Ultrafast Structural Dynamics of Cu(I)-Bicinchoninic Acid and Their

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Ultrafast Structural Dynamics of Cu(I)-Bicinchoninic Acid and Their Implications for Solar Energy Applications Kelly A. Fransted, Nicholas E. Jackson, Ruifa Zong, Michael W Mara, Jier Huang, Michael R. Harpham, Megan L Shelby, Randolph P. Thummel, and Lin X. Chen J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 11 Jul 2014 Downloaded from http://pubs.acs.org on July 12, 2014

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The Journal of Physical Chemistry

Ultrafast Structural Dynamics of Cu(I)-Bicinchoninic Acid and Their Implications for Solar Energy Applications Kelly A. Fransted1, Nicholas E. Jackson2, Ruifa Zong3, Michael W. Mara1,2,ǁ, Jier Huang1,‡, Michael R. Harpham1, Megan L. Shelby1,2, Randolph P. Thummel3,*, and Lin X. Chen1,2,* 1

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass

Avenue, Argonne, Illinois 60439-3113, United States 2

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois

60439, United States 3

Department of Chemistry, University of Houston, 4800 Calhoun Rd Houston, Texas 77204-

5003, United States

1 ACS Paragon Plus Environment


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ABSTRACT In this study, ultrafast optical transient absorption and x-ray transient absorption spectroscopy are used to probe the excited state dynamics and structural evolution of copper(I) bicinchoninic acid ([Cu(I)(BCA)2]+), which has similar, but less frequently studied biquinoline-based ligands compared to phenanthroline based complexes. The optical transient absorption measurements performed on the complex in a series of polar protic solvents demonstrate a strong solvent dependency for the excited lifetime, which ranges from approximately 40 ps in water to over 300 ps in 2-methoxy ethanol. The X-ray transient absorption (XTA) experiments showed a reduction of the prominent 1s→4pz edge peak in the excited state x-ray absorption near edge structure (XANES) spectrum, which is indicative of an interaction with a fifth ligand, most likely the solvent. Analysis of the extended X-ray absorption fine structure EXAFS spectrum shows a shortening of the metal-ligand bond in the excited state and an increase in the coordination number for the Cu(II) metal center. A flattened structure is supported by DFT calculations that show the system relaxes into a flattened geometry with a lowest energy triplet state that has a dipole forbidden transition to the ground state. While the short excited state lifetime relative to previously studied Cu(I) diimine complexes could be attributed to this dark triplet state, the strong solvent dependency and the reduction of the 1s→4pz peak in the XTA data suggest that solvent interaction could also play a role. This detailed study of the dynamics in different solvents provides guidance for modulating excited state pathways and lifetimes through structural factors such as solvent accessibility to fulfill the excited state property requirements for efficient light harvesting and electron injection. KEYWORDS


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Copper(I) bis-biquinoline complexes; ultrafast spectroscopy; excited state dynamics; time resolved x-ray spectroscopy; Copper(I) diimine complexes 1. INTRODUCTION Transition metal ligand complexes are desirable candidates for photosensitizers because of strong metal to ligand charge transfer (MLCT) absorption bands in the visible region that are capable of harvesting sunlight and driving electron transfer into semiconducting materials1,2. Under the operating principles of dye-sensitized solar cells (DSSCs), the dye molecule attached to a large bandgap semiconductor is promoted to an electronic excited state upon light absorption and subsequently injects an electron into the conduction band semiconductor film. Ultimately, the light energy is converted to electricity or the redox equivalents3,4. The hole left in the ground state of the dye sensitizer is filled by an electron generated through a redox cycle that takes electrons from a counter electrode. Ruthenium dyes, namely Ru(II) polypyridyl derivatives, traditionally have been the most commonly used dye-sensitizers because of their long excited state lifetimes in solution, intense visible absorption and emission, and high stability5. Despite these desirable properties as a photosensitizer, Ru(II) polypyridyl complexes are ultimately hindered by low abundance, and therefore high costs, as well as environmental toxicity. Copper(I) diimine complexes have emerged as a less expensive, earth abundant option as they are one of the first row transition metal compounds with excited state properties comparable to Ru(II) complexes6,7. These complexes generally absorb visible and near-ultraviolet light with high molar extinction coefficients and can exhibit relatively long lifetimes and room temperature emission6. Their usefulness, however, is hindered by the lability of the Cu(II) center8, JahnTeller distortions9, exciplex quenching10-12, and structure dependent energetics. In order to



