Chromophoric Dyads for the Light-Driven Generation of Hydrogen

Article pubs.acs.org/IC

Chromophoric Dyads for the Light-Driven Generation of Hydrogen: Investigation of Factors in the Design of Multicomponent Photosensitizers for Proton Reduction Po-Yu Ho,†,‡,§ Bo Zheng,† Daniel Mark,† Wai-Yeung Wong,*,‡,§ David W. McCamant,*,† and Richard Eisenberg*,† †

Department of Chemistry, University of Rochester, Rochester, New York 14627, United States Institute of Molecular Functional Materials, Department of Chemistry, and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, P. R. China § HKBU Institute of Research and Continuing Education, Shenzhen Virtual University Park, Shenzhen 518057, P. R. China ‡

S Supporting Information *

ABSTRACT: Two new dyads have been synthesized and studied as photosensitizers for the light-driven generation of H2 from aqueous protons. One of the dyads, Dy-1, consists of a strongly absorbing Bodipy (dipyrromethene-BF2) dye and a platinum diimine benzenedithiolate (bdt) charge transfer (CT) chromophore, denoted as PtN2S2. The two components are connected through an amide linkage on the bdt side of the PtN2S2 complex. The second dyad, Dy-2, contains a diketopyrrolopyrrole dye that is linked directly to the acetylide ligands of a Pt diimine bis(arylacetylide) CT chromophore. The two dyads, as well as the Pt diimine bis(arylacetylide) CT chromophore, were attached to platinized TiO2 via phosphonate groups on the diimine through sonication of the corresponding esters, and each system was examined for photosensitizer effectiveness in photochemical generation of H2 from aqueous protons and electrons supplied by ascorbic acid. Of the three photosensitizers, Dy-1 is the most active under 530 nm radiation with an initial turnover frequency of 260 h−1 and a total of 6770 turnovers over 60 h of irradiation. When a “white” LED light source is used, samples with Dy-2 and the Pt diimine bis(arylacetylide) chromophore, while not as effective as Dy-1, perform relatively better. A key conclusion is that the presence of a strongly absorbing organic dye increases dyad photosensitizer effectiveness only if the energy of the CT excited state lies below that of the organic dye’s lowest excited state; if not, the organic dye does not improve the effectiveness of the CT chromophore for promoting electron transfer and the light-driven generation of H2. The nature of the spacer between the organic dye and the charge transfer chromophore also plays a role in the effectiveness of using dyads to improve light-driven energy-storing reactions.

■

INTRODUCTION In the quest for abundant, sustainable carbon-free energy, the light-driven splitting of water into hydrogen and oxygen remains a most desirable target. This reaction represents the key energy-storing reaction of artificial photosynthesis (AP) in which solar photons are converted into energy stored in chemical bonds.1 As a redox reaction, water splitting can be analyzed in terms of its two half-reactions, with the cathodic half-reaction corresponding to the reduction of aqueous protons to H2 and the anodic half-reaction of water oxidation to O2.2 Each of these two half-reactions in AP may be considered as photochemical systems in parallel with what occurs in natural photosynthesis as embodied in its well-known Z-scheme and associated energetics. This paper describes efforts that concentrate on the light-driven generation of H2 which, in its simplest form, requires a light absorber or photosensitizer (PS) for electron−hole creation, a pathway for © XXXX American Chemical Society

electron−hole separation, a catalyst for collecting protons and electrons to form H2, and sources of protons (ideally from water) and electrons (often in the form of an electron donor but possibly supplied electrochemically). While studies of light-driven proton reduction in molecular systems date back more than three decades,3 recent progress has been notable, especially in terms of the design and development of new light absorbers and catalysts, and mechanistic understanding of each step of the process.2h,4 The most frequently employed photosensitizers for the lightdriven generation of H2 in molecular systems have been complexes of Ru(II) with bipyridine (bpy), terpyridine (terpy), and/or related heterocyclic ligands that possess a long-lived triplet metal-to-ligand charge transfer state (3MLCT).2c,k,5 Received: March 8, 2016

A

DOI: 10.1021/acs.inorgchem.6b00496 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Recent studies have also included analogous Ir(III) d6 systems containing phenylpyridine (ppy) ligands in place of bpy6 and Pt(II) d8 complexes that contain an anionic ligand in addition to bpy or terpy. These complexes also possess long-lived 3 MLCT states that form rapidly upon light absorption into the singlet absorbing state (1MLCT) and subsequent intersystem crossing (ISC), from which electron transfer can occur.7 Despite the success of these complexes with photoinduced electron transfer, the energies of their respective 1MLCT states are generally too large (>2.6 eV, >21000 cm−1, 450 nm.9 While these complexes were photostable, the rate of H2 evolution was disappointingly slow. One reason for this result, as well as a common feature of all metal complex photosensitizers with a 3MLCT state, is that the molar absorptivity of the corresponding singlet CT absorption is too low for efficient photon capture. Typically, these CT complexes have low energy CT absorptions with molar absorptivity constants ε of 7−15 × 103 M−1 cm−1 (7−15 × 102 m2/mol in SI units) whereas organic dyes possess visible absorption bands with ε an order of magnitude or more higher. In this regard, more strongly absorbing organic dyes with ε of ∼105 M−1 cm−1 were also examined during early studies of the light-driven generation of H2. While those dyes with heavier halogen substituents such as Rose Bengal and Eosin Y were found to promote H2 formation because of facile ISC to longlived 3ππ* states for bimolecular electron transfer, they also exhibited poor photostability, decomposing within 3−5 h.10 A hybrid approach that would take advantage of the electron transfer properties of metal−ligand charge transfer chromophores and the more intense absorption maxima of organic dyes has been undertaken over the past decade. In 2006, Ziessel and co-workers described the synthesis of a Bodipy dye linked directly to a bipyridine ligand that was then coordinated to a Ru(II) center. However, the relative energies of the excited states of the two linked chromophores led to sensitization of the Bodipy 3ππ* state of the dyad.11 In a separate study, Lazarides et al.12 synthesized two Bodipy-CT dyads that contained the PtN2S2 chromophore, one in which the dye was attached directly to the bipyridine ligand (1) in the manner reported by Ziessel, and the other in which Bodipy was linked directly to the dithiolate ligand (2 with R = H) (Figure 1). Transient absorption studies of both dyads were conducted, and the processes involved are illustrated in Scheme 1. From

Figure 1. Previously reported PtN2S2-based dyads.

Scheme 1. Processes Involved upon Excitation of Dyads 1 and 2a

a

SEnT = singlet energy transfer, ISC = intersystem crossing, TEnT = triplet energy transfer, and eT = electron transfer.

the relative rates of triplet energy transfer (TEnT) for 1 and 2, it was determined that the arrangement in 2 with Bodipy directly bound to the dithiolate would be more productive for photoinjection of an electron into TiO2 if the dyad was bound to the semiconductor. On the basis of this conclusion, Zheng et al. prepared such a dyad (2 with R = P(O)(OEt)2) and, following its attachment to platinized TiO2, examined its activity based on increased absorptivity and subsequent electron transfer for the light-driven generation of H2.13 In the present paper, we describe two other dyads to probe different aspects of this hybrid approach. One involves a different linkage between the dye and the PtN2S2 chromophore that has the potential to be more versatile in making related dyads with other dyes, and the second has a different dye (i.e., DPP (diketopyrrolopyrrole)) directly linked to a Pt diimine bis(acetylide) chromophore known to undergo electron transfer quenching. The results indicate a number of factors that must be considered in undertaking the dye−CT chromophore approach to molecular-based systems for hydrogen generation using visible light and aqueous media.

