Time-dependent Density Functional Theory Investigation Research Articles

Effect of push–Pull functional group on the ESIPT behaviour and photophysical property of thiolflavone-based compound: a TD-DFT investigation

ABSTRACT The excited-state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of three –CN-substituted (2-(4-(diethylamino)phenyl)- 3-mercapto-4H-chromen-4-one (3-NTF) compounds (3NTF-1, 3NTF-2, 3NTF-3) have been studied systematically based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. We analysed the structural parameters, infrared frequencies and electron densities of the studied compounds, and found that the intramolecular hydrogen bonds (IHBs) at tautomer form are weakened in the excited (S1) state. The weaker IHB at tautomer form is beneficial to ESIPT process. The potential energy curves (PECs) were established and showed that the ESIPT processes of 3NTF-1 and 3NTF-2 are barrierless. The ESIPT process of 3NTF-3 needs to overcome a very low energy barrier. The electron-withdrawing group (–CN) increases the energy barrier of ground state intramolecular proton transfer (GSIPT) process in 3NTF. The order of GSIPT barrier is consistent with the order of their IHB strengths of normal forms. When the electron-withdrawing group (–CN) was introduced, the absorption and fluorescence peaks red-shifted. Two –CN groups resulted in the fluorescence peak of 3NTF red-shift more than one –CN group did.

Stability improvement of UV-filter between methoxy cinnamic acid derivatives and cyclodextrins inclusion complexes based on DFT and TD-DFT investigations

Structures and UV–vis absorption spectra of the host-guest interaction of the methoxy cinnamic acid (MCA) derivatives and cyclodextrins (CDs) were performed by using the density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. All geometries of MCA derivatives (4-MCA, 245-MCA, 246-MCA), three types of CD (αCD, βCD, γCD), and five host-guest inclusion complexes between MCA and CD consisting of 4-MCA/αCD (1), 4-MCA/βCD (2), 245-MCA/βCD (3), 246-MCA/βCD (4), and 246-MCA/γCD (5) were fully optimized by using the M06-2X/6-31G (d,p) levels of theory. Two orientations (A and B) of the MCA guest molecule were considered. Upon examining the optimized geometry, five complexes of the methoxy cinnamic acid molecules are located inside the cavity of CD. Orientation B was more stable than orientation A because of the stronger intermolecular hydrogen bonds between the hydroxyl group of CD and the carboxylic group of MCA. The results indicated that the intermolecular hydrogen bond is mainly the driving force of formation between methoxy cinnamic acid and cyclodextrins. To reveal the host-guest interaction that is relevant to UV-filter compounds, the UV–vis absorption spectra were performed using TD-DFT calculations. The obtained results confirmed that orientation B is the most stable orientation and can absorb in both UVB and UVA regions which is similar to the parent MCA. Therefore, this knowledge will bring to understand the host-guest interaction between methoxy cinnamic acid and cyclodextrin complexes. The theoretical results are expected to provide valuable information for improving the stability of further UV-filter compounds.

Double-Site Twisted D-π-D′ Conjugates with Versatile Photophysical Facets for Diverse Optical Applications and Wash-Free Bioimaging of Cancer Cells

It is still a conundrum to design a single but simple organic molecule possessing numerous realistic photophysical properties such as mechanofluorochromism, aggregation-induced emission (AIE) properties, solvatochromic DSE-gens (dual-state emissive fluorogens), and viscofluorochromism. Unlike D-π-A, D-π-D′ systems are seldom explored with various applied features. This work finds how a single D-π-D′ indole-anthracene conjugate displays all these properties with real-world applications. Especially, an indole-anthracene-π-phenothiazine conjugate (IAP) appears as a solvatochromic (91 nm shift) DSE-gen that shows the longest 62 nm blueshifted enhanced emission on aggregation and the longest 45 nm blueshifted viscofluorochromic features. Furthermore, IAP adopts a fingertip-pressure-sensitive 35 nm blueshift, while a triphenylamine-linked analogue IAT demonstrates a 23 nm mechanofluorochromic redshift. Conformationally twisted backbone and π-conjugations are judiciously balanced to exhibit these optical features. Such diverse and contrasting photophysical facets are elucidated through experimental and detailed time-dependent density functional theory investigations to meet rational understandings. These fluorophores are utilized as hidden patent fingerprint memory-based platforms and software-assisted comparison studies to identify counterfeiters. Furthermore, highly sensitive fingertip-aided mechanofluorochromic features are useful for inkless security writing. Besides, wash-free bioimaging of robust oral cancer cells (FaDu) could be accomplished satisfactorily in the green (IAT) and orange-red regions (IAP) with a lower probe concentration. The anticancer property of IAT enables visualization of the progressive distortion of the dying cancer cells during wash-free bioimaging.

