Organic Mixed Ionic-Electronic Conductors Based on Tunable and Functional Poly(3,4-ethylenedioxythiophene) Copolymers.

Organic mixed ionic-electronic conductors (OMIECs) are being explored in applications such as bioelectronics, biosensors, energy conversion and storage, and optoelectronics. OMIECs are largely composed of conjugated polymers that couple ionic and electronic transport in their structure as well as synthetic flexibility. Despite extensive research, previous studies have mainly focused on either enhancing ion conduction or enabling synthetic modification. This limited the number of OMIECs that excel in both domains. Here, a series of OMIECs based on functionalized poly(3,4-ethylenedioxythiophene) (PEDOT) copolymers that combine efficient ion/electron transport with the versatility of post-functionalization were developed. EDOT monomers bearing sulfonic (EDOTS) and carboxylic acid (EDOTCOOH) groups were electrochemically copolymerized in different ratios on oxygen plasma-treated conductive substrates. The plasma treatment enabled the synthesis of copolymers containing high ratios of EDOTS (up to 68%), otherwise not possible with untreated substrates. This flexibility in synthesis resulted in the fabrication of copolymers with tunable properties in terms of conductivity (2-0.0019 S/cm) and ion/electron transport, for example, as revealed by their volumetric capacitances (122-11 F/cm3). The importance of the organic nature of the OMIECs that are amenable to synthetic modification was also demonstrated. EDOTCOOH was successfully post-functionalized without influencing the ionic and electronic transport of the copolymers. This opens a new way to tailor the properties of the OMIECs to specific applications, especially in the field of bioelectronics.

High Electronic Coupling between Cu Complexes and Oxidized Dyes Confirmed by Measurements of Driving Force Dependent Regeneration Kinetics in Minimal Electrolyte System.

We confirm fast regeneration kinetics between copper complexes and oxidized organic dyes and the major contribution of electronic coupling (HDA). The highest efficiency of dye-sensitized TiO2 solar cells has been shown by employing Cu complex redox couples. Various groups have reported a fast regeneration rate of oxidized dyes by Cu complexes giving a low driving force attributed to low reorganization energy (λ), but the effect of HDA has not been evaluated. The values of HDA and λ can be derived from driving force dependent transient absorption (TA) measurements. However, analyzing TA decay using Cu complexes is not trivial because accelerated recombination by the presence of Cu2+ complexes and biphasic TA decay often complicates the analysis. Here we employ 16 Cu1+ and Co2+ complexes and two dyes. To simplify the system, i.e., making a minimal electrolyte system, Cu2+ and Co3+ complexes and a common additive of 4-tert-butylpyridine are not used. From the driving force dependent TA decays of oxidized dyes by both Cu1+ and Co2+ complexes, λ for the combination of the Cu complexes and dyes is found to be about 0.15 eV lower than that of Co complexes. Approximately 3 to 5 times higher HDA values of Cu complexes than those of Co complexes are obtained, which is the dominant factor for faster rates. The values vary with the structure of the molecules, showing the possibility of increasing the HDA values further. The higher HDA values of a Cu complex than that of a Co complex are also reproduced by quantum chemical calculations.

Copper/Polyaniline Interfaces Confined CO2 Electroreduction for Selective Hydrocarbon Production.

Polyaniline (PANI) provides an attractive organic platform for CO2 electrochemical reduction due to the ability to adsorb CO2 molecules and in providing means to interact with metal nanostructures. In this work, a novel PANI supported copper catalyst has been developed by coupling the interfacial polymerization of PANI and Cu. The hybrid catalyst demonstrates excellent activity towards production of hydrocarbon products including CH4 and C2H4, compared with the use of bare Cu. A Faradaic efficiency of 71.8 % and a current density of 16.9 mA/cm2 were achieved at -0.86 V vs. RHE, in contrast to only 22.2 % and 1.0 mA/cm2 from the counterpart Cu catalysts. The remarkably enhanced catalytic performance of the hybrid PANI/Cu catalyst can be attributed to the synergistic interaction between the PANI underlayer and copper. The PANI favours the adsorption and binding of CO2 molecules via its nitrogen sites to form *CO intermediates, while the Cu/PANI interfaces confine the diffusion or desorption of the *CO intermediates favouring their further hydrogenation or carbon-carbon coupling to form hydrocarbon products. This work provides insights into the formation of hydrocarbon products on PANI-modified Cu catalysts, which may guide the development of conducting polymer-metal catalysts for CO2 electroreduction.

