Interplays of excited state structures and dynamics in copper(I) diimine complexes: Implications and perspectives

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Contents lists available at ScienceDirect

Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Interplays of excited state structures and dynamics in copper(I) diimine complexes: Implications and perspectives Michael W. Mara a,b , Kelly A. Fransted b , Lin X. Chen a,b,∗ a b

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, USA

Contents 1. 2.

3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structural influence/control of the MLCT state properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ground-state structure and UV–vis spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. 2.2. Ground-state structure and X-ray absorption spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structural influence on excited-state dynamics of Cu(I) diimine derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Structure and solvent-dependent emission lifetimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Jahn–Teller flattening and intersystem crossing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Structural characterization using X-ray transient absorption spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Potential energy diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Cu(I) diimine complexes in solar energy conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Light-harvesting properties of homoleptic copper diimine complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Light-harvesting properties of heteroleptic copper diimine complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

a r t i c l e

i n f o

Article history: Received 31 January 2014 Received in revised form 28 May 2014 Accepted 13 June 2014 Available online xxx Keywords: Copper diimine Transition metal complexes Excited state structure Solar energy conversion Ultrafast X-ray absorption Ultrafast optical transient absorption

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

a b s t r a c t Although they were discovered almost four decades ago, Cu(I) diimine complexes have emerged as a group of transition metal complexes that can play important roles in solar energy conversion and utilization, and have potential to replace the quintessential ruthenium polypyridyl complexes as light sensitizers, electron donors and catalytic centers. This review includes some recent photophysical studies and transient structural studies of Cu(I) diimine complexes using ultrafast optical transient absorption and emission as well as X-ray transient absorption spectroscopy. The main focus is on identifying the key structural factors that influence the excited-state properties, such as structural reorganization, intersystem crossing and solvent quenching, with these relatively new techniques on the ultrafast time scales. Ultimately, these structural factors can be used to rationally control the energetics and dynamics of the MLCT state during the light conversion processes. This insight will serve as guidance for material design using Cu(I) diimine complexes as building blocks. © 2014 Published by Elsevier B.V.

1. Introduction Copper diimine complexes, discovered decades ago [1–7], have played increasingly important roles in sustainable energy

∗ Corresponding author at: Chemical Science and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, USA. Tel.: +1 630 252 3533; Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA. Tel.: +1 847 491 3479. E-mail addresses: [email protected], [email protected] (L.X. Chen).

systems from photovoltaics to catalysis as well as molecular machine building blocks [3,8–11]. One of the reasons for their rising popularity is their potential in replacing the quintessential ruthenium polypyridyl complexes in solar energy conversion processes as both the light sensitizer and electron donor in catalysis and dye sensitized solar cells because the crystal abundancy ratio of Cu/Ru is 60,000 (60 ppm vs. 0.001 ppm) [12]. Extensive photochemical and photophysical studies carried out before the turn of the century by several pioneers have provided important photophysical insight for several copper(I) diimine complexes as

http://dx.doi.org/10.1016/j.ccr.2014.06.013 0010-8545/© 2014 Published by Elsevier B.V.

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shown in several previous reviews. The key findings from these studies are that (1) Cu(I) diimine complexes undergo the metalto-ligand-charge-transfer (MLCT) transition in the visible region similar to that of RuII tris-bipyridal complex, or [RuII (bpy)3 ]+2 , where one electron is transferred from the Cu(I) center to one of the diimine ligands, forming a transient Cu(II) species; (2) the MLCT state undergoes significant structural reorganization because its transformation from a closed shell 3d10 electronic configuration in the ground state to an open shell 3d9 electronic configuration is subject to a Jahn–Teller distortion, so that the orthogonal ligand planes in the ground-state pseudo-tetrahedral geometry flatten by decreasing the angle between the two ligand planes; (3) the coordinating solvent molecules form an “exciplex” with the MLCT state, lowering the energy gap between the ground state (GS) and the MLCT state, and shortening the excited-state lifetime. The fundamental understanding of the MLCT state structural dynamics and their correlation with the energetics and lifetimes enables further exploration of the potential applications of these Cu(I) complexes as sensitizers and electron donors in solar energy conversion devices [13–18]. Meanwhile, these studies also revealed obstacles in utilizing these Cu(I) complexes for solar fuel or electricity generation, such as a relatively short excitedstate lifetime, solvent quenching, poor photostability and relatively labile Cu(II) intermediates [19,20]. In spite of these studies, some questions on the excited-state properties remain: (1) what are the molecular conformations that prolong the MLCT state lifetime; (2) can the MLCT state be a long-lived species in strongly coordinating solvents, e.g. water, methanol, etc.; (3) what structural factors could affect the intersystem crossing rate of the MLCT state; (4) can the MLCT state inject electrons into semiconductor nanoparticles, similar to Ru(II) complex sensitizers; and (5) can the MLCT state perform catalytic functions? Ultimately, these questions can be answered through detailed investigations of the excitedstate behavior from the fundamental aspects of light-matter interactions. During the past decade, much progress has been made in understanding the excited-state structural dynamics on the ultrafast time scales (100 fs–100 ps) and interplays between the structural factors and excited-state properties [17,18,21–35], largely due to the application of femtosecond optical transient spectroscopy and the availability of pulsed X-ray sources that enable direct measurements of the MLCT state transient structures. Furthermore, advances in computational chemistry also enable quantum mechanical calculations on transition metal complexes, bringing important insight into the molecular orbital (MO) energy level, electron density distribution, and structure [21,30,36]. In this short review, we intend to summarize our and others’ recent results on excited-state structural dynamics on Cu(I) diimine complexes that are relevant to solar energy conversion, which include answers to some of the questions raised above as well as ongoing studies. The review will be focused mainly on two aspects: (1) the structural influence/control of the MLCT state properties and (2) recent development of solar energy conversion applications using Cu(I) diimine complexes. While there have been important advances in understanding the excited state properties of crystalline Cu(I) diimine complexes [37–39], this publication focuses on solution studies in order to describe the solvent interactions that are most relevant to the photochemical processes involving these complexes.

2. Structural influence/control of the MLCT state properties Unlike a typical Ru(II) complex, such as [Ru(II)(bpy)3 ]2+ that has a relatively rigid structure due to the octahedral coordination geometry, a typical Cu(I) diimine complex has a pseudo-tetrahedral coordination geometry. The flexibility of the pseudo-tetrahedral

geometry leaves room for structural reorganizations in the excited MLCT state, which can significantly influence the excited-state properties. On one hand, such reorganization could make the complex vulnerable to chemical processes that cause deligation or form other photochemical products. On the other hand, such molecular motions in the excited states afford possibilities of tuning the excited-state properties via chemical modifications that could hinder or impede the structural reorganization, and hence influence the excited-state properties. We will first describe systematic changes in the physical properties of a series of Cu(I) diimine complexes with variations in their structures and then discuss the outcome of excited-state properties due to these structural changes. 2.1. Ground-state structure and UV–vis spectrum As pointed out by the pioneering work of McMillin et al. [4,6,7,13,40–48], one of the most important structural factors influencing the excited-state properties of Cu(I) diimines is the substitution groups at the 2,9-positions that effectively block the flattening of the pseudo-tetrahedral coordination and the solvent quenching of the MLCT state. When Cu(I) ligates with various 2,9substituted phenanthroline ligands, UV–vis spectral variations are also observed as shown by examples in our studies on a series of 2,9substituted copper diimines (Fig. 1). Regardless of the substituent, the nature of the electronic transitions remains similar with systematic variations. For all Cu(I) oxidation state systems, absorption in the visible spectrum originates from MLCT transitions. Excitation at these bands depletes the electron density from the Cu(I) center, resulting in a nominally Cu(II) center in the MLCT state, while the electron density shifted to the ligands is delocalized over the two phenanthroline ligands; the degree of delocalization is partly dependent on the substituent at the 2,9 positions [49]. A typical UV–vis spectrum (Fig. 1) shows two bands assigned as: a higherenergy S2 ← S0 transition, typically located around 450 nm, and a broad S1 ← S0 shoulder out in the red portion of the spectrum. Historically, the low energy, red shoulder has been referred to as the “I band” while the main absorption peak is labeled the “IIa band” [50]. However, to be consistent with the current literature, the absorption bands will be referenced by the state that is excited in the transition. The MLCT state dynamics are similar when S1 ← S0 or S2 ← S0 transition is induced with an exception in the latter because of the additional ultrafast internal conversion from the S2 to the S1 state. The effect of the 2,9 substituents on the photophysical properties of the copper complex is primarily determined by two factors: the geometry the substituents impose on the copper center, and how well the substituents block solvent or electrolyte access to the copper center. The 2,9 substituents generally can be sorted into two types: alkyl and aryl. With alkyl substitutions, the properties of the complex are decided mainly by the size of the substituent. For example, the unsubstituted [Cu(I)(phen)2 ]+ complex has an aver˚ while the methyl-substituted age Cu N bond distance of ∼2.03 A, [Cu(I)(dmp)2 ]+ has a 2.07 A˚ average bond distance [51] and the tert-butyl-substituted [Cu(I)(dtbp)2 ]+ has a 2.11 A˚ average bond distance [33,52,53]. Furthermore, the degree of ground-state flattening decreases with an increase of the bulkiness of the groups at the 2,9 positions. However, the effect of aryl-substituents is not quite as straight-forward. Phenyl-substitutions at the 2,9 positions result in a flattened [Cu(I)(dpp)2 ]+ complex in the ground state [54]. In this case, the flattening is stabilized by ␲–␲ interactions between the phenyl rings with the opposite phenanthroline ligands [55]. Therefore, the ground-state structure of copper bisphenanthrolines is affected both by the size and shape of the 2,9 substituents. Altering the 2,9 groups does not significantly affect the ground-state electronic properties of copper diimines, but the