Fluorescent properties based on ESIPT and TICT of novel acylhydrazone-based probe and its sensing mechanism for Al3+: A TD-DFT investigation

By using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, the fluorescent process and sensing mechanism of N′-(2-hydroxybenzylidene)isoquinoline-3-carbohydrazide (NHIC) probe for Al3+ detection were explored in detail. For NHIC, twisted intramolecular charge transfer (TICT) is induced by existed state intramolecular proton transfer (ESIPT). Probe NHIC can undergo ESIPT process in the stepwise pattern, and then a stable double proton-transfer (DPT) structure is formed. There are two non-emissive TICT states (TICT1 and TICT2) existing in NHIC. TICT2 is responsible for the weak fluorescence observed experimentally. After the addition of Al3+, the coordination bond is formed between NHIC and Al3+, the TICT state is prohibited, and the strong fluorescence is switched on. It is evident that the twisted C-N single bond belonging to acylhydrazone part of NHIC results in its TICT state. We hope that our study can throw some light on the development of switch-on probe.

Heteroatom substitution controlled luminescent property and excited state intramolecular proton transfer (ESIPT) process of novel benzothiazole-based fluorophore: A TD-DFT investigation

By using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, we have systematically explored the spectral characteristics and excited state intramolecular proton transfer (ESIPT) processes of 3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methylbenzaldehyde (BTFM-OH) and its derivatives. The sulfur (S) atom in the thiazole ring of BTFM-OH was replaced by O/Se atom (denoted to BOFM-OH/BSeFM-OH) to probe the impact of heteroatom substitution on the luminescent property and ESIPT process. The simulated maximum absorption and fluorescence wavelengths of BTFM-OH are well consistent with the experimental values. From BOFM-OH to BTFM-OH and BSeFM-OH, the red-shifts of absorption and fluorescence (at normal and tautomer forms) wavelengths are found, which is in good accordance with the decreasing order of ground state and excited state HOMO-LUMO gaps of BOFM-OH < BTFM-OH < BSeFM-OH. According to the data obtained from structure, infrared vibrational frequency and electron density, it can be found that the intramolecular hydrogen bonds (IHBs) are enhanced in the S1 state, which is favorable to ESIPT process. From BOFM-OH to BTFM-OH and BSeFM-OH, the atomic electron-withdrawing ability of heteroatom is decreasing from O to S and Se, the IHB strengths become stronger in the order of BOFM-OH < BTFM-OH < BSeFM-OH, which makes the ESIPT process occur much easier along this order.

Effect of cyclodextrins inclusion complexes into absorption and emission spectra of P-methylaminobenzoate derivatives: A DFT and TD-DFT investigation.

In this study, density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of methyl 4-aminobenzoate and methyl 4- amino-2-hydroxybenzoate and their inclusions complexes with α and γ-cyclodextrins were applied to control the effect of cyclodextrins on the absorption and emission properties of P-methylaminobenzoate derivatives. Absorption and emission spectra of free and encapsulated P-methylaminobenzoate derivatives were successfully analysed and compared with experimental results. Interestingly, conformational changes between ground and excited states of both fluorophores were also studied.

