Room-Temperature Superprotonic Conductivity beyond 10-1 S cm-1 in a Co(II) Coordination Polymer.

An efficient design of crystalline solid-state proton conductors (SSPCs) is crucial for the progress of clean energy applications. Developing such materials to make them work at room temperature with a conductivity of ≥10-1 S cm-1 is of significant interest in terms of technical and commercial aspects. Utilizing the recently highlighted "coordinated-water-driven proton conduction" approach, herein, we have rationally synthesized two highly stable and scalable 1D Co(II) coordination polymers (CPs) as SSPCs, PCM-2 {[Co(bpy)(H2O)2(NO3)2]·H2O}n and PCM-3 {[Co2(bpy)2(SO4)2(H2O)6].4H2O}n, with distinct alignments in coordinated water and coordinated oxo-anions (nitrate and sulfate, respectively). The acidity of the metal-bound water molecules in PCM-2 is further enhanced through cooperative long-range continuous H bonds with coordinated Brønsted basic nitrates (proton acceptors), leading to ultrahigh superprotonic conductivities even at 25 °C (1.03 × 10-1 S cm-1 under 95% RH), and reached a maximum of 2.99 × 10-1 S cm-1 at 85 °C (95% RH). The conductivity at 25 °C is even higher than that of commercial Nafion 117 (6.74 × 10-2 S cm-1 at 100% RH). The absence of such an H-bonding interaction in PCM-3 (closed loops) resulted in a lesser conductivity of 5.87 × 10-5 S cm-1 (95% RH, 85 °C). PCM-2 represents the first example of SSPC exhibiting conductivity in the order 10-1 S cm-1 at ambient temperature (25 °C) with excellent recyclability.

Thiocarbonyl-Bridged N-Heterotriangulenes for Energy Efficient Triplet Photosensitization: A Theoretical Perspective.

Structurally-rigid metal-free organic molecules are of high demand for various triplet harvesting applications. However, inefficient intersystem crossing (ISC) due to large singlet-triplet gap ( Δ E S - T ${\Delta {E}_{S-T}}$ ) and small spin-orbit coupling (SOC) between lowest excited singlet and triplet often limits their efficiency. Excited electronic states, fluorescence and ISC rates in several thiocarbonyl-bridged N-heterotriangulene ( m ${m}$ S-HTG) with systematically increased thione content ( m = ${m=}$ 0-3) are investigated implementing polarization consistent time-dependent optimally-tuned range-separated hybrid. All m ${m}$ S-HTGs are dynamically stable and also thermodynamically feasible to synthesize. Relative energies of several low-lying singlets ( S n ${{S}_{n}}$ ) and triplets ( T n ${{T}_{n}}$ ), and their excitation nature (i. e., n π * ${n{\pi }^{^{\ast}}}$ or π π * ${\pi {\pi }^{^{\ast}}}$ ) and SOC are determined for these m ${m}$ S-HTGs in dichloromethane. Low-energy optical peak displays gradual red-shift with increasing thione content due to relatively smaller electronic gap resulted from greater degree of orbital delocalization. Significantly large SOC due to different orbital-symmetry and heavy-atom effect produces remarkably high ISC rates ( k I S C ${{k}_{ISC}}$ ~1012 s-1) for enthalpically favoured S 1 n π * → T 2 ${{S}_{1}\left(n{\pi }^{^{\ast}}\right)\to {T}_{2}}$ ( π π * ${\pi {\pi }^{^{\ast}}}$ ) channel in these m ${m}$ S-HTGs, which outcompete radiative fluorescence rates (~108 s-1) even directly from higher lying optically bright π π * ${\pi {\pi }^{^{\ast}}}$ singlets. Importantly, high energy triplet excitons of ~1.7 eV resulting from such significantly large ISC rates from non-fluorescent S 1 n π * ${{S}_{1}\left(n{\pi }^{^{\ast}}\right)}$ make these thiocarbonylated HTGs ideal candidates for energy efficient triplet harvest including triplet-photosensitization.

Tailoring Light-Harvesting in Zn-Porphyrin and Carbon Fullerene Based Donor-Acceptor Complex through Ethynyl-Extended Donor π-Conjugation.