Controlling Excited State Localization in Bichromophoric Photosensitizers via the Bridging Group.

A series of photosensitizers comprised of both an inorganic and an organic chromophore are investigated in a joint synthetic, spectroscopic, and theoretical study. This bichromophoric design strategy provides a means by which to significantly increase the excited state lifetime by isolating the excited state away from the metal center following intersystem crossing. A variable bridging group is incorporated between the donor and acceptor units of the organic chromophore, and its influence on the excited state properties is explored. The Franck-Condon (FC) photophysics and subsequent excited state relaxation pathways are investigated with a suite of steady-state and time-resolved spectroscopic techniques in combination with scalar-relativistic quantum chemical calculations. It is demonstrated that the presence of an electronically conducting bridge that facilitates donor-acceptor communication is vital to generate long-lived (32 to 45 µs), charge-separated states with organic character. In contrast, when an insulating 1,2,3-triazole bridge is used, the excited state properties are dominated by the inorganic chromophore, with a notably shorter lifetime of 60 ns. This method of extending the lifetime of a molecular photosensitizer is, therefore, of interest for a range of molecular electronic devices and photophysical applications.

Catalytic Decomposition of an Organic Electrolyte to Methane by a Cu Complex-Derived In Situ CO2 Reduction Catalyst.

Metal complexes are often transformed to metal complex-derived catalysts during electrochemical CO2 reduction, enhancing the catalytic performance of CO2 reduction or changing product selectivity. To date, it has not been investigated whether metal-complex derived catalysts also enhance the decomposition of the solvent/electrolyte components as compared to an uncoated electrode. Here, we tested the electrochemical stability of five organic solvent-based electrolytes with and without a Cu complex-derived catalyst on carbon paper in an inert atmosphere. The amount of methane and hydrogen produced was monitored using gas chromatography. Importantly, the onset potential for methane production was reduced by 300 mV in the presence of a Cu complex-derived catalyst leading to a significant amount of methane (417.7 ppm) produced at -2.17 V vs Fc/Fc+ in acetonitrile. This suggests that the Cu complex-derived catalyst accelerated not only CO2 reduction but also the reduction of the electrolyte components. This means that Faradaic efficiency (FE) measurements under CO2 in acetonitrile may significantly overestimate the amount of CH4. Only 28.8 ppm of methane was produced in dimethylformamide under an inert atmosphere, much lower than that produced under CO2 (506 ppm under CO2) at the same potential, suggesting that dimethylformamide is a more suitable solvent. Measurements in propylene carbonate produced mostly hydrogen gas while in dimethyl sulfoxide and 3-methoxypropionitrile neither methane nor hydrogen was detected. A strong linear correlation between the measured current and the amount of methane produced with and without the Cu complex-derived catalyst confirmed that the origin of methane production is solvent/electrolyte decomposition and not the decomposition of the catalyst itself. The study highlights that in a nonaqueous system, highly active catalyst in situ deposited during electrochemical testing can significantly influence background measurements as compared to uncoated electrodes, therefore the choice of solvent is paramount for reliable testing.

Effect of the Cu2+/1+ Redox Potential of Non-Macrocyclic Cu Complexes on Electrochemical CO2 Reduction.