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3

1.4 0.20 1.0 0.15 0.8 0.6 0.4

[Cu(dpp)2]

+

[Cu(dmp)2]

+

0.10 0.05 0.00

0.2 -0.05

Absorption (derivative)

Absorption (normalized)

0.25 1.2

0.0 -0.10 -0.2 8.97 8.98 8.99 9.00 9.01 9.02 9.03 9.04

Energy (keV) Fig. 2. XANES spectra of [Cu(I)(dmp)2 ]+ (blue) and [Cu(I)(dpp)2 ]+ (black), with the derivative of the spectra shown as dotted lines.

shoulder feature for the methyl-substituted [Cu(I)(dmp)2 ]+ above 500 nm indicates the dynamic nature of the ground-state structure that could have a transiently flattened conformation due to the low frequency vibrational motions that enable S1 ← S0 transition. Similarly, [Cu(I)(phen)2 ]+ , which has no alkyl groups to provide steric hindrance, exhibits an even stronger shoulder feature [57]. A much stronger shoulder feature is observed for [Cu(I)(dpp)2 ]+ [21,35] and [Cu(I)(dppS)2 ]+ [22], both of which have aryl 2,9 substituents that hinder the pseudo-tetrahedral coordination geometry. Instead, the energy-minimized structure of the molecules in the ground state is heavily-flattened, which correlates the red-shifted shoulder feature with the coordination geometry and is consistent with previous quantum mechanical calculations and suggests that the oscillator strength for the S1 ← S0 transition increases as the coordination symmetry changes from D2d (pseudo-tetrahedral) to D2 (flattened tetrahedral). 2.2. Ground-state structure and X-ray absorption spectra

Fig. 1. Top: UV–vis spectra of various Cu(I) bisphenanthrolines, showing the S2 ← S0 (∼400–500 nm) and S1 ← S0 (∼500–650 nm) MLCT transitions. Emission spectrum of [Cu(I)(dtbp)2 ]+ is also shown on the right y-axis. Bottom: Structures of the complexes shown in the spectra with labels corresponding to the line colors in the top panel.

transformation from a D2d to a D2 geometry does have a significant effect both on the electronic and structural properties of the excited state. Generally, these geometric changes shift the energy of the S2 ← S0 peak and change the intensity of the S1 ← S0 peak. The energy shift in the S2 ← S0 peak is attributed to a change in the Cu N bond distance: for instance, [Cu(I)(dtbp)2 ]+ exhibits a Cu N bond distance of 2.11 A˚ [33,52], much longer than the 2.02–2.08 A˚ distance typically observed for these complexes [51,55,56]. As a result, [Cu(I)(dtbp)2 ]+ exhibits more destabilized ground and MLCT states than the other copper complexes and ends up with a higher energy S2 ← S0 peak. The more noticeable change, however, is the difference in absorption coefficient at the lower energy S1 ← S0 peak. For a D2d complex, S1 ← S0 excitation has a transition dipole of nearly zero and hence very little oscillator strength [36]. The transition probability increases as the complex is distorted from the D2d geometry; as a result, the S1 ← S0 peak can function as a measure of the degree of flattening of the copper complex. The weakest S1 ← S0 transitions are observed for the tert-butyl substituted [Cu(I)(dtbp)2 ]+ and [Cu(I)(tbp)2 ]+ complexes [53], as these have the bulkiest 2,9 groups and retain the most pseudo-tetrahedral geometry of all of the included complexes. In comparison, a more visible

Electronic transitions indicative of the flattening distortion can not only be measured by optical methods, but also by X-ray absorption spectroscopy (XAS). While XAS is most well known for containing a wealth of direct structural information in the extended X-ray absorption fine structure (EXAFS) [58,59], electronic information regarding a specific atom can be extracted from the lower-energy X-ray absorption near-edge structure (XANES) spectrum. XANES spectra of [Cu(I)(dmp)2 ]+ and [Cu(I)(dpp)2 ]+ are displayed in Fig. 2. In particular, these Cu(I) diimine complexes exhibit a 1s → 4pz peak in the middle of the copper K-edge, which is characteristic of the coordination geometry of Cu(I) center [60]. The 4pz orbital is slightly populated due to 3d-4p mixing; however, this mixing is mitigated by distorting from D2d to D2 geometry. As a result, complexes that adopt a mostly flattened ground-state structure will exhibit a larger magnitude 1s → 4pz peak, as shown by comparing the XANES spectra of [Cu(I)(dmp)2 ]+ and [Cu(I)(dpp)2 ]+ in Fig. 2. [Cu(I)(dpp)2 ]+ exhibits a sharp, well-defined peak at 8.983 keV, while [Cu(I)(dmp)2 ]+ exhibits a much weaker peak at this energy that almost appears to be a shoulder on the edge. As stated before, replacement of the methyl groups at the 2,9 phenanthroline positions with phenyl rings results in an increased tendency to flatten in the ground state. 3. Structural influence on excited-state dynamics of Cu(I) diimine derivatives The excited-state dynamics of copper diimines have largely been studied by spectroscopic [4–6,21,22,27,35,43,50,52, 53,57,61–63] and computational methods [21,30,34,36]. As

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Fig. 3. Photochemical pathways of Cu(I) diimine complexes when the solvent molecules can access the Cu center. Complex is assumed to be in the S1 state, as internal conversion occurs on a much faster time scale than flattening and exciplex formation.

excitation into the S2 ← S0 MLCT band results in fast (<50 fs) internal conversion into the S1 state, the dynamics of these systems occur on the S1 potential energy surface. This is the case even for samples where direct S1 ← S0 excitation in the ground state is completely blocked, such as for [Cu(I)(dtbp)2 ]+ . Excitation into the MLCT states of Cu(I) diimine derivatives results in a nominally Cu(II) center with 3d9 electronic configuration. As a result, the MLCT state of these complexes exhibits a second-order Jahn–Teller distortion [27,64] and experience a further flattening distortion in the excited state as shown in Fig. 3. Meanwhile, the singlet MLCT state undergoes intersystem crossing to produce the triplet MLCT state, on a timescale of a few to >15 ps depending on the configuration of the MLCT state. According to Nozaki and coworkers [30], the spin–orbit coupling tends to decrease as the molecule flattens due to the symmetry change from D2d , which has strongly-coupled spin–orbital wavefunction overlap, to D2 , where the stabilized molecular orbitals (MOs) have reduced spin–orbital wavefunction overlap. Consequently, the intersystem crossing (ISC) time constant is a function of the angle between the two phenanthroline planes as observed in the experiments described in detail below. The overall dynamics of the MLCT state for several Cu(I) diimine complexes can be described by three time constants: (1) <1 ps, assigned as the Jahn–Teller distortion, or flattening, which is absent when the structural change is severely hindered, such as in [Cu(I)(dtbp)2 ]+ and [Cu(I)(dppS)2 ]+ , (2) 3–20 ps, assigned as intersystem crossing (ISC), which is a function of the excitedstate geometry and increases as the flattening becomes more significant, and (3) 1 ns–a few ␮s, assigned as 3 MLCT decay, which is determined by the direct interactions of the Cu center and the solvent, and depends on the solvent accessibility to the metal center. Since 3d9 Cu(II) complexes tend to form penta- or hexacoordinate complexes, the Cu(II) center of the MLCT state tends to bind a solvent or anion to form an “exciplex”. From the current experimental observations on the excited-state dynamics of these

Cu(I) diimine derivatives, when the MLCT state Cu(II) center is more exposed or a stronger electron donating polar solvent is present, a stronger “exciplex” is formed, lowering the MLCT state energy and opening up non-radiative pathways. The flattening distortion exposes the copper center, facilitating the formation of the exciplex. Exciplex formation can be prevented either by adding groups to the phenanthroline ligand that block access of solvent or other molecules to the copper, by using solvents with weaker coordinating strength, or by increasing the size of the coordinating molecule such that it becomes too bulky to attack the copper center (Fig. 4). 3.1. Structure and solvent-dependent emission lifetimes Time-resolved emission measurements have been used to determine the effect of the solvent on the MLCT state dynamics in various forms of copper diimine [5,35,50,52,57,61,65]. With exception of some carried out more recently by fluorescence upconversion on the time scale of <20 ps by us [35], as well as by Tahara and coworkers [27,63,66], most of the measurements were done by time-correlated single photon counting on the time scale of a few tens of ps–a few ␮s, when the solvent effects take place. Because the dynamics of the two shorter time constants, <1 ps and <20 ps, are mostly inner-sphere processes with minimal solvent effect, we list here separately the longer dynamic properties first and discuss the ultrafast dynamics in the next section. Table 1 provides a list of luminescence decay time constants from the 3 MLCT state on the time scale of a few tens of ps to a few ␮s as a function of solvent for a number of these complexes. While [Cu(I)(phen)2 ]+ , which has only hydrogens at the 2,9 positions of phen, exhibits virtually no luminescence at room temperature in solution, the addition of various groups to the phenanthroline ligand generally results in complexes that are emissive in non-coordinating solvents such as dichloromethane or toluene. The 2,9 positions are particularly