Organic photovoltaic efficiency though currently limited for practical applications, can be improved by means of various molecular-level modifications. Herein the role of extended donor π-conjugation through ethynyl-bridged meso-phenyl/pyridyl on the photoinduced charge-transfer kinetics is studied in noncovalently bound Zn-Porphyrin and carbon-fullerene based donor-acceptor complex using time-dependent optimally tuned range-separated hybrid combined with the kinetic rate theory in polar solvent. Non-covalent dispersive interaction is identified to primarily govern the complex stability. Ethynyl-extended π-conjugation results in red-shifted donor-localized Q-band with substantially increased dipole oscillator strength and smaller exciton binding energy, suggesting greater light-harvesting efficiency. However, the low-lying charge-transfer state below to the Q-band is relatively less affected by the ethynyl-extended π-conjugation, yielding reduced driving forces for the charge-transfer. Detailed kinetics analysis reveals similar order of charge-transfer rate constants (~1012 s-1) for all donor-acceptor composites studied. Importantly, enhanced light-absorption, smaller exciton binding energy and similar charge-transfer rates together with reduced charge-recombination make these complexes suitable for efficient photoinduced charge-separation. These findings will be helpful to molecularly design the advanced organic donor-acceptor blends for energy efficient photovoltaic applications.

Mechanistical Insights into the Ultrasensitive Detection of Radioactive and Chemotoxic UO22+ Ions by a Porous Anionic Co-Metal-Organic Framework.

Development of a simple, cost-efficient, and portable UO22+ sensory probe with high selectivity and sensitivity is highly desirable in the context of monitoring radioactive contaminants. Herein, we report a luminescent Co-based metal-organic framework (MOF), {[Me2NH2]0.5[Co(DATRz)0.5(NH2BDC)]·xG}n (1), equipped with abundant amino functionalities for the selective detection of uranyl cations. The ionic structure consists of two types of channels decorated with plentiful Lewis basic amino moieties, which trigger a stronger acid-base interaction with the diffused cationic units and thus can selectively quench the fluorescence intensity in the presence of other interfering ions. Furthermore, the limit of detection for selective UO22+ sensing was achieved to be as low as 0.13 µM (30.94 ppb) with rapid responsiveness and multiple recyclabilities, demonstrating its excellent efficacy. Density functional theory (DFT) calculations further unraveled the preferred binding sites of the UO22+ ions in the tubular channel of the MOF structure. Orbital hybridization between NH2BDC/DATRz and UO22+ together with its significantly large electron-accepting ability is identified as responsible for the luminescence quenching. More importantly, the prepared 1@PVDF {poly(vinylidene difluoride)} mixed-matrix membrane (MMM) displayed good fluorescence activity comparable to 1, which is of great significance for their practical employment as MOF-based luminosensors in real-world sensing application.

Nature and energetics of low-lying excited singlets/triplets and intersystem crossing rates in selone analogs of perylenediimide: A theoretical perspective.

The structural rigidity and chemical diversity of the highly fluorescent perylenediimide (PDI) provide wide opportunities for developing triplet photosensitizers with sufficiently increased energy efficiency. Remarkably high intersystem crossing (ISC) rates with a complete fluorescence turn-off reported recently for several thione analogs of PDI due to substantially large spin-orbit coupling garners huge attention to develop other potential analogs. Here, several selone analogs of PDI, denoted as mSe-PDIs (m = 1-4) with varied Se content and positions, are investigated to provide a comprehensive and comparative picture down the group-16 using density functional theory (DFT) and time-dependent DFT implementing optimally tuned range-separated hybrid in toluene dielectric. All mSe-PDIs are confirmed to be dynamically stable and also thermodynamically feasible to synthesize from their oxygen and thione congeners. The first excited-state singlet (S1) of mSe-PDI with relatively low Se-content (m = 1, 2) is of nπ* character with an expected fluorescence turn-off. Whereas, the ππ* nature of the S1 for 3Se-PDI and 4Se-PDI suggests a possible fluorescence turn-on in the absence of any other active nonradiative deactivation pathways. However, ∼4-6 orders greater ISC rates (∼1012-1014 s-1) than the fluorescence ones (∼108 s-1) for all mSe-PDIs signify highly efficient triplet harvest. Importantly, significantly higher ISC rates for these mSe-PDIs than their thione congeners render them efficient triplet photosensitizers.