Cu2+/1+ complexes facilitate the reduction of CO2 to valuable chemicals. The catalytic conversion likely involves the binding of CO2 and/or reduction intermediates to Cu2+/1+, which in turn could be influenced by the electron density on the Cu2+/1+ ion. Herein we investigated whether modulating the redox potential of Cu2+/1+ complexes by changing their ligand structures influenced their CO2 reduction performance significantly. We synthesised new heteroleptic Cu2/1+ complexes, and for the first time, studied a (Cu-bis(8-quinolinolato) complex, covering a Cu2+/1+ redox potential range of 1.3 V. We have found that the redox potential influenced the Faradaic efficiency of CO2 reduction to CO. However, no correlation between the redox potential and the Faradaic efficiency for methane was found. The lack of correlation could be attributed to the presence of a Cu-complex-derived catalyst deposited on the electrodes leading to a heterogeneous catalytic mechanism, which is controlled by the structure of the in situ deposited catalyst and not the redox potential of the pre-cursor Cu2+/1+ complexes.

Systematic Tuning of Electronic Ground and Excited States in Donor-Acceptor Dyes; Steps toward Designer Compounds for Modern Technologies.

The vibrational and electronic properties of six systematically altered donor-acceptor dyes were investigated with density functional theory (DFT), spectroscopy, and electrochemical techniques. The dyes incorporated a carbazole donor connected to a dithieno[3,2-b:2',3'-d]thiophene linker at either the C2 (m) or C3 (p) position. Indane-based acceptors contained either dimalononitrile (IndCN), ketone and malononitrile (InOCN) or diketone (IndO) electron accepting groups. Molecular geometries modeled by DFT using the BLYP functional and def2-TZVP basis set showed planar geometries containing large, extended π-systems and produced Raman spectra consistent with the experimental data. Electronic absorption spectra had transitions with π-π* character at wavelengths below 325 nm and a charge transfer (CT) transition region from 500 to 700 nm. The peak wavelength was dependent on the donor and acceptor architecture, with each modulating the HOMO and LUMO levels, respectively, supported by TD-DFT estimates using the LC-ωPBE* functional and 6-31g(d) basis set. The compounds showed emission in solution with quantum yields ranging from 0.004 to 0.6 and lifetimes of less than 2 ns. These were assigned to either π-π* or CT emissive states. Signals attributed to CT states exhibited positive solvatochromism and thermochromism. The spectral emission behavior of each compound trended with the acceptor unit moieties, where malononitrile units lead to greater π-π* character and ketones exhibited greater CT character.

Cu-THQ-EFG Composite for Highly Selective Electrochemical CO2 Reduction to Formate at Low Overpotentials.

Metal organic framework (MOFs) are promising materials for electrocatalysis. However, the active sites of bulk MOFs crystal normally cannot be fully utilized because of the slow reagent penetration of pores and blockage of active sites. Herein, we report a facile way to deposit copper-benzoquinoid (Cu-THQ) on the edge-functionalized graphene (EFG) which prevented material's aggregation. EFG used as a substrate provides higher electrical conductivity and stability in water than previously utilized graphene oxide (GO). Besides, the plate-like morphology of EFG proved to be more beneficial to support the MOF, because of the functional groups on its edge regions and much lower resistance compared to the sheet GO. Therefore, EFG can boost the resultant material's catalytic activity for CO2 electroreduction (CO2RR). Furthermore, Cu-THQ exhibits high selectivity for formate formation in CO2RR. Representing as the only CO2 reduced liquid product, formate can be separated from gaseous products and further extracted from the electrolyte for practical use. The electrocatalytic results of Cu-THQ-EFG indicate the composite exhibits a higher current density of -3 mA/cm2 and faradaic efficiency of -0.25 V vs. RHE, corresponding to 50 mV of overpotential. Moreover, it features a less negative on-set potential of -0.22 V vs. RHE, which is close to the equilibrium potential of CO2RR (-0.2 V vs. RHE) and is 0.16 V more positive than the on-set potential of Cu-THQ-GO (-0.38 V vs. RHE).

Investigation of the Geometric and Spectroscopic Properties of Four Twisted Triphenylpyridinium Donor-Acceptor Dyes.