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effectively implement Cu(I) diimine complexes in solar energy applications, we must understand the relationship between their structural components and excited state dynamics. To this end, much research has been done on mapping the structure-function relationship of Cu(I) diimine complexes. The most extensively studied of these complexes has been cuprous bis-phenanthroline compounds, as these complexes demonstrate a wide array of dynamics determined by the structure of the ligand. The parent compound, [Cu(I)(phenanthroline)2]+, is completely non-emissive and shows an excited lifetime of less than 200 ps. Disubstitution by adding either alkyl or aryl groups at the 2,9-positions increases the excited lifetime by stabilizing the Cu(I) state and increasing the transition energy, which can lead to long lived, room temperature emission13. The most well characterized phenanthroline complex is [Cu(I)(2,9dimethyl-1,10-phenanthroline)2]+ ([(Cu(I)(dmp)2]+)14-20, and the excited state structural dynamics have been established using both optical spectroscopy9,21-23 and x-ray transient absorption (XTA)24-26. The current picture of the excited state dynamics of this complex is as follows: upon absorption of a photon, the complex is excited via a MLCT transition from the S0 to the S2 state changing from a Cu(I) metal center to nominally Cu(II). Because the electron configuration goes from a closed shell 3d10 to an open shell 3d9 configuration, the complex is susceptible to a JahnTeller distortion. This distortion causes flattening of the pseudo-tetrahedral geometry in the Franck-Condon state as the complex relaxes via internal conversion from the S2 to S1 state on a sub-ps time scale. The system then undergoes intersystem crossing in 5-15 ps. Once in the triplet state, there are two pathways to the ground state. First, the triplet can be a long-lived state and radiatively decay via phosphorescence. Alternatively, depending on the degree of flattening and the solvent interaction with the copper center, a solvent molecule can bind to the Cu(II) metal center, lowering the energy of the excited state and increasing the non-radiative decay rate. 4 ACS Paragon Plus Environment

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Depending on the strength of the interaction with the solvent, the non-radiative relaxation either happens immediately or, if it is a weak interaction with a non-coordinating solvent, the complex relaxes to a lower energy intermediate before finally returning to the ground state. As demonstrated with aryl or alkyl substituents at the 2,9 positions, the excited state dynamics can be finely tuned with the choice of substituents on the parent phenanthroline compound. Studies on bis (2,9-di-tert-butyl-1,10-phenanthroline) copper (I) ([Cu(I)(dtbp)2]+) show that the bulky substituents at the 2,9-positions can completely block the flattening from occurring in the excited state, leading to a long lived emission (i.e. 3µs) and complete elimination of the sub-ps time component from the kinetic traces27. Large phenyl groups on bis (2,9-diphenylphenanthroline) copper (I) ([Cu(I)(dpp)2]+) force a flattened geometry in the ground state to accommodate π-stacking of the phenyl groups with the opposite phenanthroline moiety, but the substituents are also bulky enough that they prevent the solvent from ever accessing the Cu(I) metal center28. The excited state lifetime is therefore solvent independent. A sulfonated version of this complex showed not only a long-lived MLCT state, but also efficient electron transfer when attached to titania nanoparticles29. Further studies of a 3,8 substituted ligand demonstrate that even very bulky groups at other positions on the phenanthroline compound do little to block the solvent access to the copper center and prevent fast decay from occurring, demonstrating that tuning on the 2,9-positions is key in controlling the photodynamics30. While phenanthroline ligands have been the most commonly studied, other diimine ligands can bind easily to Cu(I) metal centers. Cu(I) complexes with bipyridine31-34 and biquinoline ligands35-37 have demonstrated strong, visible MLCT transitions and have been used for solar energy applications. A series of copper (I) complexes with bridged 2,2′-biquinoline ligands demonstrated structurally dependent absorption and oxidative properties36, but the photophysics 5 ACS Paragon Plus Environment


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of even simple cuprous bis-biquinoline complexes have yet to be extensively studied. The photophysics of these complexes must also be investigated to establish which structures can make the excited state properties of Cu(I) diimine complexes suitable for solar energy applications. One such ligand, 2,2′-biquinoline-4,4′-dicarboxylic acid or bicinchoninic acid (BCA), binds to Cu(I) and forms a water soluble complex that absorbs strongly at 562 nm (See Scheme 1).

Scheme 1: [Cu(I)(2,2′-biquinoline-4,4′-dicarboxylic)2]PF6 or Cu(I) bis-bicinchoninic acid hexafluorophosphate ([Cu(I)(BCA)2]PF6). Because of its strong binding affinity and absorption, BCA is most commonly used in a protein assay38, but its red-shifted absorption relative to most previously studied Cu(I)-diimine complexes makes it a desirable candidate as a photosensitizer for DSSCs. The carboxylic acid groups can be used to anchor the dye to semi-conductor nanoparticles, providing a direct route for electron injection. Furthermore, the fused benzo-rings of the biquinoline parent compound make the system bulky compared to the phenanthroline ligand, suggesting it may aid in solvent blocking. BCA has been used as part of a heteroleptic copper(I) complex for DSSCs 39. In addition, Wills et al. have recently attached [Cu(I)(BCA)2]+ to titania nanoparticles40 and have shown with calculations and electrochemical studies that the energy of the first excited state of