A Water-Stable Cationic SIFSIX MOF for Luminescent Probing of Cr2 O7 2- via Single-Crystal to Single-Crystal Transformation.

The sensing and monitoring of toxic oxo-anion contaminants in water are of significant importance to biological and environmental systems. A rare hydro-stable SIFSIX metal-organic framework, SiF6 @MOF-1, {[Cu(L)2 (H2 O)2 ]·(SiF6 )(H2 O)}n , with exchangeable SiF6 2- anion in its pore is strategically designed and synthesized, exhibiting selective detection of toxic Cr2 O7 2- oxo-anion in an aqueous medium having high sensitivity, selectivity, and recyclability through fluorescence quenching phenomena. More importantly, the recognition and ion exchange mechanism is unveiled through the rarely explored single-crystal-to-single crystal (SC-SC) fashion with well-resolved structures. A thorough SC-SC study with interfering anions (Cl- , F- , I- , NO3 - , HCO3 - , SO4 2- , SCN- , IO3 - ) revealed no such transformations to take place, as per line with quenching studies. Density functional theory calculations revealed that despite a lesser binding affinity, Cr2 O7 2- shows strong orbital mixing and large driving forces for electron transfer than SiF6 2- , and thus enlightens the fluorescence quenching mechanism. This work inaugurates the usage of a SIFSIX MOF toward sensing application domain under aqueous medium where hydrolytic stability is a prime concern for their plausible implementation as sensor materials.

Electronic Structures and Charge Mobilities of Several Regioisomeric B2N2-Substituted Perylenediimides.

Tunable and rich electronic properties of perylenediimide (PDI), an n-type semiconductor together with its synthetic ease and processibility, make it suitable for various optoelectronic and field-effect transistor applications. The electronic structures, spectroscopic properties, and charge mobilities for a few isoelectronic BN-substituted PDIs (B2N2-PDIs) with varied BN-patterning are studied using density functional theory (DFT) and time-dependent DFT employing optimally tuned range-separated hybrid. Two substitutional doping patterns, namely, BNNB and NBBN with zero dipole and also BNBN, the one with a finite dipole, are considered to explore the changes in the PDI properties due to different B2N2-substitutions. All three B2N2-PDIs are found to be dynamically stable and lie within a small energy window of ca. ∼1.7 kcal mol-1. An increased electronic gap due to charge localization produces a similar but slightly blue-shifted low-lying optical peak compared to the pristine PDI, in good agreement with the experimental observations. Additionally, differently considered BN patterns result in only slightly varied charge mobilities due to mainly differences in electronic couplings with larger electron mobilities found for the experimentally synthesized BNNB-PDI crystal. On the other hand, small reorganization energy and relatively large coupling for the hole transport produce greater hole mobilities for the NBBN-PDI. Varied nuclear reorganization and electronic coupling are understood by analyzing Huang-Rhys factors associated with normal modes and frontier molecular orbitals, respectively. These results serve as complementary to understanding the recently reported experimental findings and also provide new insights into the impact of different BN patterns on modulating the PDI electronic and charge-transport properties.

Does the Intersystem Crossing Rate of ß-Iodinated Phosphorus Corrole Depend on Iodine Numbers and/or Positions?

The heavy-atom effect is known to enhance the intersystem crossing (ISC) in organic molecular systems. Effects of iodine numbers and positions on the ISC rate of a few meso-difluorophenyl substituted ß-iodinated phosphorus corroles (PCs) with axially ligated fluorine atoms (mI-FPC; m = 1-4) are studied using a time-dependent optimally tuned range-separated hybrid. Solvent effects are accounted for through a polarizable continuum model with a toluene dielectric. Calculations suggest similar thermodynamic stability for all mI-FPCs and also reproduce the experimentally measured 0-0 energies for some of the freebase phosphorus corrole (FPC) systems studied here. Importantly, our results reveal that all mI-FPCs display 10 times larger ISC rate (∼109 s-1) than the fluorescence rate (∼108 s-1), and the higher ISC rate stems from the improved spin-orbit coupling (SOC) introduced by lighter heteroatoms like central P and biaxial F rather than the I heavy-atom effect. However, an enhanced SOC is found with increasing I content for El-Sayed forbidden ISC channels. Research findings reported in this study unveil the impact of light heteroatoms and heavy atoms in promoting ISC in several iodinated PCs, which help in designing visible-light-driven efficient triplet photosensitizers.