■

RESULTS AND DISCUSSION Synthesis and Characterization. The syntheses for the two dyads described in this report, Dy-1 and Dy-2, are shown in Schemes 2 and 3, respectively. In the synthesis of Dy-1, the precursor of the dithiolate ligand linked to the Bodipy moiety via an amide bond was prepared using a mixed anhydride reaction.14 This compound was then reacted with KOtBu (2 mol equiv) in order to deprotect the dithiolate ligand in situ, after which it was reacted with Pt(4,4′-(P(O)(OEt)2)bpy)Cl2 to obtain Dy-1 (ca. 40% yield). For convenience, the ligand 4,4′-(P(O)(OEt)2)bpy will be represented in the rest of the paper as “bpyP2”. The previously reported Pt(bpyP2)Cl2 B


Article

Inorganic Chemistry Scheme 2. Synthetic Route for Dyad Dy-1

Scheme 3. Synthetic Route for Dyad Dy-2 and Pt(bpyP2)(CCPh)2

silica gel, and characterized by 1H NMR spectroscopy (Figures S4−S6), mass spectrometry, and elemental analyses. UV−Vis Absorption Studies. The UV−vis absorption spectra of the dyads as well as the complex Pt(bpyP2)(CCPh)2 and the dye DPP (2,5-dioctyl-3,6-di(thiophen-2-yl)pyrrolo[3,4c]pyrrole-1,4(2H,5H)-dione; Scheme 3) in dichloromethane at ambient temperature are summarized in Figure 2 and Table 1. The absorption spectrum of Dy-1 is basically the sums of the constituent chromophores (Figures S7 and S8), including bands in the 300−450 nm region corresponding to ligandbased transitions, a sharp band at 524 nm that is the Bodipy S0 → S1 (1ππ*) transition, and a broad peak at 610 nm that is the mixed metal/ligand-to-ligand′ charge transfer (MMLL′CT) of the PtN2S2 moiety.16 This last absorption feature is seen in the

complex was prepared by the reaction of cis-Pt(DMSO)2Cl2 with the corresponding bipyridine derivative.12 In the synthesis of Dy-2, the diketopyrrolopyrrole dye was derivatized by bromination and converted into a TMS-protected ethynyl ligand by Sonogashira coupling,15 and then, following deprotection, was reacted with Pt(bpyP2)Cl2 to obtain Dy-2 using CuI-catalyzed metathesis. The metathesis reaction gave a >80% product yield. For subsequent comparisons with Dy-2 in terms of spectroscopy and photochemical H2 generation, the complex Pt(bpyP2)(CCPh)2 was also prepared using CuI catalysis of the reaction of a Pt dichloride complex with phenylacetylene (Scheme 3). The dyads Dy-1 and Dy-2, as well as Pt(bpyP2)(CCPh)2, were purified by chromatography on C


Article

Inorganic Chemistry

Figure 2. UV−vis absorption spectra of photosensitizers Dy-1 and Dy2, as well as the complex Pt(bpyP2)(CCPh)2 and the dye DPP labeled in Scheme 3, in dichloromethane at ambient temperature.

Table 1. Absorption Maxima for Dy-1, Dy-2, Pt(bpyP2)(CCPh)2, and DPP in Dichloromethane at Ambient Temperature photosensitizer Dy-1 Pt(bpyP2)(CCPh)2 Dy-2 DPP

absorption λmax, nm (molar absorptivity ε, M−1 cm−1) 318 (30171), 524 300 (37995), 438 304 (51818), 397 339 (12819), 354 549 (32333)

(59666), 610 (6232) (9104) (24122), 584 (65689) (11596), 511 (26569),

spectrum of Pt(bdt)(bpyP2) in acetonitrile reported previously.13 For the complex Pt(bpyP2)(CCPh)2, the higher energy bands (λ < 350 nm) with molar absorptivities around 40000 M−1 cm−1 correspond to diimine- and arylacetylide-based intraligand transitions, whereas the lower energy band (ca. 440 nm) that is observable until 550 nm is the 1MLCT excitation from a filled Pt d orbital to a vacant π*diimine as previously assigned.17 In contrast to Pt(bpyP2)(CCPh)2, dyad Dy-2 with two diketopyrrolopyrrole-based aryl acetylide ligands gives an intense peak at 584 nm with ε ∼70000 M−1 cm−1, a shoulder at ∼555 nm, and an absorption onset near 700 nm. By comparison of this absorption with that of the alkylated DPP, the intense band at 584 nm is assigned as the HOMO → LUMO transition of the thienyl-DPP chromophore, which has a significantly enhanced absorption coefficient and extended conjugation, both of which are generated by the coordination with Pt.18 The substantially lower energy of this strong absorption relative to that of DPP may be explained by the more negative character of the acetylide ligand and its consequent effect on the energy of the highest filled thienyl orbitals.18 Dyad Dy-2 also exhibits higher energy bands having λ < 400 nm that arise from diimine- and acetylide-based intraligand transitions, as well as the π → π* transition of the DPP-based chromophore.19 Electrochemical Characterization. Photosensitizers Dy-1 and Dy-2 as well as complex Pt(bpyP2)(CCPh)2 were investigated by cyclic voltammetry (CV), and their cyclic voltammograms and electrochemical data are shown in Figure 3 and Table 2, respectively. All potentials are reported versus SCE. For those irreversible peaks, the peak potential is given. The photosensitizer Dy-1 exhibits two reversible reduction processes with peaks at −0.97 and −1.35 V, respectively, that resemble similar reductions in the CV of compound 2 (R =

Figure 3. Cyclic voltammograms of (a) Dy-1, (b) Pt(bpyP2)(CCPh)2, and (c) Dy-2 in dry dichloromethane using 0.1 M tetrabutylammonium hexafluorophosphate as supporting electrolyte with a scan rate of 100 mV/s.

Table 2. Summary of Oxidation and Reduction Potentials from Cyclic Voltammogramsa E1/2, V vs SCE photosensitizer

oxidation

reduction

Dy-1 Pt(bpyP2)(CCPh)2 Dy-2

0.76 (irrev); 1.00 0.84 (irrev) 0.82 (irrev)

−0.97; −1.35 −0.99 −0.89; −1.24

a

Redox potentials were measured by cyclic voltammetry in dry dichloromethane containing 0.1 M of tetrabutylammonium hexafluorophosphate as a supporting electrolyte and a photosensitizer concentration of ∼1 × 10−3 M. A saturated calomel electrode (SCE) was used as reference. Under these conditions, the reversible oxidation of ferrocene was E1/2 = 0.43 V. All solutions were deaerated prior to measurement.

P(O)(OEt)2) within the same range.13 The first reduction wave results from the reversible PtN2S2 moiety reduction whereas the more cathodic reduction wave is attributed to the reversible Bodipy-based reduction.11,20 In addition, Dy-1 possesses one irreversible and one reversible oxidation peaks at 0.76 and 1.00 V (within the anodic boundary of 1.5 V). The two oxidations in the CV are similar to two oxidations of the parent compound 2 (R = P(O)(OEt)2). The first oxidation wave corresponds to irreversible PtN2S2 oxidation8e,12,13 while the more anodic oxidation wave comes from the reversible Bodipy oxidation with the formation of its radical cation.11,20 With regard to Pt(bpyP2)(CCPh)2, one reversible reductive peak occurs at −0.99 V, and one irreversible oxidative peak is seen at 0.84 V. D


