Design of Efficient Photoinduced Charge Separation in DonorâCopper

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Design of Efficient Photo-Induced Charge Separation in Donor-Copper(I)-Acceptor Triad Martina Sandroni, Antoine Maufroy, Mateusz Rebarz, Yann Pellegrin, Errol Blart, Cyril Ruckebusch, Olivier Poizat, Michel Sliwa, and Fabrice Odobel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp507984s • Publication Date (Web): 10 Nov 2014 Downloaded from http://pubs.acs.org on November 15, 2014

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The Journal of Physical Chemistry

Design of Efficient Photo-Induced Charge Separation in Donor-Copper(I)-Acceptor Triad Martina Sandroni†,#,¶, Antoine Maufroy†,¶, Mateusz Rebarz‡,¶, Yann Pellegrin†, Errol Blart†, Cyril Ruckebusch‡, Olivier Poizat‡, Michel Sliwa*,‡, Fabrice Odobel*,† †

CEISAM, Université de Nantes, CNRS, 2 rue de la Houssinière, 44322 Nantes Cedex 3, France

‡

Laboratoire de Spectrochimie Infrarouge et Raman (LASIR), CNRS UMR 8516/Université

Lille Nord de France, Université Lille1 – Sciences et Technologies/Chemistry Department, bât C5, 59655 Villeneuve d’Ascq Cedex, France KEYWORDS: charge transfer, Cu, donor-acceptor systems, synthesis design, time-resolved spectroscopy ABSTRACT A pure and stable copper(I)-based Donor−Cu(I)−Acceptor triad was synthesized featuring an efficient stepwise photo-induced charge separation upon excitation of the copper(I) MLCT excited state. The heteroleptic copper(I) complex is composed of two phenanthrolines, one substituted by a naphthalene bisimide (NDI) as electron acceptor and the other by a ferrocene (Fc) as electron donor. The synthesis of two dyads with different spacers between the electron acceptor and Cu(I) center and the charge separation mechanism and dynamics were determined by electrochemical and femtosecond transient experiments which show that two parallel electron

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transfer routes occur from the unrelaxed 1MLCT and flattened 3MLCT states with time constants of 540 fs and 162 ps, respectively. The final charge separated state Fc+−Cu(I)−NDI- has a 34 ns lifetime in acetonitrile and is formed with a quantum yield of 90% upon excitation on the MLCT transition of the copper(I) complex. INTRODUCTION The transduction of light energy into a long-lived charge-separated state with a high quantum yield is a fundamental step for the development of artificial photosynthetic devices.1-2 In this context, multi-component systems such as triads “D-S-A” composed of a sensitizer (S) connected to an electron donor ligand (D) and an electron acceptor ligand (A) represent the generally well-accepted molecular design to reach long-lived charge-separated states.3-8 Tris(2,2’-bipyridine)ruthenium(II) ([Ru(bpy)3]2+) and its derivatives have been widely used as sensitizers to prepare countless examples of dyads and triads.6, 8-10 The optimal organization of the components in a polyad is certainly the linear geometry as it maximizes the distance between the photo-generated charges on D and A. The utilization of [Ru(bpy)3]2+ sensitizer to build up triads is a complicated task because the octahedral geometry of this complex inevitably induces the formation of different isomers, which makes it difficult to control the distance and the orientation between the units.10-11 The use of terpyridine ligands solves this problem, but the MLCT excited-state of bis(terpyridine)ruthenium(II) complexes is usually too short-lived to be of a significant interest to trigger photoinduced charge separation.6,

10, 12-15

Besides, the

ruthenium presents significant drawbacks such as high cost, poor abundance (0.001 ppm in the earth’s crust) and toxicity. An attractive alternative is to use copper(I) diimine complexes, because they also exhibit similar MLCT excited-state properties16-17 with an absorption band localized in the visible range. Copper is also more abundant (60 ppm) and therefore much


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cheaper, while being less toxic than ruthenium. Another important distinction between ruthenium(II) and copper(I) complexes lies in the fact that ruthenium(II) complexes are generally six-coordinated in an octahedral geometry, while copper(I) complexes are typically fourcoordinated and tetrahedral. This feature is particularly useful for the design of rod-like molecular arrays. However despite their similarities to [Ru(bpy)3]2+ there is only one recent example reported by Elliott and co-workers of photo-induced charge separation in a donorheteroleptic Cu(I)-acceptor triad.18 This is mainly due to the intrinsic lability of the Cu-N bonds in diimine complexes which generally leads to a mixture of three non separable complexes in equilibrium. The recent molecular triad reported by Elliott and co-workers consists in an electron donor (phenanthiazine), a bis(phenanthroline) copper(I) sensitizer and an electron acceptor (viologen). The triad undergoes multistep photoinduced charge separation leading to a diradical cation charge-separated state (CSS) with ca. 50% quantum yield and a 100 ns lifetime.18 However, this system was in fact a mixture of homoleptic and heteroleptic complexes, whose practical interest is limited. Furthermore, details on the ultrafast formation of CSS could not be achieved. Elucidating the explicit mechanism for future photosynthetic devices based on copper was thus not completely possible. The stability issue of heteroleptic phenanthroline Cu(I) complexes was solved by Schmittel and co-workers who reported the elegant HETPHEN concept (HETeroleptic PHENanthroline).19 The HETPHEN strategy has been used to prepare a whole range of metallo-supramolecular architectures,20 such as heteroleptic nanoladders, nanoboxes, nanogrids, nanoracks; surprisingly, it has almost never been used to develop heteroleptic complexes for photo-induced charge separation.21

Chart 1. Structures of the triad and the parent complexes.


