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

Energy-Level Alignment of Zn-Phthalocyanine-Physisorbed Graphitic Carbon Nitride: Effects of Corrugation

Visible-light-absorbing photosensitizers surface-adsorbed onto two-dimensional (2D) graphitic carbon nitride (g-C3N4) often promote photoinduced interfacial charge transfer (CT) and thereby show huge potential in photovoltaics and photocatalytic applications. Here, an electron-donating Zn-phthalocyanine (ZnPc) photosensitizer physisorbed on a heptazine-based g-C3N4 acceptor is studied for exploring and better understanding the electronic band alignment using dispersion and short-range-corrected density functional theory (DFT) for the extended sheets and also dispersion and long-range corrected DFT for the finite-size composites. The physically relevant corrugated 2D g-C3N4 sheet is found to be energetically more stable (by ∼22.2 kcal mol–1 per heptazine unit) than the corresponding planar analogue. The out-of-plane distortion due to the pseudo-Jahn–Teller effect and repulsive interactions among peripheral N lone pairs cause the corrugation. However, almost similar binding affinity for ZnPc is found for both the planar and corrugated sheets. Importantly, corrugation produces a type-II band alignment for the ZnPc@g-C3N4 blend, independent of the ZnPc adsorption configuration, which is beneficial for efficient charge separation. Further, the presence of low-lying CT electronic states close to the ZnPc Q-band as revealed by time-dependent DFT calculations for finite-size composites offers the possibility of photoinduced CT. These findings shed valuable insights on the energy-level alignment and the interfacial charge separation between the ZnPc donor and the planar/corrugated g-C3N4 acceptor, showing routes to develop high-performance photovoltaic materials and efficient photocatalysts for carbon dioxide reduction and water splitting

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

Thieno Analogues of RNA Nucleosides: A Detailed Theoretical Study

We use first-principles density functional theory calculations to investigate the structural, energetic, bonding aspects, and optical properties of recently synthesized thieno-analogues of RNA nucleosides. The results are compared against the findings obtained for both the natural nucleosides as well as available experimental data. We find that the modified nucleosides form the hydrogen bonded Watson–Crick (WC) base pairing with similar H-bonding energy as obtained for the natural nucleosides. We have calculated and compared the charge transfer integrals for the H-bonded natural and thieno-modified nucleosides. We find that the thieno modification of these nucleosides strongly affects the charge transfer integrals due to the difference in extent of orbital delocalization in these two types of nucleosides. We also find that the degree of reduction of charge transfer integrals is larger for the H-bonded A–U pair than in the G–C pair. We also focus on the optical absorption properties of these thieno-modified nucleosides and their WC H-bonded base pairs in gas phase as well as with implicit water. Our calculated results show that the low energy peaks in the absorption spectra mainly arise because of the π–π* electronic transition for both the nucleosides, and the observed red shift for thieno-nucleosides compared to natural nucleosides are consistent with the calculated decrease in electronic gaps. Our results demonstrate that the thieno modification of natural nucleosides significantly modifies their electronic and optical properties, although the basic structural and bonding aspects remained the same. It also gives a microscopic understanding of the experimentally observed optical behaviors

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

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

Unraveling the Mechanism of Photoinduced Charge Transfer in Carotenoid–Porphyrin–C₆₀ Molecular Triad

Photoinduced charge transfer (CT) plays a central role in biologically significant systems and in applications that harvest solar energy. We investigate the relationship of CT kinetics and conformation in a molecular triad. The triad, consisting of carotenoid, porphyrin, and fullerene is structurally flexible and able to acquire significantly varied conformations under ambient conditions. With an integrated approach of quantum calculations and molecular dynamics simulations, we compute the rate of CT at two distinctive conformations. The linearly extended conformation, in which the donor (carotenoid) and the acceptor (fullerene) are separated by nearly 50 Å, enables charge separation through a sequential CT process. A representative bent conformation that is entropically dominant, however, attenuates the CT, although the donor and the acceptor are spatially closer. Our computed rate of CT at the linear conformation is in good agreement with measured values. Our work provides unique fundamental understanding of the photoinduced CT process in the molecular triad

Calculating High Energy Charge Transfer States Using Optimally Tuned Range-Separated Hybrid Functionals

Recently developed optimally tuned range-separated hybrid (OT-RHS) functionals within time-dependent density functional theory have been shown to address existing limitations in calculating charge transfer excited state energies. The RSH success in improving the calculation of CT states stems from enforcing the correspondence of the frontier molecular orbitals (FMOs) to physical properties, where the highest occupied MO energy relates to the ionization potential and the lowest unoccupied MO energy relates to the electron affinity. However, in this work, we show that a less accurate description of CT states that involves non-FMOs is afforded by the RSH approach. In order to achieve a high quality description of such higher energy CT states, the parameter tuning procedure, which lies at the foundation of the RSH approach, needs to be generalized to consider the CT process. We demonstrate the need for improved description of such CT states in donor–acceptor systems, where the optimal tuning parameter is accounting for the state itself

Quantitative Prediction of Optical Absorption in Molecular Solids from an Optimally Tuned Screened Range-Separated Hybrid Functional

We show that fundamental gaps and optical spectra of molecular solids can be predicted quantitatively and nonempirically within the framework of time-dependent density functional theory (TDDFT) using the recently developed optimally tuned screened range-separated hybrid (OT-SRSH) functional approach. In this scheme, the electronic structure of the gas-phase molecule is determined by optimal tuning of the range-separation parameter in a range-separated hybrid functional. Screening and polarization in the solid state are taken into account by adding long-range dielectric screening to the functional form, with the modified functional used to perform self-consistent periodic-boundary calculations for the crystalline solid. We provide a comprehensive benchmark for the accuracy of our approach by considering the X23 set of molecular solids and comparing results obtained from TDDFT with those obtained from many-body perturbation theory in the GW-BSE approximation. We additionally compare results obtained from dielectric screening computed within the random-phase approximation to those obtained from the computationally more efficient many-body dispersion approach and find that this influences the fundamental gap but has little effect on the optical spectra. Our approach is therefore robust and can be used for studies of molecular solids that are typically beyond the reach of computationally more intensive methods

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