A time-dependent density functional theory investigation of the atmospheric absorption spectra of polycyclic aromatic hydrocarbons (PAHs) and their derivatives (Alkyl-PAHs, oxygenated-PAHs, and Nitrated-PAHs) over an urban area in China

Polycyclic aromatic hydrocarbons (PAHs) and their oxygenated (OPAHs) and nitrated (NPAHs) derivatives are main chromophores of the carbonaceous aerosol brown carbon (BrC), which is linked with radiative forcing. Here, we investigated the atmospheric absorption spectra of 64 PAHs, OPAHs, and NPAHs directly over the Chinese megacity of Xi’an, by employing a time-dependent density functional theory (TD-DFT) computational approach and correcting the results for the experimentally determined atmospheric concentration of the studied molecules. The obtained data showed that these molecules contribute more to radiative forcing by absorbing light in the UVA and (sub)visible region of the spectrum. Investigating “daily” absorption spectra revealed major seasonal variation in the intensity of light absorption, but little changes in the shape of the absorption spectra. The observed light absorption can be explained mainly by contributions from PAHs and to a lesser extent by carbonyl-OPAHs, with relatively low contributions of the other OPAHs and NPAHs. Among them, benzo[b+j+k]fluoranthenes, benzo[e]pyrene, benzo[a]pyrene, benzo[ghi]perylene, indeno[1,2,3-cd]pyrene, 6H-benzo[cd]pyren-6-one, 7H-benz[de]anthracen-7-one, and benz[a]anthracene-7,12-dione are highlighted as potentially problematic contributors for radiative forcing over Xi’an.

Cyclometalated Spirobifluorene Imidazolylidene Platinum(II) Complexes with Predominant 3LC Emissive Character and High Photoluminescence Quantum Yields.

Two novel bidentate C^C*spiro cyclometalated platinum(II) complexes comprising a spiro-conjugated bifluorene ligand and different β-diketonate auxiliary ligands are synthesized and characterized. Their preparation employs a robust and elaborate synthetic protocol commencing with an N-heterocyclic carbene precursor. Structural characterization by means of NMR techniques and solid-state structures validate the proposed and herein presented molecular scaffolds. Photophysical studies, including laser flash photolysis methods, reveal an almost exclusively ligand-centered triplet state, governed by the C^C*spiro-NHC ligand. The high triplet energies and the long triplet lifetimes in the order of 30 μs in solution make the complexes good candidates for light-emitting diode-driven photocatalysis, as initial energy transfer experiments reveal. In-depth time-dependent density functional theory investigations are in excellent accordance with our spectroscopic findings. The title compounds are highly emissive in the bluish-green color region with quantum yields of up to 87% in solid-state measurements.

Coupling between Plasmonic and Molecular Excitations: TDDFT Investigation of an Ag-Nanorod/BODIPY-Dye Interaction.

A time-dependent density functional theory (TDDFT) computational approach is employed to study the optical coupling between a plasmonic system (a Ag50 nanorod) and a fluorescent dye (BODIPY). It is found that the BODIPY dye can interact with a plasmonic system in a rather different and selective way according to the mutual orientation of the fragments. Indeed, (i) the plasmon excitation turns out to be sensitive to the presence of the BODIPY transition and (ii) this can lead to amplify or suppress the resonance accordingly to the relative orientation of the corresponding transition dipoles. To understand the coupling mechanism, we analyze the shape of the induced density in real space and the Individual Component Map of the Oscillator Strength (ICM-OS) plots and achieve a simple rationalization and insight on the origin and features of the coupling. The resulting possibility of understanding plasmon/fluorophore interactions by simple qualitative arguments opens the way to a rational design of hybrid (plasmon + dye) systems with the desired optical behavior.

Effect of double bond on electronic and optical properties of coelenteramide: A time-dependent density functional theory investigation

Coelenteramide has got attention for the great latent application in biologically inspired organic light-emitting devices. However, only blue light was emitted by coelenteramide. In order to obtain a wide distribution of light, the effect of double bond on electronic and optical properties of coelenteramide were studied by CAM-B3LYP/6-31+G(d) method. The results showed that the location of double bond was very important. For 1a, 2a, 2b and 3a, the double bond was inserted between amidopyrazine core and phenol group, which had strong π-conjugation effect, effectively improved the HOMO and LUMO energy levels, the capabilities of holes and electrons injection. The absorption and emission wavelengths generate a large bathochromic shift compared with native coelenteramide. In contrast, for 1b, 1c, 2c, double bond between amidopyrazine core and p-cresol, or between amidopyrazine core and benzyl group have little impact on HOMO and LUMO energy levels, energy gap, ionization potentials, electron affinities and absorption spectra.