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dbp

phen

dmp

dbd

dbtm

mp

5

dop

dsbp

dtbp

dnpp

p

dpp

bcp

tpp

dptm

dpd mp

p

dmp

dnp

tmbp

p

detp Fig. 4. Ligands for Cu(I) diimine complexes listed in Table 1.

important for extending the excited-state lifetime, as ligands at these positions help to impede solvent access to the copper center. While [Cu(I)(detp)2 ]+ has rather bulky ligands at the 3,8 positions of the phenanthroline ligand, the 2,9 positions are bare, resulting in an extremely short lifetime and small emission quantum yield [57]. Similarly, [Cu(I)(dmp)2 ]+ with the methyl groups at the 4,7 positions exhibits no luminescence, while [Cu(I)(dmp)2 ]+ with those same groups at the 2,9 positions does exhibit luminescence [61]. To a large extent, both the emission lifetime and the emission quantum yield will increase as the bulkiness of the 2,9 substituents increases. For example, if the alkyl groups of [Cu(I)(dmp)2 ]+ are increased in length, the quantum yield and emission lifetime increase accordingly; for the series of [Cu(I)(dmp)2 ]+ , [Cu(I)(dbp)2 ]+ , and [Cu(I)(dop)2 ]+ in dichloromethane, the quantum yields go as 4 × 104 , 9 × 104 , and 10 × 104 , and the emission lifetimes as 90 ns, 150 ns, and 155 ns, respectively [5]. These values

can be extended further by adding branching groups to the alkyl chains, as seen for [Cu(I)(dnpp)2 ]+ and [Cu(I)(dsbp)2 ]+ ; the branching groups further protect the copper center in the excited state, increasing the quantum yields and emission lifetimes to 16 × 104 and 260 ns and 45 × 104 and 400 ns, respectively [5]. While substitution at the 2,9 positions is imperative for observing luminosity, substitution at the other phenanthroline positions still has a significant effect on luminescence. For example, while both [Cu(I)(dbp)2 ]+ and [Cu(I)(dbdmp)2 ]+ have comparable emission time constants, adding two more methyl groups at the 3,8 positions in [Cu(I)(dbtmp)2 ]+ increases the lifetime constant by a factor of ∼6 [61]. A similar effect is observed for the 2,9-phenyl substituted equivalents of these complexes, though the enhancement effect is less dramatic. Addition of phenyl rings to the 4,7 positions of the phenanthroline ligand results in a slight decrease in emission lifetime for [Cu(I)(dmp)2 ]+ and [Cu(I)(dpp)2 ]+ ; however,

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6

Table 1 Excited-state luminescence lifetimes of various copper complexes. Complex

Acetonitrile  (ns)

[Cu(I)(phen)2 ]+ [Cu(I)(dmp)2 ]+ [Cu(I)(dpp)2 ]+ [Cu(I)(dnpp)2 ]+ [Cu(I)(dbp)2 ]+ [Cu(I)(tpp)2 ]+ [Cu(I)(dpdmp)2 ]+ [Cu(I)(dbdmp)2 ]+ [Cu(I)(dptmp)2 ]+ [Cu(I)(dbtmp)2 ]+ [Cu(I)(detp)2 ]+ [Cu(I)(bcp)2 ]+ [Cu(I)(dnp)2 ]+ [Cu(I)(tmbp)2 ]+ [Cu(I)(dmpp)2 ]+ [Cu(I)(dop)2 ]+ [Cu(I)(dsbp)2 ]+ [Cu(I)(dtbp)2 ]+ a b c d e f g

0.016a 1.6b 120d 100b 35b

260d 440d 0.014a

50b 130b

max (nm) 700 740 685 690

735 690

690 670

Dichloromethane

Tetrahydrofuran

 (ns)

 (ns)

0.14a 90c 270d 260b 150b 230e 310d 145d 480d 920d 0.2a 80e 250e 18e 237f 155b 400b 3260g

max (nm) 690 720 665 670 745 720 710 715 670 770 715 775 720 673 650 595

Methanol max (nm)

<10c 190d 140b 50b

700 730 675 680

350d 630d

725 670

55b 200b

 (ns)

max (nm)

120

720

683 660

Indicates Ref. [57]. Indicates Ref. [5]. Indicates Ref. [35]. Indicates Ref. [61]. Indicates Ref. [50]. Indicates Ref. [65]. Indicates Ref. [52]

such a substitution does slightly increase the emission quantum yield, red-shifts the maximum emission wavelength, and increases the maximum absorption coefficient in the ground state [50]. It is likely that the red-shifting of the emission is a result of conjugation between the 4,7 phenyls and the phenanthroline ligands, lowering the energy of the phenanthroline ␲* orbital. Most Cu(I) diimines exhibit a Jahn–Teller flattening distortion in the excited state, with only a few exceptions in the cases of complexes with extreme structural constraints [33,52,53]. Typically this flattening distortion is manifest in the transient absorption spectrum as a sub-picosecond rise time in the excited-state absorption. Alternatively, the ultrafast fluorescence upconversion by Tahara and coworkers identified such structural transformation by the emission spectrum as a function of delay time [27,66]. They observed clear evidence of signature fluorescence signals for the initial conformation and a flattened conformation with a spectral evolution described well with a two-conformation model [27,66]. Even [Cu(I)(dpp)2 ]+ , which is already heavily flattened in the ground state, exhibits additional sub-picosecond Jahn–Teller flattening, but its analog with two additional sulfonate groups at the meta or para positions of the phenyl rings, however, shows no sub-ps time constant in the excited-state dynamics. One of the few complexes that remains pseudo-tetrahedral in the excited state is the severely constrained [Cu(I)(dtbp)2 ]+ ; this complex exhibits the weakest S1 ← S0 band, as the bulky tert-butyl groups at the 2,9 phenanthroline positions prevent any dynamic flattening in the ground state [33,52,53]. As expected, the MLCT state exhibits no sub-ps dynamics due to the absence of the excited-state flattening, resulting in an extended excited-state lifetime according to the energy gap law. In contrast, its single substituted complex, with tert-butyl at 2 or 9 positions of phen, displays the sub-ps dynamics, suggesting the flattening dynamics because only one side of the phen has the bulky group, which is insufficient for blocking the flattening pathway. 3.2. Jahn–Teller flattening and intersystem crossing Optical transient absorption (TA) spectroscopy is one of the most common techniques used for measuring the ultrafast

excited-state dynamics of these systems. TA spectra of [Cu(I)(dmp)2 ]+ are shown in Fig. 6. The ultrafast spectra are quite similar regardless of whether the low or high energy MLCT bands are excited, with the primary difference being the S1 ← S2 internal conversion process exhibited when exciting the highenergy peak. Two primary transformations in the TA spectra are readily observed. First, the excited-state absorption evolves from broad and featureless to having multiple peaks. These peaks are vibronic in nature and are due to the localization of the MLCT electron on one of the phenanthroline ligands. Second, the spectra blue-shift on the 10–15 ps timescale, indicative of intersystem crossing. While there are some exceptions, the majority of Cu(I) diimine TA spectra exhibit these two features. Kinetics traces probing the MLCT excited-state absorption at the red side of the spectra in Fig. 6 can be fit by a sum of three exponential terms Ai e−t/␶i (i = 1–3) where Ai is the weight of the ith component, and  i is the ith time constant. Kinetic traces for [Cu(I)(dmp)2 ]+ and [Cu(I)(dpp)2 ]+ in acetonitrile are shown in Fig. 7. Among most of the Cu(I) diimine complexes studied so far,  1 is a sub-picosecond rise time constant related to a Jahn–Teller distortion, an inner-sphere process,  2 is a 10–15 ps time constant related to ISC, and  3 a >1 ns to a few ␮s time constant related to 3 MLCT state decay influenced strongly by Cu-solvent interactions. Fluorescence upconversion measurements as shown in Fig. 5 and presented by Shaw et al. [35] observed evidence for the assignment of  1 and the TA measurements with Cu(I) diimine complexes with 2,9 substituents with different steric hindrance confirmed this assignment. When the flattening is completely blocked by the bulky groups at 2,9 positions,  1 is absent, as seen in [Cu(I)(dtbp)2 ]+ [53] and [Cu(I)(dppS)2 ]+ [22]. The assignment of  2 to the time constant of ISC is based on the results from fluorescence upconversion and time-correlated single photon counting [27,30,35]. ISC occurs much more slowly in these systems than in ruthenium polybipyridyl complexes in part because of the lower Z value of copper compared to ruthenium. However, in addition to this effect, structural factors also play an important role. According to the calculations of Nozaki and coworkers, the energies of the MOs in [Cu(I)(dmp)2 ]+ vary as a function of the angle between the two ligand planes, with the set of MOs with high spin–orbit coupling coefficient destabilizing and