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We report herein the first design of a pure and stable Donor-Sensitizer-Acceptor (D-S-A) triad in which a heteroleptic bis-phenanthroline copper(I) sensitizer assembles a naphthalene diimide (NDI) and a ferrocene (Fc) playing the role of electron acceptor and electron donor respectively (Chart 1). It has to be pointed out that the use of a coordination complex as the chromophore is particularly interesting, because it offers a modular approach, in which the donor and the acceptor groups are assembled at the end of the synthesis. This potentially provides the possibility of screening different combination of donor and acceptor groups with a minimal synthetic effort.10,

14-15

The final coordination step has thus the double role of forming the

chromophore and bringing together the donor and acceptor groups. The electron acceptor selected in this study was naphthalene diimide (NDI), because this moiety was used for the construction of a number of donor-acceptor arrays, and displays extremely useful features, such as a relatively anodic reduction potential (around -0.6 V vs. SCE) compatible with electron transfer from excited copper(I)-diimine complexes (ECu(II)/Cu(I)* ~ -1.1 eV vs. SCE).22-23 In addition, the excited singlet and triplet states as well as the reduced radical display characteristic spectral signatures, which facilitate their detection by transient absorption spectroscopy.23 In this paper, we describe the synthesis of dyads D1 and D2 and of the triad, the full characterization of their optical and electrochemical properties, and the detailed analysis of their excited-state and


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charge-transfer dynamics by laser flash photolysis and femtosecond transient absorption measurements. These results demonstrate that visible light excitation of the copper complex in the triad triggers the formation, of a charge separated state composed of a reduced NDI and an oxidized Fc, having a lifetime of 34 ns with a quantum efficiency of 90% in acetonitrile. We believe that these results should set the path for future design of new stable triad based on copper complexes for solar energy conversion applications. EXPERIMENTAL SECTION Synthesis Phen (1,10-phenanthroline) and dmp (2,9-dimethyl-1,10-phenanthroline or neocuproine) were purchased from Aldrich and used without further purification. MesPhen (2,9-dimesityl1,10-phenanthroline),24

2-(4-nitrophenyl)-5,5-dimethyl-1,3-dioxane

1,25

N-octyl-1,4,5,8-

naphthalenetetracarboxylic monoanhydride 2,26 2,9-dibutyl-1,10-phenanthroline-5,6-dione 5,24 2,9-dimethyl-5-amino-1,10-phenanthroline 7,27 2,9-dimesityl-1,10-phenanthroline-5,6-dione 824 and [Cu(CH3CN)4]PF628 were prepared as described previously. Details of the synthesis and characterization, 1H,

13

C NMR spectroscopy, mass spectrometry and electrochemistry for the

different complexes and ligands are given in Supporting Information. Spectroscopic measurements. All the solvents used in spectroscopic measurements were provided by Sigma-Aldrich with spectroscopic grade purity and were used without further purification. Absorption spectra were measured by Cary-100 spectrometer (Agilent Technologies). Steadystate fluorescence spectra were recorded by Fluorolog 3 spectrofluorometer (Horiba Jobin Yvon).


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Nanosecond transient absorption experiments were carried out using a laser flash photolysis apparatus described elsewhere.29 Briefly excitation pulses (460 nm, fwhm 4 ns, 1 mJ, 0.5 Hz) were provided by a 10-Hz Nd:YAG laser coupled to an OPO. The probe light was provided by a pulsed Xe lamp. The transmitted light was dispersed by a monochromator and analyzed with a photomultiplier coupled to a digital oscilloscope. The recorded traces were averaged for several pulses and repeated for different wavelengths to reconstruct the spectra afterward. The deconvolution procedure of the individual decays with experimentally measured instrument response function (IRF of about 4 ns) for single wavelengths analyses of the transient absorption data were performed using Igor Pro 6.20. Samples were contained in a quartz cell (10×10 mm2 section) at an adjusted concentration (10-4 mol.dm-3) to get an absorption value of about 0.5 at the pump excitation wavelength. The solutions were deoxygenated by bubbling argon for at least 20 min before the measurements. The femtosecond transient absorption setup used in this study was also already described in detail elsewhere.30 Briefly, in our experiments, a 1-kHz Ti:sapphire laser system delivers 100 fs (0.8 mJ) pulses at 800 nm. The 400 nm excitation pulses were obtained from the second harmonic of the amplifier output. Pump pulse energy at the sample was about 3 μJ with a diameter about 0.5 mm (1.5 mJ/cm2). The white light continuum probe beam is generated by focusing the fundamental beam in a 1 mm CaF2 rotating plate. The pump-probe polarization configuration is set at the magic angle (54.7°) and the probe pulse is delayed in time relative to the pump pulse using an optical delay line. The white light continuum is split into a probe beam (with pump) and a reference beam (without pump). The transmitted light of the probe and reference beam is recorded on two different channels of a multichannel spectrograph equipped with a CCD camera (Princeton Instrument) and the transient spectra are computed. The


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instrumental response function of about 200 femtosecond was estimated measuring the stimulated Raman amplification signal from the solvent. All experimental data were corrected from the group velocity dispersion and global decay analysis was used to get time constant. The sample cell path length was 1 mm. The studied compounds were diluted in acetonitrile at a concentration corresponding to an absorption of about 1 at the pump wavelength. Stability of the sample was checked after each experiment. Multivariate curve resolution – alternating least squares (MCR-ALS) Among the computational and statistical methods used to solve mixture analysis problems, self-modeling curve resolution describes a set of chemometric tools for estimating pure component spectra and concentration profiles from data matrices of mixture spectra recorded from an evolving chemical system (any chemical system that change in systematic way as a function of time, temperature, pH, etc.). Self-modeling curve resolution does not require any assumptions except a bilinear model for the data. In a bilinear model, a spectroscopic mixture represented by a matrix D containing m time-dependent spectra (rows) registered at n wavelengths (columns) can be described according to equation

D = CST + E where the

matrices C(m x N) and ST(N x n) contain the time-dependent concentration profiles and the characteristic spectra of the N absorbing transient species in the mixture, respectively. The matrix E(m x n) contains the residual signal, which is mostly due to experimental noise. MCRALS31-32 is one of the most widely used self-modeling methods to decompose two-way data. Only a brief description of the main steps of the method for the application to spectrophotometric data is given in the following. More comprehensive information can be found in the literature where the potential of the method for femtosecond transient experiments has been reported,33-36 The number N of components required to describe the maximum of the variance in D is first


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estimated, e.g. from singular value decomposition.37 The iterative alternating least squares (ALS) optimization then starts from initial estimates of C or ST and, in each cycle, the matrices C and ST are calculated under constraints. Constraints are applied to enforce some generic knowledge about the concentration and spectra profiles during the ALS optimization, such as non-negativity of the concentration values.

