@article{mukherjee_bowman_jakubikova_2015, title={Cyclometalated Fe(II) Complexes as Sensitizers in Dye-Sensitized Solar Cells}, volume={54}, ISSN={["1520-510X"]}, DOI={10.1021/ic502438g}, abstractNote={Dye-sensitized solar cells (DSSCs) often utilize transition metal-based chromophores for light absorption and semiconductor sensitization. Ru(II)-based dyes are among the most commonly used sensitizers in DSSCs. As ruthenium is both expensive and rare, complexes based on cheaper and more abundant iron could serve as a good alternative. In this study, we investigate Fe(II)-bis(terpyridine) and its cyclometalated analogues, in which pyridine ligands are systematically replaced by aryl groups, as potential photosensitizers in DSSCs. We employ density functional theory at the B3LYP/6-31G*,SDD level to obtain the ground state electronic structure of these complexes. Quantum dynamics simulations are utilized to study interfacial electron transfer between the Fe(II) photosensitizers and a titanium dioxide semiconductor. We find that cyclometalation stabilizes the singlet ground state of these complexes by 8-19 kcal/mol but reduces the electron density on the carboxylic acid attached to the aryl ring. The results suggest that cyclometalation provides a feasible route to increasing the efficiency of Fe(II) photosensitizers but that care should be taken in choosing the substitution position for the semiconductor anchoring group.}, number={2}, journal={INORGANIC CHEMISTRY}, author={Mukherjee, Sriparna and Bowman, David N. and Jakubikova, Elena}, year={2015}, month={Jan}, pages={560–569} }
 @article{mara_bowman_buyukcakir_shelby_haldrup_huang_harpham_stickrath_zhang_stoddart_et al._2015, title={Electron Injection from Copper Diimine Sensitizers into TiO2: Structural Effects and Their Implications for Solar Energy Conversion Devices}, volume={137}, ISSN={["0002-7863"]}, DOI={10.1021/jacs.5b04612}, abstractNote={Copper(I) diimine complexes have emerged as low cost replacements for ruthenium complexes as light sensitizers and electron donors, but their shorter metal-to-ligand-charge-transfer (MLCT) states lifetimes and lability of transient Cu(II) species impede their intended functions. Two carboxylated Cu(I) bis-2,9-diphenylphenanthroline (dpp) complexes [Cu(I)(dpp-O(CH2CH2O)5)(dpp-(COOH)2)](+) and [Cu(I)(dpp-O(CH2CH2O)5)(dpp-(Φ-COOH)2)](+) (Φ = tolyl) with different linker lengths were synthesized in which the MLCT-state solvent quenching pathways are effectively blocked, the lifetime of the singlet MLCT state is prolonged, and the transient Cu(II) ligands are stabilized. Aiming at understanding the mechanisms of structural influence to the interfacial charge transfer in the dye-sensitized solar cell mimics, electronic and geometric structures as well as dynamics for the MLCT state of these complexes and their hybrid with TiO2 nanoparticles were investigated using optical transient spectroscopy, X-ray transient absorption spectroscopy, time-dependent density functional theory, and quantum dynamics simulations. The combined results show that these complexes exhibit strong absorption throughout the visible spectrum due to the severely flattened ground state, and a long-lived charge-separated Cu(II) has been achieved via ultrafast electron injection (<300 fs) from the (1)MLCT state into TiO2 nanoparticles. The results also indicate that the TiO2-phen distance in these systems does not have significant effect on the efficiency of the interfacial electron-transfer process. The mechanisms for electron transfer in these systems are discussed and used to develop new strategies in optimizing copper(I) diimine complexes in solar energy conversion devices.}, number={30}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Mara, Michael W. and Bowman, David N. and Buyukcakir, Onur and Shelby, Megan L. and Haldrup, Kristoffer and Huang, Jier and Harpham, Michael R. and Stickrath, Andrew B. and Zhang, Xiaoyi and Stoddart, J. Fraser and et al.}, year={2015}, month={Aug}, pages={9670–9684} }
 @misc{jakubikova_bowman_2015, title={Fe(II)-Polypyridines as Chromophores in Dye-Sensitized Solar Cells: A Computational Perspective}, volume={48}, ISSN={["1520-4898"]}, DOI={10.1021/ar500428t}, abstractNote={Over the past two decades, dye-sensitized solar cells (DSSCs) have become a viable and relatively cheap alternative to conventional crystalline silicon-based systems. At the heart of a DSSC is a wide band gap semiconductor, typically a TiO2 nanoparticle network, sensitized with a visible light absorbing chromophore. Ru(II)-polypyridines are often utilized as chromophores thanks to their chemical stability, long-lived metal-to-ligand charge transfer (MLCT) excited states, tunable redox potentials, and near perfect quantum efficiency of interfacial electron transfer (IET) into TiO2. More recently, coordination compounds based on first row transition metals, such as