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the complex should be sufficient for electron injection into the TiO2. The performance of the DSSC, however, proved to be insufficient as the incident photon to charge carrier efficiency spectrum showed a quantum efficiency no greater than 1% when scanning pertinent visible wavelengths. This finding suggests that there is little electron transfer from the complex to the nanoparticles, and the authors point out that this could be due to relaxation to a short excited state lifetime or to a lower energy, non-emissive state. To understand the photophysics underlying the ultrafast dynamics of [Cu(I)(BCA)2]+, we present optical transient absorption (TA) and x-ray transient absorption (XTA) studies on [Cu(I)(BCA)2]+ in solution in four different solvents. Biquinoline compounds are a new class of ligands that have yet to be fully tested in their ability to form Cu(I) complexes that can act as photosensitizers in solar energy applications. Optical TA measurements probe the excited state dynamics to map out the excited state lifetimes and available visible transitions. In a similar way, XTA uses a pump-probe methodology, where the pump is a visible laser pulse, and the probe is an x-ray pulse, to map the structural changes that occur as the dynamics take place41,42. Using this method, we can probe both the electronic environment of the metal center as well as the local structure surrounding the absorbing metal. Our results demonstrate that like previous Cu(I) phenanthroline complexes, this complex bearing biquinoline ligands is still susceptible to a large degree of flattening in the excited state, and therefore undergoes exciplex quenching. Our results suggest that the biquinoline ligand must also be finely tuned either to prevent the flattening from occurring or to fully sterically block the Cu(I) center from the solvent, as the bulkiness of the biquinoline parent compound is not enough to prevent exciplex formation. 2. EXPERIMENTAL METHODS



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2.1 Synthesis of [Cu(I)(BCA)2]+ Complex An aqueous solution of 2,2'-biquinoline-4,4'-dicarboxylic acid dipotassium salt trihydrate (108 mg, 0.228 mmol) in 4 mL of water was added to a solution of Cu(NO3)2·3H2O (29 mg, 0.120 mmol) in 4 mL of water to give a green suspension at room temperature. The mixture was stirred for 30 min. Then 30 mg of ascorbic acid in 4 mL of water was added, producing a purple solution. The reaction mixture was stirred at room temperature for 40 min. Excess HCl solution (2N) was added to afford a precipitate. The solid was collected, washed with methanol (2 mL) and ether (3 mL) and dried in the air to give a solid (75 mg, 83%). The PF6- salt of the complex was obtained by adding excess NH4PF6 to a methanol solution of the chloride salt, followed by passing the mixture through a Sephadex column. The purple fraction was collected and evaporated. 1H NMR (400 MHz, CD3OD) δ 9.17 (s, 4H), 8.68 (d, J = 8.70 Hz, 4H), 7.79 (d, J = 8.70 Hz, 4H), 7.57 (pseudo t, J = 7.33 Hz, 4H), 7.38 (pseudo t, J = 7.79 Hz, 4H); MALDI-MS m/z = 751 for [M-Cl]+. 2.2 Steady State Absorption and Emission Measurements Steady-state absorption spectra were taken at ambient temperature using a Shimadzu UV-1601 spectrophotometer. Steady-state fluorescence measurements were performed but no room temperature emission was detected between 450 and 900 nm. The spectra of the complex in both ethanol and 2-methoxy ethanol were measured after bubbling the solvent with nitrogen gas. 2.3 Optical Transient Absorption Spectroscopy Transient absorption measurements were performed on an apparatus based on a commercial Ti:Sapphire regenerative amplifier. A 1 kHz pulse train of 100 fs, 2.9 mJ pulses centered at 830 nm was generated by a Spitfire Pro regenerative amplifier (Spectra-Physics Lasers) pumped by 8 ACS Paragon Plus Environment

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an Empower Nd:YLF Q-switched laser (Spectra-Physics Lasers). The regenerative amplifier was seeded by a Mai Tai Oscillator also pumped by an Empower (Spectra-Physics Lasers). The amplifier output was used to generate both the pump and probe beams. The 527 nm pump pulses were generated by a home built, white light seeded, two-pass optical parametric amplifier (OPA). A small percentage of the regen output is focused into a sapphire disc to generate the white light continuum, which is mixed with two passes of a 400 nm beam generated by doubling the regen output in a BBO crystal. The TA experiments were performed using a commercial Helios pump probe spectrometer (Ultrafast Systems LLC) using a fiber optic detector. The probe beam was generated by focusing the remaining portion of the regen output into a sapphire crystal in order to form a white light continuum ranging from 450 nm to 800 nm. The sample was pumped with 527 nm, 100 µJ pulses chopped at 500 Hz and the instrument response function for the entire pump-probe set up was approximately 300 fs. The transient absorption changes at a given wavelength were analyzed by fitting the data with a multiexponential decay convolved with a Gaussian function. Using the Surface Xplorer software (Ultrafast Systems LLC), the data set was chirp corrected by analyzing the non-resonant response of the solvent. Experiments were performed at room temperature in a 2 mm cuvette with the sample mixing via a magnetic stir bar for the duration of the experiment. Samples were prepared by dissolving a small amount of the complex in the chosen solvent (water, methanol, ethanol or 2-methoxy ethanol), sealing with a septum, and then bubbling with nitrogen to remove oxygen from the sample. All solvents were spectrograde and used as received. Sample integrity was verified by checking the absorbance spectrum before and after the TA experiment. 2.4 X-ray Transient Absorption Spectroscopy



