Rapid photoinduced charge injection into covalent polyoxometalate–bodipy conjugates

A series of redox tunable polyoxometalate–bodipy conjugates display variable charge transfer dynamics occuring down to 54 ps.


Introduction
The direct generation of chemical fuels from sunlight is a major scientic challenge for the development of a sustainable economy and production of hydrogen through water splitting. In green plants, photosynthesis operates in four steps: (1) light collection, (2) charge separation, (3) charge accumulation and (4) conversion to chemical fuel. Charge accumulation is a key step as most of the redox reactions for fuel generation such as H 2 production from water are multi-electronic processes. 1 While various noble-metal-based complexes (e.g. Pd, Pt.) have been widely employed as reduction catalysts in articial photosynthetic systems, polyoxometalates have recently emerged in this eld owing to their electron reservoir abilities and the activity of their reduced forms in the hydrogen evolution reaction. [2][3][4] POMs form a remarkable class of well-dened molecular nano-scale oxoclusters of the early transition metals with an unmatched diversity of structures and properties. 5 Their implementation in articial photosynthetic devices requires their association with a visible-range antenna since POMs only absorb in the UV region of the solar spectrum.
Owing to their anionic nature, POMs have mainly been associated with photosensitizer (PS) through electrostatic interactions. [6][7][8][9][10][11] By comparison, few covalent organic-inorganic POMbased hybrids have been developed. [12][13][14][15][16][17] While the covalent functionalization of POMs is synthetically more demanding, this approach enables ne control between the different subunits of the system, which is required for tuning the kinetics of photoinduced electron transfer between the excited chromophore and the POM. 3 We previously reported the synthesis and photophysical properties of different POM-PS conjugates. [18][19][20][21][22][23] Among them heteroleptic carbocyclometalated iridium(III)-POM dyads offered the most promising photophysical performance. 23 In these compounds, photoinduced charge-separated excited states of various lifetimes (ranging from nanoseconds to hundreds of nanoseconds, the longest values reported for covalently bonded photosensitized POMs) were observed by transient absorption spectroscopy. Furthermore, in the presence of a sacricial electron donor (triethylamine) and a proton source (acetic acid), the system is capable of photoaccumulating two electrons on the POM and produces hydrogen. 20 However, the POM-Ir(III) system suffered from the presence of a noble metal (Ir) and the low stability of the iridium complex in the presence of strong acid, which is necessary to improve the electron reservoir properties of the POM.
Furthermore, in order to incorporate the POM-PS hybrids into a photocathode, the functionalization of the PS by an appropriate anchoring group (carboxylic acid, phosphonate.) is a required step, which has been very scarcely developed with carbocyclometalated iridium(III) complexes. 24 In this context, bodipy uorophores have been identied as adequate candidates because of their tunable photophysical properties, high chemical stability, easy and versatile chemical modication such as the addition of a graing function. 25 Here, we present the rst examples of Keggin-type POMs that are covalently bound to one or two bodipy units via organic tethers of different lengths (Scheme 1). The series of three hybrids display distinct redox behaviour both on the POM and on the bodipy units. The photophysical properties of these hybrids are reported and discussed on the basis of the required kinetics that should be achieved for their implementation in a photoelectrocatalytic device.

Synthesis of the different POM-bodipy conjugates
We previously developed three Keggin-based polyoxometalate hybrid platforms bearing one or two remote iodo aryl functions. These hybrids vary according to the nature of the metal ion (polyoxotungstate vs. polyoxomolybdate) and the nature of the primary functionalization (silyl vs. tin). 21