The geometric and spectroscopic properties of four cationic N-aryl-2,4,6-triphenylpyridinium-based donor-acceptor dyes─1-[4-(9H-carbazol-9-yl)phenyl]-2,4,6-triphenylpyridinium, 1-[4-(N,N-diphenylamino)phenyl]-2,4,6-triphenylpyridinium, 1-(9-phenyl-9H-carbazol-3-yl)-2,4,6-triphenylpyridinium, and 1-(9-ethyl-9H-carbazol-3-yl)-2,4,6-triphenylpyridinium─are reported. The four dyes exhibited a twisted, quasi-perpendicular geometry about the central donor-acceptor bond, shown by X-ray crystallography and supported by Raman spectroscopy and DFT calculations. The electronic absorption spectra show weak charge transfer (CT) transitions at about 400 nm (ε ∼ 3000 L mol-1 cm-1). Time dependent (TD) DFT supported the nature of the CT transition, displaying an 89-97% shift in electron density from the donor to the acceptor upon electronic excitation. Excited state geometry calculations revealed significant geometry changes upon electronic excitation. Enhancement of vibrational modes attributable to this transition was also recognized in the resonance Raman spectra. Emission spectroscopies showed two distinct emission bands. The lower energy band, resulting from radiative decay of the CT excited state, exhibited large anomalous Stokes shifts of ∼9000 cm-1. Much of the Stokes shift was a consequence of geometry changes between the ground and excited states. This was confirmed by variable temperature emission studies, with Stokes shifts reducing by up to 3000 cm-1 upon cooling from 293 to 80 K. Additionally, a high energy aggregation induced emission band was present for two of the dyes, resulting from the inhibition of excited state geometry reorganization and supported by solid-state emission spectra. These phenomena exemplify the importance of geometry in short range donor-acceptor dyes such as these.

Enhanced interfacial electron transfer kinetics between Co2+/3+ complexes and organic dyes with free space near their backbone.

Fast electron transfer (ET) between surface-bound dye molecules and electron donor molecules dissolved in electrolytes with simultaneous reduction in recombination rates are crucial to improve the photon-to-electron conversion efficiency of photo-electrochemical technologies. Here, the electron transfer characteristics of a new dye molecule PX47 with only two alkyl chains placed in the anti configuration of the π-conjugated quarterthiophene backbone is studied. It is anticipated that the appropriate free space between the alkyl chains allowed the approach of the Co(c1-bpy)3 redox mediator to near the backbone of the dye anchored to a TiO2 electrode even at complete coverage of the TiO2 surface, thereby enhancing electronic coupling. The ET kinetics measured by transient absorption spectroscopy were enhanced by a factor of six between PX47 and Co(c1-bpy)3 as compared to the structurally similar MK2 dye with four alkyl chains. The ET rates between PX47 and the larger nonyl-substituted (Co(c9-bpy)3 or tert-butyl substituted Co bipyridine (Co(dtb-bpy)3) were reduced by half and one third as compared to Co(c1-bpy)3, respectively, indicating a blocking effect of longer or bulky substituents on the redox mediators. For MK2 with four alkyl chains near the backbone, the ET rate was very similar between Co(c1-bpy)3 and Co(dtb-bpy)3 and was enhanced for (Co(c9-bpy)3, the latter explained by trapping the mediator inside the dye layer due to alkyl-alkyl interactions. Unexpectedly, two distinctly different recombination rates were measured between the oxidized Co(c1-bpy)3 mediators and TiO2 electrons in the PX47-TiO2 samples, which is explained by two possible arrangements of the PX47 on the TiO2 surface. The dominant arrangement allowed the adsorption of Co(c1-bpy)3 on the TiO2 surface enhancing recombination. The findings suggest that by strategically placing alkyl chains around the electronic units of dye molecules, the redox mediator can be intercalated into the dye layer increasing proximity and better electronic coupling. Although this work presents an effective strategy to enhance ET rates by designing structurally complementary electron donor-acceptor pairs, additional design strategies to further reduce recombination in such open dye structures are needed.

Reactive Extrusion Printing for Simultaneous Crystallization-Deposition of Metal-Organic Framework Films.