Assessing Intersystem Crossing Rates in Donor- and/Acceptor-Functionalized Corroles: A Computational Study.

Small singlet-triplet gap (ΔES-T) and large spin-orbit coupling (SOC) between the low-lying excited spin singlet and triplet states greatly promote the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes that are keys to harvest the triplet population. The electronic structure of a molecule, which strongly depends on its geometry, governs the ISC/RISC. Herein, we have studied visible-light-absorbing freebase corrole and its electron donor/acceptor functional derivatives to explore and understand the effect of homo/hetero meso-substitution in the modulation of corrole photophysical characteristics using time-dependent density functional theory implementing optimally tuned range-separated hybrid. Dimethylaniline and pentafluorophenyl are considered as the representative donor and acceptor functional groups, respectively. Solvent effects are accounted for using a polarizable continuum model with the dichloromethane dielectric. Calculations reproduce the experimentally measured 0-0 energies for some of the functional corroles studied here. Importantly, results reveal that both the homo- and hetero-substituted corroles including the unsubstituted one show substantial ISC rates (∼108 s-1) that are comparable to the fluorescence rates (∼108 s-1). On the other hand, while homo-substituted corroles exhibit modest RISC rates (∼104 - 106 s-1), hetero-substituted ones show relatively lower RISC rates (∼103 - 104 s-1). These results together suggest that both homo- and hetero-substituted corroles could act as triplet photosensitizers, which is also evident from some experimental reports on modest singlet oxygen quantum yield. Calculated rates are analyzed with respect to the variation of ΔES - T and SOC, and their dependence on the molecular electronic structure was evaluated in detail. The research findings reported in this study will add to understanding rich photophysical properties of functional corroles and also aid in devising molecular-level design strategies for developing heavy-atom free functional corroles or related macrocycles for applications in lighting, photocatalysis, photodynamic therapy, etc.

Understanding remarkably high triplet quantum yield in thione analogs of perylenediimide: A detailed theoretical study.

The diverse and tunable electronic structures of perylenediimide (PDI), together with its high thermal and chemical stability, make the compound suitable for applications in bioimaging, electrical, and optical devices. However, a large singlet-triplet gap (ΔES-T) and almost zero spin-orbit coupling (SOC) between the lowest excited singlet (S1) and triplet (T1) restrict intersystem crossing (ISC) in highly fluorescent pristine PDI, yielding a near zero triplet quantum yield (ΦT). Interestingly, a thione analogs of PDI with varied S content (mS-PDIs, m = 1-4) was experimentally shown to yield ΦT ∼ 1.0 through efficient ISC. Time-dependent optimally-tuned range-separated hybrid calculations are performed to rationalize the experimentally observed red-shifted optical absorption and also the remarkably high ISC with almost zero radiative fluorescence reported for these mS-PDIs. To this end, the relative energies of low-lying excited singlets Sn (n = 1, 2) and a few triplets Tn(n = 1-3), along with their nature (nπ* or ππ*), are assessed for each of the mS-PDIs studied in chloroform. To our surprise and contrary to the earlier reports, both S1 and T1 are found to be of the same ππ* character, originating from the highest occupied to lowest unoccupied orbital transition, which, therefore, leads to a still large ΔES-T and vanishingly small SOC, as expected from the identical wavefunction symmetry. Increasing S content lowers S1(ππ*) due to a greater extent of π-delocalization, which well complements and supports the observed red-shift. More importantly, the T2 (or T3) closely lying to the S1 is of nπ* and, therefore, produces a relatively smaller ΔES-T and larger SOC. Detailed kinetics analysis suggests S1(ππ*) → T2(nπ*) is the primary ISC channel for all mS-PDIs, which is responsible for the remarkably high ΦT observed. In addition, comparable SOC and ΔES-T produce similar ISC rates for all mS-PDIs.

Potential of a pH-Stable Microporous MOF for C2H2/C2H4 and C2H2/CO2 Gas Separations under Ambient Conditions.

