Electrochemically‐Switched 2nd Order Non‐Linear Optical Response in an Arylimido‐Polyoxometalate with High Contrast and Cyclability

Abstract Electrochemically switched 2nd order non‐linear optical responses have been demonstrated for the first time in polyoxometalates (POMs), with an arylimido‐derivative showing a leading combination of high on/off contrast (94 %), high visible transparency, and cyclability. Spectro‐electrochemical and TD‐DFT studies indicate that the switch‐off results from weakened charge transfer (CT) character of the electronic transitions in the reduced state. This represents the first study of an imido‐POM reduced state, and demonstrates the potential of POM hybrids as electrochemically activated molecular switches.

Since their first observation in the 1960s, [1] non-linear optical (NLO) effects have become essential to current and future technologies involving generation and manipulation of laser light: for example telecommunications, optical and electro-optical computing, optical data storage and biological imaging. [2] For advanced applications in telecommunications and computing, the fast 2 nd order NLO responses of molecular charge transfer chromophores promise advantages over traditional inorganic salts, and have led to development of many organic and organometallic NLO chromophores. [2][3][4] Molecular 2 nd order NLO performance is assessed by measurement of the first hyperpolarizability b, with static (zero-frequency) values b 0 used to facilitate comparison. Yet, while materials with extremely high b 0 values (@ 10 À 27 esu) have been obtained, [5] only dimethylaminostilbazolium tosylate (DAST), based on the DAS + cation (b 0 = 25 × 10 À 30 esu), [3a,b] is in current technological use.
Two key challenges for molecular NLO materials are (i) developing chromophores that combine high activity with adequate transparency, [6] and (ii) producing materials whose properties can be switched. [7][8][9] Regarding (i), structural modifications that increase b usually involve stronger electron donor/acceptor pairs or extended π-systems that increase conjugation-these lower HOMO-LUMO gaps and thus tend to increase absorption of visible and nearinfrared light, leading to lower device efficiency and reduced photostability. For (ii), most molecular NLO switches have relied on redox reagents or other chemical stimuli (pH) which are impractical for application in devices, [7] or photoisomerization [8] which due to structural change is slower and more vulnerable to side reactions than electron transfer. Electrochemical oxidation of a donor or reduction of an acceptor potentially provides a rapid and device compatible means of switching NLO responses by turning on and off charge transfer transitions, [9] with less risk of degradation. However, the few existing examples are all based on materials with strong low energy metal-to-ligand charge transfer transitions (MLCT), whose activity is turned off by oxidizing the metal center. Reabsorption of second harmonic light by such MLCT transitions is intrinsically problematic for eventual construction of stable, efficient molecule-based NLO switches.
Herein, we describe a new strategy based on electrochemical switching of higher energy ligand-to-polyoxometalate CT (LPCT) transitions. Polyoxometalates (POMs) are well known for fast, stable redox chemistry, as they accept electrons with almost no structural change. [10] Moreover, owing to push-pull CT effects, we previously demonstrated that NLO chromophores based on arylimido-derivatized Lindqvist POM acceptors ([Mo 6 O 18 NAr] 2À ) show high static first hyperpolarizabilities b 0 , combined with high transparency, giving better transparency/non-linearity trade-offs than typical purely organic systems. [11] Here, introducing bulky groups (iPr) around the Mo=N bond has enabled us to stabilize and characterise the [Mo 6 O 18 NAr] 3À state for the first time, revealing the strongest overall performance of any switchable NLO chromophore to date: β 0 , zzz = 89 × 10 À 30 esu (by hyper-Rayleigh Scattering, HRS), high transparency beyond 500 nm, 94 % on/off contrast, and the best cyclability yet demonstrated-4 complete cycles in electrochemistry-HRS, 16 by linear spectroelectrochemistry, with minimal degradation.