RESULTS AND DISCUSSION Synthesis of the dyads and triad. The preparation of the triad calls for the initial synthesis of the phenanthroline ligands L1, L2 and L3 substituted by a NDI or a ferrocene (Fc) moieties (Schemes 1-3). Scheme 1. Synthesis of the ligands L1. Reagents and conditions: a) Imidazole, Zn(OAc)2, 140°C; b) TFA, CHCl3, r.t.; c) NH4OAc, CH3COOH, CHCl3, reflux; d) hexyl iodide, Cs2CO3, DMF, 100°C.

Two Ligands L1 and L2 bearing the NDI moiety differ by the nature of the linker between the electron acceptor and the phenanthroline ligand. Both structures are rigid, and allow us to investigate the effect of phenyl imidazole spacer (a classical group used for the functionalization of phenanthroline ligands)38-42 on the electron transfer kinetics. The preparation of L1 follows a


8

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convergent synthesis using a NDI-substituted benzaldehyde intermediate 4, which was coupled with the dione 5 in the last reaction step to provide the first ligand L1 (Scheme 1). Towards this objective, the known aniline derivative 125 was reacted with naphthalene monoimide 243 in presence of a stoichiometric quantity of zinc acetate in molten imidazole as solvent. After purification, the aldehyde was deprotected by trifluoroacetic acid in chloroform in quantitative yield. The construction of the imidazole heterocycle was then performed according to a modification of the Steck and Day protocol,38, 44 using milder conditions and extra dried reagents to favor the formation of the imidazole instead of the oxazole derivative.26, 45 Since the imidazole derivative 6 is very polar, and tends to stack on silica gel, the crude reaction mixture was engaged in the alkylation step without further purification. Towards this goal, the imidazole unit was deprotonated with caesium carbonate and alkylated with hexyl iodide to afford ligand L1 in 29% yield (2 steps). Furthermore, this modification avoids dealing with a mixture of different species in equilibrium in solution since the imidazole proton in such ligand is quite acidic and therefore easily prone to deprotonation.46-47 The modest yield can be certainly ascribed to the formation of a minority but non negligible percentage of the oxazole derivative, and to the several purification steps required to separate the two molecules. The synthesis of ligand L2 was carried out in three steps starting from the commercially available neocuproine (2,9-dimethyl-1,10-phenanthroline) which underwent nitration.48 In a second step, the nitro group was reduced with hydrazine using palladium 10% on charcoal as catalyst (Scheme 2).27 The amine 8 was then heated in DMF with naphthalene imide mono anhydride 2 to furnish ligand L2 in 23% yield. Attempts to increase the yield by using different reaction conditions (microwave activation, use of molten imidazole as the solvent) did not lead to substantial improvement. The amine in 8 is hindered and deactivated by the electron


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withdrawing capacity of the phenanthroline core accounting thus for the low nucleophilicity of the amine. Scheme 2. Synthesis of the ligand L2. Reagents and conditions: a) HNO3, H2SO4, reflux; b) hydrazine monohydrate, 10% Pd/C, EtOH, reflux; c) DMF, 120°C.

Ligand L3 is composed of a 2,9-dimesityl-1,10-phenanthroline unit connected to a ferrocene moiety through an imidazole linker. It was synthesized in two steps starting from 2,9-dimesityl1,10-phenanthroline-5,6-dione 924 and ferrocene carboxaldehyde 10 with a modified Steck and Day protocol and obtained in a modest yield of 37%.38 Finally, the alkylation step was performed using the protocol described for L1, and afforded L3 as a beige-yellow solid in 83% yield. All the above ligands were characterized by

1

H and

13

C-NMR, and high-resolution mass

spectrometry confirmed the structures (cf. Supporting Information). Scheme 3. Synthesis of ligand L3. Reagents and conditions: a) NH4OAc, CH3COOH, CHCl3, reflux, Ar; b) hexyl iodide, Cs2CO3, DMF, 110°C.


10

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Scheme 4. Synthetic route to the dyads D1 and D2 and the triad.

Bu N CuI N

N

Hex N

N

N

L1

O

O

N

N Oct

O

O

Bu PF6-

N

+

[Cu(CH3CN)4]PF6

D1

CH2Cl2, RT

O

N

C8H17

N

L2 O

12

N

N Cu

N

O N O

N

PF6-

D2 PF6-

CH2Cl2, RT

N

N

N

C6H13 N

N

N

Cu

+ [Cu(CH3CN)4]PF6 + L3

N

N

Fe

D3 O C8H17

PF6-

N O

O

CH2Cl2, RT L2 + L3 + [Cu(CH3CN)4]PF6

N O

N

N

C6H13 N

N

N

Cu N

Fe

The preparation of the copper(I) complexes is quite straightforward and follows the HETPHEN strategy.19,

24, 43

It consists in mixing one equivalent of each phenanthroline ligand and one

equivalent of copper(I) tetrakisacetonitrile and affords nearly pure heteroleptic complexes which


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were, however, purified by column chromatography on silica gel (Scheme 4).19,