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X-ray transient absorption spectra of [Cu(I)(BCA)2]+ in ethanol (1.5 mM) were acquired at Beamline 11ID-D of the Advanced Photon Source at Argonne National Laboratory. Briefly, a steady stream of an ethanolic solution of the sample of approximately 0.7 mm diameter was illuminated with 527 nm, 5 ps laser pulses at 1 kHz repetition rate, which were generated by second harmonic generation of the output of an Nd:YAG laser. The sample was bubbled with nitrogen for the duration of the experiment to eliminate oxygen from the sample. The illuminated sample was probed with the x-ray beam under the standard timing mode where 24 bunches of ~80-100 ps width were equally distributed around the storage ring with interbunch separation of 154 ns. One bunch was synchronized with the pump pulse at a delay of 100 ps using a programmable delay generator (PDL-100A-20NS, Colby Instruments). X-ray absorption spectra were collected in fluorescence mode with two custom built avalanche photodiode (APD) detectors utilizing a Soller slit/Z-1 filter to minimize elastic scattering. A third APD upstream was used to normalize incoming intensity. Approximately 40 scans were collected over the course of 5 hours per sample for a total of 15 hours. 2.5 XTA Analysis X-ray absorption fine structure (XAFS) analysis was performed using the FEFF and IFEFFIT packages43. The data preprocessing was done in Athena44. The scans were calibrated using a Cu reference foil, and the edge energies were set as the zero intercept of the second derivative. The background was subtracted using Athena’s Autobk algorithm with a k-weight of 2 and an Rbkg value of 0.8545. The copper K-edge XAFS spectra were processed and analyzed in Artemis for fitting in R-space44. The calculated ground state and lowest energy excited state geometries for [Cu(I)(BCA)2]+ (see section 2.6) were used as the model structures for the system, and the scattering paths were generated using the FEFF 6.0 program built into Artemis. The S02 and ∆E 10 ACS Paragon Plus Environment

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parameters were set to the same values for all the fits in each data set, and the same scattering paths were included for each fit. Multiple scattering paths were used to fit higher level shells. 2.6 DFT Calculation Methods Calculations were performed using the ADF2012.01 suite of programs 46-48. The B3LYP hybridexchange functional with a zeroth order regular approximation (ZORA)49-51 scalar relativistic correction was employed for the electronic structure of the [Cu(I)(BCA)2]+ complex. A double-ζ polarized (DZP) basis set was used to describe the C, N, and H atoms, and a triple-ζ polarized (TZP) basis set was used to describe the atomic orbitals of copper. [Cu(I)(BCA)2]+ ground state geometry optimizations were performed using a D2 symmetry constraint, and normal-mode analysis confirmed that converged geometries were true energy minima. Vertical singlet and triplet excitation energies were calculated at the optimized ground-state geometry using timedependent density functional theory (TDDFT)52, and the excited-state geometry of the lowestlying triplet states was relaxed with a B3LYP geometry optimization under D2 symmetry constraints. 3. RESULTS 3.1 UV-VIS Spectra The steady state absorption spectra of [Cu(I)(BCA)2]+ display two strong peaks at 560 nm and 620 nm (Figure 1). Based on previous absorption spectra for Cu(I) diimine complexes, both the main absorption peak and the red shoulder can be assigned to MLCT transitions53,54. The absorption maximum of this complex is red shifted compared to most Cu(I) diimine complexes, most likely due to increased delocalization of the BCA ligand. The solvent has little effect on the position of the main absorption. Based on past literature53,54, the red shoulder is attributed to 11 ACS Paragon Plus Environment


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the MLCT transition from the flattened pseudo-tetrahedral geometry. While the solvent has little effect on the absorption maximum, it does change the relative intensity of the red shoulder compared to the main absorption feature. The absorption spectra of [Cu(BCA)2]+ in water and methanol are nearly identical, but the intensity of the red shoulder decreases in the bulkier solvents ethanol and 2-methoxy ethanol.

Figure 1: UV-Vis absorption spectra of [Cu(I)(BCA)2]+ in water (pink), methanol (red), ethanol (green), and 2-methoxy ethanol (blue). 3.2 Optical Transient Absorption


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The optical TA spectra for [Cu(I)(BCA)2]+ in ethanol are presented in Figure 2. Immediately after excitation, three separate peaks appear in the spectrum: a ground state bleach (GSB) feature at

Figure 2: Transient absorption spectra for [Cu(I)(BCA)2]+ in ethanol at 500 fs, and 1, 5, 10, 20, 50, 100, and 220 ps. The spike at 527 nm is from scattered intensity from the pump beam. the main absorption peak, an excited state absorption (ESA) feature blue shifted from the GSB, and an ESA at lower energy, or redshifted, from the GSB. The GSB is broad and featureless while the blue shifted ESA peak has a slight shoulder, which is most likely due to the vibronic nature of the excited state species. The corresponding blue ESA was not observed in previously studied Cu(I) diimine complexes because of the spectral detection limit. The ground state 13 ACS Paragon Plus Environment


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absorption of [Cu(I)(BCA)2]+ is about 100 nm red shifted from the absorption spectra of previously studied Cu(I) diimine complexes, thus enabling the observation of the blue ESA feature. The lower energy ESA peak is now more difficult to discern as its low intensity coincides with the weakest portion of the white light probe spectrum. The TA spectra show virtually no spectral shift as the peaks appear immediately after the excitation and maintain their spectral shape and position as they decay. Previous work on Cu(I) diimine complexes showed a blue shift in the ESA spectrum for the first 20 ps, and this shift was attributed to vibronic cooling. Solvent τ1/ps (% amplitude) τ2/ps (% amplitude) Water 4.1 ± 0.2 (44%) 45.5 ± 2.2 (56%) Methanol 6.8 ± 0.3 (67%) 69 ± 1 (33%) Ethanol 8.1 ± 0.3 (26%) 213 ± 3 (74%) 2-methoxy ethanol 10.2 ± 0.6 (35%) 314 ± 10 (65%) Table 1: Summary of Excited State Lifetimes for [Cu(I)(BCA)2]+ in Solution for Traces at 560 nm.