Cost-effective adsorption-based C2H2/C2H4 and C2H2/CO2 gas separations are extremely important in the industry. Herein, a pH-stable three-dimensional (3D) metal-organic framework (MOF), IITKGP-25, possessing exposed functional sites is presented, which facilitates such separations with excellent ideal adsorbed solution theory (IAST) selectivity (4.61 for C2H2/C2H4 and 3.93 for C2H2/CO2) under ambient conditions (295 K, 100 kPa, 50:50 gas mixtures) and a moderate affinity toward C2H2 (26.6 kJ mol-1). Interestingly, IITKGP-25 can maintain structural integrity in water and in aqueous acidic/alkaline (pH = 2-10) medium because of the higher coordination numbers around the metal center and the hydrophobicity of the ligand. The adsorption capacity for C2H2 remains unchanged for a minimum of up to five consecutive cycles and 15 days of exposure to 97% relative humidity, which are the prerequisites of an adsorbent for practical gas separation application. Density functional theory (DFT) calculations reveal that the open Cd(II) sites and carboxylate oxygen-coordinated Cd(II) corner of the triangle-shaped one-dimensional (1D) channel are the enthalpically more preferred binding sites for C2H2, which stabilize the adsorbed C2H2 through nonlocal stronger H-bonding and also pπ-dπ and CH-π interactions.

Electronic Coupling and Electrocatalysis in Redox Active Fused Iron Corroles.

Conjugated arrays composed of corrole macrocycles are increasingly more common, but their chemistry still lags behind that of their porphyrin counterparts. Here, we report on the insertion of iron(III) into a ß,ß-fused corrole dimer and on the electronic effects that this redox active metal center has on the already rich coordination chemistry of [H3tpfc] COT, where COT = cyclo-octatetraene and tpfc = tris(pentafluorophenyl)corrole. Synthetic manipulations were performed for the isolation and full characterization of both the 5-coordinate [FeIIItpfc(py)]2COT and 6-coordinate [FeIIItpfc(py)2]2COT, with one and two axial pyridine ligands per metal, respectively. X-Ray crystallography reveals a dome-shaped structure for [FeIIItpfc(py)]2COT and a perfectly planar geometry which (surprisingly at first) is also characterized by shorter Fe-N (corrole) and Fe-N (pyridine) distances. Computational investigations clarify that the structural phenomena are due to a change in the iron(III) spin state from intermediate (S = 3/2) to low (S = 1/2), and that both the 5- and 6-coordinated complexes are enthalpically favored. Yet, in contrast to iron(III) porphyrins, the formation enthalpy for the coordination of the first pyridine to Fe(III) corrole is more negative than that of the second pyridine coordination. Possible interactions between the two corrole subunits and the chelated iron ions were examined by UV-Vis spectroscopy, electrochemical techniques, and density functional theory (DFT). The large differences in the electronic spectra of the dimer relative to the monomer are concluded to be due to a reduced electronic gap, owing to the extensive electron delocalization through the fusing bridge. A cathodic sweep for the dimer discloses two redox processes, separated by 230 mV. The DFT self-consistent charge density for the neutral and cationic states (1- and 2-electron oxidized) reveals that the holes are localized on the macrocycle. A different picture emerges from the reduction process, where both the electrochemistry and the calculated charge density point toward two consecutive electron transfers with similar energetics, indicative of very weak electron communication between the two redox active iron(III) sites. The binuclear complex was determined to be a much better catalyst for the electrochemical hydrogen evolution reaction (HER) than the analogous mononuclear corrole.

Understanding High Fluorescence Quantum Yield and Simultaneous Large Stokes Shift in Phenyl Bridged Donor-π-Acceptor Dyads with Varied Bridge Lengths in Polar Solvents.