Electrochemistry
The redox properties of the POM-bodipy dyads, their related POM hybrids and bodipy references were investigated by cyclic voltammetry and, in cases of irreversible redox processes, by differential pulse voltammetry in deoxygenated dichloromethane (DCM) with tetrabutylammonium hexa-uorophosphate (TBAPF 6 ) as the supporting electrolyte in a standard three-electrode cell, composed of a glassy carbon working electrode, a platinum counter electrode, and a saturated calomel reference electrode (SCE) ( Fig. 1 and Table 1).
First, it is noted that the redox potentials of the starting POM-based platforms are considerably shied in DCM compared to acetonitrile, 3 which outlines a very important effect of the organic solvent in the reduction processes of the , it can be seen that the latter is more difficult to reduce by more than half a volt due to a one-charge difference between both oxoclusters. BOD 1 -TMS and BOD 2 -TMS both feature reversible monoelectronic oxidation and reduction processes. While the oxidation potential for each are rather similar, reduction of BOD 2 -TMS occurs at a signicantly more positive potential than reduction of BOD 1 -TMS (DE red ¼ 300 mV). Indeed, in BOD 1 -TMS the phenyl unit is twisted for steric reasons, which decouples it from the p system of the bodipy unit, 29 while in BOD 2 -TMS, the p system of the bodipy unit extends over the alkynyl moieties, favouring its reduction. displays two quasi-reversible processes followed by an irreversible process. The rst one is attributed of the reduction of the bodipy unit while the two others correspond to monoelectronic reductions of the POM framework. In the reduction part, K Sn Mo [BOD 2 ] displays two apparent quasi-reversible processes. The rst one at À0.57 V vs. SCE is attributed to the reduction of the polyoxomolybdate framework, while the second at À1.04 V vs. SCE is attributed to quasi simultaneous reduction of the bodipy and the one-electron reduced POM (note that in differential pulse voltammetry experiment these two reduction processes are slightly separated with a maximum at À1.04 V and a shoulder at ca. À1.00 V vs. SCE attributed to the one-electron reduced POM and bodipy reduction respectively, Fig. S14

Electronic absorption and photophysical properties
The electronic absorption spectra for bodipy reference compounds and POM-bodipy hybrids dissolved in DCM are shown in Fig   a The standard potentials of irreversible processes were determined by differential pulse voltammetry experiments (Fig. S7-S14).  , the absorption prole is red-shied due to an increase of the p system resulting from the presence of the alkynyl group at the meso-position. 60% relative to BOD 2 -TMS, suggesting charge-transfer (if operating) from bodipy to POM is less efficient (note that the presence of the aryl tin unit that participates into the p system of the bodipy unit slightly modies its electronic properties and may account for the decrease in quantum yield of K Sn W [BOD 2 ]).
To probe the fate of the bodipy excited state in these hybrids, transient optical and infrared absorption spectroscopy was performed on DCM solutions of the hybrids. The results are summarised in Table 3. To extract the intermediate, global analysis of the spectra was performed using the program OPT-MUS, in which the transients at all detection wavelengths are analysed simultaneously with a single set of exponential functions. 31 The transient absorption spectra of K Si W [BOD 1 ] at various time delays aer excitation at 540 nm are shown in Fig. 3. At short delay times, a bleach centered at 520 nm forms, consistent with the depletion of the bodipy ground state. The rst transient species resembles the excited bodipy (spectra for BOD 1 -TMS and BOD 2 -TMS are provided in the ESI for comparison, Fig. S16 and 17 †) and decays with s 1 ¼ 54 ps, as a second transient species with a sharp absorption at 400 nm grows in over the same timescale and decays with s 2 ¼ 4.8 ns. The absorption prole of this second transient is consistent with the oxidised bodipy generated by spectroelectrochemical methods (Fig. S15 †). Furthermore, this transient also features a broad absorption starting at 600 nm and extending past 700 nm, which is characteristic of a reduced polyoxometalate, and is hence assigned to the charge-separated state, [BOD + -POM(+1 e À )].
The transient spectra for K Sn Mo [BOD 2 ] in DCM were more complex since a three component model was needed to accurately t the relaxation dynamics of this hybrid. The rst component (s 1 ¼ 180 ps) in the evolution-associated difference spectra (EAS) has the same general shape as the BOD 2 -TMS excited state (Fig. S17 †), and contains a negative signal from the stimulated emission. The second component (s 2 ¼ 520 ps) is red-shied (by 6 nm) relative to the rst one and does not contain a contribution from the stimulated emission. We have assigned this to the charge-separated state, [BOD + -POM(+1 e À )] since the electron transfer from the BOD 2 to the polyoxomolybdate core of K Sn