Article

Inorganic Chemistry

yielded energy transfer time constants on the order of 0.24−5.8 ps.12,23 The longer FRET time constant for Dy-1 is attributed to the longer distance between the Bodipy and bdt than that in compound 2 with R = P(O)(OEt)2.23 Importantly, when Dy-1 is excited with a 615 nm pump pulse, the system shows no bleaching of the Bodipy absorption band. Excitation of Dy-1 with 615 nm light leads directly to the 1MMLL′CT state, after which it undergoes intersystem crossing to the 3MMLL′CT state without further triplet energy transfer to Bodipy’s 3ππ* state. As described previously, the lack of triplet energy transfer back to Bodipy indicates that the 3MMLL′CT state lies below that of Bodipy’s 3ππ* state because of the electron withdrawing phosphonate substituents that lower the energy of the bpy π* orbital.23 In order to probe the feasibility of reductive quenching of the Dy-1* excited states by ascorbic acid, as may be the case in actual hydrogen production conditions, additional transient absorption experiments were performed with Dy-1 in the presence of 0.5 M ascorbic acid. Dimethyl sulfoxide (DMSO) was used to allow solubility of both Dy-1 and ascorbic acid. In these experiments, the transient absorption kinetics and spectra matched those in DCM without ascorbic acid and did not vary whether or not the ascorbic acid was present. This indicated that none of the singlet or triplet states of Dy-1 are reductively quenched by ascorbic acid before 2 ns, the longest time probed. The dynamics of Dy-2 and the dye DPP were also investigated with TA spectroscopy. The spectra of DPP are shown in Figure S9 and exhibit a ground state bleach (GSB) from 460 to 565 nm and a broad excited state absorption (ESA) from 650 to 750 nm. Both the ESA and GSB decay occur with the same time constants, indicating relaxation back to the DPP ground state in ∼7 ns (Figure 5b), which is consistent with other reported measurements.24 Figure 5 shows the TA spectra of Dy-2 and compares the kinetics of Dy-2 to those of DPP. Similar to DPP, a GSB is observed between 485 and 615 nm for Dy-2, and a broad ESA is observed at λ > 615 nm. Figure 5b displays the differences in DPP kinetics when the dye is free and attached to the Pt bpy bis(acetylide) moiety. In contrast to the DPP’s long-lived excited state, the dynamics of the excited state of the dye when attached as in Dy-2 are dramatically shortened. The kinetics of Dy-2’s GSB and ESA were fit well with a rapid loss of intensity in 2−4 ps. As the bleach recovers, though, it acquires narrower features than those that appear in the UV−vis spectrum. After the initial 2−4 ps process is completed, no change is observed in the TA signal for the subsequent 1500 ps, indicating that the species formed in 2−4 ps is much longer lived than a simple monomeric DPP singlet excitation. We assign the rapid dynamics in Dy-2 to ISC to the DPP triplet state occurring with a time constant of 2−4 ps. This ISC is much faster than the ∼135 ps formation time of T1 in the DPP−Pt(acac) system from Goswami and the ∼300 ps ISC in Pt(Bodipy-bpy)Cl2 from Lazarides (the Bodipy-bpy ligand is shown in 1).12,18 Calculations have shown that there is significant electron density on the Pt atom in the HOMO and LUMO of similar DPP-containing Pt bis(arylacetlyide) species, that will greatly enhance the rate of ISC in Dy-2 relative to previously studied systems.18,25 The kinetics of Dy-2 in Figure 5b clearly exclude the possibility of energy or electron transfer from the DPP to the Pt(bpy) species, which would be necessary for sensitization of the semiconductor in systems with Dy-2 bound to TiO2. Instead, the triplet state of DPP is the terminal excited state

The reduction wave indicates the addition of an electron to an orbital on the bipyridine ligand21 while the oxidation wave corresponds to oxidation of a phenyl ring to form a radical cation.22 In contrast, Dy-2 possesses two reversible reductive peaks at −0.89 and −1.24 V, and one irreversible oxidative peak at 0.82 V in the CV. The oxidative wave can be assigned to the formation of the radical cation of thienyl DPP-based moiety based upon a comparison of the cyclic voltammogram of TDPP-Hex (2,5-dihexyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) in the literature.19 The first reductive peak for Dy-2 corresponds to the addition of an electron to a bpy π* orbital while the second reduction wave is assigned to radical anion formation in the thienyl DPP-based part of the dyad in accord with the CV result of Dhar et al.19 Transient Absorption. Figure 4 summarizes transient absorption (TA) spectroscopy experiments performed on Dy-1

Figure 4. Transient absorption (a) spectral and (b) kinetic traces of Dy-1 in dichloromethane. The absorption spectrum of Dy-1 (black dashed trace) is shown in part a as a reference. The kinetics are characterized by a ∼4 ps rapid reorganization followed by a dominant 40−50 ps decay.

with a 510 nm excitation pulse. Initially, the spectral signal displays a broad positive feature below 460 nm, attributed to an excited state absorption (ESA), a ground state bleach (GSB) from 460 to 530 nm, and a stimulated emission (SE) band from 530 to 600 nm. All of these signals have been previously observed and attributed to the population of the 1ππ* (S1) state of Bodipy.12 The characteristics of the Bodipy 1ππ* state decay away with a 42 ps time constant, revealing residual signals on the hundreds of picoseconds time scale that are consistent with excitation of the 3MMLL′CT state of the PtN2S2 species.23 The 42 ps time constant most likely measures the time scale for fluorescence resonance energy transfer (FRET) from the Bodipy to the Pt chromophore. These kinetics are significantly slower than previous measurements on similar complexes that E


Article

Inorganic Chemistry

phosphonate.13 In the present study, comparisons of the UV− vis spectra in solution of the dyads Dy-1 and Dy-2, as well as Pt(bpyP2)(CCPh)2 and 2 with R = P(O)(OEt)2, before and after sonication in the presence of platinized TiO2 (Figure S10) showed a great decline in absorption band intensities from which it was possible to estimate the extent of attachment to the TiO2 nanoparticles (Dy-1, 98%; Dy-2, 94%). The complex Pt(bpyP2)(CCPh)2 was observed to be 94% bound to TiO2 whereas compound 2 with R = P(O)(OEt)2 had previously been reported to be 97% bound. The UV−vis spectra for Dy-2 in dichloromethane solution (50 μM) before and after dye attachment are illustrated in Figure 6 as an example in estimating the dye-loading value onto platinized TiO2 (the corresponding spectra for Dy-1 and Pt(bpyP2)(CCPh)2 are exhibited in Figures S11 and S12).

Figure 5. Transient absorption (a) spectral traces of Dy-2 in dichloromethane at various time delays from −0.5 ps to 1.59 ns. Part b shows the kinetic traces of DPP (blue) and Dy-2 (red). The absorption spectrum of Dy-2 (black dashed trace) is shown in part a as a reference. Figure 6. UV−vis spectra of Dy-2 in dichloromethane solution (2.5 mL, 50 μM) (blue line) and after sonication with platinized TiO2 nanoparticles (20 mg) and followed by the removal of nanoparticles (black line). Absorbance was taken in 2 mm cuvettes.

species, persisting unperturbed past 2 ns. Clearly, the DPPbased excited state is unable to sensitize the Pt diimine bis(acetylide) CT chromophore as a consequence of the relative energies of their lowest singlet excited states and the result that SEnT in Dy-2 is energetically unfavorable. The relative energies of the two chromophores in Dy-2 are seen in the UV−vis absorption spectra of Dy-2 and Pt(bpyP2)(CCPh)2. Attachment of Photosensitizers onto TiO2. The attachment of dyes and other photosensitizers onto TiO2 in dyesensitized solar cells for efficient and rapid electron injection into the semiconductor has been done using a variety of anchoring groups including carboxylate, phosphonate, catecholate, hydroxamate, sulfonate, salicylate, and acetylacetonate.26 In the present study, dyads Dy-1 and Dy-2 and the bis(acetylide) complex Pt(bpyP2)(CCPh)2 each contain two diimine-bound phosphonate groups that can be used for attachment to TiO2 by sonication following a procedure recently reported.13 The advantage of this procedure was established when it was found that ester hydrolysis in a separate step prior to attachment led to significant decomposition of the Pt chromophore. The effectiveness of the procedure was determined in the report by Zheng et al., in which the extent of attachment was determined to be greater than 90% following sonication for 20 min and the dyes were observed to still be bound to the semiconductor after extensive illumination. The path to using sonication first became clear to us when a previous study revealed only partial hydrolysis of a phosphonate-ester-containing dye when stirred in the presence of TiO2.27 Subsequently, similar observations were also made for dyes and photosensitizers containing carboxylate esters, but binding to TiO2 of the carboxylate linker has been shown elsewhere not to be as robust as binding of