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Figure 3: TA kinetic traces of [Cu(I)(BCA)2]+ in 2-methoxy ethanol (red), ethanol (green), methanol (violet), and water (blue) at 560 nm. The closed circles represent the data while the black lines indicate the bi-exponential fits to the traces. The traces have been individually normalized to 1, but a vertical offset has been introduced for clarity. The intensity of the TA spectrum was monitored at a single wavelength to generate the kinetic traces in Figure 3. The resultant kinetic traces were fit to a sum of exponentials convolved with a Guassian instrument response function (IRF), which was approximately 300 fs in all cases (Table 1). The traces were taken from the largely GSB feature at 560 nm and for all four solvents the dynamics follow the same pattern: a shorter decay on the order of a few picoseconds and a longer decay on the order of 100 ps. Kinetic traces from the blue shifted ESA peak also



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showed the same pattern and were fit to similar time components. Traces from the red shifted ESA peak could not be accurately modeled as the signal to noise was low. In the case of the shorter time constant, the solvent effect is small as it varies from 4 ps in water to 10 ps in 2methoxy ethanol. The longer time decay, however, showed a strong solvent dependence. For [Cu(I)(BCA)2]+ in water, methanol, ethanol, and 2-methoxy ethanol, the exponential decay varied as 45 ps, 70 ps, 213 ps, and 314 ps, respectively, showing that the time constant increases with bulkiness of the solvent. Surprisingly, unlike previous Cu(I) diimine complexes, [Cu(I)(BCA)2]+ shows no sub-ps time component usually attributed to the flattening distortion of the excited state. The lack of an ultrafast rise component will be discussed later in the paper. Despite the lack of the sub-ps time component, the other two time components fit the established picture of excited state dynamics for Cu(I) diimine complexes. Based on this picture, the shorter time component seen here reflects the intersystem crossing while the longer time component can be attributed to relaxation to the ground state. 3.3 XANES and XAFS Spectra


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Figure 4: XANES spectra of [Cu(I)(BCA)2]+ in ethanol for both the ground state and laser excited configurations. Measurements were performed at Cu K-edge. Laser excitation was at 527 nm and the X-ray probe was at a delay of 100 ps. The sample concentration was ~1.5 mM. The prominent features discussed in the text are indicated by arrows. The XANES region of both the ground state and the laser excited configuration of [Cu(I)(BCA)2]+ in ethanol is shown in Figure 4. Three main spectral changes occur upon excitation with the laser pulse: an edge shift to higher energy, the appearance of a pre-edge feature near 8981 eV, and a reduction of the edge peak located at 8987 eV. The pre-edge feature arises from the 1s to 3d transition that can only take place when Cu(II) is generated. Similarly, 17 ACS Paragon Plus Environment


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the edge shift to higher energy is due to the added energy needed to pull an electron from a more positive Cu(II) center. Based on previous studies, the edge peak is assigned to the 1s→4pz transition55,56. The edge peak was utilized in estimating the percent ground state of the laser excited spectrum, and the peak “disappeared” when the spectrum was considered 50% excited state. A similar procedure was used in fitting the XAFS data. The S02 parameter is a multiplicative factor that accounts for amplitude loss due to inelastic electron-electron scattering, and therefore determines the amplitude of a given path in the XAFS spectrum57 . For the fitting to the ground state structure, it was set to 0.8026 for all scattering paths. When fitting the laser excited spectrum, the amplitudes for the ground state and excited state paths were adjusted until the goodness of fit parameter was optimized. In this case, it was 0.47 for the ground state paths, and 0.34 for the excited state paths, suggesting an excited state population of about 42%. We therefore estimate that the laser excited spectrum is between 40% and 50% excited state.

Figure 5: XAFS spectra of [Cu(I)(BCA)2]+ in ethanol in (a) k-space and (b) R-space. A kweight of 3 and a range of 1.5-8.25 were used for the Fourier Transform (Hanning window) and the range is indicated by the dashed black lines in (a).