Reactive extrusion printing (REP) is demonstrated as an approach to simultaneously crystallize and deposit films of the metal-organic framework (MOF) Cu3 btc2 (btc=1,3,5-benzenetricarboxylate), also known as HKUST-1. The technique co-delivers inks of the copper(II) acetate and H3 btc starting materials directly on-surface and on-location for rapid nucleation into films at room temperature. The films were analyzed using PXRD, profilometry, SEM and thermal analysis techniques and confirmed high-quality Cu3 btc2 films are produced in low-dispersity interconnected nanoparticulate form. The porosity was examined using gas adsorption which showed REP gives Cu3 btc2 films with open interconnected pore structures, demonstrating the method bestows features that traditional synthesis does not. REP is a technique that opens the field to time-efficient large-scale fabrication of MOF interfaces and should find use in a wide variety of coating application settings.

Exohedral Functionalization of Fullerene by Substituents Controlling of Molecular Organization for Spontaneous C60 Dimerization in Liquid Crystal Solutions and in a Bulk Controlled by a Potential.

A study was carried out on the possibility of orderly and spontaneous dimerization at room temperature of C60 cages in fullerene liquid crystal fullerene dyads (R-C60). For this purpose, dyads with a structural elements feature supporting π-stacking and Van der Waals interactions were tested, due to the presence of terthiophene donors linked through an α-position or dodecyloxy chains. In addition, this possibility was also tested and compared to dyads with shorter substituents and the pristine C60. Research has shown that only in dyads with the features of liquid crystals, π-dimerization of C60 units occurs, which was verified by electrochemical and spectroelectrochemical (ESR) measurements. Cyclic voltammetry and differential voltammetry studies reveal π-dimerization in liquid crystal dyad solution even without the possibility of previous polymerization (cathodic or anodic) under conditions in the absence of irradiation and without the availability of reaction initiators, and even with the use of preliminary homogenization. These dyads undergo six sequential, one-electron reductions of π-dimer (R-C60···C60-R), where two electrons are added successively to each of the two fullerene cages and first form two radical anion system (R-C60)•-(R-C60)•- without pairing with the characteristics of two doublets. Similarly, the second reductions of π-dimer occur at potentials that are close to the reduction potential for the conversion to a system of two triplet dianions (R-C60)2-(R-C60)2-. Electron paramagnetic resonance spectra indicate a significant interaction between C60 cages. Interestingly, the strength of intermolecular bonds is so significant that it can overcome Coulombic repulsion, even with such highly charged particles as dianions and trianions. Such behavior has been revealed and studied so far only in covalently bonded C60 dimers.

The impact of insufficient time resolution on dye regeneration lifetime determined using transient absorption spectroscopy.

Dye regeneration lifetimes of a combination of dyes and redox mediators were determined by two transient absorption (TA) spectrometers with 0.5 ns (sub-ns) and 6 ns (ns) time resolutions to elucidate the impact of insufficient time resolution on the measurements of dye regeneration kinetics in dye-sensitised semiconductor electrodes. Due to the disordered nature of the dye-sensitised electrodes, the dye regeneration lifetime is often characterised by half-decay time (τ1/2) of the initial signal magnitude. Alternatively, τ1/2,S is calculated from stretched-exponential lifetime (τww) and the distribution of lifetimes characterised by the stretch parameter (ß). Stretched-exponential functions were numerically modelled, showing that to keep the error in τ1/2 ≤ 10%, τww needs to be at least 20 times longer than the time resolution in case of non-dispersive transients (ß = 0.9) but at least 870 times longer when dispersive (ß = 0.5). To test the predictions, TA decays of a combination of organic and porhyrin dyes and three cobalt-complex mediators are analysed, spanning a range of τww and ß. These examples show that a 262% error in τ1/2 is possible if the time resolution of the TA setup is only 13 times faster than τww and smaller ß results in larger error when τww is similar. Determining τ1/2,S by stretched-exponential fitting generally reduces the error compared to that determined directly from the graph. However, if the stretched-exponential function does not correctly describe the early signal transient, even a larger error by stretched-exponetial fitting is introduced. The key requirement for accurate measurement is to have a fast-enough TA setup to resolve the initial plateau of the TA signal. To demonstrate the impact of the measured errors, the measured regeneration lifetimes are plotted versus the driving force of the reaction and modelled using Marcus theory. Erroneous regeneration rates lead to an underestimated electronic coupling term by 2.2 times in case of a series of porphyrin dyes matched with Co complex electrolytes, a significant impact when the interpretation of factors affecting electron transfer at dye-sensitised semiconductor/electrolyte interface is discussed.