Spectroscopic, NBO, DFT, and TD-DFT investigation of 7-(Dimethylamino)-4-(Trifluoromethyl)Coumarin (C152)

In different solvents, the electronic UV-Vis absorption and emission spectra of 7-(Dimethylamino)-4-(Trifluoromethyl)Coumarin (C152) were monitored. For the C152 molecular structure, the impact of solvent polarity on fluorescence quantum yield is explored. What is more, the critical micelle concentration (cmc) magnitude was calculable employing a plot of fluorescence intensity against cetrimonium bromide (CTAB) and sodium dodecyl sulfate concentrations (SDS). What is more, the photostability of the C152 model structure exploitation utterly varied solvents like acetone, methyl isobutyl ketone, and amyl alcohol in terms of approach for calculating half-life methodology (pumping with nitrogen laser 337.1 nm), yet because of the range of pulses needed to cut back the studied molecule intensity to 50% of its original value. The B3LYP/6-311G(d) level of theory was wont to optimize the C152 molecular structure. Natural bond orbital (NBO) analysis is additionally wont to explore the electronic hyperconjugation of the C152 modeling structure. B3LYP/6-311G++(d, p) is employed to get NBO studies for the molecule under investigation. The computational electronic UV-Vis absorption spectra of the C152 molecule were computed exploitation time-dependent density functional theory (TD-DFT) at the B3LYP/6-311G++(d, p) level in gas and several other solvents. Theoretical findings were contrasted with experimental findings. The results reveal that the computational optical characteristics of the investigated molecular structure accord with the experimental findings.

Impact of Charge-Resonance Excitations on CT-Mediated J-Type Aggregation in Singlet and Triplet Exciton States of Perylene Di-Imide Aggregates: A TDDFT Investigation

The modulation of intermolecular interactions upon aggregation induces changes in excited state properties of organic molecules that can be detrimental for some optoelectronic applications but can be exploited for others. The time-dependent density functional theory (TDDFT) is a cost-effective approach to determining the exciton states of molecular aggregates, and it has been shown to provide reliable results when coupled with the appropriate choice of the functional. Here we apply a general procedure to analyze the aggregates’ exciton states derived from TDDFT calculations in terms of diabatic states chosen to coincide with local (LE) and charge-transfer (CT) excitations within a restricted orbital space. We apply the approach to study energy profiles, interstate couplings, and the charge-transfer character of singlet and triplet exciton states of perylene di-imide aggregates (PDI). We focus on the intermolecular displacement along the longitudinal translation coordinate, which mimics different amounts of slip-stacking observed in PDI crystals. The analysis, in terms of symmetry-adapted Frenkel excitations (FE) and charge-resonance (CR) states and their interactions, discloses how the interchange of the H/J character for small longitudinal shifts, previously reported for singlet exciton states, also occurs for triplet excitons.

DFT and TDDFT investigation of four triphenylamine/phenothiazine-based molecules as potential novel organic hole transport materials for perovskite solar cells

Hole transport materials (HTMs) based on conductive organic molecules are crucial components to prepare highly efficient perovskite solar cells (PSCs). To overcome the limitations arising from the use of the most common Spiro-OMeTAD, hindering large-scale PSCs production, alternative HTMs are highly desirable. Therefore, several fully organic molecules mainly based on triphenylamine moieties have already been proposed. However, there is still room for the development of more efficient and easily obtainable HTMs. Within this framework, the in silico design of four novel triphenylamine/phenothiazine-based HTMs (HTM1-4) is presented here. Their electronic and molecular properties have been investigated by means of Density Functional Theory (DFT) and Time Dependent DFT (TDDFT) methods and the results have been compared to those of Spiro-OMeTAD and a previously reported phenothiazine-based HTM (PTZ2). The analysis suggests that HTM1-4 fulfil the requirements that enable hole extraction and transport processes in PSCs. Therefore, they should be considered as possible alternatives to Spiro-OMeTAD for the construction of potentially efficient PSCs.