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Fig. 5. The fluorescence upconversion results shown in Ref. [66] where the conversion of the two conformations, pseudo-tetrahedral Frank-Condon MLCT state and flattened MLCT state, is identified for three Cu(I) bisdiimine complexes, (a) [Cu(phen)2 ]+ , (b) [Cu(dmp)2 ]+ , (c) [Cu(dpp)2 ]+ . Reproduced from [66] with permission of the Royal Society of Chemistry.

the other set with low spin–orbit coupling stabilizing at a flattened geometry [30]. This important result implies that one can modulate the angle between the two phen ligand planes to alter the ISC rate. Such a hypothesis based on the DFT calculations has been supported by our experimental results where the shortest ISC time constant  2 is less than 4 ps in the strictly pseudo-tetrahedral [Cu(I)(dtbp)2 ]+ while all other complexes that can flatten at the MLCT state or are flattened in the ground state display  2 > 10 ps ISC time constants [22,35,53]. Such relatively long ISC time constants suggest that if one can transfer an electron or energy on a time scale much faster than  2 , it is possible to gain additional driving force in the 1 MLCT state for photochemical processes, which will be demonstrated by

1.4 ps 20 ps

ΔA

0.01

2.0 ns

0.00

-0.01

-0.02

our recent study on dye sensitized solar cell (DSSC) mimics to be presented later. Both  1 and  2 are largely solvent independent and hence are associated with largely inner-sphere processes. The longest time constant  3 as discussed above and listed in Table 1 is assigned to solvent quenching of the MLCT state, which requires direct access of the solvent to the copper center to form a strongly or weakly bound “exciplex”. Time-dependent density functional theory (TD-DFT) has also been used to calculate the ground and excited-state properties of these systems. Zgierski showed that all of the absorption bands in the visible region do in fact correspond to MLCT transitions; he also showed that the lower energy absorption into the S1 state is symmetry forbidden for D2d symmetry but becomes more

λpump = 417 nm 450

500

550

500

550

600

600

650

650

Probe Wavelength (nm) Fig. 6. Transient absorption spectra for [Cu(I)(dmp)2 ]+ following MLCT excitation at 417 nm, probing excited-state absorption. Arrows in inset point to the early rise (0.3–1.2 ps) of the TA spectrum.

Fig. 7. Optical transient absorption kinetics traces of [Cu(I)(dmp)2 ]+ and [Cu(I)(dpp)2 ]+ , probing the excited-state absorption. Experiments were performed with acetonitrile as solvent.

Reprinted with permission from [35]. Copyright 2007 American Chemical Society.

Reprinted with permission from [35]. Copyright 2007 American Chemical Society.

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allowed due to contributions from b1 vibrational modes as the complex adopts the flattened geometry [36]. In addition, several low frequency normal modes have been identified for being responsible in initiating the Jahn–Teller effect that flattens the pseudo-tetrahedral coordination for some of the Cu(I) diimine complexes as well as distortions in other dimensions [36,67]. TD-DFT calculations by Siddique et al. mapped out the potential energy diagram for these complexes as functions of the distortion along certain reaction pathways, such as the flattening of the tetrahedral geometry. Specifically, their results demonstrated that the excited-state energy, spin–orbit coupling, and phosphorescence rate constant all decrease as the copper complex adopts a more flattened structure [30]. Finally, DFT calculations were also used to calculate the geometry of the 5-coordinate exciplex (with HCN as the fifth ligand), and also showed that the charge of the copper in the excited state is quite similar to that of the Cu(II) oxidation state [34]. The calculated exciplex structure had a slight contrac˚ as tion of the Cu N bonds to the phenanthroline ligands of 0.01 A, well as a decrease in the flattening angle of ∼2◦ ; however, the HCN was only able to approach within ∼3 A˚ of the copper center. In a more recent calculation, Smolentsev et al. [67] found that additional rocking motion of the phenanthroline ligands must be involved to accommodate the ligation of the fifth ligand, such as acetonitrile. Therefore, if the rocking motion of the MLCT state is inhibited, the “exciplex” will not form. 3.3. Structural characterization using X-ray transient absorption spectroscopy For more than a decade, X-ray transient absorption (XTA) spectroscopy [37,68–72] has been used to directly characterize the electronic and geometric structural reorganizations in the excited state of transition metal complexes, such as the currently discussed Cu(I) diimine complexes. XTA, from an experimental stand point, is analogous to optical TA (OTA) using the same “pump-probe” approach. The “pump” is a laser pulse that induces a valence electronic transition to initiate a chemical or physical process. The “probe” in OTA is the second laser pulse that probes the absorbance changes of the sample as a function of the delay time from the pump pulse and detects excited-state dynamics, electronic coherence, and energies of the transient species. The “probe” in XTA is an X-ray pulse that detects XANES and EXAFS spectra as a function of the delay time with the pump laser pulse, which is well-suited for a direct observation of the MLCT state structural evolution, such as the Cu(I) to Cu(II) conversion and the formation of the “exciplex” between the 3 MLCT state and the solvent molecules. The first XTA measurements were performed on [Cu(I)(dmp)2 ]+ using X-ray pulses from a synchrotron facility, which have an intrinsic temporal width of 80–100 ps; as a result, the dynamics described by  1 and  2 will be too fast to be captured, while only the structure of the 3 MLCT state with lifetime  3 > 100 ps can be extracted. The surprising result from the XTA spectra of the [Cu(I)(dmp)2 ]+ 3 MLCT state (Fig. 8) in both toluene (non-coordinating solvent) and acetonitrile (coordinating solvent) suggested that the “exciplex” state formed even in toluene. This conclusion is obtained by observed similarity of XANES spectra between the electrochemically generated [Cu(II)(dmp)2 ]+ with the 3 MLCT state, because the former forms the penta-coordinated complex with the additional ligand from a solvent molecule. Recently, higher-quality XTA data was obtained for [Cu(dmp)2 ]+ in acetonitrile and toluene that has made the previous results more accurate regarding the “exciplex” formation. The new date revealed minor differences between the MLCT state XANES and [Cu(dmp)2 ]+2 has been observed in Fig. 8B which suggest that the “exciplex” formation in either acetonitrile or toluene may not be as stable and complete as those in the ground state [Cu(dmp)2 ]+2 .

The new data with higher quality also indicated that the average Cu N for [Cu(dmp)2 ]+ in toluene does in fact decrease (Fig. 9 right) similar to that exhibited for acetonitrile. The 3 MLCT state spectrum exhibits an edge shift of ∼3 eV higher than the ground state [Cu(I)(dmp)2 ]+ due to the electron loss from the Cu(I) center accompanied by the energy upshift of the 1s → 4pz peak, as well as a smooth rising edge due to the ligation of the Cu center with the solvent. Upon conversion from Cu(I) to Cu(II), a new feature also occurs below the edge energy that corresponds to a 1s → 3d quadrupole-allowed transition and takes place when the MLCT transition shifts one of the Cu 3d electrons (either the dxy or dyz orbitals) [36] onto the phenanthroline ligands, resulting in a 3d9 copper center. These XANES spectra (Fig. 8) suggest two main findings. First, the MLCT state of Cu(I) diimine complexes tends to form the “exciplex” with the solvent as long as the copper center is exposed to the solvent, regardless of whether the solvent is “coordinating” or “non-coordinating”. The crystal structures of Cu(II) diimine complexes with the solvent/anion [73,74] verified that the smooth rising edge is an indication of the pentacoordinated structure, which was also observed in other first row transition metal complexes such as in metalloporphyrins. Second, the “exciplex” will not form if the copper center is not sufficiently exposed to the solvent to form interactions that are capable of sustaining the ligation. As seen in [Cu(I)(dpp)2 ]+ case, the Cu(II) species does not exhibit a smooth rising edge and has a distinct 1s → 4pz peak (Fig. 8E), representing the XANES edge features when only the Cu(I) → Cu(II) transformation occurs but the “exciplex” formation does not take place. The distinctly different XANES features in the Cu(II) and the MLCT states with and without the “exciplex” formation shown in Fig. 8 clarifies the misinterpretation of the experimental data relying on the calculated results from quantum mechanical calculations and molecular dynamics simulations of the experimental data in the literature [75]. Based on all the experimental results from both optical and X-ray transient absorption measurements, it is clear that the solvent accessibility to the copper center is the determinant factor for the formation of the “exciplex”, rather than the interactions of the ligands with peripheral solvent molecules as described in Ref. [76]. If the copper center is exposed to solvent to allow the interactions between them to be sufficiently strong, the “exciplex” will form. Otherwise, there will be no exciplex formation. In other words, the formation of the “exciplex” is solvent independent. However, if the “exciplex” is capable of forming, the strength of the interactions between the copper and the solvent determines the extent of the excited-state quenching. Such a conclusion was reached only after comparison of the MLCT state dynamics in different solvents for a series of Cu(I) diimine complexes with different solvent accessibilities. Nevertheless, we cannot rule out the possible coexistence of exciplex and the unligated MLCT state species as a result of a chemical equilibrium. The primary structural information from EXAFS measurements comes from the extended X-ray absorption fine structure (EXAFS) portion of the spectrum, which is the oscillatory part of the spectrum above the edge energy. Fourier transforms of these oscillations result in a pseudo radial distribution function corresponding to the distance between the absorbing (in this case, copper) and the surrounding atoms. The EXAFS spectrum of ground and laser excited [Cu(I)(dmp)2 ]+ (∼25% MLCT state + ∼75% ground state mixture) is shown in Fig. 9. The EXAFS spectra indicated a decrease in the average Cu N bond distance, consistent with the results from the TD-DFT calculations. The increase in peak intensity also indicates an increase in coordination number in the first shell peak, indicating the formation of the exciplex with an acetonitrile forming a formal bond with the copper. When XTA experiments were performed in toluene, there was again an increase in peak intensity, but in this case the average first shell distance became larger. This is likely an indication of the formation of the exciplex in toluene;