Fe(II)-polypyridines, gained some attention as potential sensitizers in DSSCs due to their low cost and abundance. While such complexes can in principle sensitize TiO2, they do so very inefficiently since their photoactive MLCT states undergo intersystem crossing (ISC) into low-lying metal-centered states on a subpicosecond time scale. Competition between the ultrafast ISC events and IET upon initial excitation of Fe(II)-polypyridines is the main obstacle to their utilization in DSSCs. Suitability of Fe(II)-polypyridines to serve as sensitizers could therefore be improved by adjusting relative rates of the ISC and IET processes, with the goal of making the IET more competitive with ISC. Our research program in computational inorganic chemistry utilizes a variety of tools based on density functional theory (DFT), time-dependent density functional theory (TD-DFT) and quantum dynamics to investigate structure-property relationships in Fe(II)-polypyridines, specifically focusing on their function as chromophores. One of the difficult problems is the accurate determination of energy differences between electronic states with various spin multiplicities (i.e., (1)A, (1,3)MLCT, (3)T, (5)T) in the ISC cascade. We have shown that DFT is capable of predicting the trends in the energy ordering of these electronic states in a set of structurally related complexes with the help of appropriate benchmarks, based either on experimental data or higher-level ab initio calculations. Models based on TD-DFT and quantum dynamics approaches have proven very useful in understanding IET processes in Fe(II)-polypyridine-TiO2 assemblies. For example, they helped us to elucidate the origin of "band selective" sensitization in the [Fe(bpy-dca)2(CN)2]-TiO2 assembly (bpy-dca = 2,2'-bipyridine-4,4'-dicarboxylic acid), first observed by Ferrere and Gregg [ Ferrere , S. ; Gregg , B. A. J. Am. Chem. Soc. 1998 , 120 , 843 . ]. They also shed light on the relationship between the linker group that anchors Fe(II)-polypyridines onto the TiO2 surface and the speed of IET in Fe(II)-polypyridine-TiO2 assemblies. More interestingly, our results show that the IET efficiency is strongly correlated with the amount of electron density on the linker group and that one can obtain insights into the IET in dye-semiconductor assemblies based on ground state electronic structure calculations alone. This may be useful for quick screening of a large number of complexes for use as potential sensitizers in DSSCs, especially if followed up by TD-DFT and quantum dynamics simulations for selected target compounds to confirm efficient sensitization. While our focus over the past few years has been exclusively on Fe(II)-polypyridines, the computational strategies outlined in this Account are applicable to a wide variety of sensitizers.}, number={5}, journal={ACCOUNTS OF CHEMICAL RESEARCH}, author={Jakubikova, Elena and Bowman, David N.}, year={2015}, month={May}, pages={1441–1449} }
 @article{nance_bowman_mukherjee_kelley_jakubikova_2015, title={Insights into the Spin-State Transitions in [Fe(tpy)2]2+: Importance of the Terpyridine Rocking Motion}, volume={54}, ISSN={0020-1669 1520-510X}, url={http://dx.doi.org/10.1021/acs.inorgchem.5b01747}, DOI={10.1021/acs.inorgchem.5b01747}, abstractNote={Iron(II) polypyridine complexes have the potential for numerous applications on a global scale, such as sensitizers, sensors, and molecular memory. The excited-state properties of these systems, particularly the intersystem crossing (ISC) rates, are sensitive to the choice of ligands and can be significantly altered depending on the coordination environment. We employ density functional theory and Smolyak's sparse grid interpolation algorithm to construct potential energy surfaces (PESs) for the photophysically relevant states ((1)A, (3,5)MC, and (1,3)MLCT) of the [Fe(tpy)2](2+) (tpy = 2,2':6',2"-terpyridine) complex, with the goal of obtaining a deeper understanding of the ground- and excited-state electronic structure of this system. The three dimensions that define our adiabatic PESs consist of equatorial and axial metal-ligand bond length distortions and a terpyridine ligand "rocking angle", which has not previously been investigated. The intersection crossing seams and minimum energy crossing points (MECPs) between surfaces are also determined. Overall, we find that the PESs of all electronic excited states investigated are characterized by low-energy valleys along the tpy rocking-angle coordinate. This results in the presence of large low-energy areas around the MECPs on the intersection seams of different electronic states and indicates that inclusion of this third coordinate is crucial for an adequate description of the PESs and surface crossing seams of the [Fe(tpy)2](2+) complex. Finally, we suggest that tuning the energetics of the tpy ligand rocking motion could provide a way to control the ISC process in this complex.}, number={23}, journal={Inorganic Chemistry}, publisher={American Chemical Society (ACS)}, author={Nance, James and Bowman, David N. and Mukherjee, Sriparna and Kelley, C. T. and Jakubikova, Elena}, year={2015}, month={Nov}, pages={11259–11268} }