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The ground state and laser excited XAFS spectra of [Cu(I)(BCA)2]+ in ethanol are shown in Figure 5, where the k-space (a), and R-space (b) spectra are presented. The R-space spectra were generated by performing a Fourier transform over the window of k = 1.5-8.25 Å-1 with a k3 weighting. The k-space spectra show a slight shift to higher k for the laser excited sample. The signal to noise decreases at higher k, as expected, so the window was chosen to maximize number of oscillations available for fitting while minimizing the noise in the Fourier transform. The R-space fitting was performed over a window of 1 to 3.11 Å for both the ground and laser excited spectra. These fits are shown in Figure 6 (a) and (b), respectively. The magnitude R spectra reveal that upon excitation, the bond lengths generally shorten. The magnitude also suggests a decrease in coordination number, although it seems unlikely that the Cu(I) center would lose a coordinating molecule in the excited state based on the optical TA data. It is important to note that the laser excited spectrum is fit using a combination of the ground state spectrum and excited state spectrum from the DFT calculations. The peak position of the individual configurations is dependent on both the ∆E value, which is the experimental change relative to the theoretical wavenumber grid, and the bond distance from the Cu(I). While the ∆E value is expected to stay the same, the bond distance from the Cu center should change upon excitation, meaning the single peak on the magnitude R spectrum is most likely a combination of two underlying peaks. If these peaks are shifted from one another in R, the total amplitude of the laser excited spectrum could be less than the amplitude of the ground state spectrum, resulting in a wider but shorter peak.



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Figure 6: Fourier transform (solid) and best fit (dashed line) for the magnitude (black) and imaginary (red) R-space spectra of [Cu(I)(BCA)2]+ for the ground state (a) and laser excited (b) populations.

Atom-atom scattering path

atom-atom distance (Å ±0.02 Å)

Debye-Waller factor (Å2 ±0.001 Å2)

Ground State Excited State Ground State Excited State 1.98 1.91 0.005 0.01 Cu-N 2.55 2.51 0.005 0.01 Cu-C (a) 2.80 2.72 0.005 0.01 Cu-C (b) 3.26 3.17 0.005 0.01 Cu-C (c) Table 2: XAFS Fitting Parameters in Ethanol, with ∆E = -4.6 eV and S01 = 0.8.

Scheme 2: Atomic Centers Indicated in XAFS Data Fitting


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The results from the R-space fits are shown in Table 2 and pertinent carbon pathways are labeled in Scheme 2. While multiple scattering paths were included, their contributions were minor and only in the higher shell fits. Thus, only the single carbon and nitrogen paths are included. The fits support what was shown in just the magnitude R-spectra: the bond lengths all shorten upon laser excitation. The bond length decrease suggests that the ligands move closer to the Cu(II) metal center, and previous results have also suggested that this decrease is indicative of a strongly interacting solvent24. The Debye-Waller factors increase with the MLCT, suggesting that added electron density on the ligand increases the overall disorder in the bond lengths. It should also be noted that the laser excited spectrum was fit using a coordination number of 5 for the nitrogen scattering path, indicating that the coordination of Cu center actually increases upon excitation. This coordination number was determined based on the goodness of fit parameter and the physical interpretation of the fits. When a coordination number of 4 was used, the S02 values indicated that the excited state component of the spectrum should actually be greater than 50%, which is not physically plausible. 3.4 DFT Calculation Results

Figure 7: Calculated geometries for [Cu(I)(BCA)2]+ in vacuum. The geometries correspond to the lowest energy (a) ground state, (b) singlet excited state, and (c) triplet excited state.



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Ground State 2.068 125.4 80.3 90.3

Singlet State 2.055 108.5 81.2 62.4

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Triplet State 2.007 110.9 82.5 62.4

Cu-N (Å) Cu-N-Cu angle (interbiquinoline) Cu-N-Cu angle (intrabiquinoline) biquinoline-biquinoline flattening angle Table 3: Relevant DFT Parameters for [Cu(I)(BCA)2]+. All angles are in degrees.

The calculated geometries of the [Cu(I)(BCA)2]+ structure in the ground state, lowest energy singlet state, and lowest energy excited state are shown in Figure 7 (a), (b), and (c), respectively. The corresponding parameters are presented in Table 3. The excited state geometries support the current picture of Cu(I) diimine dynamics. The ground state is calculated as perfectly tetrahedral with the angle between the ligands at 90 degrees. As the geometry changes in the excited state, the complex flattens and the angle between the ligands decreases. The lowest energy excited state is a triplet state, indicating that an intersystem crossing occurs. The angle between the ligands already decreases to 62.4 degrees even by the lowest energy singlet state, which is the same as the angle in the triplet state. The inter-ligand plane angle is smaller than that for the MLCT state of [Cu(I)(dmp)2]+ is about 68°, reflecting even less hindered flattening.24 The transition energies were calculated to be 567 nm and 652 nm, which are very close the absorption peaks shown in the UV-Vis spectra. The orbital calculations (See SI) indicate that the two visible transitions are indeed MLCTs. These results are in agreement with the calculations done by Wills et al. Finally, the calculations suggest that the triplet state is actually a dark state due to a dipole forbidden transition, suggesting another reason for no observed emission. 4. DISCUSSION The extensive research on cuprous bis-phenanthroline complexes demonstrates that the two most important aspects of the complex influencing the MLCT dynamics are the flattening distortion