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24

All the

complexes are completely stable in solution and on silica gel during column chromatographies and were satisfyingly characterized by 1H,

13

C NMR spectroscopy and mass spectrometry (see

Supporting information). Steady-state absorption and luminescence spectra. The electronic absorption spectra of the three dyads D1, D2, and D3, and the triad along with the reference complexes C1 and C2 (see Chart 1) are given in Figure 1 and the related data are collected in Table 1. Complexes C1 and C2 show two characteristic bands: a first weak band at around 460 nm assigned to the usual MLCT transition and a more intense one, located below 300 nm, corresponding to π-π* transitions centered on the phenantroline. Absorption spectra of dyads D1 and D2 exhibit the characteristic absorption bands of the reference complex C1 and C2 and of the NDI moiety (cf. Figure S2). The latter is characterized by vibrationally structured bands with maxima at 340, 357 and 378 nm attributed to the π-π* transitions.22 This indicates that the ground-state electronic interaction of the phenanthroline moiety with the appended NDI is relatively weak in D1 and D2. This was much expected since frontier molecular orbitals have a node on the nitrogen atom of the NDI, interrupting thus the electronic communication.49 In dyad D3, the MLCT band of the copper complex is slightly more intense than in the other dyads owing to the superimposition of the ferrocene d-d transitions which occur in the same region.50 The UV-Visible spectrum of the triad is a linear combination of the spectra of dyads D2 and D3 and confirms the weak electronic perturbation upon attachement of the NDI and ferrocene to the phenanthroline ligands. The steady-state emission properties after MLCT excitation (460 nm excitation) of the dyads and triad were then measured in acetonitrile, in order to evaluate the possibility of a charge transfer between the excited state of the bis-phenthroline Cu(I) complex and the nearby electron


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acceptor (naphthalene diimide) or the electron donor (ferrocene). In dyads D1 and D2, given the highly reductive properties of excited copper(I)-diimine complexes, the MLCT should be oxidatively quenched by photoinduced electron transfer towards the NDI moiety (see below).

Figure 1. (a) Absorption and (b) luminescence (λex = 460 nm) spectra of Triad and its precursors recorded in CH3CN. Complexes C1 and C2 emit in the 700-800 nm region (Figure 1). It is attributed to the radiative decay of the triplet MLCT state, which corresponds to a charge transfer transition from copper(I) to a phenanthroline ligand.16-17 Dyad D1 has nearly identical features when excited at 460 nm, unravelling that no further photoinduced process takes place upon excitation of the copper complex. Interestingly, in the same conditions the MLCT emission of dyad D2 is strongly


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quenched, which is consistent with a the presence of an efficient photoinduced electron transfer to NDI. Table 1. UV-Visible absorption and emission (λex = 460 nm) data of the complexes recorded in CH3CN. Complex C1 C2

D1

D2

D3

Triad

a

λabs(nm)a 460 276 462 276 463 380 360 339 278 467 381 361 341 276 467 296 263 475 378 358 338 284 263

ε (M-1cm-1)b 4.5 × 103 5.0 × 104 5.5 × 103 4.9 × 104 4.9 × 103 3.7 × 104 3.5 × 104 3.1 × 104 8.9 × 104 5.0 × 103 2.6 × 104 2.3 × 104 1.6 × 104 4.4 × 104 6.4 × 103 5.0 × 104 6.7 × 104 6.6 × 103 3.3 × 104 3.2 × 104 2.9 × 104 6.3 × 104 6.5 × 104

λem (nm)

τ (ns)

~ 740

28 ± 5

~ 745

21 ± 4

~ 740

30 ± 5

n.d.

~ 750

9±3

n.d.

estimated error on λabs ±1 nm. bestimated error on ε ±15%. n.d. – not detectable.

Shining light in the MLCT band of dyad D3, barely shows very weak luminescence, indicating a quenching of the excited state. As the excited state of copper(I) diimine complexes is a quite poor oxidant, it seems unlikely to have a reductive quenching by electron transfer from the ferrocene to the excited Cu(I) moiety. On the other hand, ferrocene possesses low-lying d-d states which can act as efficient energy acceptors from the MLCT state of the complex.50 This


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quenching process could compete with the electron transfer towards the acceptor in the triad, but its rate constant is much slower than the oxidative quenching with the NDI (see time-resolved spectroscopy results below). Finally, as expected from dyad D2, excitation in the MLCT absorption band of the triad, evidences not detectable emission signal supporting thus an efficient quenching of the Cu(I) MLCT excited state and the formation of charge separated state. Electrochemistry and driving forces for charge transfer reactions. The electrochemical properties of the complexes were investigated by cyclic voltammetry (Figure S1) and by square wave voltammetry to determine the redox potentials of the copper(I), the NDI and of the ferrocene subunits (Table 2). Table 2. Electrochemical properties of the complexes recorded in dry degassed dichloromethane with TBAPF6 0.1 M as supporting electrolyte and referenced versus SCE. E1/2 (NDI0/-)

∆E

E1/2 (Fc+/0)

∆E

E1/2 (CuII/I)

∆E

V

(mV)

V

(mV)

V

(mV)

C1

─

─

─

─

0.99

140

C2

─

─

─

─

0.90

89

D1

-0.59

99

─

─

0.99

99

D2

-0.55

150

─

─

0.97

170

D3

─

0.62

87

0.90

89

Triad

-0.55

0.60

89

0.93

89

Complex

a

90

estimated errors on E1/2 and ∆E are ±20 mV The reduction of NDI in the dyads D1, D2 and triad occurs respectively at -0.59 V, -0.55 V,

and -0.55 V, which is in agreement with previously reported values for NDI-centered reduction.23,

43

The weak influence of the substituents on the reduction potential of the NDI

moiety is consistent with the presence of nodes in the HOMO and LUMO located on the nitrogen


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atom of the bisimide groups.43,