Photophysical properties of electron donor-π-acceptor (D-π-A) dyads for a given pair of D and A highly depend on the π-bridge type and length and also on the solvent polarity. In this work, first-principles calculations with optimally tuned range-separated hybrids are implemented to explore and understand the optical absorption and emission properties of recently synthesized novel D-π-A dyads with 1,2-diphenylphenanthroimidazole (PPI) as D and 1,2,4-triazolopyridine (TP) as A with varied phenyl π-bridge lengths (denoted as PPI-Pn-TP, n = 0-2 considered here) in solvents of different dielectrics. All three D-π-A dyads display almost an unaltered low-lying optical peak position and a red-shifted emission with increasing solvent polarity, corroborating well with the reported experimental observations. The observed emission shift was attributed to the stabilization of an intramolecular charge-transfer (ICT) state by the polar solvent. Contrastingly, our calculations reveal no ICT; rather the shift is essentially originated from the substantial excited-state relaxation involving primarily rotation of the PPI phenyl ring directly linked to the π-bridge, leading to an almost planarized emissive state. Further, the greater frontier molecular orbital delocalization-driven high fluorescence rate together with increased structural rigidity of the emissive state rationalize the observed high fluorescence quantum yield. The present research findings not only are helpful to better understand the reported experimental observations but also show routes to molecularly design functional D-π-A molecules for advanced optoelectronic, sensing, and biomedical applications.

Origins of Molecular-Twist-Triggered Intersystem Crossing in Functional Perylenediimides: Singlet-Triplet Gap versus Spin-Orbit Coupling.

Singlet-triplet gap (ΔES - T) and spin-orbit coupling (SOC) primarily govern intersystem crossing (ISC)-mediated photo- and electro-luminescence processes. Structural-twist in organic molecules is known to improve ISC efficiency. However, how and to what extent a twist affects the ΔES - T and SOC are not yet fully understood. In this work, the impact of molecular-twist on these energetics governing ISC is unveiled in a series of highly fluorescent prototype perylenediimides (PDIs) in dichloromethane implementing reliable quantum-chemical calculations. While S1 → T1 ISC remains suppressed with increasing twist, a relatively larger decrease in ΔES - T together with a modest increase in SOC results in enhanced S1 →T2 ISC. Significantly modulated ISC rates are predicted in a few experimentally relevant -CN- and -Br-substituted PDIs, where twist of varied extent arises naturally depending on substituent's chemical nature, numbers, and positions. This study uncovers the critical role of molecular-twist in tailoring ISC and thereby helps designing functional organic triplet-generating materials.

Tailoring intersystem crossing of perylenediimide through chalcogen-substitution at bay-position: A theoretical perspective.

Molecular-scale design strategies for promoting intersystem crossing (ISC) in small organic molecules are ubiquitous in developing efficient metal-free triplet photosensitizers with high triplet quantum yield (ΦT). Air-stable and highly fluorescent perylenediimide (PDI) in its pristine form displays very small ISC compared to the fluorescence due to the large singlet-triplet gap (ΔES-T) and negligibly small spin-orbit coupling (SOC) between the lowest singlet (S1) and triplet state (T1). However, its ΦT can be tuned by different chemical and mechanical means that are capable of either directly lowering the ΔES-T and increasing SOC or introducing intermediate low-lying triplet states (Tn, n = 2, 3, …) between S1 and T1. To this end, herein, a few chalcogen (X = O, S, Se) bay-substituted PDIs (PDI-X2) are computationally modeled aiming at introducing geometrical-strain at the PDI core and also mixing nπ* orbital character to ππ* in the lowest singlet and triplet excited states, which altogether may reduce ΔES-T and also improve the SOC. Our quantum-chemical calculations based on optimally tuned range-separated hybrid reveal the presence of intermediate triplet states (Tn, n = 2, 3) in between S1 and T1 for all three PDI-X2 studied in dichloromethane. More importantly, PDI-X2 shows a significantly improved ISC rate than the pristine PDI due to the combined effects stemming from the smaller ΔES-T and the larger SOC. The calculated ISC rates follow the order as PDI-O2 < PDI-S2 < PDI-Se2. These research findings will be helpful in designing PDI based triplet photosensitizers for biomedical, sensing, and photonic applications.

Theoretical insights on tunable optoelectronics and charge mobilities in cyano-perylenediimides: interplays between -CN numbers and positions.