Light-Driven Hydrogen Generation Studies. Dyad photosensitizers Dy-1 and Dy-2, as well as complex Pt(bpyP2)(CCPh)2, were examined for their ability to promote H2 generation when attached to platinized TiO2 nanoparticles and irradiated using either 530 nm or “white” light generated from LED sources (see Experimental Section). The photochemical experiments were conducted on aqueous solutions at pH 4.0 with ascorbic acid (AA, 0.5 M) serving as the sacrificial electron donor. The “white” LED light, the spectrum of which is shown in Supporting Information (Figure S13), was employed with different power outputs (130, 190, and 260 mW) to probe the influence of light intensity on system components at specified concentrations. As has been described in previous reports,4c,d,28 5 mL samples in 40 mL vials were irradiated in a locally constructed temperature-controlled 16well LED photolysis apparatus with the pressure of each sample measured in real time, and the head space of each sample was analyzed by GC analysis at the end of the irradiation period. The GC analysis (100 μL) for H2 produced was done after 60 h of irradiation with methane used as an internal standard to calibrate each H2 determination. Hydrogen production curves (vs time) for samples containing the Dy-1 and Dy-2 dyads and the complex Pt(bpyP2)(CCPh)2 are displayed in Figure 7 (for 130 mW power) and Figure S14 for greater white LED intensities. The corresponding data, including turnover number (TON), turnover frequency (TOF with respect to moles of photosensitizer attached to the platinized TiO2), initial TOF F


Article

Inorganic Chemistry

with a TON of 6770, while systems with dyad Dy-2 and Pt(bpyP2)(CCPh)2 as photosensitizers yield only 1560 and 860 TONs, respectively. Under white-light LED irradiation at 130 mW power, systems with the two dyads gave closer values, with TONs for Dy-1 and Dy-2 of 4030 and 2190, respectively, while the system containing Pt(bpyP2)(CCPh)2 as photosensitizer produced 2090 TON. Examination of H2 production under white LED illumination was also conducted at higher intensities (190 and 260 mW), and gave TONs that were ca. 40−50% larger than at 130 mW power and in the same approximate order. On the basis of the results for both TON and TOFi obtained under 530 nm irradiation, the system with Dy-1 as a photosensitizer is much more effective in converting photon energy into H2 than those of Dy-2 and Pt(bpyP2)(CCPh)2. A key difference between these systems is that, in Dy-1, the CT chromophore is a Pt bipyridine dithiolate, whereas in the other two photosensitizers the CT chromophore is a Pt bipyridine bis(acetylide) moiety. From previous studies, it is known that the Pt(bpy)(dithiolate) chromophore possesses a much lower energy CT than Pt(bpy)(CCAr)2 complexes with the HOMO of the former being of mixed metal and dithiolate character whereas in the latter the HOMO is mainly Pt in character. The respective absorptive states of the two chromophores are identified as 1MMLL′CT and 1MLCT, and as is characteristic in the dynamics of 4d and 5d metal complex excited states, intersystem crossing occurs rapidly to the corresponding triplet excited states. What is significant in the initial analysis of the results for light-driven H2 generation is that the 1MLCT state of the Pt(bpy)(CCAr)2-containing complexes is at higher energy than the 530 nm radiation of the green LEDs, whereas the 1 MMLL′CT state of Dy-1 is not. When “white” LED light is employed (see Figure S13 for the source spectrum), the performance of the systems containing the Pt(bpyP2)(CCPh)2 CT moiety increases relative to that with Pt(bpyP2)(benzenedithiolate), consistent with the higher

Figure 7. Hydrogen generation curves with respect to different photosensitizers under irradiation of (a) green LED (530 nm) at 130 mW and (b) white LED (410−800 nm) at 130 mW. Each sample consisted of 5 mL of 0.5 M AA in water at pH 4.0 to which was added 20 mg of photosensitizer−Pt−TiO2.

(TOFi, based on the first 2 h of irradiation), and the absolute percentage difference between TOF and TOFi are shown in Table 3. Under 530 nm irradiation at 130 mW power, all of the photosensitizers in the present study promote the light-driven generation of H2 (>2 mL) to varying extents relative to the system without any photosensitizer (this control produced 500 nm light strongly, the charge transfer state of Pt(bpyP2)(CCPh)2 that would be used for photoinduced electron transfer into platinized TiO2 and eventual H2 formation appears to be inaccessible from the lower energy DPP* excited state. While the activity of Dy-1 is the highest of the three photosensitizers described in this report, it is essential to compare its activity with those of other photosensitizers having the same PtN2S2 CT chromophore. The specific compounds of interest are the previously reported compound 2 with R = P(O)(OEt)2 and the Pt(bdt)(bpyP2) complex without an attached dye.13 Comparison of the systems containing these compounds shows that the system with 2 does substantially better than those with Dy-1 or the Pt(bdt)(bpyP2) complex (see Table 3). The latter two systems are within error the same in terms of both TOFi and overall TON over 60 h. While the greater activity for the earlier reported compound 2 relative to Pt(bdt)(bpyP2) under 530 nm irradiation indicates the value of having a more intensely absorbing organic dye directly linked to the CT chromophore, the absence of any significant improvement of the system with Dy-1 relative to that with Pt(bdt)(bpyP2) indicates that other factors are involved. Although the Bodipy dye in Dy-1 is absorbing 530 nm light, this absorption is not leading to effective hydrogen production as it does in dyad 2. The nature of the attachment between the two chromophores in each dyad including the number and types of bonds may also play a role in dyad effectiveness. Specifically, in terms of hydrogen evolution, the direct linking of the Bodipy dye to the bdt benzene ring in 2 functions better than the phenylamide linkage in Dy-1. While the latter linkage offers more versatility in the construction of dyads with other organic dyes and the PtN2S2 CT chromophore, the longer and more flexible bridge between the dye and the CT chromophore may work against effective energy transfer within the dyad. Analysis of this difference between Dy-1 and compound 2 begins with a straight line difference in length between centers of the Bodipy and PtN2S2 chromophores in these two dyads. In 2, the distance is 9.94 Å, but in Dy-1, the amide and phenyl groups increase this distance to 15.5 Å (calculated via density functional theory with B3LYP/LANL2MB). The other FRET parameters remain the same between the two dyads (Bodipy Φfl = 0.60 and R0 = 41.7 Å) except for the orientational term, κ2, which was vanishingly small in 2 but can be estimated to be 2 /3 in Dy-1 because of rotational averaging.29 The increased distance accounts for the slower energy transfer time in Dy-1 (42 ps) compared to that of 2 (0.7 ps). However, since the energy transfer time is so much faster than the natural lifetime of Bodipy (3.5 ns), the energy transfer will still be nearly 100% efficient. In fact, the TA spectra show that FRET from Bodipy to PtN2S2 is nearly 100% efficient in Dy-1, just as it is in 2.23 Given this fact, it is unclear why Dy-1 does not improve hydrogen production to the same extent as compound 2. It appears that, on the surface of TiO2 under hydrogen production conditions, there is some mechanism at play that prevents light absorption by Bodipy in Dy-1 from effectively leading to electron transfer of the semiconductor. Hence, Dy-1

is no more effective than the Pt(bdt)(bpyP2) complex. Determination of the precise mechanism by which this occurs will require performing transient absorption experiments on Dy-1 bound to TiO2 and, in the presence of Pt nanoparticles and AA, a challenging experiment that is beyond the scope of this work. Photochemical experiments with white LED light show that although Dy-1 exhibits only 60% of the TON than it does under 530 nm light, systems containing Dy-2 and Pt(bpyP2)(CCPh)2 yield greater TONs (up to ca. 2100) under white light relative to 530 nm irradiation. The increases for systems with Dy-2 and Pt(bpyP2)(CCPh)2 are very similar, and indicate that, with the white-light source, only higher energy photons are used effectively upon MLCT absorption in Dy-2 and Pt(bpyP2)(CCPh)2, and not from lower energy photons that would be absorbed by the DPP chromophore of Dy-2. In other words, the lower energy absorption localized on DPP is only weakly effective for TiO2 sensitization. For the system containing Dy-1, only the higher energy fraction of the white light is harvested successfully for photoinduced electron transfer and subsequent hydrogen production. When the power of the white light is increased from 130 to 190 mW, a significant increase (ca. 30−50%) is observed in the amounts of hydrogen generated for systems with Dy-1, Dy-2, and Pt(bpyP2)(CCPh)2 as photosensitizers, but a further increase to 260 mW power results in essentially no change in H2 TON. It is conjectured that, at this light flux, the light absorbers are becoming saturated.