Rapid spatially-resolved post-synthetic patterning of metal-organic framework films.

Reactive inkjet printing was used for fast and facile spatially-controlled post-synthetic patterning of metal-organic framework films. Here, we report use of the reactive inkjet printing technique to rapidly produce patterned electroactive MOF films by covalent attachment of redox-responsive ferrocenyl groups to UiO-66-NH2 on FTO glass. This study paves the way for the wide applicability of reactive printing to MOF film modification.

Electrochemical and Spectroelectrochemical Studies on the Reactivity of Perimidine-Carbazole-Thiophene Monomers towards the Formation of Multidimensional Macromolecules versus Stable π-Dimeric States.

During research on cross-linked conducting polymers, double-functionalized monomers were synthesized. Two subunits potentially able to undergo oxidative coupling were used-perimidine and, respectively, carbazole, 3,6-di(hexylthiophene)carbazole or 3,6-di(decyloxythiophene)carbazole; alkyl and alkoxy chains as groups supporting molecular ordering and 14H-benzo[4,5]isoquinone[2,1-a]perimidin-14-one segment promoting CH⋯O interactions and π-π stacking. Electrochemical, spectroelectrochemical, and density functional theory (DFT) studies have shown that potential-controlled oxidation enables polarization of a specific monomer subunit, thus allowing for simultaneous coupling via perimidine and/or carbazole, but mainly leading to dimer formation. The reason for this was the considerable stability of the dicationic and tetracationic π-dimers over covalent bonding. In the case of perimidine-3,6-di(hexylthiophene)carbazole, the polymer was not obtained due to the steric hindrance of the alkyl substituents preventing the coupling of the monomer radical cations. The only linear π-conjugated polymer was obtained through di(decyloxythiophene)carbazole segment from perimidine-di(decyloxythiophene)-carbazole precursor. Due to the significant difference in potentials between subsequent oxidation states of monomer, it was impossible to polarize the entire molecule, so that both directions of coupling could be equally favored. Subsequent oxidation of this polymer to polarize the side perimidine groups did not allow further crosslinking, because rather the π-π interactions between these perimidine segments dominate in the solid product.

Amphiphilic Zinc Porphyrin Single-Walled Carbon Nanotube Hybrids: Efficient Formation and Excited State Charge Transfer Studies.

Herein, the microscopic and spectroscopic characterization of a novel non-covalent electron donor-acceptor system, in which three different metalloporphyrins (1, 2, and 3) play the dual role of light harvester and electron donor with SWCNTs as electron acceptor, is described. To this end, microscopy, that is, atomic force microscopy (AFM) and transmission electron microscopy (TEM) corroborate the formation of 1-SWCNT, 2-SWCNT, and 3-SWCNT. Spectroscopy by means of Raman, fluorescence, and transient absorption spectroscopy confirmed efficient charge-transfer interaction from photoexcited metalloporphyrins to SWCNTs in the ground and excited state of 1-SWCNT, 2-SWCNT, and 3-SWCNT. The complementary use of spectroelectrochemical and transient absorption measurements substantiates the formation of one-electron oxidized metalloporphyrins after photoexcitation. Multiwavelength global analysis provides insights into the charge-separation and recombination processes in 1-SWCNT, 2-SWCNT, and 3-SWCNT upon photoexcitation. Notably, both the charge-separation and recombination dynamics are fastest in 2-SWCNT. Importantly, the strongest interactions in the steady-state experiments are associated with the fastest excited state decay in the time-resolved measurements.