Mo
[BOD 2 ] is thermodynamically favourable and the absorption prole of this second transient is consistent with the oxidised bodipy (Fig. S15 †) (Fig. 4).
This second species decays to form a long-lived component (s 3 ¼ 495 ns), attributed to a bodipy triplet excited state. We have previously observed this behaviour for a triphenylaminebodipy conjugate, arising from photoinduced chargeseparation followed by recombination to the triplet state. 32 The transient absorption spectra of K Sn W [BOD 2 ] in DCM were much simpler (Fig. 5) Table 3 Driving force for charge-separation (DG CS ) and recombination (DG rec ) for POM-bodipy hybrids. DG CS ¼ E(BOD*/BOD + ) À E(POM/POM À ), DG rec ¼ E(POM/POM À ) À E(BOD + /BOD). (BOD*/ BOD + ) ¼ E(D/D + ) À E 0-0 (Note that the work terms for electrostatic interactions are neglected since they are estimated to be below 0.1 eV)  To compare our results with those we obtained previously for all-organic bodipy systems, 32,33 we also performed time-resolved infrared spectroscopy (Fig. S22 †). The dynamics agree with those recorded in the visible region. In the spectra for K Sn -Mo [BOD 2 ] a sharp, long lived band at 1530 cm À1 is present. The shape and the lifetime of this band are in agreement with our previous work and supports our assignment of the long-lived species as the bodipy triplet excited state. 33 The fact that the charge-separated state recombines via a bodipy-centered triplet state in K Sn Mo [BOD 2 ] but directly to the ground state in K Si -W [BOD 1 ] is probably due to the slightly higher energy of the CS state (Fig. 6) 65 eV) in a molecular dyad system incorporating a structurally similar bodipy to BOD 1 appended to zinc terpyridine. 34 Assuming that the triplet energy is similar, this places it 150-200 meV higher than the charge-separated state in K Si W [BOD 1 ].
As the POM and the bodipy are considerably electronically decoupled, it is possible to separate processes centered on each component of the POM-bodipy hybrids and determine the driving forces for charge-separation and recombination, as presented in Table 3.
For [BOD 2 ] the p system of the bodipy unit extends over the aryl tin ring i.e. at the vicinity of the polyoxomolybdate core. According to Marcus theory, the rate constant for an electron transfer process of a supramolecular system in the non-adiabatic limit (i.e. when its different elements are poorly electronically coupled) can be expressed by the following equation: 35,36 where n is an electronic factor that is proportional to the overlap between the electronic wavefunctions of the donor and acceptor units.    sensitized NiO-based photocathodes are limited by the fast (<ns) rate of charge recombination at the oxide-dye interface. 37,38 The tune-ability of the electronic properties of the photosensitizer and POM subunits to control the chargetransfer dynamics is, therefore, extremely attractive for implementing photosensitized POM-based hybrids in molecular photocathodes.

Conclusions
New POM-bodipy conjugates were synthesized through postfunctionalization of organosilyl and organotin POM derivatives. An advantage of these systems over e.g. dye-sensitized TiO 2 , is that the electronic properties of both the donor and acceptor can be specically tuned. In these photoactive hybrids the redox properties of the POM, the bodipy and the spacer length were modied in order to evaluate the effect of these parameters on the kinetics of photoinduced electron transfer. The transient absorption spectroscopy unequivocally indicates the occurrence of photoinduced electron transfer from the bodipy to the POM for hybrids displaying the best electron accepting properties, with kinetics up to ca. 2 Â 10 10 s À1 , constituting the rst example of charge-separated state on noble metal-free covalent POM-PS conjugates. While POMs are drawing a growing attention in the eld of articial photosynthesis and molecular electronics, fundamental insights on their kinetics of intramolecular electron injection into the POM unit within POM-PS conjugates are scare. 18,23,39,40 For instance the effects of the solvent and their associated counter-ions on their reorganization energy remain unexplored, the only experimental studies being yet limited to outer-sphere chemical and electrochemical reduction of POMs. 41,42 The present system should allow lling this shortfall owing to their modular design. The long lifetime of the charge separated state is also an exciting prospect for integrating these systems into photocathodes, since electron-hopping between POM units or chargetransfer to a substrate or catalyst would be competitive with recombination.

Conflicts of interest
There are no conicts to declare.