Air-stable perylenediimide (PDI) and its derivatives, in particular the cyano-functionalized ones, have attracted great research attention for their potential use in flexible optoelectronics, organic field-effect-transistors (OFETs) as n-type transport materials and also as non-fullerene acceptors in organic photovoltaics (OPVs). Herein we provide a detailed theoretical study on the optical, electrochemical and charge-transport properties (electron and hole mobilities) in a few CN-substituted PDIs with varied number of -CN at different positions (both symmetric and asymmetric di- and tetra-CN derivatives) using density functional theory (DFT) and time-dependent DFT implementing optimally tuned screened range-separated hybrid (OT-SRSH) combining with kinetic rate theory. All cyano-PDIs studied here are energetically stable and form stable π-stacked structures similar to the pristine one, and also act as better electron acceptors. No significant changes in the PDI optical properties are found with the different ways of CN-functionalization, but, this strongly affects the π-stacked geometry, and thereby the electronic coupling, which greatly modulates the PDI intrinsic carrier mobility. Calculated room-temperature electron mobility for the pristine PDI is in excellent agreement with the reported OFET value (∼0.1 cm2 V-1 s-1). Interestingly, relatively large electronic couplings together with small reorganization energies of the symmetrically substituted tetra-CN PDI result in very large charge mobilities (0.4 cm2 V-1 s-1 for electrons and 5.6 cm2 V-1 s-1 for holes) among the systems studied. Therefore, this may serve as a potential ambipolar transport material and hence, naturally calls for experimental demonstration. This detailed and comprehensive study sheds light on the complex interplays between the -CN numbers and the positions for tailored optoelectronic and charge-transport in several functional PDIs, and also shows routes to molecularly design potential n-type materials.

Molecular-scale engineering of the charge-transfer excited states in non-covalently bound Zn-porphyrin and carbon fullerene based donor-acceptor complex.

The low-lying charge-transfer (CT) excited-state plays an unprecedented role in promoting charge separation processes in organic photovoltaic (OPV) materials typically made of electron donor and acceptor building blocks. A Zn-porphyrin donor non-covalently bound to a fullerene derivative PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) acceptor displays low-lying CT excited states close to the donor absorbing state, and this unique donor-acceptor (D-A) complex is considered to be a potential candidate material for harvesting solar energy. Chemical tuning is expected to alter the CT energetics and their nature as well, which may affect the charge-separation efficiency. In this study, we computationally explore the possibility of tailoring the CT excited states of this novel composite by molecular-scale means via selective pyrrole ring hydrogenation of the Zn-porphyrin macrocycle donor using dispersion-corrected density functional theory (DFT) and time-dependent DFT methods employing an optimally tuned range-separated hybrid functional. Three representative donors are considered: Zn-porphyrin (no pyrrole ring hydrogenation), Zn-chlorin (one hydrogenated pyrrole ring) and Zn-bacteriochlorin (two hydrogenated diagonal pyrrole rings) that differ by the degree of pyrrole ring hydrogenation. Predictions are thoroughly compared to available theoretical and experimental data. Our results suggest that all three D-A complexes are energetically stable and show slightly increasing binding affinity with the extent of ring hydrogenation. This is ascribed to only a slight increase in the ground-state CT and significant van der Waals dispersion interactions. On the other hand, all three D-A complexes exhibit considerably tuned low-lying donor-localized π-π* absorbing and donor-to-acceptor CT states in their excited electronic states, which strongly affect the CT rates calculated in polar solvent. Among the three complexes studied, the one with the Zn-chlorin donor blended with the PCBM acceptor reveals energetics (such as driving force, reorganization energy and electronic coupling) strongly favouring the forward CT process with significantly reduced backward CT, and therefore, it turns out to be the best performer. This study sheds light on the fundamentals of molecular-scale engineering of excited-state properties in novel D-A complexes, which strongly affects the CT rates in polar solvent, and, thereby, opens up possible synthetic routes for tailoring and optimizing the performance level of OPV devices.

Structural, Electronic, and Spectral Properties of Metal Dimethylglyoximato [M(DMG)2; M = Ni2+, Cu2+] Complexes: A Comparative Theoretical Study.