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Fig. 8. XANES spectra [Cu(I)(dmp)2 ]+ ground state and laser excited in toluene and acetonitrile showing similar changes in both solvents (A); the MLCT states from (A) plotted with electrochemically generated [Cu(II)(dmp)2 ]+2 species in toluene and acetonitrile (B); [Cu(I)(dpp)2 ]+ ground and the MLCT states in toluene and acetonitrile showing similar changes in both solvents (C); static Cu(I) and Cu(II) species for less sterically hindered [Cu(dmp)2 ]+/+2 and [Cu(phen)2 ]+/+2 with a smooth rising edge as an indication of the solvent ligation (D); and static Cu(I) and Cu(II) species for sterically hindered [Cu(dpp)2 ]+/+2 with a distinct peak of 1s → 4pz transition (E).

however, since toluene is weakly coordinating, the interactions with the copper center are likely much weaker, resulting in the toluene resting farther away from the metal and therefore increasing the average first shell distance [76]. While EXAFS analysis is the most direct method for measuring the structure of a system, the structure can also be extracted by fitting the XANES spectrum. The exciplex structures of [Cu(I)(dmp)2 ]+ [32,57,77] and [Cu(I)(dbtmp)2 ]+ [32] in acetonitrile were interrogated using the XANES fitting program FitIt [78] in conjunction with TD-DFT calculations. For [Cu(I)(dmp)2 ]+ , the XANES features are most accurately replicated by the penta-coordinate exciplex complex with an average phenanthroline Cu N distance of 2.04 A˚ ˚ There are also and an acetonitrile Cu N bond distance of 2.00 A. additional “rocking” distortions involving the ligand planes that are associated with the binding of the solvent ligand [67]. However, the bulkier butyl ligands of [Cu(I)(dbtmp)2 ]+ prevent exciplex formation, and so the XANES features are most accurately replicated by

a flattened structure without the rocking distortions or the ligation of a fifth ligand. As a result, this complex has a much longer excited-state lifetime in acetonitrile than [Cu(I)(dmp)2 ]+ (181 ns vs. 1.8 ns). The optical TA results suggest that the bulkiness of the phenyl rings in [Cu(I)(dpp)2 ]+ also prevent exciplex formation; to confirm this and determine the structural properties of aryl-substituted copper bisphenanthrolines in general, XTA measurements were also performed on this complex [21]. The XANES spectra shown in Fig. 10 exhibit a shift in edge energy and the appearance of the 1s → 3d peak below the edge energy that are consistent with that observed for [Cu(I)(dmp)2 ]+ ; the 1s → 4pz peak is still prevalent on the edge, which as mentioned above is an indication that no exciplex is formed. EXAFS fits of the [Cu(I)(dpp)2 ]+ excited-state in toluene and acetonitrile result in the same first-shell coordination number, indicating that no exciplex formation is exhibited for [Cu(I)(dpp)2 ]+ .

0.10 Ground state Laser excited MLCT state

1.5 1.0

0.08

Ground State MLCT State

FT[k χ(k)]

0.06

2

0.0

2

k χ(k)

0.5

-0.5 -1.0

0.04

0.02

-1.5 1

2

3

4

5 -1

k (Å )

6

7

8

0.00 0

1

2

3

4

5

6

R (Å)

Fig. 9. EXAFS spectra of ground and laser excited [Cu(I)(dmp)2 ]+ in toluene: (left) EXAFS spectra in vector k-domain; (right) FT-EXAFS spectra without phase correction.

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Fig. 10. (Top) XANES spectra of [Cu(I)(dpp)2 ]+ (black), [Cu(II)(dpp)2 ]+2 (blue), and photoexcited [Cu(I)(dpp)2 ]+ (red) 100 ps following photoexcitation. Solvent is acetonitrile. (Bottom) HOMO and LUMO of [Cu(I)(dpp)2 ]+ determined from TD-DFT calculations. Adapted with permission from [21]. Copyright 2013 American Chemical Society.

Where the phenyl-substituted complexes differ most from the alkyl-substituted complexes is in the rotation of the phenyl rings. As stated before, a heavily flattened ground state is stabilized for [Cu(I)(dpp)2 ]+ by phenyl-phenanthroline ␲–␲ interactions [55], resulting in a more allowed S1 ← S0 transition and therefore a broader UV–vis spectrum. Excitation at the MLCT bands results in a transferred electron from the copper center to the phenanthroline ligand. Following excitation, the phenyl rings rotate ∼10◦ into the plane of the phenanthroline ligand. However, the XTA results show that, in the photoexcited state, the phenyl rings are rotated too far out of plane from the phenanthroline ligands for conjugation across the phenyl rings to occur, resulting in a LUMO where the electron density is totally localized on the phenanthroline ligand with little to no electron density on the phenyl rings. This conclusion was confirmed by TD-DFT calculations, which are also shown in Fig. 10 [21]. 3.4. Potential energy diagrams Potential energy diagrams for [Cu(I)(dmp)2 ]+ , [Cu(I)(dpp)2 ]+ , and [Cu(I)(dtbp)2 ]+ are shown in Fig. 11 and were constructed based on theoretical calculations and both optical and X-ray spectroscopic measurements. These diagrams only show the intramolecular processes that occur upon photoexcitation, and as such, potential energy curves from the formation of the exciplex are omitted. The diagram for [Cu(I)(dmp)2 ]+ is typical for most alkyl-substituted copper(I) diimines. These complexes exhibit a slightly distorted D2d symmetry and are most stabilized with a nearly 90◦ flattening angle; that is, it adopts a nearly perfect tetrahedral geometry. Excitation for these complexes is typically into the symmetry allowed S2 state. In this case, fast internal conversion (<50 fs [27,63]) to the S1 state is then followed by a sub-picosecond Jahn–Teller flattening distortion. This flattening distortion weakens spin–orbit coupling, resulting in a 10–15 ps ISC time. The return of the 3 MLCT state to the ground-state occurs on the nanosecond time scale and is solvent dependent as discussed earlier (Fig. 12). The much bulkier [Cu(I)(dtbp)2 ]+ exhibits quite different dynamics, despite also being an alkyl-substituted complex. The size of the tert-butyl groups significantly raises the potential barrier for flattening; as a result, no dynamics flattening occurs in the ground state. Furthermore, flattening is not allowed in the excitedstate, and so the D2d symmetry is retained upon photoexcitation. Preventing the flattening distortion has two significant effects on the dynamics of this system. First, the spin–orbit coupling is stronger than for other copper bisphenanthrolines, shortening the

Fig. 11. The potential energy diagram as a function of the angle between the two ligand planes for a ground-state Cu(I) bisphenanthroline with a tetrahedral (black) or heavily flattened (blue) ground-state structure.

intersystem crossing time constant to 4–8 ps. Second, there is a rather large difference in energy between the ground and the 3 MLCT states, resulting in a very long lifetime (>2 ␮s) according to the energy gap law. Finally, the bulky ligands prevent any solvent or counterion access to the copper center, and so exciplex formation cannot occur. However, these ligands make the complex much more labile, and dissolving this system in a strongly coordinating solvent such as acetonitrile will result in loss of one of the ligands. Therefore it is impossible to directly test the dynamics of this system in coordinating solvent. The primary difference in the potential energy diagram of [Cu(I)(dpp)2 ]+ from the alkyl-substituted systems lies in the ground-state. The ground state of [Cu(I)(dpp)2 ]+ is stabilized by ␲–␲ interactions between the phenyl rings and the phenanthroline ligands, and the complex tends to heavily flatten to accommodate these interactions. Therefore, the ground state has a potential minimum at a flattening angle of <70◦ [21,55] instead of near 90◦ . Jahn–Teller flattening will still occur, shifting the flattening angle to ∼63◦ [21]. The small amount of Jahn–Teller flattening should result in a larger energy gap between the ground and 3 MLCT states than that observed for [Cu(I)(dmp) ]+ , which loses 2 much more energy in the excited state than [Cu(I)(dpp)2 ]+ due

Fig. 12. Schematic of a dye sensitized solar cell showing electron transfer through the various energy levels of the system components. Downward vertical arrows represent relaxation within the same component of the DSSC. The figure is adapted from O’Regan and Grätzel [79]. Reprinted with permission from [79]. Copyright 1991 Macmillan Publishers Limited.