49

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In bis-phenantholine copper(I) complexes, it is generally

accepted that the first oxidation corresponds to the reversible removal of an electron from a copper(I) ion t2 orbital which has an antibonding character.16-17, 51-52 This potential is anodically shifted in complexes containing the bulkier butyl substituents on the phenanthroline (C1 and D1) compared to those with methyl substituents (C2, D2 and triad) owing to the restricted flattening of the complex upon copper(II) formation, which destabilizes the preferred square based pyramidal geometry of the latter. In general, Cu(I) complexes give rise to slow electron transfer processes, due to the large reorganization energy involved in the geometry change that accompanies the oxidation of Cu(I) to Cu(II). This leads to a difference between the oxidation and reduction peak potentials that is larger than the 60 mV expected for an ideally reversible monoelectronic process. In the dyad D3 and in the triad, the oxidation of ferrocene occurs around 0.60 V, which is more anodic than that recorded for the unsubstituted ferrocene (0.47 V vs.SCE in our conditions). This certainly reflects the electron withdrawing character of phenanthroline which removes electron density on the appended ferrocene. The weak variations of the redox potential measured for the same electroactive groups in the dyads and triad indicate weak electronic electronic between the components of the system. This is in agreement with the conclusions from the electronic absorption spectra. Taking into account the data from emission and electrochemistry, it becomes possible to determine the Gibbs free enthalpies of the charge transfer processes between the three electroactive groups. The oxidation E1/2(S+/S*) and the reduction E1/2(S*/S-) potential for the MLCT excited state of the copper(I) sensitizer can be estimated from equations 1 and 2 indicated below: E1/2(S+/S*) = E1/2(S+/S) - E00

eq. 1


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E1/2(S*/S-) = E1/2(S/S-) + E00

eq. 2

with E00 standing for the zero-zero energy of the triplet MLCT excited state, which was calculated with the wavelength at the intersection of the absorption and emission spectra (1.9 eV for complex C2). Let us begin by considering the photoinduced electron transfer to the NDI in the dyads D1 and D2 and triad, whose driving force (∆Get) can be calculated according to the equation 3: ∆Get = E1/2(S+/S*) - E1/2(NDI0/NDI-)

eq. 3

In both dyads D1-D2 and in the triad, the photoinduced electron transfer from the MLCT of the copper(I) complex to NDI is thermodynamically allowed because there is a substantial driving force around -0.4 eV within these three molecules. Conversely, the ferrocene subunit cannot quench the MLCT in dyad D3 and triad by photoinduced charge transfer process, because the excited state reduction potential of the copper(I) photosensitizer is below 0.4 V vs. SCE (since the phenanthroline-centered reduction is known to occur below -1.5 V vs. SCE53). For this reason, the most reasonable emission quenching process of MLCT in dyad D3 is ascribed to energy transfer from the excited-state copper(I) complex to d-d state of ferrocene, which was already reported in literature for other systems.50 Finally, in the triad the two electron transfer steps leading to the formation of the final charge-separated state Fc+-[CuI]-NDI•─ presented below (reactions 1 and 2) are energetically favorable: Fc-[CuI]*-NDI → Fc-[CuII]-NDI•─

(react. 1)

Fc-[CuII]-NDI•─ → Fc+-[CuI]-NDI•─

(react. 2)


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∆Ghs = E1/2(Fc+/Fc0) - E1/2(CuII/I)

Page 18 of 36

eq. 4

Indeed, using equations 3 and 4, it is possible to demonstrate that reactions 1 and 2 are thermodynamically allowed with the respective driving forces of -0.4 eV and -0.33 eV. Photo-dynamics of charge separation. Having demonstrated the thermodynamic feasbility of the stepwise electron transfer from the copper MLCT to NDI and then the hole shift to ferrocene, and based on the luminescence results, it can be presumed that D2 and the triad excited in the MLCT absorption band lead to the formation of NDI radical anion and CSS, as confirmed by the following nanosecond flash photolysis results. Photo-excitation of dyad D1 (Figure 2) and complex C1 (Figure S3) in acetonitrile at 460 nm yields a transient species characterized by a spectrum displaying a negative bleach band at the position of the ground state MLCT absorption (450 nm) and positive bands at 525, 575, and 380 nm. This spectrum features the typical signature of the 3MLCT excited state, which decays homogenously with a time constant of 30 ns (D1) and 28 ns (C1). Electron transfer towards the NDI unit in D1 is not perceptible in spite of its significant driving force, certainly because it is too slow due to the weak electronic coupling and the longer distance between the copper centre and the NDI. This result is consistent with the similar steady-state emission properties found for C1 and D1. Similarly, the nanosecond transient absorption spectra of dyad D3 (Figure S5) shows the characteristic signature of the 3MLCT excited state as for the reference complex C2 (Figure S4), with a faster decay time constant in D3 (9 ns) than in C2 (21 ns), in agreement with a quenching by energy transfer from the ferrocene unit as discussed previously. As expected, the transient absorption spectra of D2 and the triad excited at 460 nm (Figure 2) reveal the appearance of the typical spectral signature of NDI•─ with a strong band around 470 nm and an absorption extending up to 700 nm with a second distinct band at 605 nm.22 The bleaching of the ground


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state absorption of copper(I) diimine complexes in the visible region is not observed as its extinction coefficient (Table 1) is much smaller than that of the overlapped NDI radical anion band (ε ≈ 23 000 mol-1.L.cm-1)23 A bleach of the ground state absorption of the NDI moiety is also seen below 400 nm. For D2, the short distance between the sensitizer and the electron acceptor accounts for a short charge-separated state lifetime (≤ 4 ns, within the instrumental response). On the other hand, for the triad, the NDI ground state is recovered with a time constant of 34 ns (Figure 2). Such decrease of the back electron transfer rate by a factor of at least 10 times is attributed to the formation of a new, extended NDI•─-Cu(I)-Fc•+ CSS from the initial NDI•─-Cu(II)-Fc CSS by hole migration from the copper(II)-diimine moiety to the Fc group. Indeed, the increase in lifetime observed for the charge-separated state can only be explained by a larger distance between the two photogenerated charges.