Dimethylglyoxime (DMG) usually forms thermodynamically stable chelating complexes with selective divalent transition-metal ions. Electronic and spectral properties of metal-DMG complexes are highly dependent on the nature of metal ions. Using range-separated hybrid functional augmented with dispersion corrections within density functional theory (DFT) and time-dependent DFT, we present a detailed and comprehensive study on structural, electronic, and spectral (both IR and UV-vis) properties of M(DMG)2 [M = Ni2+, Cu2+] complexes. Ni(DMG)2 results are thoroughly compared with Cu(DMG)2 and also against available experimental data. Stronger H-bonding leads to greater stability of Ni(DMG)2 with respect to isolated ions (M2+ and DMG-) compared to Cu(DMG)2. In contrast, a relatively larger reaction enthalpy for Cu(DMG)2 formation from chemically relevant species is found than that of Ni(DMG)2 because of the greater binding enthalpy of [Ni(H2O)6]2+ than that of [Cu(H2O)6]2+. In dimers, Ni(DMG)2 is found to be 6 kcal mol-1 more stable than Cu(DMG)2 due to a greater extent of dispersive interactions. Interestingly, a modest ferromagnetic coupling (588 cm-1) is predicted between two spin-1/2 Cu2+ ions present in the Cu(DMG)2 dimer. Additionally, the potential energy curves calculated along the O-H bond coordinate for both complexes suggest asymmetry and symmetry in the H-bonding interactions between the H-bond donor and acceptor O centers in the solid-state and in solution, respectively, well corroborating with early experimental findings. Interestingly, a lower proton transfer barrier is obtained for the Ni(DMG)2 compared to its Cu-analogue due to stronger H-bonding in the former complex. In fact, relatively weaker H-bonding in Cu(DMG)2 results in blue-shifted O-H stretching modes compared to that in Ni(DMG)2. On the other hand, qualitatively similar optical absorption spectra are obtained for both complexes with red-shifted peaks found for the Cu(DMG)2. Finally, computational models for axial mono- and diligand (aqua and ammonia) coordinated M(DMG)2 complexes are predicted to be energetically feasible and stable with relatively greater binding stability obtained for the ammonia-coordination.

Covalently Assembled Monolayers of Homo- and Heteroleptic FeII -Terpyridyl Complexes on SiOx and ITO-Coated Glass Substrates: An Experimental and Theoretical Study.

Well-defined FeII -terpyridyl monolayers were fabricated on SiOx and conductive ITO-coated glass substrates through covalent-bond formation between the metallo-organic complexes and a preassembled coupling layer. Three different homo- and heteroleptic complexes with terminal pyridyl, amine, and phenyl groups were tested. All the films were found to be densely packed and homogeneous, and consist of molecules standing upright. They exhibited high thermal (up to ≈220 °C) and temporal (up to 5 h at 100 °C) stability. The UV/Vis spectra of the monolayers showed pronounced metal-to-ligand charge-transfer bands with a significant redshift compared with the solution spectra of the metallo-ligands with a pendant pyridyl group quaternized with the coupling layer, whereas the shift was significantly smaller when the coupling layer was bonded to the primary amine (-NH2 ) group of the complex. Cyclic voltammograms of the monolayers showed reversible, one-electron redox behavior and suggested strong electronic coupling between the confined molecules and the underlying substrate. Analysis of the electrochemistry data allowed us to estimate the charge-transfer rate constant between the metal center and the substrate. Additionally, detailed quantum-chemical calculations were performed to support and rationalize the experimentally observed photophysical properties of the FeII -terpyridyl complexes both in the solution state and when bound to a SiOx -based substrate.

Photoinduced Homolytic Bond Cleavage of the Central Si-C Bond in Porphyrin Macrocycles Is a Charge Polarization Driven Process.

Photoinduced cleavage of the bond between the central Si atom in porphyrin macrocycles and the neighboring carbon atom of an axial alkyl ligand is investigated by both experimental and computational tools. Photolysis and electron paramagnetic resonance measurements indicate that the Si-C bond cleavage of Si-phthalocyanine occurs through a homolytic process. The homolytic process follows a low-lying electronic excitation of about 1.8 eV that destabilizes the carbide bond of similar bond dissociation energy. Using electronic structure calculations, we provide insight into the nature of the excited state and the resulting photocleavage mechanism. We explain this process by finding that the electronic excited state is of a charge transfer character from the axial ligand toward the macrocycle in the reverse direction of the ground state polarization. We find that the homolytic process yielding the radical intermediate is energetically the most stable mechanistic route. Furthermore, we demonstrate using our computational approach that changing the phthalocyanine to smaller ring system enhances the homolytic photocleavage of the Si-C bond by reducing the energetic barrier in the relevant excited states.