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to flattening. Therefore one would predict that, in the absence of quenching by exciplex formation, [Cu(I)(dpp)2 ]+ should exhibit a longer excited-state lifetime than [Cu(I)(dmp)2 ]+ . As shown in Table 1, not only is the excited-state lifetime of [Cu(I)(dpp)2 ]+ in acetonitrile longer than that of [Cu(I)(dmp)2 ]+ due to blocking solvent attack to the copper center, the lifetime is also longer in the non-coordinating dichloromethane, consistent with the change in the energy gap. Finally, since the excited state flattening angle of [Cu(I)(dpp)2 ]+ is similar to that of [Cu(I)(dmp)2 ]+ , one would expect similar spin–orbit coupling strengths and therefore similar intersystem crossing lifetimes, and both complexes have 10–15 ps ISC time constants. These structure-dependent excited-state properties are important for controlling the photochemical reactions and to make copper diimine complexes that are useful in a wide range of solar energy conversion applications. 4. Applications of Cu(I) diimine complexes in solar energy conversion The search for an earth abundant alternative to use in solar energy conversion applications drives much of the Cu(I) diimine based research presented above. Cu(I) diimine complexes can act as photosensitizers in photocatalytic processes, where they convert solar energy into redox equivalents that perform the necessary half reactions for water splitting. In dye-sensitized solar cells (DSSCs) pioneered by O’Regan and Grätzel [79], a photosensitizer is attached to semi-conductor nanomaterials, such as TiO2 nanoparticles (See Fig. 12). When these complexes absorb a photon, an electron is injected into the conduction band of the nanoparticles where charge transport occurs. For the cycle to continue, the dye must be regenerated via electron transfer from a redox species, which is reduced at the counter electrode. A successful photosensitizer will have a long-lived excited state after absorbing visible light, and the excited state should be highly energetic such that electron transfer can occur. The excited state must also be a strong oxidizing or reducing agent, and the oxidation or reduction must be reversible such that the ground state can be regenerated [80]. For uses related to DSSC’s, the photosensitizer should also have a way to covalently attach to the semi-conductor [81]. While transition metal complexes (TMC) made with rare-earth metals such as ruthenium exhibit such characteristics, properties of Cu(I) diimine complexes can complicate their use in photocatalysis and DSSCs. As discussed in the previous sections, the MLCT state can be created by visible photons from sunlight, and the newly formed 3d9 metal center is susceptible to Jahn–Teller distortions. The distortions cause low energy, short-lived excited states that can be easily quenched by exciplex formation with coordinating solvents or counter ions. To avoid such complications, much effort has been put into designing ligands that sterically prevent Jahn–Teller distortions or that sterically block solvent access to the copper center [53]. 4.1. Light-harvesting properties of homoleptic copper diimine complexes Pioneering work by Sakaki et al. [82] demonstrated the potential of Cu(I) complexes use in dye-sensitized solar cells. Using the Grätzel cell, which consisted of ruthenium(II) complexes tethered to titanium dioxide nanoparticles, as inspiration, Sakaki synthesized [Cu(tmdcbpy)2 ]+ (tmdcbpy = 4,4 ,6,6 -Tetramethyl-2,2 bipyridine-5,5 -dicarboxylic acid, see Fig. 13) for use in a DSSC. Motivated by their two previous attempts to use earth-abundant transition metal complexes to form solar cells, the authors designed the complex with carboxylic acid moieties added to the 2,2 bipyridine framework for attachment to the nanoparticles and

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Fig. 13. 4,4 ,6,6 -Tetramethyl-2,2 -bipyridine-5,5 -dicarboxylic acid (tmdcbpy). Reproduced from [82] with permission of the Royal Society of Chemistry.

electron injection into the conduction band of TiO2 , and methyl groups on both the 4,4 and 5,5 positions for an increase in the excited-state lifetime. The absorption spectrum of [Cu(tmdcbpy)2 ]+ attached to TiO2 closely matched that of the complex in solution, suggesting that the complex was indeed tethered to the nanoparticles. Electron injection was confirmed with a clear dependency of the IPCE on the absorption spectrum of the Cu(I) complex. An unsuccessful attempt to form an effective solar cell using a Cu(I) complex with a 1,10-phenanthroline-based ligand with no carboxylic acid moieties demonstrated that the carboxylate groups are necessary for the attachment to the nanoparticles, and for the electron injection. Overall, this solar cell did not perform as well as the Grätzel cells, but it far exceeded the two previously attempted Cu(I) and Fe solar cells. Castellano and coworkers [80] have developed a robust, long-lived cuprous photosensitizer by adding substituents to 1,10-phenanthroline ligand. The ligands for [Cu(dsbtmp)2 ]+ , where dsbtmp is 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,1phenanthroline, have sec-butyl and methyl substituents that cooperatively interact for steric control of the molecule. Based on the work of McMillin [5], who showed that methyl groups in the 3 and 8 positions enhance the steric effect of subsituents at 2,9 positions of the phenanthroline, Castellano et al. chose to add branched sec-butyl chains in order to increase the sterics without making the ligands thermodynamically compromised. This Cu(I) compound therefore shows very little distortion from the pseudotetrahedral D2d symmetry in the ground state, as evident by no red shoulder appearing in the absorption spectrum. At room temperature, [Cu(I)(dsbtmp)2 ]+ has a ␮s photoluminescence lifetime in both coordinating and noncoordinating solvents, and the electrochemistry shows that the complex can be both reversibly oxidized and reduced, thus making [Cu(I)(dsbtmp)2 ]+ a good candidate for a photosensitizer. Further work by Castellano and coworkers [83] showed that [Cu(I)(dsbtmp)2 ]+ can participate in hydrogen evolution catalysis (Fig. 14). When [Cu(I)(dsbtmp)2 ]+ is coupled with the waterreducing compound, Co(III)(dmgH)2 (py)Cl, and the sacrificial electron source N,N-dimethyl-p-toluidine (DMT), hydrogen is produced. In this process, the excited state of the Cu(I) complex is oxidatively quenched by the cobalt compound, which starts the photocatalytic cycle. The DMT donates one electron to the newly photogenerated copper(II) metal center of the [Cu(II)(dsbtmp)2 ]2+ to regenerate the photosensitizer. Due to the inhibited distortion of the complex, [Cu(I)(dsbtmp)2 ]+ is active even after 5 days of hydrogen production under visible irradiation. Yuan and others developed a cationic Cu(I) complex that upon attachment to TiO2 nanoparticles also demonstrated utility in both a DSSC as well as a photocatalyst [84]. In the synthesis of the ligand, methyl groups were appended at the necessary 6,6 -positions of a 2,2 -bipyridine parent compound in order to protect the labile Cu(I) center (see Fig. 15). Because they have been extensively used as an efficient acceptor unit, cyanoacetic acid moieties were added at the end position to act as anchoring groups for binding to the TiO2 nanoparticles. The complex in solution in methanol shows a strong MLCT band at 496 nm, and DFT calculations show that the LUMO for this transition resides primarily on the ␲-system of the bpy moieties and on the anchoring units. The presence of the

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Fig. 14. Structure of proposed cuprous photosensitizer and schematic of hydrogen production. Reprinted with permission from [80]. Copyright 2013 American Chemical Society.

Fig. 16. Schematic of [Cu(I)(dppS)2 ]+ attached to TiO2 nanoparticles. Reproduced from [22] with permission of Wiley-VCH. Fig. 15. Structure of the bipyridine based cationic complex used by Yuan et al. Reproduced from [84] with permission of the Royal Society of Chemistry.

LUMO on the ligand, especially on the aromatic rings, may facilitate electron injection because it provides mixing between the bpy orbital and the TiO2 acceptor orbital. IR spectroscopy confirmed the attachment of the Cu(I) complex to the nanoparticles, and the photocurrent density voltage characteristic curves illustrate efficient sensitization of nanocrystalline TiO2 . The performance of the solar cell was lower than that of a standard ruthenium dye under the same conditions, but the results clearly imply that the synthesized complex works properly for harvesting light in the solar cell. Prompted by the promising results of their solar cell, the authors subsequently showed that the Cu(I) complex-TiO2 nanoparticle system was effective as a photocatalyst. CO2 photoreduction was established in a solid–gas reaction system that contained the copper complex/P-25 and water vapor over a 0.1 g of solid sample. The copper complex/P-25 selectively reduced CO2 to CH4 and although the performance was lower than for previous noble metal complexes, it outperformed a comparable iridium complex sensitizer, demonstrating that it is possible to use an earth-abundant metal complex for solar cells and photocatalysts. Continued work in our group by Huang et al. sought to understand the structural relationship of Cu(I) complexes to electron injection dynamics by observing electron injection from [Cu(I)(dppS)2 ]+ , where dppS = 2,9-diphenyl-1,10phenanthrolinedisulfonic acid disodium salt to TiO2 nanoparticles [22]. The sulfonated phenyl groups play a prominent role in directing the dynamics of the complex. They force a flattened ground-state geometry, which weakens spin–orbit coupling, and

they block the copper center from solvent access. The structural constraints provided by the sulfonated phenyl groups allow for a long-lived MLCT excited state. Furthermore, the sulfonate groups are used to link to the TiO2 nanoparticles (See Fig. 16). A combination of electronic paramagnetic resonance (EPR), optical TA experiments, and XTA experiments confirm the fast injection of electrons from the excited [Cu(I)(dppS)2 ]+ to the nanoparticles, with electron transfer occurring in 0.4 and 12 ps from the 1 MLCT and 3 MLCT states, respectively.