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Figure 2. (a) Transient absorption spectra of D1 and D2 recorded in CH3CN 4 ns after 460 nm excitation and (b) corresponding kinetic traces. (c) Transient absorption spectra of the triad recorded in CH3CN at several times after 460 nm excitation. (d) Comparison of the kinetic traces of D2, C2, D3 and the triad recorded in CH3CN. In order to assess the time constants associated with the formation kinetics of the CSS in the dyad D2 and in the triad, femtosecond transient absorption measurements in acetonitrile were carried out. As reported in previous works, bis-phenanthroline copper(I) complexes present specific photophysical properties compared to ruthenium(II) polypyridyl complexes because the initial tetrahedral D2d geometry of the ground-state flattens into a D2 symmetry in the relaxed MLCT triplet excited state.51,

54-56

Ultrafast spectroscopic studies showed that the flattening

occurs within a few hundreds of femtosecond and intersystem crossing (ISC) within 10 picoseconds.54-55,57


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Figure 3. (a) Transient absorption spectra for C2 and D3 recorded at different time delays after 400 nm femtosecond excitation in CH3CN. (b) Kinetic trace of transient absorption at 600 nm and 580 nm fitted with a sum of exponential functions. These time constants are almost intrinsic and characteristic for all the homoleptic bisphenanthroline copper(I) complexes having substituents on the 2 and 9 positions. The observed long ISC time constant, which was initially discovered by Nozaki and co-workers,58 and investigated and rationalized recently,51,

54-57

is much slower than the usual hundreds of

femtoseconds values reported for ruthenium(II) polypyridyl complexes.59 First, femtosecond transient absorption measurements were carried out on the reference compounds C2 and D3 in


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Page 22 of 36

order to determine the short-time dynamics of the MLCT excited state as only homoleptic bisphenanthroline copper(I) complexes studies were reported in the literature.51, 54-55 A 100 fs pulse excitation at 400 nm of complex C2 in acetonitrile leads to the appearance of a negative bleach band in the 430–500 nm domain with a maximum at 460 nm, and a featureless broad absorption band covering the 500 to 650 nm domain (Figure 3). The latter grows up homogeneously until 5 ps, whereas the bleach band remains constant. Finally, the positive signal evolves in few tens of picoseconds toward a final spectrum with two maxima at 532 nm and 572 nm and a less extended red tail. After 50 ps, the spectrum remains unchanged and matches the one obtained by nanosecond transient absorption experiments (SI Fig. S4). The rise and decay of the 500 to 650 nm absorption band were analyzed by a global analysis method (Table S1) with 2 time constants of 360 fs and 10.6 ps, respectively. Referring to previous studies on homoleptic bisphenanthroline copper(I) complexes,51, 54-55 the 360 fs kinetic can be assigned to the flattening process in the 1MLCT excited state and the 10.6 ps to the intersystem crossing toward the final 3

MLCT excited state. The intensity of the bleach band does not change from 0.3 ps to 1 ns,

which indicates that there is no ground state repopulation during this timescale and implies a high quantum yield of formation of the 3MLCT as in ruthenium polypyridyl complexes. Indeed from the intensity of the bleach signal measured just after the excitation in nanosecond transient absorption (SI Fig. S4), a minimum value of 85% for the quantum yield of the formation of the 3

MLCT state can be estimated on the basis of the measured reference signal for the triplet state

of ruthenium trisbipyridine and the known stationary extinction coefficients of C2.30 For D3 (Figure 3) two similar time constants were also retrieved (350 fs and 14.5 ps, Table S1) as for C2. The second time constant assigned to ISC is characterized by a decay of the positive absorption band in the visible region, mainly in the red region part above 575 nm (Figure 3).


22

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However, for D3 the entire positive absorption band decays, which can be explained by a lower rigidity of the complex, as suggested by the absence of vibrational structure in the 3MLCT absorption spectrum for D3 compared to C2.

Figure 4. Transient absorption spectra for D2 at different time delays after 400 nm femtosecond excitation in CH3CN (a) and time-dependent concentration profiles (d) with the corresponding transient absorption spectra (c) obtained by the MCR-ALS (b). The transient absorption spectra of D2 measured in acetonitrile following femtosecond excitation at 400 nm within the MLCT absorption band are shown in Figure 4. Similar to C2 and D3, a broad positive band from 460 to 700 nm is growing with the excitation pulse and assigned in agreement with recent studies55,

57

to the convolution of the undistorted Franck-Condon S2

singlet MLCT excited state and the undistorted S1 singlet MLCT excited state after a relaxation


23


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Page 24 of 36

in less than 100 femtoseconds. Within several hundreds of femtoseconds this broad band evolves to a new trace with three maxima at 474 nm, 532 nm and 606 nm. In comparison with the literature, the flattening occurs with similar time constant and these bands are thus assigned to the contribution of flattened S1 singlet MLCT state (532 nm) and of the NDI•─ radical anion (474, 606 nm). However the instrumental response function of 200 femtoseconds does not allow us to clearly distinguish spectrally the ultrafast relaxation from the undistorted singlet MLCT S2 to S1, the change of structure to flattened singlet MLCT state, and the formation of NDI•─ radical anion. The growing of NDI•─ radical anion stops after the formation of flattened singlet MLCT state and Cu(II)-NDI•─ CSS is thus formed from the undistorted 1MLCT state. Then, in a few picoseconds, the flattened 1MLCT state absorption decreases as for D3 with the concomitant appearance of the fourth maximum at 572 nm (see the 15 ps spectrum), which can be clearly assigned to the triplet MLCT state populated by intersystem crossing. Finally, the NDI•─ radical anion spectrum grows again in a hundred of picoseconds, indicating that the Cu(II)-NDI•─ CSS is also produced from the 3MLCT state (about 60%, see Table S1). Scheme 5. Photophysical pathway for the formation of the charge separated state in dyad D2.