■

CONCLUSIONS Two new dyads containing a strongly absorbing organic dye and a Pt complex-based charge transfer chromophore have been synthesized and studied as photosensitizers for the lightdriven generation of H2 from aqueous protons. The dyad Dy-1 containing a Bodipy dye and a platinum diimine benzenedithiolate (bdt) charge transfer (CT) chromophore shows a slight increase in effectiveness relative to the PtN 2 S 2 chromophore alone in the light-driven generation of H2. However, this dyad in which the two components are connected through an amide linkage on the bdt side of the PtN2S2 complex is not as effective as the previously reported compound 2 in which there is a direct bond between the Bodipy dye and the benzenedithiolate ligand. The second dyad, Dy-2, which contains a diketopyrrolopyrrole dye that absorbs strongly at λ > 550 nm and a Pt diimine bis(arylacetylide) CT chromophore with a higher energy CT state, shows relatively little enhancement of activity relative to that of a system in which just the Pt diimine bis(arylacetylide) complex is used as the photosensitizer. The results illustrate that (1) the effectiveness of a dyad relative to that of a CT chromophore alone depends critically on the relative energies of the excited states of the dyad components and (2) longer linkages between the strongly absorbing organic dye and the metal CT complex should be avoided because they can slow the energy transfer rate to the point that it competes less effectively with other quenching pathways, thereby decreasing photosensitizer performance.

■

EXPERIMENTAL SECTION

Chemicals. All reactions were conducted under a N2 atmosphere. All chemicals were purchased from Sigma-Aldrich and TCI, and were used without further purification. All solvents were dried and degassed before use. 3,4-Dimercaptobenzoic acid,30 10-(4-aminophenyl)-2,8-

H


Article

Inorganic Chemistry

eluent to afford a pale brown solid (40 mg, 83% yield). 1H NMR (CD2Cl2, 400 MHz): 9.87−9.85 (m, 2H, Ar), 8.53 (d, 2H, J = 13.2 Hz, Ar), 7.93 (dd, 2H, J = 12, 5.2 Hz, Ar), 7.53−7.51 (m, 4H, Ar), 7.32− 7.28 (m, 4H, Ar), 7.23−7.19 (m, 2H, Ar), 4.30−4.14 (m, 8H, alkyl), 1.39−1.35 (m, 12H, alkyl) ppm. ESI-MS, m/z: [M + K]+ 864.13 (expected), 864.2 (found). Anal. Calcd for C34H36N2O6P2Pt: C, 49.46; H, 4.39; N, 3.39. Found: C, 49.81; H, 4.19; N, 3.23. 2,5-Dioctyl-3-(thiophen-2-yl)-6-(5-((trimethylsilyl)ethynyl)thiophen-2-yl)pyrrolo-[3,4-c]pyrrole-1,4(2H,5H)-dione (DPPContaining TMS-Protected Ethynyl Ligand). To a flask containing 3-(5-bromothiophen-2-yl)-2,5-dioctyl-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (82 mg, 0.136 mmol), Pd(PPh3)2Cl2 (5 mg, 0.007 mmol), and CuI (3 mg, 0.014 mmol) that had been evacuated and refilled with N2 three times was added a mixture of triethylamine (2 mL) and THF (20 mL). To this solution was added trimethylsilylacetylene (40 mg, 0.408 mmol). The solution was heated at 40 °C overnight, and then the solvents were removed by evaporation. The residue was purified via column chromatography using hexane/dichloromethane (1:1, v/v) as the eluent to yield a purple solid (50 mg, 59% yield). 1H NMR (CD2Cl2, 400 MHz): 8.92 (d, 1H, J = 3.6 Hz, Ar), 8.81 (d, 1H, J = 3.6 Hz, Ar), 7.68 (d, 1H, J = 4.8 Hz, Ar), 7.34−7.28 (m, 2H, Ar), 4.05−3.98 (m, 4H, alkyl), 1.71 (br, 4H, alkyl), 1.40−1.28 (m, 20H, alkyl), 0.88−0.86 (m, 6H, alkyl), 0.29 (s, 9H, −Si(CH3)3) ppm. 3-(5-Ethynylthiophen-2-yl)-2,5-dioctyl-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (Dy-2 Precursor). To a flask containing the ethynyl precursor (50 mg, 0.081 mmol) and K2CO3 (56 mg, 0.403 mmol) was added 25 mL of THF/MeOH (4:1, v/v). The solution was allowed to stir at room temperature for 3 h. The solvents were removed by evaporation under reduced pressure, and the residue was extracted with dichloromethane and washed with water (3 × 20 mL). The organic layer was evaporated under reduced pressure, and then purified by column chromatography on silica gel using a mixture of hexane/dichloromethane (1:2, v/v) as eluent to obtain a purple solid (23 mg, 52% yield). 1H NMR (CDCl3, 400 MHz): 8.96 (d, 1H, J = 3.6 Hz, Ar), 8.83 (d, 1H, J = 4 Hz, Ar), 7.65 (d, 1H, J = 4.8 Hz, Ar), 7.38 (d, 1H, J = 4 Hz, Ar), 7.29−7.28 (m, 1H, Ar), 4.08−4.01 (m, 4H, alkyl), 3.58 (s, 1H, CCH), 1.73−1.69 (m, 4H, alkyl), 1.41−1.26 (m, 20H, alkyl), 0.88−0.85 (m, 6H, alkyl) ppm. Dyad Dy-2. Under a N2 atmosphere, the DPP-based ethynyl ligand (23 mg, 0.043 mmol) and Pt(4,4′-(P(O)(OEt)2)bpy)Cl2 (14 mg, 0.020 mmol) were added to a mixture of NEt3 (1 mL) and chloroform (20 mL) in the presence of a catalytic amount of CuI (1 mg, 0.006 mmol). The reaction mixture was stirred at room temperature overnight. The solvent was removed under reduced pressure, and then the crude product was purified by column chromatography over silica gel using dichloromethane, and then chloroform/methanol (25:1, v/v) as eluent to obtain a purple-blue solid (34 mg, 99% yield). 1H NMR (CD2Cl2, 400 MHz): 9.82−9.80 (m, 2H, Ar), 9.01 (d, 2H, J = 4 Hz, Ar), 8.89 (d, 2H, J = 2.8 Hz, Ar), 8.62 (d, 2H, J = 13.6 Hz, Ar), 8.02 (dd, 2H, J = 12, 5.2 Hz, Ar), 7.67 (d, 2H, J = 4.8 Hz, Ar), 7.31−7.28 (m, 4H, Ar), 4.29−4.20 (m, 8H, alkyl), 4.09−4.06 (m, 8H, alkyl), 1.75−1.73 (m, 8H, alkyl), 1.41−1.27 (m, 52H, alkyl), 0.88−0.82 (m, 12H, alkyl) ppm. MALDI-TOF MS, m/z: [M + H]+ 1718.59 (expected), 1718.58 (found). Anal. Calcd for C82H104N6O10P2PtS4: C, 57.29; H, 6.10; N, 4.89. Found: C, 57.41; H, 6.08; N, 4.59. Transient Absorption. The laser system for femtosecond transient absorption has been described in detail previously.23,27 In brief, the pump was generated by a home-built NOPA, and the probe was a white-light continuum generated in sapphire, which was detected at 1 kHz by a CCD (Princeton Instruments). All samples were held in a 2 mm quartz cuvette, and all measurements were carried out at room temperature with magic angle (54.7°) polarization between the pump and probe. All TA experiments were taken while continuously translating the sample vertically (1 mm/s). Samples were pumped by NOPA pulses centered around the dyes’ main absorption bands, and the OD values at pump wavelengths were kept between 0.3 and 0.8. Pump powers were kept at 60−80 nJ/pulse. The absorbance of the dyes was monitored after TA experiments to ensure dye integrity. TA