4.2. Light-harvesting properties of heteroleptic copper diimine complexes The design of stable heteroleptic Cu(I) complexes has also been a key advancement in the development of effective copper-based DSSC’s [85]. Heteroleptic complexes have the useful advantage of being able to accurately tune the photophysical and photoredox properties of the Cu(I) complex in order to optimize their performance in the solar cell. Usually this approach consists of synthesizing an anchoring ligand with the commonly-used phosphonic or carboxylate acid groups to attach to the semiconductor nanoparticles. The ancillary ligand is then functionalized to reach the suitable electronic properties to maximize the injection of electrons into semiconductor nanoparticles. While the heteroleptic approach has been commonly used in ruthenium(II)-based DSSC’s with great success, the lability of the Cu(I) metal center makes forming and isolating heteroleptic Cu(I) complexes difficult. Much of the emerging research discussed below involves the design and

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Fig. 17. Synthesis of ligand 3 by Bozic-Weber et al. Ligand 3 properties were explored as the ligand in a Cu(I) homoleptic complex as well as in a heteroleptic with a phosphonic anchoring ligand. Reproduced from [86] with permission of the Royal Society of Chemistry.

optimization of heteroleptic Cu(I) complexes in order to make them more stable for use in DSSC’s. Heteroleptic Cu(I) complexes with a phosphonic acid anchoring ligand were developed by Bozic-Weber et al. and are effective sensitizers in a TiO2 solar cell [86]. The authors optimized performance of the heteroleptic complex by introducing 6,6 -dialkyl substituents to again stabilize the Cu(I) oxidation state, a triphenyl amino-substituent to extend the ␲-conjugation and red shift the absorbance spectrum, and a long chain substituent which suppresses intermolecular aggregation (Fig. 17). The anchoring ligands contain phosphonic acid anchors, which stabilize the surface bound complex with respect to the carboxylate system and increase the energy conversion efficiencies. Complexes of Cu(I) with just the ancillary ligand have an absorption spectrum that extends all the way to 600 nm, with the main MLCT peak being centered at 476 nm. The DSSC’s were constructed by first applying a scattering TiO2 layer to mesoporous TiO2 followed by treatment with H2 O-TiCl4 . These cells were soaked in a solution of the anchoring ligand, and the surface anchored heteroleptic complex was formed with subsequent soaking in solutions of [Cu(3)2 ][PF6 ]. DSSC’s using I− /I3 − electrolyte were characterized and the solar cells showed no ripening effects and slight performance improvement with increased concentrations of H2 O-TiCl4 . Replacement of the I− /I3 − electrolyte with a [Co(bpy)3 ]2+ /[Co(bpy)3 ]3+ electrolyte led to solar cells with comparable performance, demonstrating that a stable iodine-free Cu(I) solar cell can be constructed. The HETPHEN concept was applied by Sandroni and coworkers [87] to generate new heteroleptic Cu(I) diimine complexes and utilize them as photosensitizers in DSSC’s. Competition with the formation of homoleptic compounds often inhibits the formation of the pertinent heteroleptic complex, and the

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HETPHEN approach was developed by Schmittel and coworkers to overcome the thermodynamic stability of homoleptic complexes. The HETPHEN concept uses steric and electronic effects of bulky substituents on the bisimine sites of phenanthroline ligands to direct the coordination chemistry [88]. As previously mentioned, heteroleptic compounds are a desirable option in creating effective solar cells because the multiple ligands allow for fine tuning of the thermodynamics of the photo-induced processes, and the ligands can be tuned for efficient electron injection into the conduction band. In this case, the tuning is done via the connecting moiety of the anchoring ligand and electron enriching groups on the ancillary ligand. In this work, 2,2 -biquinoline-4,4 -dicarboxlic acid (dcbqH2 ) was used as the anchoring ligand to form heteroleptic complexes with either 2,9-dimesityl-1,10-phenanthroline (L0) or N-hexyl-2,9-dimesityl1,10-phenanthroline-[a:b]imidazo-(4-dianisylaminophenyl) (L1). The combination of dcbqH2 with L0 gives rise to the Cu(I) complex C1 while dcbqH2 combined with L1 produces C2 (Fig. 18). The diimine ancillary ligands were used to red shift the absorbance spectrum of the complex to increase the light harvesting efficiency while the strong electron accepting nature of the anchoring ligand favors electron injection into the conduction band of TiO2 nanoparticles. Furthermore, the bulk of the ligands should block any exciplex formation with the Cu(I) metal center. The UV–vis spectra of the complexes in solution confirmed the red shifting of the spectrum as both complexes show broad MLCT bands around 560 nm which extend all the way to 700 nm. The electrochemistry, in combination with DFT calculations, suggests that the HOMO is located on the d-orbitals of Cu(I) while the LUMO is localized on the dcbqH2 . This scenario is desirable for electron injection. No photoluminescence is detected, but a reversible oxidation process in acetonitrile suggests that this is due to the energy gap law rather than exciplex formation. The complexes were attached to TiO2 electrodes, and both showed poor photovoltaic performance because of weak potential output and short circuit current. The authors surmise that the small Voc is due to favored charge recombination with electrolyte and/or fast geminate charge recombination. The weak short circuit current is probably due to the weak visible absorption, and therefore low LHE, and feeble injection driving forces. To prevent a low molar extinction coefficient of the MLCT band and ligand switching around the Cu(I) center, Pellegrin and co workers [89] applied the same design philosophy, HETPHEN, to synthesize six new heteroleptic Cu(I) complexes (see Fig. 19). The complexes were based on 2,9-dimesityl-2-(4 bromophenyl)imidazo[4,5-f][1,10]phenanthroline (L1) or 3,6-din-butyl-11-bromodipyrido[3,2-a:2 ,3 -c]phenazine (L2) combined with complementary matching ligands 2,9-R2 -1,10 phenanthroline with R H, methyl, n-butyl, or mesityl groups. The crystal structure of C3 exhibited a distorted tetrahedral symmetry, with Cu N bond lengths that are typical of Cu(I) bisphenanthroline complexes and similar N Cu N bite angles. Strong deviation from the tetrahedral geometry toward a heavily distorted trigonal pyramid is probably due to evidence of ␲-stacking between just one mesityl substituent of the ligand mes-phen and the adjacent L2 phenanthroline. Subsequently, the structures of C2, C3, C4, and C6 are very similar; C1 and C5 differ in that the Cu N bonds are closer and the N Cu N angles are smaller. The difference is probably attributed to the steric hindrance of the n-Bu and ␲-stacking mesityl groups toward n-BuPhen ligand. The UV–vis absorbance spectra suggest that with the exception of C5, the structure of the complexes only slightly impact the MLCT energy. For C5, the absorbance shows a broad shoulder, most likely due to the ␲-stacking between the phenanthroline and mesityl groups. The electrochemistry and photoluminescence both support the claim of a sterically hindered Cu(I) metal center. Additional chemistry on the model compound C6 shows that the complexes are relatively robust to a cross coupling Suzuki reaction,

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Fig. 18. Structures of the complexes used by Sandroni et al. in their heteroleptic Cu(I) diimine complexes. Reproduced from [87] with permission of the Royal Society of Chemistry.

but C6 is susceptible to dissociating and forming the homoleptic complex in excess n-Bu-phen. The formation of the homoleptic compound is likely due to its thermodynamic stability and shows the importance of introducing the reactants in the correct order to form the heteroleptic compound. Creating long-lived charge transfer states is an integral step in solar energy conversion processes. Elliott and coworkers [90] have found a way to compete with fast back electron transfer in a single chromophore system by creating triad assemblies that consist of an electron donor, a Cu(I) chromophore, and an electron acceptor (Fig. 20). In their heteroleptic Cu(I) bis(phenanthroline) triad, they covalently attached an electron donor in the 2,9 positions to one phenanthroline ligand and attached an electron acceptor to the other phenanthroline ligand. This arrangement enables a charge separation state because the covalently attached electron donor of the donor ligand is in the pertinent position for steric hindrance of the exciplex formation while solvent blocking methyl groups are attached to the 2,9 positions of the acceptor ligand compound, and

having the donor/acceptor groups directly attached prevents fast back electron transfer. In the specific case of the electron donors, phenothiazine (PTZ)-type donors are attached in a way that enables -stacking with the phenanthroline ligand. Transient absorption experiments confirm that a charge separated state is formed after excitation of the Cu(I) complex, representing the first time a photoinduced multistep intramolecular CSS is formed using just a Cu(I) photosensitizer. Heteroleptic compounds consisting of a bipyridine ligand and a dipyrrin ligand were constructed by Hewat and coworkers [91]. Five complexes were formed by combining (4,4-(R)-6,6 -(CH3 )-bipyridine), where R = CH3 or CO2 Et, with either 1,3,7,9-tetra-methyldipyrromethene, 1,13-diphenyl-6,8-diisoindolemethene, or 1,13-diphenyl-3,11di(tri-fluoromethyl)-6,8-diisoindolemethene (Fig. 21). Of the five complexes, only three (complexes 1–3) were fully characterized since complexes 4 and 5 were unstable and had a low yield. The absorbance spectra were shifted to lower energies with the added

Fig. 19. Heteroleptic compounds synthesized by Pellegrin et al. Reprinted with permission from [89]. Copyright 2011 American Chemical Society.