24

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The thorough analysis of femtosecond data was performed by multivariate curve resolution – alternating least squares35 (MCR-ALS, see experimental section) to extract the time-dependent concentration profiles as well as spectra of the different components. The results of concentration profile and corresponding spectra obtained for three species extracted during the resolution are shown in Figure 4. The comparison with C2 enables us to clearly assign the contribution of the following intermediates: 1MLCT (blue), 3MLCT (green) and Cu(II)-NDI•─ (red). As mentioned previously, the 1MLCT spectrum is a mixture of undistorted S2, S1 and flattened S1 excited states, leading thus to two time constants observed for the blue concentration profile, flattening and ISC steps respectively. The analysis of time-dependent concentration profiles confirms definitely that Cu(II)-NDI•─ has two precursors, the undistorted 1MLCT and 3MLCT states, and that flattened 1MLCT is the precursor of 3MLCT. A global analysis of time-dependent concentration profiles leads to the following three time constants (Scheme 5): 420 fs for the parallel formation of flattened 1MLCT and Cu(II)-NDI•─ 1CSS from the undistorted

1

MLCT

state (S2 and S1); 10 ps for the ISC, i.e., formation of flattened 3MLCT; and 147 ps for the


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Page 26 of 36

formation of Cu(II)-NDI•─ 3CSS from 3MLCT. No spectral difference is found for 3CSS and 1

CSS, however it cannot be excluded that the second step of 10 ps can also be assigned to the

ISC from 1CSS to 3CSS and that the decrease of signal interperted as a minor back electron transfer pathway from the 1CSS. Employing [Ru(bpy)3]2+ as an actinometer, a global quantum yield formation of CSS of about 93% could be precisely determined for D2. Secondly from the pre exponential factor found at 475 nm quantum yields of 60% and 40% can be deduced for the flattening and electron transfer steps, respectively (Scheme 5). From these values and the time constant of the first step, a rate constant of about 1.4 × 1012 s-1 (700 fs) is found for flattening. This increase of the characteristic time for flattening in comparison to C2 (360 fs) is in agreement with the recent studies from Tahara and co workers who showed that flattening is concomitant with rotation of the group substituted on the phen ligands and increases until 920 ps for copper(I) bisdiimine complexes with 2,9-diphenyl-1,10-phenanthroline ligands.57 Finally, the back electron transfer time constant in D2 was also evaluated precisely to 2.7 ns. It should be noted that the observed electron transfer is ultrafast (k1et = 1.0 × 1012 s-1) when occurring from the undistorted 1MLCT geometry compared to the one from the flattened 1MLCT and 3MLCT states (k2et = 6.8 × 109 s-1). This result can be related to recent report of an ultrafast electron injection rate in TiO2 conduction band from the

1

MLCT of copper bis-phenanthroline

sensitizer.60 It underlines the fact that, in such bis-diimine copper(I) complexes, the CSS formation kinetics is strongly dependent on the MLCT excited state geometry dynamics. One can argue that 400 nm excitation leads to higher excited states and that electron transfer occurs from S2 state but not from S1 state. To infirm this hypothesis, experiments with 500 nm excitation were conducted and similar time constant were measured (580 fs, 6.5 ps, 136 ps, see Table S2, SI Fig. S6) with an increase of quantum yield for flattening pathway from undistorted 1MLCT with


26

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a value of about 70%. This could be assigned to a slower electron transfer due to a lowering of the driving force due to a lower energy lying S1 excited-state. Altogether the reason for the strong decrease of electron transfer from flattened geometry in comparison to the undistorted one is an experimental result which needs further theoretical calculation to rationalize it. One hypothesis that needs to be confirmed is that the geometry has important influence regarding the orbital electronic coupling and electron transfer efficiency. Indeed it seems that a linear geometry such as the one for D1 and for flattened MLCT is not favorable to fast electron transfer.

Figure 5. (a) Transient absorption spectra for the triad recorded at different time delays after 400 nm femtosecond excitation in CH3CN. (b) Kinetic trace of transient absorption at 475 nm fitted with a sum of exponential functions. Transient absorption spectra of the triad upon femtosecond excitation at 400 nm are shown in Figure 5. The spectral evolution is essentially similar to that found for dyad D2 and no spectral signature of the Fc•+ species can be identified, as expected because of the very weak absorption of ferrocenium.61 Based on the driving force of each electron transfer reaction, the first process is


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Page 28 of 36

expected to be the formation of the NDI•─-Cu(II)-Fc, followed by hole shift from copper (II) to Fc to yield the NDI•─-Cu(I)-Fc•+ extended CSS. As the distance between Fc and the copper atom is short, the hole shift is expected to be ultrafast, in such a way that the intermediate NDI•─Cu(II)-Fc CSS cannot be the detected. From global decay analysis three time constants were identified for the triad as for D2 (an example of single kinetic trace is shown in Figure 5) and a precise photo-dynamical scheme can be drawn (Scheme 6), characterized by the formation of NDI•─-Cu(I)-Fc•+ in two parallel, rate-limiting steps of 540 fs and 162 ps associated with the population of the intermediate NDI•─-Cu(II)-Fc 1CSS and 3CSS from the undistorted 1MLCT state and 3MLCT state, respectively. However, since the final hole transfer is too fast to be resolved, the spectra appearing with time constants of 540 fs and 162 ps must characterize the NDI•─-Cu(I)-Fc•+ extended CSS. Similarly to D2 there is no signature for the ISC from 1CSS to 3

CSS, 500 nm excitation gave the same time constant (SI Fig. S7) and from pre exponential

factor quantum yield of 50% (SI Table S1) were found for electron transfer and flattening from 1

MLCT, which leads to a rate constant of about 0.9 × 1012 s-1 (1.1 ps). The increase of the

flattening time constant in the triad in comparison to D2 (700 fs) and C2 (360 fs) can be accounted for the fact that substituted group needs more time for rotation. More interestingly, the ground state bleach signal does not change within 1 ns, indicating that the quantum yield for CSS formation is about 100%. Indeed, employing [Ru(bpy)3]2+ as an actinometer, a global quantum yield formation of CSS of about 90% could be precisely determined for the triad. Scheme 6. Photophysical pathway for the formation of the charge separated state in the triad.