Substrate-Dependent Electron-Transfer Rate of Mixed-Ligand Electrolytes: Tuning Electron-Transfer Rate without Changing Driving Force.

To meet various requirements for electron transfer (ET) at the substrate/electrolyte interface, mixed redox couples assigned to different functions have been applied. While in all studies the mixed redox species had different redox potentials, such redox systems inherently lose energy by ET between the species. We report interfacial ET kinetics employing mixed-ligand electrolytes based on Co2+/3+ complexes with mixtures of dimethyl- and dinonyl-substituted bipyridyl (bpy) ligands with the same redox potential. The ET rates of the mixed electrolytes decrease with the increasing ratio of the dinonyl-bpy ligand, with substrates adsorbed by molecules without alkyl chains due to a blocking effect. However, when the molecules on substrates have four alkyl chains, the ET rate between the molecules and the electrolytes with increasing ratio of the dinonyl-bpy ligand is enhanced. The substrate-dependent behavior is explained by selective intermolecular interactions. The results open design flexibility for mixed-redox electrolyte systems to control ET at multi-substrate interfaces and provide a novel means to tune ET rates simultaneously for various ET processes in a system without losing energy by the ET.

Second-order programming the synthesis of metal-organic frameworks.

Herein we report a new second-order coordinate-covalent programming strategy for metal-organic framework synthesis. We show controlled heterofunctional copolymerisation turns on 'in lattice' linking to deliver highly porous frameworks in a single step process.

Investigation of Ferrocene Linkers in ß-Substituted Porphyrins.

A series of ß-ferrocene-modified zinc porphyrins, with various electron-withdrawing units appended to the ferrocene, were synthesized, and their electronic properties were investigated. The ferrocene was able to be modified with the substituents, with its oxidation potential increased by up to 0.3 V, without significantly perturbing the porphyrin core. A small red-shift of the strongest absorption band (B band) occurred upon the addition of the electron-withdrawing substituents (270 cm-1), occurring alongside a broadening of the band. The singlet state is unaffected by the ferrocene substitution; however, the triplet state lifetimes are decreased by 10.4-10.6 µs from that of the unsubstituted ferrocene porphyrin (18.1 µs). Computational studies showed that the changes in the optical properties are due to a loss of degeneracy of the porphyrin lowest unoccupied molecular orbitals; this is supported by resonance Raman spectroscopy studies, which show different enhancement patterns when probing the high- and low-energy edges of the B band.

Excited-State Switching Frustrates the Tuning of Properties in Triphenylamine-Donor-Ligand Rhenium(I) and Platinum(II) Complexes.

The photophysical properties of a series of rhenium(I) tricarbonyl and platinum(II) bis(acetylide) complexes containing a triphenylamine (TPA)-substituted 1,10-phenanthroline ligand have been examined. The complexes possess both metal-to-ligand charge-transfer (MLCT) and intraligand charge-transfer (ILCT) transitions that absorb in the visible region. The relative energies and ordering of the absorbing CT states have been successfully controlled by changing the metal center and modulating the donating ability of the TPA group through the addition of electron-donating methoxy and electron-withdrawing cyano groups. The ground-state properties behave in a predictable manner as a function of the TPA substituent and are characterized with a suite of techniques including electronic absorption spectroscopy, resonance Raman spectroscopy, electrochemistry, and time-dependent density functional theory calculations. However, systematic control over the ground-state properties of the complexes does not extend to their excited-state behavior. Unexpectedly, despite variation of both the MLCT and ILCT state energies, all of the luminescent complexes displayed near-isoenergetic emission at 298 K, yet the emissive lifetimes of the complexes vary from 290 ns to 3.9 µs. Excited-state techniques including transient absorption and transient resonance Raman, combined with a suite of quantum-chemical calculations, including scalar relativistic effects to elucidate competitive excited-state relaxation pathways, have been utilized to aid in assignment of the long-lived state in the complexes, which was shown to possess differing 3MLCT and 3ILCT contributions across the series.