Characterization of [NBu 4 ] 2 [1] by cyclic voltammetry (CV, Figure S2) finds a [1] 2À /3À reduction peak with near ideal reversibility (Table 1), negatively shifted by ca. 60 mV from that previously reported for [1H] 2À /3À . [11b] This indicates a more electron rich POM cluster, resulting from inductive donation from the i Pr groups ortho to the imido group. A second, irreversible reduction is observed at À 1.92 V vs Fc/ Fc + , while quasi-reversible oxidation of the amine was seen at 0.30 V vs Fc/Fc + . UV/Vis spectroscopy reveals a ligandto-POM (LPCT) charge transfer peak at 431 nm ( Figure 3, Table 1), slightly redshifted from [1H] 2À (424 nm). This is ascribed to inductive donation from the i Pr groups raising the level of the HOMO, which DFT calculations (ωB97X-D/ 6-311G(d)/LanL2TZ, MeCN solvation by IEFPCM) indicate is spread across the entire arylimido moiety (Figure 4a), more than the POM based LUMO. TD-DFT calculations reproduce the LPCT peak well, although the (vertical excitation) energy is over-estimated slightly (0.3 eV) vs experiment ( Figure S4). The LPCT character of the lowestenergy band is stronger for the oxidised than the reduced form, as shown by the excitation-induced electron density differences ( Figure S5). Subsequently, the change of dipole moment upon excitation is slightly smaller for the reduced form than the oxidised one (8.0 versus 9.6 D).
To probe the stability of the [1] 3À reduced state, and its suitability for redox-switched HRS, we investigated the first reduction of [1] 2À by bulk electrolysis. Investigation of unprotected derivatives such as [1H] 2À in similar conditions has indicated solvolysis of the reduced state leads to loss of ligand and conversion of � 80 % of the derivative into [Mo 6 O 19 ] 2À /3À . [17] However, for sterically protected [NBu 4 ] 2 - [1], CV peak currents measured after 20 minutes reduction at À 1.2 V vs Fc/Fc + indicate minimal loss of the imido compound ( Figure S3). Subsequent spectroelectrochemical investigation showed that upon reduction the low energy λ max blue shifts 33 nm (0.24 eV), to 398 nm, and loses intensity ( Figure 3) as the POM acceptor weakens-DFT calculated LUMOs are ca. 0.8 eV higher in energy for the reduced state, consistent with the electrochemically observed spacing of ca. 0.8 V between first and second   Figure S7). This is consistent with DFT calculations showing a SOMO (slightly stabilized over the α and β HOMOs) based predominantly on the Mo opposite the imido group (Figure 4b), and is logical in showing the electron localizing away from the imido-donor.
A specially designed electrochemical cell ( Figure S8, SI) enabled coupled electrochemistry/HRS measurements, which showed a near 94 % decrease in second harmonic signal upon reduction of [1] 2À , and a b 0;zzz of only 7 × 10 À 30 esu for the [1] 3À reduced state-a very high contrast switching effect (Figures 5, S9). Reoxidation recovered 90 % of the initial signal, which was retained over a further three off/on cycles. The smaller switch-off in later cycles is likely a result of potential drift, as chronoamperometry data reveal less charge was passed and so less complete (ca. 92 %) reduction was achieved. The dispersion corrected TD-DFT computed dynamic b 1064;HRS value, of 149 × 10 À 30 esu for [1] 2À , is a good match for experiment, the calculated reduced state b is a large overestimate, but does reveal an attenuation consistent with the experimental trend. Stability over more cycles has been demonstrated by spectroelectrochemistry, (Figure S10) which shows 95 % retention of the absorbance    at λ max after 16 cycles. Almost all of the decline occurs in the first 8 cycles, with the absorbance remaining almost constant afterwards. The improved stability in later scans may result from consumption of adventitious water in the first cycles, presence of decomposition products shifting equilibria in favour of [1] 2À /3À , and also reflects establishment of equilibria between [1] 2À and [1] 3À after the initial cycles, which may not be completely overcome electrochemically. In any case, the results show that [1] 2À is an electrochemically addressable, molecular 2 nd order NLO switch with very high contrast and leading multi-cycle stability. [18] In conclusion, we have shown for the first time that arylimido-Lindqvist reduced states can be effectively stabilized by placing sterically bulky groups around the Mo=N bond. Although the reduced state SOMO is located away from the imido group, reduction dramatically weakens CT to the POM, producing a reversible 94 % switch-off in 2 nd order NLO activity. Combined with leading cyclability and transparency, this gives [1] 2À the best overall performance of any molecular NLO switch to date. Future work will focus on translating these results into thin films, [9b] to produce bulk materials capable of electrochemical-NLO switching. More broadly, the work highlights the strong potential of imido-POM hybrids as molecular switches, which we are continuing to explore in new structures.