Tuning optoelectronic properties of carbazole through substituent effect: TD-DFT investigation

π-conjugated organic materials such as carbazoles have attracted much attention because of their applications in electronic devices such as OLED’s, solar cells and sensors. Due to their optoelectronic properties, high charge carrier mobility, suitable band gaps and orbital energies, carbazoles have received immense attention to serve as a potent photosensitizer for obtaining high performance in DSSC. The present QSPR study of substituted carbazole and carbazole anion with electron donor (NH2) and acceptor (NO2) substituents at various positions is done by the density functional theory (DFT) calculations at B3LYP/6-31G(d,p) level. The longest maximum absorption wavelengths (λmax) in vacuum as well as in polar (acetonitrile) and non-polar (benzene) solvents are studied by using the time-dependent density functional theory (TD-DFT). For the singly substitution of NH2 (at C3) and NO2 (at C4) groups to the carbazole and its anion, the λmax values are observed to be shifted to the longest wavelength from 290.56 nm to 325.73 and 375.77 nm than the other respective positions. Whereas, the respective λmax values for carbazole-anion are found at 384.28 and 590.53 nm respectively. For disubstituted carbazole with NH2 and NO2 at C3 and C4 positions, the longest λmax shifts to 477.15 nm and for carbazole-anion the longest λmax obtained at C4 (NH2) and C5 (NO2) positions with λmax value 694.61 nm. Similar observations are also found in acetonitrile and benzene solvents. Further, it is observed that as the λmax increases, the HOMO-LUMO energy gap (EHL) value decreases accordingly which can be attributed to intramolecular charge transfer from NH2 to NO2 groups. A very good correlation of λmaxwith EHL is observed with correlation coefficient between 0.91 to 0.95 in vacuum as well as in acetonitrile and benzene solvents. Present study may provide valuable guidelines for the choice of suitable substituent to design carbazole moieties as efficient photosensitizer in DSSC. Copyright © 2017 VBRI Press.

Hydrogen-bonded 3D like structures of Cu(II)/Zn(II) coordination polymers: photoluminescence, TDDFT study and semiconductive properties

Two Cu/Zn complexes [Cu(4,4'-bipy)(H2O)2(ClO4)2] nn(4,4'-bipy) (1) and [Zn(4,4'-bipy)(H2O)4]3 n 6n(4,4'-bipy)6n(ClO4)·2nH2O (2) (4,4'-bipy = 4,4'-bipyridine) have been prepared via hydrothermal reactions and characterized by single-crystal X-ray diffraction. Both complexes feature an infinite 1-D chain-like structural motif with the metal ions surrounded by two nitrogen atoms and four oxygen atoms to yield a octahedral configuration. Both complexes have a 3-D coordination polymer network formed by the rich hydrogen-bonding interactions. Powder photoluminescence studies reveal that they have an emission in the blue (1) or red (2) region of the light spectrum. Time-dependent density functional theory investigation discover that the nature of the emission is resulted from the ligand-to-metal charge transfer for 1, as well as ligand-to-ligand charge transfer for 2. Narrow optical band gaps of 1.43 eV (1) and 2.24 eV (2) are found by the solid-state UV/vis diffuse reflectance spectra.

Evaluation of the excited state dynamics, photophysical properties, and the influence of donor substitution in a donor-$$\pi$$-acceptor system