diethyl-5,5-difluoro-1,3,7,9-tetramethyl-5H-dipyrrolo[1,2-c:2′,1′-f ][1,3,2]diazaborinin-4-ium-5-uide (Bodipy-phenyl-NH2),31 Pt(4,4′-(P(O)(OEt)2)bpy)Cl2,13 2,5-dioctyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione,32 and 3-(5-bromothiophen-2-yl)-2,5-dioctyl-6-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione32 were synthesized according to the reported procedures. Characterization. 1H NMR spectra were recorded on a Bruker Advance 400-MHz spectrometer and referenced to either tetramethylsilane as an internal standard or a residual proton resonance of the deuterated solvent. Elemental analyses were performed using a PerkinElmer 2400 Series II Analyzer. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy was conducted on an Autoflex Bruker MALDI-TOF system. Alternatively, the mass of compounds was determined using an LTQ VELOS Thermo LCMS (positive mode). Cyclic voltammetry (CV) measurements of the complexes were performed with a CHI 680D potentiostat using a one-compartment cell with a glassy-carbon working electrode, a glassy-carbon auxiliary electrode, and an SCE reference electrode. Tetrabutylammonium hexafluorophosphate was used as the supporting electrolyte. UV−vis spectra were recorded on a Cary 60 UV−vis spectrophotometer, using a 2 mm path length quartz cuvette in dichloromethane or acetonitrile. Synthesis of Ligands and Complexes. 10-(4-(3,4-Bis(pivaloylthio)benzamido)-phenyl)-2,8-diethyl-5,5-difluoro1,3,7,9-tetramethyl-5H-dipyrrolo[1,2-c:2′,1′-f ][1,3,2]diazaborinin-4-ium-5-uide (Dy-1 Precursor). This compound was synthesized by a modified published procedure.33 3,4-Dimercaptobenzoic acid (42 mg, 0.223 mmol) was dissolved in dry THF (8 mL), to which was added dropwise triethylamine (0.13 mL) and pivaloyl chloride (0.094 mL, 0.764 mmol) while cooling in an ice bath. The reaction mixture was allowed to warm up to room temperature and stirred overnight. The solvent and volatile chemicals were removed by evaporation under reduced pressure, yielding a white solid (105 mg). This product was dissolved in 5 mL of dry THF (5 mL), to which was added in a dropwise manner Bodipy-phenyl-NH2 (88 mg, 0.223 mmol) in a 1:1 mixture of DCM:THF (10 mL) and then stirred for overnight under room temperature. A bright orange solid (100 mg, 61% yield) was obtained after solvent removal under reduced pressure, and was used for the next step without further purification. 1H NMR (acetone-d6, 400 MHz): 10.00 (br, 1H, R−NH−C(O)−), 8.17 (s, 1H, Ar), 8.17−8.07 (m, 3H, Ar), 7.74 (d, 1H, J = 8 Hz, Ar), 7.36 (d, 2H, J = 8.4 Hz, Ar), 2.50 (s, 6H, alkyl), 2.38−2.33 (m, 4H, alkyl), 1.42 (s, 6H, alkyl), 1.33 (s, 18H, alkyl), 1.00−0.97 (m, 6H, alkyl) ppm. Dyad Dy-1. The Dy-1 precursor (30 mg, 0.041 mmol) was suspended in 5 mL of methanol, followed by addition of potassium tert-butoxide (9 mg, 0.082 mmol) in methanol (5 mL). After 30 min, to this reaction mixture was added Pt(4,4′-(P(O)(OEt)2)bpy)Cl2 (28 mg, 0.041 mmol) in THF solution (5 mL) using a syringe. The reaction mixture was stirred at room temperature under N2 overnight. Solvents were removed by evaporation under reduced pressure and then purified by column chromatography on silica gel using initially pure dichloromethane, and then pure acetonitrile as eluent. The purple band was collected and evaporated under reduced pressure to yield the purple solid (19 mg, 39% yield). 1H NMR (CD2Cl2, 400 MHz): 9.71 (br, 1H, R−NH−C(O)−), 9.25 (s, 1H, Ar), 9.11 (s, 1H, Ar), 8.11− 8.05 (m, 3H, Ar), 7.88−7.77 (m, 3H, Ar), 7.58 (s, 1H, Ar), 7.31 (d, 3H, J = 8 Hz, Ar), 7.07 (d, 1H, J = 8 Hz, Ar), 4.38−4.28 (m, 4H, alkyl), 4.00−3.87 (m, 4H, alkyl), 2.52−2.49 (m, 6H, alkyl), 2.38−2.34 (m, 4H, alkyl), 1.49−1.38 (m, 18H, alkyl), 1.05−1.01 (m, 6H, alkyl) ppm. MALDI-TOF MS, m/z: [M + H]+ 1185.29 (expected), 1185.29 (found). Anal. Calcd for C48H56BF2N5O7P2PtS2: C, 48.65; H, 4.76; N, 5.91. Found: C, 48.70; H, 4.70; N, 5.57. Pt(bpyP2)(CCPh)2. Under a N2 atmosphere, ethynylbenzene (15 mg, 0.115 mmol) and Pt(4,4′-(P(O)(OEt)2)bpy)Cl2 (40 mg, 0.058 mmol) were added to a mixture of NEt3 (0.5 mL) and dichloromethane (10 mL) in the presence of a catalytic amount of CuI (1 mg, 0.006 mmol). The reaction mixture was stirred at room temperature overnight. The solvent was removed under reduced pressure, and then the crude product was purified by column chromatography over silica gel using dichloromethane, and then dichloromethane/methanol (27:1, v/v) as I


Article

Inorganic Chemistry data were processed with Igor Pro (WaveMetrics Inc.). The chirp of the white-light probe was corrected manually in the software and further corrected by allowing time zero to be wavelength dependent in the kinetic fits. The kinetics were then fit with a sum of exponential decays convoluted with the ∼200 fs Gaussian instrument response; errors in the fitting parameters are ±1σ unless otherwise specified. In Situ Attachments of Platinum(II) Photosensitizers onto TiO2. A 2.5 mL portion of a solution of the photosensitizer in dichloromethane (50 μM) (or in acetonitrile for Dy-1) was added to a 20 mL vial containing 20 mg of platinized TiO2. The sample vial was sonicated for 20 min in the dark, and then the photosensitizer-attached platinized TiO2 particles were separated by centrifugation and dried under vacuum for 30 min. The photosensitizer−TiO2−Pt sample was then transferred to a 40 mL vial to which 5 mL of aqueous ascorbic acid solution (0.5 M, pH 4.0) was added. Photocatalytic Hydrogen Generation Studies. The 40 mL sample vials (irradiation surface area: 5.72 cm2) were placed into a temperature-controlled block at 15 °C, and each vial was sealed with an airtight cap fitted with a pressure transducer and a rubber septum. The samples were then purged with a mixture of gas containing nitrogen/methane (79:21 mol %). The methane present in the gas mixture serves as an internal standard for GC analysis at the end of the experiment. The samples were irradiated from below in a locally built 16-well photolysis apparatus with Philips LumiLED Luxeon Star Hex Green (530 nm) 700 mA LEDs and Philips LumiLEDs Luxeon ES Cool White (410−800 nm) 700 mA LEDs, on top of an orbital shaker. The light power of each LED was set to 130, 190, and 260 mW, and measured with an L30 A Thermal sensor and a Nova II power meter (Ophir-Spiricon LLC). The pressure changes in the vials were recorded using a Labview program from a Freescale semiconductor sensor (MPX4259A series). At the end of the experiment, the headspaces of the vials were characterized by gas chromatography to verify that the measured pressure change was a consequence of hydrogen generation, and to confirm the amount of hydrogen generated. The amounts of hydrogen evolved were determined by using gas chromatography (Shimadzu GC-17A with a molecular sieve 5 Å column and TCD detector) at the end of the radiation period and were quantified using a calibration plot of the integrated amount of H2 relative to the internal methane standard.