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Fig. 20. Structures of proposed donor and acceptor ligands. Reprinted with permission from [90]. Copyright 2012 American Chemical Society.

conjugation of the dipyrrin ligand. The absorbance spectra of complexes 2 and 3 looked similar to the free dipyrrin ligands and this observation, in combination with an observed fluorescence as opposed to phosphorescence and calculations showing HOMO’s localized on the dipyrrin ligands, suggests that the absorption band of the complexes is ligand based and does not involved the Cu(I) metal center. Electrochemical studies showed that these complexes were easily oxidized, but in situ solar studies did show that the complexes could nonetheless function as dye-sensitizers. Though the efficiencies were low, it is one of the first times that the use of a chromophoric ancillary ligand helped increase the light harvesting capability of the Cu(I) complex. Hole-transport functionalized ligands with triphenylaminedendrons were synthesized by Bozic-Weber et al. and then utilized in heteroleptic Cu(I) complexes attached to TiO2 nanoparticles for use in dye-sensitized solar cells [92]. The two ancillary ligands (1 and 2) were synthesized from 2,2 -bipyridine ligands and had first and second generation dendrons for hole transport and 6,6 -dimethyl substitutions for stabilization of the copper center. The electrochemistry of the homoleptic complexes [Cu(1)2 ]+ and [Cu(2)2 ]+ shows that each complex has a copper-centered and ligand-centered oxidation process. Solar cells were prepared by functionalizing TiO2 with the anchoring ligands (Fig. 22), and then metallating the surface bound ligands by an exchange reaction with the labile homoleptic copper complexes. The authors used I− /I3 − as the electrolyte and measured the light-to-power conversion efficiency under illumination 1, 3, 6, and 47 days after sealing. Complexes with the anchoring ligand 3 performed the best, supporting a previous argument that phosphonic anchors perform better than carboxylate groups. A slight improvement in efficiency in going from anchoring ligand 4 to 5 suggests that a phenyl spacer may be a means of enhancing the performance of DSSC’s. These studies demonstrated the use of hole-transport functionalized Cu(I)

Fig. 22. Anchoring ligands synthesized and used for heteroleptic compounds with hole-transport functionalized ancillary ligands. Reproduced from [92] with permission of the Royal Society of Chemistry.

complexes that can be implemented and further optimized in DSSC’s (Fig. 23). Ashbrook and Elliott [93] successfully prepared an earth abundant DSSC using a Cu(I) bis(2,9-dimethyl-1,10-phenanthroline) complex as a photosensitizer and a cobalt-based mediator (Fig. 23). To attached the Cu(I) photosensitizer to TiO2 nanoparticles, the authors synthesized diethyl 2,2 -((2,9-dimethyl-1,10phenanthroline-5,6-diyl)bis(oxy))diacetic acid (KbindDMP), which has carboxylate groups that easily attached to the titania nanoparticles. The Cu(I) binds to the KbindDMP compound and is then bound by the synthesized DMP based compounds used for the DSSCs. The molecular design attempted to incorporate carboxylate groups for easy attachment to TiO2 nanoparticles, lipophilic components that would orient the attached ligands away from TiO2 for easy binding to Cu(I), and secondary electron donor moieties that would aid in rapid dye regeneration. The best performing ligand, 10,10 -(((2,9dimethyl-1,10-phenanthroline-5,6-diyl)-bis(oxy))bis(butane-4,1diyl))bis(2,4,6,8-tetramethylphenothiazine) (tmpDMP) shows promising J–V curves. When TA experiments are run with and without the Co mediator, there is a large difference in the lifetime; reduction of the TMPTZ+ portion of the Cu(I) complex occurs much more quickly than recombination with the electrons in the conduction band. These results demonstrate that Cu(I)-based dyes can act with a Co based mediator to make a completely earth-abundant DSSC without the use of the more common, but highly toxic I− /I3 − electrolyte. Constable, Housecraft, and coworkers [81] explored the steric effects of a series of heteroleptic compounds where they tuned both the ancillary ligands and the anchoring ligands attached to

Fig. 21. Bipyridine and dipyrrin ligands used to form heteroleptic Cu(I) complexes synthesized by Hewat and used as photosensitizers in DSSC’s [91]. Reproduced from [91] with permission of the Royal Society of Chemistry.

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highest energy-conversion efficiencies were achieved when the 6,6 -substitution was either the iso-butyl or phenyl group and the anchoring ligand contained a spacer. 5. Summary

Fig. 23. Schematic of DSSC system studied by Ashbrook and Elliott. Reprinted with permission from [93]. Copyright 2013 American Chemical Society.

TiO2 nanoparticles (Fig. 24). The ancillary ligands were modified at the 6,6 positions of 2,2-bipyridine ligands; substituents included methyl, butyl, iso-butyl, hexyl, and phenyl to give ligands 1 through 5. The anchoring ligands were 6,6 -dimethyl-2,2 -bipyridine ligands with phosphate groups to attach to the nanoparticles. A simple alkyl chain was introduced to one of the anchoring ligands to act as a spacer for potentially sterically demanding ancillary ligands. Crystal structures of [Cu(1)2 ][PF6 ], [Cu(2)2 ][PF6 ], and [Cu(3)2 ][PF6 ] show distorted tetrahedral coordination while ␲-stacking between a phenyl substituent and a pyridine ring of the adjacent ligand causes [Cu(5)2 ][PF6 ] to have a flattened structure. When the heteroleptic complexes were attached to the nanoparticles, the

Structural dependent MLCT state properties in Cu(I) diimine complexes have been extensively studied by ultrafast optical absorption/emission and X-ray absorption spectroscopies to reveal the interplay between the ground and excited-state structures with the excited-state properties, such as structural reorganization, intersystem crossing and solvent ligation. The minimization of the direct solvent accessibility to the copper center at the MLCT state is the key to prolonging the MLCT state lifetime and to avoiding the formation of the “exciplex” state as well as the solvent quenching. The coordination symmetry of the Cu(I) center in terms of the relative ligand plane orientation is another key factor to modulate the intersystem cross rate, as flattening to the D2 symmetry prolongs the singlet MLCT state lifetimes, gaining sufficient driving force for photochemical reactions. This fundamental understanding serves as guidance in materials design and solar energy conversion applications, where new synthetic designs have generated Cu(I) complexes that are longer lived and less susceptible to exciplex formation. By carefully tuning the substituents of the ligands, greater steric hindrance and electron donating abilities have been achieved. Furthermore, new heteroleptic approaches have allowed for greater thermodynamic tunability and electron donor/acceptors to be incorporated within the same ligand. The complexes that have emerged from this fundamental research demonstrate the great potential to create useful systems from earth abundant materials, such as copper. Acknowledgements We thank the support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DEAC02-06CH11357. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The authors would like to thank Drs. Klaus Attenkofer (now at NSLS-II, Brookhaven National Laboratory), Guy Jennings, Xiaoyi Zhang and Mr. Charles Kurtz of the Advanced Photon Source for their contributions to the Beamline 11ID-D facility at the APS. LXC would like to thank her collaborators in the XTA team since 2002, Drs. G. B. Shaw, E. C. Wasinger, J. V. Lockard, M. R. Harpham, A. B. Stickrath, J. Huang, Ms. M. L. Shelby for their contributions to the results included in the review. Our other collaborators contributions and scientific exchanges are also appreciated, Drs. G. Smolentsev, Kristoffer M. Haldrup, Profs. A. Sotadov, G. J. Meyer, F. Castellano, P. Coppens, and many others. The instrumentation supports (to LXC et al.) from the US Department of Energy for purchasing lasers, detectors and other related equipment enabling the initiation and upgrade of XTA experiments at Beamline 11IDD are greatly appreciated. References

Fig. 24. Structures of studied heteroleptic compounds attached to TiO2 nanoparticles. Reproduced from [81] with permission of the Royal Society of Chemistry.

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