼ 425 nm.Excitation into the Q bands of 1 leads to the S 1 emissions at 607 and 659 nm.
The emission spectrum of compound 3 does not display an S 2 uorescence.Excitation into either the Soret or Q bands leads to emission at 613 and 660 nm.Interestingly it is possible to detect porphyrin uorescence even upon exciting into the tpy bands (l exc ¼ 285 and 320 nm).Since the tpy absorption is well separated from the porphyrin absorption bands, this is a clear indication of intramolecular energy transfer.The energetics of the system are favourable for an energy transfer from the (p-p*) tpy excited states (upper lying levels) to the S 2 state (lower level), followed by internal conversion to S 1 and radiative decay.
Excitation spectra monitored at 560 and 600 nm conrm the presence of a broad peak centred at 285 nm, in agreement with the involvement of a tpy absorption in the population of the S 1 state.The ruthenium(II) complex [Ru(3)( 4)][PF 6 ] 2 exhibits an emission behaviour similar to that of 3. Excitation into the tpy absorption bands (l exc ¼ 284 and 310 nm) results in porphyrin uorescence (l max em ¼ 613 and 661 nm).and (c) Flamigni et al. 60 complex was recorded at room temperature.Upon excitation in the MLCT band (l exc ¼ 532 nm), the transient absorption spectrum obtained resembles the characteristic triplet-triplet spectrum of [Zn(TPP)], rst predicted by Gouterman 52 and later reported by Holten and coworkers. 53Gouterman predicted two possible transitions from the porphyrin T 1 state: an intense allowed transition that would result in a doubly excited conguration and consist of two absorption peaks to lower energy of the Soret band, and a weak, forbidden transition in the near infrared (IR) leading to a highly excited singlet conguration.In the transient absorption spectrum of [Ru(3)( 4)][PF 6 ] 2 (Fig. 12), the loss of the ground state porphyrin is clearly indicated by bleaching of the Soret band (425 nm) and of the Q(0, 1) and Q(0, 0) bands (560 and 600 nm).The broad bands at 470 and 500 nm are associated with absorption of the porphyrin T 1 state and creation of the doubly excited conguration.A broad absorption is present at lower energies, extending from 600 nm to the NIR, in agreement with the literature spectrum. 53he near-IR (NIR) transient absorption spectra of [Ru(3)( 4)] [PF 6 ] 2 are shown in Fig. 13.The band at 820 nm is associated with the forbidden, higher energy singlet which appears at 832 nm for [Zn(TPP)] 53 in CH 2 Cl 2 .For [Ru(3)( 4)][PF 6 ] 2 , the NIR absorption was detected only in deaerated solution.The recovery of the ground state was monitored at 385, 425 and 470 nm for the aerated solution and at 385, 425, 470 and 820 nm for the deaerated one.As expected for a triplet state, the lifetimes ranged from hundreds of nanoseconds for the aerated solution to tens of microseconds for the deaerated one, due to the exclusion of a non-radiative deactivation pathway through reaction with triplet O 2 .The decay curves are consistent with a mono-exponential decay in all cases; the lifetimes for ground state recovery are: s 385 ¼ 441 AE 44 ns and 59 AE 6 ms, s 425 ¼ 418 AE 42 ns and 52 AE 5 ms, s 470 ¼ 435 AE 44 ns and 48 AE 5 ms, s 820 ¼ 49 AE 5 ms.Since all the lifetimes are consistent within experimental error, it is reasonable to assume that all observed transitions originate from a single chemical species which we propose to be the porphyrin T 1 state.Furthermore, we conclude that upon MLCT excitation a triplet-to-triplet energy transfer occurs from the 3 MLCT level to T1, the latter being the lowest accessible level for [Ru(3)( 4)][PF 6 ] 2 .
The literature contains a number of molecular triads and dyads related to [Ru(3)( 4)][PF 6 ] 2 (Scheme 3). 54When the triad (Scheme 3a) reported by Benniston et al. 55 is excited in the MLCT band, the 3 MLCT emission is quenched in favour of a triplet-to-triplet energy transfer to the porphyrin T 1 state.The lifetime of T 1 was determined to be 65 AE 5 ms with a triplet-totriplet energy transfer rate constant of 8 Â 10 10 s À1 .A Dexter type mechanism of energy transfer 56 was proposed, and it is signicant that excitation into the Q band resulted in a decreased S 1 uorescence.Benniston et al. 55 rationalize this in terms of singlet (S 1 ) to triplet ( 3 MLCT) energy transfer (k ¼ 4 Â 10 8 s À1 ), involving an endergonic Dexter type mechanism (a process that is spin-forbidden).In the osmium-containing dyad in Scheme 3b, excitation in the Soret band leads to direct transfer to the 3 MLCT state 57,58 followed by triplet-totriplet energy transfer to T 1 .A high rate constant for porphyrin uorescence quenching again accounts for complete energy transfer.In oligopyridine complexes, the excited MLCT state is localized on one of the ligands, 59 and Benniston et al. argue that the energy ows from S 1 to the tpy domain directly connected to the porphyrin.In addition, if the second tpy in the complex lacks an extended p-system in the 4-position of the central pyridine ring, the electronic energy can be considered to reside on the porphyrin-bearing tpy because the intra-ligand energy transfer would not be as fast as that of energy transfer to the porphyrin triplet state.The overall effect is an intersystem crossing involving the porphyrin unit, involving the {Ru(tpy)} 2+ domain.For the triad reported by Flamigni et al. (Scheme 3c), 60 the energy transfer rate constant for the triplet-to-triplet transfer is >5 Â 10 10 s À1 , consistent with a fast and quantitative quenching of the ruthenium-containing manifold.Once again the uorescence originating from S 1 in quenched in favour of population of the 3 MLCT.Triplet-to-triplet energy transfer takes place with a rate constant >2 Â 10 10 s À1 (Fig. 14b), leading to the {Zn(TPP)} T 1 state.By deactivation of this state the ground state is recovered.