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CONCLUSIONS This study presents the first synthesis of a pure and stable Cu(I)-based Donor−Cu(I)−Acceptor triad featuring efficient stepwise photo-induced charge separation upon excitation of the copper(I) MLCT excited state or NDI acceptor group. Two different spacers between the electron acceptor and Cu(I) entities were studied through the synthesis and electrochemical characterization of two dyads. Although the formation of charge separation state (CSS) is thermodynamically allowed, the phenyl imidazole spacer kinetically inhibits it. The reason behind this effect may be related to the weak electronic coupling. This is also the first report on ultrafast photo-dynamics for heteroleptic bis-phenanthroline copper(I) complexes. The detailed analysis of femtosecond transient absorption data for MLCT excitation allowed us to assign three time constants involved in the formation of charge separation state CSS with an overall quantum yield of formation of about 90%. Another important result is that the CSS formation time constant is strongly dependent on the initial flattening occurring in the excited state for bisdiimine copper(I) complexes. The use of substituents at 2,9 position of phenanthroline ligands of


29


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Page 30 of 36

various bulkiness could be a strategy to control the efficiency of CSS. Altogether a thorough final scheme for the formation of CSS can be drawn for the first time for a Donor−Cu(I)−Acceptor triad. These results are unique and should set the path to design future tunable multicomponent molecular systems based on bis-diimine copper(I) complexes for photoinduced charge separation. The use of earth abundant d-block metals represents an important step to establish the viability of solar energy systems without the constraints of materials cost and sustainability. ASSOCIATED CONTENT Supporting Information. Detailed synthetic procedures and characterizations of the complexes (NMR data, mass spectrometry data, cyclic voltammograms, transient absorption data). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * E-mails: [email protected] (M.S.), [email protected] (F.O.) Present Addresses # CEMCA UMR 6521 – Université de Bretagne Occidentale – 6, avenue Victor Le Gorgeu – 29238 Brest, France Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ¶ These authors contributed equally.


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The Agence Nationale de Recherche (ANR) is gratefully acknowledged for the financial support through the ANR Blanc “HeteroCop” (ANR- 09-BLAN-0183-01). F.O. warmly thanks Benoit Colasson (Université Paris Descartes) for fruitful and inspiring discussions about the present work. REFERENCES (1) Concepcion, J. J.; House, R. L.; Papanikolas, J. M.; Meyer, T. J. Chemical approaches to artificial photosynthesis. Proc. Natl. Acad. Sci. USA 2012, 109, 15560-15564. (2) Gust, D.; Moore, T. A.; Moore, A. L. Solar Fuels via Artificial Photosynthesis. Acc. Chem. Res. 2009, 42, 1890-1898. (3) Wang, M.; Na, Y.; Gorlov, M.; Sun, L. Light-driven hydrogen production catalysed by transition metal complexes in homogeneous systems. Dalton Trans. 2009, 6458-6467. (4) Rau, S.; Walther, D.; Vos, J. G. Inspired by nature: light driven organometallic catalysis by heterooligonuclear Ru(ii) complexes. Dalton Trans. 2007, 9, 915-919. (5) Balzani, V.; Credi, A.; Venturi, M. Photochemical Conversion of Solar Energy. ChemSusChem 2008, 1, 26-58. (6) Campagna, S.; Puntoriero, F.; Nastasi, F.; Bergamini, G.; Balzani, V. Photochemistry and Photophysics of Coordination Compounds: Ruthenium. Top. Curr. Chem. 2007, 280, 117-214. (7) Gust, D.; Moore, T. A.; Moore, A. L. Mimicking Photosynthetic Solar Energy Transduction. Acc. Chem. Res. 2000, 34, 40-48. (8) Wenger, O. S. Long-range electron transfer in artificial systems with d6 and d8 metal photosensitizers. Coord. Chem. Rev. 2009, 253, 1439-1457. (9) Scandola, F.; Chiorboli, C. Electron transfer in chemistry; Weinheim, 2001. (10) Sauvage, J. P.; Collin, J. P.; Chambron, J. C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Ruthenium(II) and Osmium(II) Bis(terpyridine) Complexes in Covalently-Linked Multicomponent Systems: Synthesis, Electrochemical Behavior, Absorption Spectra, and Photochemical and Photophysical Properties. Chem. Rev. 1994, 94, 993-19. (11) Hankache, J.; Niemi, M.; Lemmetyinen, H.; Wenger, O. S. Photoinduced Electron Transfer in Linear Triarylamine-Photosensitizer-Anthraquinone Triads with Ruthenium(II), Osmium(II), and Iridium(III). Inorg. Chem. 2012, 51, 6333-6344. (12) Abrahamsson, M.; Jager, M.; Kumar, R. J.; Osterman, T.; Persson, P.; Becker, H.-C.; Johansson, O.; Hammarstrom, L. Bistridentate ruthenium(II)polypyridyl-type complexes with


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Design of Efficient Photoinduced Charge Separation in DonorâCopper

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Design of Efficient Photoinduced Charge Separation in DonorâCopper