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and the potential for exciplex formation. In order to bring the photophysics of cuprous bisbiquinoline complexes into the discussion, we use the presented optical TA, XTA, and calculated results to analyze both aspects of [Cu(I)(BCA)2]+ in solution. Previous transient absorption studies on Cu(I)-diimine complexes have consistently shown a sub-ps time component, which has been attributed to the flattening distortion of the complex in the excited state. The surprising lack of a sub-ps component in these data suggests that either [Cu(I)(BCA)2]+ does not flatten, or the flattening dynamics occur more quickly than can be resolved by the 300 fs IRF of the TA spectrometer. First the lack of sub-ps component and potential for flattening will be addressed, followed by a discussion of the possibility of exciplex formation in solution. 4.1 Analysis of the Flattening Distortion The ground state electronic spectra of [Cu(I)(BCA)2]+ in all four solvents show a strong absorption near 550 nm and a weaker shoulder at approximately 650 nm. The shoulder band has been attributed to a static or dynamic flattening distortion that induces a D2d to D2 symmetry transformation58-60. The relative intensity of these two features is consequently a measure of the degree of symmetry breaking distortion, with a higher relative intensity of the red shoulder indicative of a larger degree of flattening. The energy splitting between these two features has also been linked to the dihedral angle distortion59. The flattening from the pseudo tetrahedral geometry effectively splits the near degenerate Cu 3dxz and 3dyz HOMO orbitals, which translates to increased energy splitting between the two transitions. In the absorbance spectra presented in Figure 1, the shoulder at 650 nm indicates that there is some potential for a dihedral angle distortion, and this capability is slightly dependent on the solvent. The shoulder intensity is decreased for the bulkier solvents compared to water and methanol. The spectra show almost



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no shift in the energy splitting, suggesting that whatever solvent dependency of the dihedral angle there is, it is only minor. Previous research in this group on Cu(I)-diimine complexes with 2,9-disubstituted phenanthroline ligands has shown kinetic data with no sub-ps component in only two cases. First, in [Cu(I)(dtbp)2]+, the ligands in the 2,9-position were so bulky that the complex was locked in a tetrahedral geometry even upon excitation and never flattened as it relaxed to the ground state. This geometrical constraint was indicated by the absence of a low energy shoulder in the absorbance spectrum. Second, [Cu(I)(dpp)2]+ showed an intense low energy shoulder in the absorbance spectrum, which was attributed to a deviation from the tetrahedral geometry to accommodate bulky phenyl groups. The bulkiness of the phenyl groups consequently blocks the Cu metal center from the solvent following excitation, preventing exciplex formation. Hence, the low energy shoulder in the [Cu(I)(BCA)2]+ in the absorbance spectra demonstrate that there is a potential for flattening in the excited state, which is only slightly influenced by the solvent, but the bulkiness of the ligands is not so great as to block solvent accessibility. Given the evidence of flattening potential in the absorbance spectra and the solvent dependency of the excited state lifetime, a lack of flattening in the excited state is unlikely. This conclusion is further supported by the DFT calculations which clearly indicate flattening in both the lowest energy singlet and triplet excited states. The flattening, then, must occur on a time scale faster than can be resolved by our instrument. A recent paper from the Tahara group helps explain why this flattening would occur on a faster time scale than has been observed previously22. Iwamura and coworkers performed ultrafast emission spectroscopy on several Cu(I) diimine complexes with 2,9-disubstituted phenanthroline ligands. Their results from ultrafast femtosecond emission spectroscopy experiments indicate a sub-ps component on the order of 50 fs, which is attributed 24 ACS Paragon Plus Environment

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to relaxation from the S2 to the S1 state, in addition to the well discussed 200 fs-1ps time component that has been attributed to the flattening of the ligands in the excited state. These early time dynamics cannot be explained by simple transition state theory due to the nonequilibrium nature of non-linear processes. In this case, they propose that a nuclear wavepacket on a multi-dimensional excited state potential energy surface (PES) must be used to explain the excited state generated by the photoexcitation. Their picture looks as follows: as the complex flattens, the substituents in the 2- and 9- positions must move close together, and because of the repulsion, the flattening must be accompanied by a rotation of the substituents. In cases where the substituents are bulky and must rotate substantially, the trajectory on the PES can be complicated, resulting in a long time component. In cases where there are no substituents in the 2- and 9- positions, and therefore no rotational degree of freedom, the relevant vibrational coordinate is only the flattening. The simpler mechanism results in a faster flattening time component. In the case of [Cu(I)(BCA)2]+, the situation is similar to the Cu(I) bisphenanthroline example. While there are rings fused to the analogous 2- and 9- positions of the phenanthroline ligands, the ligand is nearly planar and the rings cannot rotate. The lack of a rotational degree of freedom would also suggest that the only coordinate that matters, then, is the flattening angle, resulting in a fast time component. Since the flattening component of the Cu(I)phen compound is on the order of 200 fs, it is not surprising that the flattening of Cu(I)BCA would be faster than what could be observed with a 300 fs IRF. 4.2 Exciplex Formation with a Fifth Coordinating Molecule The intensity of the edge feature in the XANES spectrum varies for different Cu(I) coordination complexes, since the 4pz orbital is susceptible to changes in the metal coordination environment. The intensity of this peak, therefore, is highly indicative of the geometry of the Cu(I) complex 25 ACS Paragon Plus Environment