 @article{bowman_chan_jakubikova_2015, title={Investigating interfacial electron transfer in highly efficient porphyrin-sensitized solar cells}, volume={1196}, journal={Photoinduced processes at surfaces and in nanomaterials}, author={Bowman, D. N. and Chan, J. and Jakubikova, E.}, year={2015}, pages={169–188} }
 @article{bowman_mukherjee_barnes_jakubikova_2015, title={Linker dependence of interfacial electron transfer rates in Fe(II)-polypyridine sensitized solar cells}, volume={27}, number={13}, journal={Journal of Physics. Condensed Matter}, author={Bowman, D. N. and Mukherjee, S. and Barnes, L. J. and Jakubikova, E.}, year={2015} }
 @article{bowman_bondarev_mukherjee_jakubikova_2015, title={Tuning the Electronic Structure of Fe(II) Polypyridines via Donor Atom and Ligand Scaffold Modifications: A Computational Study}, volume={54}, ISSN={["1520-510X"]}, DOI={10.1021/acs.inorgchem.5b01409}, abstractNote={Fe(II) polypyridines are an important class of pseudo-octahedral metal complexes known for their potential applications in molecular electronic switches, data storage and display devices, sensors, and dye-sensitized solar cells. Fe(II) polypyridines have a d(6) electronic configuration and pseudo-octahedral geometry and can therefore possess either a high-spin (quintet) or a low-spin (singlet) ground state. In this study, we investigate a series of complexes based on [Fe(tpy)2](2+) (tpy = 2,2';6',2″-terpyridine) and [Fe(dcpp)2](2+) (dcpp = 2,6-bis(2-carboxypyridyl)pyridine). The ligand field strength in these complexes is systematically tuned by replacing the central pyridine with five-membered (N-heterocyclic carbene, pyrrole, furan) or six-membered (aryl, thiazine-1,1-dioxide, 4-pyrone) moieties. To determine the impact of ligand substitutions on the relative energies of metal-centered states, the singlet, triplet, and quintet states of the Fe(II) complexes were optimized in water (PCM) using density functional theory at the B3LYP+D2 level with 6-311G* (nonmetals) and SDD (Fe) basis sets. It was found that the dcpp ligand scaffold allows for a more ideal octahedral coordination environment in comparison to the tpy ligand scaffold. The presence of six-membered central rings also allows for a more ideally octahedral coordination environment relative to five-membered central rings, regardless of the ligand scaffold. We find that the ligand field strength in the Fe(II) polypyridines can be tuned by altering the donor atom identity, with C donor atoms providing the strongest ligand field.}, number={17}, journal={INORGANIC CHEMISTRY}, author={Bowman, David N. and Bondarev, Alexey and Mukherjee, Sriparna and Jakubikova, Elena}, year={2015}, month={Sep}, pages={8786–8793} }
 @article{bowman_blew_tsuchiya_jakubikova_2013, title={Elucidating Band-Selective Sensitization in Iron(II) Polypyridine-TiO2 Assemblies}, volume={52}, ISSN={["1520-510X"]}, DOI={10.1021/ic4007839}, abstractNote={Iron(II) polypyridines represent a cheaper and nontoxic alternative to analogous Ru(II) polypyridine dyes successfully used as photosensitizers in dye-sensitized solar cells (DSSCs). We employ density functional theory (DFT) and time-dependent DFT (TD-DFT) to study ground and excited state properties of [Fe(bpy)(CN)4](2-), [Fe(bpy-dca)(CN)4](2-), and [Fe(bpy-dca)2(CN)2] complexes, where bpy = 2,2'-bipyridine and dca = 4,4'-dicarboxylic acid. Quantum dynamics simulations are further used to investigate the interfacial electron transfer (IET) between the excited Fe(II) dyes and a TiO2 nanoparticle. All three dyes investigated display two bands in the visible region of the absorption spectrum, with the major transitions corresponding to the metal-to-ligand charge transfer states. The calculated IET rates from the particle states created by the excitation of the lower-energy absorption band are comparable to or slower than the rate of the excited state decay into the nonemissive, metal-centered states of the Fe(II) dyes (∼100 fs), indicating that the IET upon the excitation of this band is unlikely. Several particle states in the higher-energy absorption band display IET rates at or below 100 fs, suggesting the possibility of the IET between the Fe(II)-sensitizer and TiO2 nanoparticle upon excitation with visible light. Our results are consistent with the previous experimental work on Fe(II) sensitizers (Ferrere, S. Chem. Mater. 2000, 12, 1083) and elucidate the band-selective nature of the IET in these compounds.}, number={15}, journal={INORGANIC CHEMISTRY}, author={Bowman, David N. and Blew, James H. and Tsuchiya, Takashi and Jakubikova, Elena}, year={2013}, month={Aug}, pages={8621–8628} }