Axially Bound Ruthenium Phthalocyanine Monolayers on Indium Tin Oxide: Structure, Energetics, and Charge Transfer Properties

Abstract

The efficiency of charge collection at the organic/transparent conducting oxide (TCO) interface in organic photovoltaic (OPV) devices affects overall device efficiency. Modifying the TCO with an electrochemically active molecule may enhance OPV efficiency by providing a charge-transfer pathway between the electrode and the organic active layer, and may also mitigate surface recombination. The synthesis and characterization of phosphonic acid-ruthenium phthalocyanine (RuPcPA) monolayer films on indium tin oxide (ITO), designed to facilitate charge harvesting at ITO electrodes, is presented in this work. The PA group was installed axially relative to the Pc plane so that upon deposition, RuPcPA molecules were preferentially aligned with the ITO surface plane. The tilt angle of 22° between the normal axes to the Pc plane and the ITO surface plane, measured by attenuated total reflectance (ATR) spectroscopy, is consistent with a predominately in-plane orientation. The effect of surface roughness on RuPcPA orientation was modeled, and a correlation was obtained between experimental and theoretical mean tilt angles. Based on electrochemical and spectroelectrochemical studies, RuPcPA monolayers are composed predominately of monomers. Electrochemical impedance spectroscopy (EIS) and potential modulated-ATR (PM-ATR) spectroscopy were used to characterize the electron-transfer (ET) kinetics of these monolayers. A rate constant of 4.0 × 10 s was measured using EIS, consistent with a short tunneling distance between the chromophore and the electrode surface. Using PM-ATR, k values of 2.2 × 10 and 2.4 × 10 s were measured using TE and TM polarized light, respectively; the similarity of these values is consistent with a narrow molecular orientation distribution and narrow range of tunneling distances. The ionization potential of RuPcPA-modified ITO was measured using ultraviolet photoelectron spectroscopy and the results indicate favorable energetics for hole collection at the RuPcPA/ITO interface, indicating that this type of TCO modification may be useful for enhancing charge collection efficiency in OPV devices.

Exploring the Optimal Synthetic Pathways towards µ‐Carbido Diruthenium(IV) Bisphthalocyaninates

“…tert-Butyl-and tetra-crownsubstituted complexes had relatively narrow Q-bands at 619 and 615 nm with a shoulder at 640-650 nm, which originates from excitonic coupling between Pc macrocycles. The shape of Q-band of the low-symmetry complex [(15C5)(BuO) 6 PcRu] 2 (μ-C) was somewhat intermediate between the shapes of symmetrical counterparts [(BuO) 8 PcRu] 2 -(μ-C) and [(15C5) 4 PcRu] 2 (μ-C). The shape of Q-band of the low-symmetry complex [(15C5)(BuO) 6 PcRu] 2 (μ-C) was somewhat intermediate between the shapes of symmetrical counterparts [(BuO) 8 PcRu] 2 -(μ-C) and [(15C5) 4 PcRu] 2 (μ-C).…”

“…Gratifyingly, the use of the base with non-coordinating cation NMe 4 OH helped to overcome this limitation allowing for the preparation of the target [(15C5) 4 Pc] 2 (μ-C) in the excellent yield of 86 %. Similarly, the treatment of [(15C5)(BuO) 6 PcRu](CO) with NMe 4 OH in combination with CHCl 3 in iPrOH/toluene mixture afforded the corresponding μ-carbido complex in a 76 % yield. Thus, the critical influence of the nature of the base on the synthesis of dimeric phthalocyanines emphasizes the role of supramolecular interactions which can strongly affect the properties of crownsubstituted phthalocyanines.…”

“…[42] The method was based on interaction of monomeric complex HPcRuCl 2 with CHCl 3 in the presence of excess of KOH in refluxing 2-propanol. [(tBu) 4 PcRu] 2 (μ-C) 72 [a] [(15C5)(BuO) 6 PcRu] 2 (μ-C) 15 76 The formed species [PcRu] -· and CCl 2 recombine in the sequence of redox reactions with the formation of target μcarbido dimer [PcRu] 2 (μ-C).…”

“…However, this method could not be successfully applied when mono-crown-substituted complex [(15C5)(BuO) 6 PcRu]-(CO) was treated with CHCl 3 /KOH. The target μ-carbido complex was isolated only in 15 % yield.…”

“…The preparation of the μ-nitrido diruthenium bisphthalocyaninate has been published but no data on its catalytic properties is still available. In the present work we have extended our studies to metalation of three symmetrical phthalocyanines H 2 [(tBu) 4 Pc], H 2 [(15C5) 4 Pc] and H 2 [(MesO) 8 Pc] as well as a low-symmetry ligand H 2 [(15C5)-(BuO) 6 Pc] (Scheme 1). In the present work we have extended our studies to metalation of three symmetrical phthalocyanines H 2 [(tBu) 4 Pc], H 2 [(15C5) 4 Pc] and H 2 [(MesO) 8 Pc] as well as a low-symmetry ligand H 2 [(15C5)-(BuO) 6 Pc] (Scheme 1).…”