■

(FRG2/03-04/083). Additionally, P.-Y.H. was supported by the Mr. Kwok Yat Wai and Madam Kwok Chung Bo Fun Graduate School Development Fund during his stay at the University of Rochester. D.W.M. is grateful for support as an Alfred P. Sloan Research Fellow.

■

(1) Schultz, M. G.; Diehl, T.; Brasseur, G. P.; Zittel, W. Science 2003, 302, 624−627. (2) (a) Gregory, R. P. F. Biochemistry of Photosynthesis, 2nd ed.; Wiley-Interscience: New York, 1977. (b) Grätzel, M. Acc. Chem. Res. 1981, 14, 376−384. (c) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163− 170. (d) Hara, M.; Kondo, T.; Komoda, M.; Ikeda, S.; Kondo, J. N.; Domen, K.; Hara, M.; Shinohara, K.; Tanaka, A. Chem. Commun. 1998, 357−358. (e) Khaselev, O.; Turner, J. A. Science 1998, 280, 425−427. (f) Lawlor, D. W. Photosynthesis, 3rd ed.; BIOS Scientific Publishers: Oxford, 2001. (g) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. J. Am. Chem. Soc. 2004, 126, 13406−13413. (h) Alstrum-Acevedo, J. H.; Brennaman, M. K.; Meyer, T. J. Inorg. Chem. 2005, 44, 6802− 6827. (i) Chakraborty, S.; Wadas, T. J.; Hester, H.; Schmehl, R.; Eisenberg, R. Inorg. Chem. 2005, 44, 6865−6878. (j) Eisenberg, R.; Nocera, D. G. Inorg. Chem. 2005, 44, 6799−6801. (k) Youngblood, W. J.; Lee, S. H.; Kobayashi, Y.; Hernandez-Pagan, E. A.; Hoertz, P. G.; Moore, T. A.; Moore, A. L.; Gust, D.; Mallouk, T. E. J. Am. Chem. Soc. 2009, 131, 926−927. (l) Blankenship, R. E. Molecular Mechanisms of Photosynthesis; Blackwell: Oxford, 2002. (3) (a) Lehn, J. M.; Sauvage, J. P. Nouv. J. Chim. 1977, 1, 449−451. (b) Kalyanasundaram, K.; Kiwi, J.; Grätzel, M. Helv. Chim. Acta 1978, 61, 2720−2730. (c) Moradpour, A.; Amouyal, E.; Keller, P.; Kagan, H. Nouv. J. Chim. 1978, 2, 547−549. (d) Kirch, M.; Lehn, J.-M.; Sauvage, J.-P. Helv. Chim. Acta 1979, 62, 1345−1384. (e) Keller, P.; Moradpour, A.; Amouyal, E.; Kagan, H. J. Mol. Catal. 1980, 7, 539−542. (f) Borgarello, E.; Kiwi, J.; Pelizzetti, E.; Visca, M.; Grätzel, M. Nature 1981, 289, 158−160. (g) Furlong, D. N.; Grieser, F.; Hayes, D.; Hayes, R.; Sasse, W.; Wells, D. J. Phys. Chem. 1986, 90, 2388−2396. (h) Palmans, R.; Frank, A. J. J. Phys. Chem. 1991, 95, 9438−9443. (4) (a) Han, Z.; Eisenberg, R. Acc. Chem. Res. 2014, 47, 2537−2544. (b) Eckenhoff, W. T.; McNamara, W. R.; Du, P. W.; Eisenberg, R. Biochim. Biophys. Acta, Bioenerg. 2013, 1827, 958−973. (c) Das, A.; Han, Z.; Haghighi, M. G.; Eisenberg, R. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 16716−16723. (d) Han, Z.; Qiu, F.; Eisenberg, R.; Holland, P. L.; Krauss, T. D. Science 2012, 338, 1321−1324. (e) Eckenhoff, W. T.; Eisenberg, R. Dalton Trans. 2012, 41, 13004− 13021. (f) Esswein, A. J.; Nocera, D. G. Chem. Rev. 2007, 107, 4022− 4047. (g) Wang, M.; Na, Y.; Gorlov, M.; Sun, L. Dalton Trans. 2009, 6458−6467. (5) (a) Li, L.; Duan, L.; Xu, Y.; Gorlov, M.; Hagfeldt, A.; Sun, L. Chem. Commun. 2010, 46, 7307−7309. (b) Brimblecombe, R.; Koo, A.; Dismukes, G. C.; Swiegers, G. F.; Spiccia, L. J. Am. Chem. Soc. 2010, 132, 2892−2894. (c) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85−277. (d) Collin, J. P.; Guillerez, S.; Sauvage, J. P.; Barigelletti, F.; De Cola, L.; Flamigni, L.; Balzani, V. Inorg. Chem. 1991, 30, 4230− 4238. (6) Cline, E. D.; Adamson, S. E.; Bernhard, S. Inorg. Chem. 2008, 47, 10378−10388. (7) (a) Du, P.; Eisenberg, R. Chem. Sci. 2010, 1, 502−506. (b) Jarosz, P.; Du, P.; Schneider, J.; Lee, S.-H.; McCamant, D.; Eisenberg, R. Inorg. Chem. 2009, 48, 9653−9663. (c) Du, P.; Knowles, K.; Eisenberg, R. J. Am. Chem. Soc. 2008, 130, 12576−12577. (8) (a) Geary, E. A. M.; Yellowlees, L. J.; Jack, L. A.; Oswald, I. D. H.; Parsons, S.; Hirata, N.; Durrant, J. R.; Robertson, N. Inorg. Chem. 2005, 44, 242−250. (b) Islam, A.; Sugihara, H.; Hara, K.; Singh, L. P.; Katoh, R.; Yanagida, M.; Takahashi, Y.; Murata, S.; Arakawa, H.; Fujihashi, G. Inorg. Chem. 2001, 40, 5371−5380. (c) Paw, W.; Cummings, S. D.; Adnan Mansour, M.; Connick, W. B.; Geiger, D. K.; Eisenberg, R. Coord. Chem. Rev. 1998, 171, 125−150. (d) Connick, W. B.; Gray, H. B. J. Am. Chem. Soc. 1997, 119, 11620−11627.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00496. 1 H NMR spectra, chemical structure of BodipyP, UV−vis spectra, TA spectra, kinetics details, preparation of platinized TiO2, emission spectra, and H2 production among different photosensitizers under white-light irradiation (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Science Foundation Collaborative Research Grant CHE-1151789. W.-Y.W. is thankful for financial support from Areas of Excellence Scheme of HKSAR (AoE/P-03/08), Hong Kong Research Grants Council (HKBU12302114), National Natural Science Foundation of China (51573151), and Hong Kong Baptist University J


