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.

Molecular-Scale Design of Azulene-Based Triplet Photosensitizers: Insights from Time-Dependent Optimally Tuned Range-Separated Hybrid.

Metal-free triplet photosensitizers are ubiquitous in photocatalysis, photodynamic therapy, photovoltaics, and so forth. Their photosensitization efficiency strongly depends on the ability of the low-lying excited spin-triplet to be populated through intersystem crossing. Small singlet-triplet gaps and considerable spin-orbit coupling between the excited spin-singlet and spin-triplet facilitate efficient intersystem crossing. Azulene (Az), a classic example of Anti-Kasha's blue emitter with considerable fluorescence quantum yield, holds great promise because of its chemical stability, rich electronic properties, and high structural rigidity. Here, we provide computationally modeled Az-derived photosensitizers, namely, Az-CHO and Az-CHS, implementing polarization consistent time-dependent optimally tuned range-separated hybrid. Calculations reveal energetic reordering of low-lying ππ* and nπ* singlet states between Az-CHO and Az-CHS and, thereby, rendering the latter to a nonfluorescent one. Importantly, a small singlet-triplet gap and large spin-orbit coupling for Az-CHX with X = O and S produce remarkably high intersystem crossing rates. Furthermore, strong nonadiabatic coupling between the S1(nπ*) and S2(ππ*) in Az-CHS due to substantially smaller energy gap causes enhanced S1 population via fast internal conversion. These research findings provide new insights into the development of functional Az and or related heavy-atom-free small organic molecule-based triplet photosensitizers.

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.

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.

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.

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.

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.

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.

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.

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.