Fig. 6
Fig. 6 Spectroelectrochemical data for the oxidative cycle of 3 (z1 mM in CH 2 Cl 2 , [ n Bu 4 N][PF 6 ] supporting electrolyte).(a) Absorption spectra before (blue line) and after (red line) the oxidative cycle.(b) A spectrum was recorded every 0.1 V, starting from 0 V (first blue line at the front) to +1.8 V (last blue line) and back from +1.8 V (first red line) to 0 V (last red line).The potential is referenced with respect to the Fc/Fc + redox couple with the same cell under the same experimental conditions.

Fig. 7
Fig. 7 Spectroelectrochemical data for the oxidative cycle of [Ru(3)(4)] [PF 6 ] 2 (z1 mM in MeCN, [ n Bu 4 N][PF 6 ] supporting electrolyte).(a) Absorption spectra before (blue line) and after (red line) the oxidative cycle.(b) A spectrum was recorded every 0.1 V, starting from 0 V (first blue line at the front) to +1.5 V (last blue line) and back from +1.5 V (first red line) to 0 V (last red line).See caption to Fig. 6 for referencing to Fc/Fc + .

Fig. 8
Fig. 8 Spectroelectrochemical data for the reductive cycle of [Ru(3)(4)][PF 6 ] 2 (z1 mM in MeCN, [ n Bu 4 N][PF 6 ] supporting electrolyte).(a) Absorption spectra before (blue line) and after (red line) the reductive cycle.(b) A spectrum was recorded every 0.1 V, starting from 0 V (first blue line at the front) to À1.8 V (last blue line) and back from À1.8 V (first red line) to 0 V (last red line).See caption to Fig. 6 for referencing to Fc/Fc + .
It was not possible to detect S 2 uorescence by exciting into the shoulder of the Soret band.Transient absorption spectra of [Ru(3)(4)][PF 6 ] 2 In order to further probe the emission behaviour of [Ru(3)(4)][PF 6 ] 2 , the transient absorption spectrum of the