Article

Inorganic Chemistry (e) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949−1960. (f) Zuleta, J. A.; Bevilacqua, J. M.; Eisenberg, R. Coord. Chem. Rev. 1991, 111, 237−248. (g) Zuleta, J. A.; Burberry, M. S.; Eisenberg, R. Coord. Chem. Rev. 1990, 97, 47−64. (9) Zhang, J.; Du, P.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc. 2007, 129, 7726−7727. (10) (a) Misawa, H.; Sakuragi, H.; Usui, Y.; Tokumaru, K. Chem. Lett. 1983, 12, 1021−1024. (b) Mau, A. W. H.; Johansen, O.; Sasse, W. H. F. Photochem. Photobiol. 1985, 41, 503−509. (c) Shimidzu, T.; Iyoda, T.; Koide, Y. J. Am. Chem. Soc. 1985, 107, 35−41. (d) Hashimoto, K.; Kawai, T.; Sakata, T. Nouv. J. Chim. 1984, 8, 693−700. (11) Galletta, M.; Puntoriero, F.; Campagna, S.; Chiorboli, C.; Quesada, M.; Goeb, S.; Ziessel, R. J. Phys. Chem. A 2006, 110, 4348− 4358. (12) Lazarides, T.; McCormick, T. M.; Wilson, K. C.; Lee, S.; McCamant, D. W.; Eisenberg, R. J. Am. Chem. Soc. 2011, 133, 350− 364. (13) Zheng, B.; Sabatini, R. P.; Fu, W.-F.; Eum, M.-S.; Brennessel, W. W.; Wang, L.; McCamant, D. W.; Eisenberg, R. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, E3987−E3996. (14) Zhu, X. X.; Yu, Q. S.; Greig, N. H.; Flippen-Anderson, J. L.; Brossi, A. Heterocycles 2003, 59, 115−128. (15) (a) Sonogashira, K. 2.4-Coupling Reactions Between sp2 and sp Carbon Centers. In Comprehensive Organic Synthesis; Fleming, B. M. T., Ed.; Pergamon: Oxford, 1991; pp 521−549. (b) Sonogashira, K. J. Organomet. Chem. 2002, 653, 46−49. (c) Sonogashira, K. CrossCoupling Reactions to sp Carbon Atoms. In Metal-Catalyzed CrossCoupling Reactions; Wiley-VCH Verlag GmbH, 2007; pp 203−229. (16) (a) Zuleta, J. A.; Bevilacqua, J. M.; Proserpio, D. M.; Harvey, P. D.; Eisenberg, R. Inorg. Chem. 1992, 31, 2396−2404. (b) Karolin, J.; Johansson, L. B. A.; Strandberg, L.; Ny, T. J. Am. Chem. Soc. 1994, 116, 7801−7806. (17) Chan, C.-W.; Cheng, L.-K.; Che, C.-M. Coord. Chem. Rev. 1994, 132, 87−97. (18) Goswami, S.; Winkel, R. W.; Alarousu, E.; Ghiviriga, I.; Mohammed, O. F.; Schanze, K. S. J. Phys. Chem. A 2014, 118, 11735− 11743. (19) Dhar, J.; Venkatramaiah, N.; A, A.; Patil, S. J. Mater. Chem. C 2014, 2, 3457−3466. (20) (a) Ziessel, R.; Allen, B. D.; Rewinska, D. B.; Harriman, A. Chem. - Eur. J. 2009, 15, 7382−7393. (b) Burghart, A.; Kim, H.; Welch, M. B.; Thoresen, L. H.; Reibenspies, J.; Burgess, K.; Bergström, F.; Johansson, L. B. Å. J. Org. Chem. 1999, 64, 7813−7819. (21) Ohsawa, Y.; Hanck, K. W.; DeArmond, M. K. J. Electroanal. Chem. Interfacial Electrochem. 1984, 175, 229−240. (22) McGarrah, J. E.; Eisenberg, R. Inorg. Chem. 2003, 42, 4355− 4365. (23) Sabatini, R. P.; Zheng, B.; Fu, W. F.; Mark, D. J.; Mark, M. F.; Hillenbrand, E. A.; Eisenberg, R.; McCamant, D. W. J. Phys. Chem. A 2014, 118, 10663−10672. (24) Alsulami, Q. A.; Aly, S. M.; Goswami, S.; Alarousu, E.; Usman, A.; Schanze, K. S.; Mohammed, O. F. J. Phys. Chem. C 2015, 119, 15919−15925. (25) Beljonne, D.; Wittmann, H. F.; Köhler, A.; Graham, S.; Younus, M.; Lewis, J.; Raithby, P. R.; Khan, M. S.; Friend, R. H.; Brédas, J. L. J. Chem. Phys. 1996, 105, 3868−3877. (26) (a) Zhang, L.; Cole, J. M. ACS Appl. Mater. Interfaces 2015, 7, 3427−3455. (b) Bae, E.; Choi, W. J. Phys. Chem. B 2006, 110, 14792− 14799. (c) Bae, E.; Choi, W.; Park, J.; Shin, H. S.; Kim, S. B.; Lee, J. S. J. Phys. Chem. B 2004, 108, 14093−14101. (27) Sabatini, R. P.; Eckenhoff, W. T.; Orchard, A.; Liwosz, K. R.; Detty, M. R.; Watson, D. F.; McCamant, D. W.; Eisenberg, R. J. Am. Chem. Soc. 2014, 136, 7740−7750. (28) (a) McNamara, W. R.; Han, Z.; Yin, C. J.; Brennessel, W. W.; Holland, P. L.; Eisenberg, R. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15594−15599. (b) Han, Z.; Shen, L.; Brennessel, W. W.; Holland, P. L.; Eisenberg, R. J. Am. Chem. Soc. 2013, 135, 14659−14669. (c) Das, A.; Han, Z.; Brennessel, W. W.; Holland, P. L.; Eisenberg, R. ACS Catal. 2015, 5, 1397−1406. (d) McNamara, W. R.; Han, Z.; Alperin, P.

J.; Brennessel, W. W.; Holland, P. L.; Eisenberg, R. J. Am. Chem. Soc. 2011, 133, 15368−15371. (29) (a) Lazarides, T.; McCormick, T.; Du, P.; Luo, G.; Lindley, B.; Eisenberg, R. J. Am. Chem. Soc. 2009, 131, 9192−9194. (b) Sabatini, R. P.; Lindley, B.; McCormick, T. M.; Lazarides, T.; Brennessel, W. W.; McCamant, D. W.; Eisenberg, R. J. Phys. Chem. B 2016, 120, 527−534. (30) (a) Liénard, B. M. R.; Selevsek, N.; Oldham, N. J.; Schofield, C. J. ChemMedChem 2007, 2, 175−179. (b) Mahendran, A.; Vuong, A.; Aebisher, D.; Gong, Y.; Bittman, R.; Arthur, G.; Kawamura, A.; Greer, A. J. Org. Chem. 2010, 75, 5549−5557. (c) Little, M. A.; Loughrey, J. J.; Santoro, A.; Halcrow, M. A.; Hardie, M. J. Tetrahedron Lett. 2014, 55, 2530−2533. (31) (a) Azov, V. A.; Diederich, F.; Lill, Y.; Hecht, B. Helv. Chim. Acta 2003, 86, 2149−2155. (b) Song, H.; Rajendiran, S.; Kim, N.; Jeong, S. K.; Koo, E.; Park, G.; Thangadurai, T. D.; Yoon, S. Tetrahedron Lett. 2012, 53, 4913−4916. (32) Gao, H.; Li, Y.; Wang, L.; Ji, C.; Wang, Y.; Tian, W.; Yang, X.; Yin, L. Chem. Commun. 2014, 50, 10251−10254. (33) Okamura, T.-a.; Tatsumi, M.; Omi, Y.; Yamamoto, H.; Onitsuka, K. Inorg. Chem. 2012, 51, 11688−11697.

K


Chromophoric Dyads for the Light-Driven Generation of Hydrogen

Recommend Documents