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and the flattening angle between the ligands and of the coordinating environment of the Cu metal center55. When the Cu center is in a four coordinate pseudo-tetrahedral geometry, the 4pz orbital is nearly empty and localized, giving rise to an intense 1s to 4pz transition. When this geometry is flattened in the MLCT state, the 3d-4p mixing reduces and the 4p orbital becomes even emptier, hence increasing the intensity of the 1s→4pz peak. If the metal center becomes penta or hexa coordinated, the lone pair electrons from the ligating nitrogen or oxygen cause the energies of the 4pz orbital to be progressively shifted to higher values, resulting in an attenuated 1s →4pz peak. As observed in the XANES spectra shown in Figure 4, the 1s→4pz edge feature is prominent in the ground state spectrum but is substantially weaker in the laser excited spectrum. The behavior of the edge feature then suggests that there is some flattening in the ground state, as evidenced by this intense transition, and that upon laser excitation, a fifth molecule coordinates with the Cu(II) metal center, leading to a weakening of the 1s to 4pz transition. The hypothesis of a fifth ligand binding to the metal center is supported by the fits to the XAFS spectra in that the fit required an increase in coordination number for the first shell scattering pathway. Furthermore, those results shown in Table 2 indicate that the average bond length shortens upon laser excitation. In previous [Cu(I)(dmp)2]+ studies, the XTA data showed a decrease in bond length when in strongly coordinating acetonitrile but an increase in bond length in weakly coordinating toluene. The decrease in bond length was attributed to a stronger interaction with the solvent, and the decrease in bond length observed here is consistent with that conclusion. The optical TA data also suggest interaction of the Cu metal center with a fifth ligand. As shown in Table 1, the shorter TA time constants are not strongly dependent on solvent, as only a slight increase is observed with the bulkiness of the solvents. These results suggest that the early time dynamics are inner sphere processes, and that the flattening is not affected by an interaction with 26 ACS Paragon Plus Environment

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the solvent. Little spectral motion of the peaks is also observed in all four solvents. Since there is no time dependent energy shift of the peaks, whatever vibrational cooling or vibrational relaxation processes might occur are faster than the time resolution of our instrument. No strong dependency on the solvent is observed until the decay to the ground state, which is presumably from the triplet state where the complex is completely flattened. The excited state lifetime, however, increases dramatically with the bulkiness of the solvent, going from nearly 50 ps in water to over 300 ps in 2-methoxy ethanol. The change in geometry makes the Cu(I) center more accessible, and the excited state lifetime increases with the bulkiness, and therefore the inaccessibility, of the solvent. The solvent dependency, which occurs after the complex is completely flattened, is strong evidence for exciplex formation. 5. CONCLUSIONS The optical TA, XTA, and calculated results presented here demonstrate that like cuprous bisphenanthroline complexes, Cu(I) complexes with biquinoline ligands can also undergo excited state flattening and exciplex quenching. Although there is no sub-ps time component in the kinetic traces, the ground state absorbance spectra suggest that there is still a flattening potential. The DFT calculations on the excited state geometry clearly show that this is indeed the case, and the lowest energy excited state has a reduced dihedral angle. The XANES spectra show that the 1s→4pz transition peak feature, which is indicative of the coordination environment of the Cu metal center, disappears upon laser excitation. Coordination with a fifth molecule would attenuate the intensity of the transition associated with this peak, providing good evidence that a fifth molecule is indeed binding to the photogenerated Cu(II) center. This conclusion is corroborated by the strong solvent dependency of the excited state lifetime, suggesting that the flattening distortion provides access of the solvent to the metal. The excited state flattening and 27 ACS Paragon Plus Environment


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coordination suggest the low efficiency measurements observed for DSSCs using [Cu(I)(BCA)2]+ as the photosensitizer are a consequence of the short excited state lifetime due to exciplex formation. It is apparent from these data that fine tuning of the biquinoline ligand is still necessary to shield the metal center from the solvent access in order to effectively block the exciplex formation. These steps would increase the excited state lifetime and increase the potential for electron injection from the MLCT state to the semiconducting nanoparticles. ACKNOWLEDGMENTS We would like to acknowledge support from the Division of Chemical Sciences, Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-AC0206CH11357 (KAF and LXC), as well as DE-AC02-06CH11357 and DE-FG02-07ER15888 (RZ and RPT) for support of this work. RZ and RPT also thank the Robert A. Welch Foundation (grant E-621). The authors would like to thank Xiaoyi Zhang of 11-ID-D at the Advanced Photon Source for her help with the XTA measurements. Use of beamline 11-ID-D at the Advanced Photon Source was supported by the U.S. DOE under Contract No. DE-AC0206CH11357. The authors would also like to thank Dr. Dugan Hayes for his advice on experimental design. ASSOCIATED CONTENT Supporting Information Additional materials, characterization methods, 1H NMR, mass spec, and DFT calculations results is available in the supplementary information. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Authors *Phone: 713.743.2734. E-mail: [email protected] *Phone: 630-252-3533 or 847-491-3479. E-mail: [email protected] or [email protected] Present Address ǁ

Department of Chemistry, Stanford University, Stanford, California 94305-4401, United States.

‡

Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-181, United

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TOC Graphic


Ultrafast Structural Dynamics of Cu(I)-Bicinchoninic Acid and Their

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