There have been numerous attempts for the theoretical design of a better donor-[Formula: see text]-acceptor structural framework with improved absorption and emission properties. However, for effective dye designing, it is necessary to understand the electronic and photophysical properties of the dye systems. In this work, we report a detailed density functional theory (DFT) and time-dependent density functional theory (TD-DFT) investigations of the excited state characteristics and the influence of various groups (-HCO, =CH2, (-CH3)2, (HCO)2, and (-OCH3)2) attached to the donor group (-NH2) in a p-nitroaniline D-[Formula: see text]-A system which are symbolized respectively as p-nitroaniline (A), N,N-dimethylnitroaniline (A2), N,N-dicarbonylnitroaniline (A3), N-methylenenitroaniline (A4), and N,N-dimethoxynitroaniline (A5). The first principles DFT and TD-DFT calculations from the ground state (S0) to the first five excited states: (S0→S1), (S0→S2), (S0→S3), (S0→S4), and (S0→S5) were utilized to explore the reactivity of D-[Formula: see text]-A system using the conceptual DFT approach, characterization of electron excitation using the hole-electron analysis, visual study of the various real space functions in the hole-electron framework, density of states (DOS), measurement of charge transfer (CT) length of electron excitation ([Formula: see text]), measurement of the overlapping degrees of hole and electron of electron excitation ([Formula: see text]), interfragment charge transfer (IFCT) during electron excitation, and the second-order perturbation energy analysis from the natural bond orbitals (NBO) computation. Results of the excitation studies show that all the studied compounds exhibited an n→[Formula: see text]* localized type for first excitations (S0→S1) on -NO2 group in A, A2, A4, and A5 and -NCl2 in A3. [Formula: see text]→[Formula: see text]* charge transfer excitations were confirmed for S0→S2/S4/S5 in A and A2, S0→S3/S4/S5 in A3 and A5, and S0→S4/S5 in A4. The NBO second-order perturbation energy analysis suggest that the most significant hyperconjugative interactions were [Formula: see text] (54.43kcal/mol), [Formula: see text] (40.82kcal/mol), [Formula: see text] (11.67kcal/mol), [Formula: see text] (29.52kcal/mol), [Formula: see text] (11.55kcal/mol), [Formula: see text] (23.40kcal/mol), and [Formula: see text] (24.88kcal/mol) [Formula: see text](24.64kcal/mol), which respectively corresponds to the A, A2, A3, A4, and A5 D-[Formula: see text]-A systems under investigation, and these strong interactions stabilize the systems.

Synthesis, characterization, surface analysis, optical activity and solvent effects on the electronic absorptions of Schiff base-functionalized amino thiophene derivatives: Experimental and TD-DFT investigations

New derivatives of 3,3′-di-(((E)-(5-substituted-thiophen-2-yl)methylene)amino)-N-methylpropylamines (3a,b) were synthesized from the dehydration of N-methyl-diaminopropylamine and 2-thielylcarboxaldehyde derivatives; spectroscopically characterized by 1H- and 13C-NMR, IR, LC-MS, UV-Vis and elemental analysis. The nature of the electronic transitions of the SB compounds was investigated using Time-Dependent Density-Functional Theory (TD-DFT). Surface analysis and influence of solvent polarity on spectral properties were examined and established consequently. The molecular electrostatic potential (MEP) revealed that the two geometrical structures were found to be quite similar in term of electronic distributions. The presence of different electrophilic and nucleophilic sites located on the surfaces was suggested to stabilize the structures via classical H-bond and non-classical C-H…π interactions, in addition to interact with assorted solvent molecules. On the other hand, the solvatochromism of compound 3b revealed a gradual shift to the red region through the increase of the solvent polarity, recording a 12 nm of bathochromic shift. The solvation relationship between the experimental λmax and Gutmann's donicity numbers displayed a sense of positive linear behavior with a fluctuation, which was ascribed to a week interaction between molecules of solute and selected solvent. The band gap energy of compound 3a was evaluated experimentally and computationally. Using optical absorption spectra, a value of ‒ 3.801 eV was estimated following Tauc approach, while ‒ 3.720 eV was resulting from TD-DFT simulation.

The Photochemistry of Fe2(S2C3H6)(CO)6(µ-CO) and Its Oxidized Form, Two Simple [FeFe]-Hydrogenase CO-Inhibited Models. A DFT and TDDFT Investigation

FeIFeI Fe2(S2C3H6)(CO)6(µ-CO) (1a–CO) and its FeIFeII cationic species (2a+–CO) are the simplest model of the CO-inhibited [FeFe] hydrogenase active site, which is known to undergo CO photolysis within a temperature-dependent process whose products and mechanism are still a matter of debate. Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) computations, the ground state and low-lying excited-state potential energy surfaces (PESs) of 1a–CO and 2a+–CO have been explored aimed at elucidating the dynamics of the CO photolysis yielding Fe2(S2C3H6)(CO)6 (1a) and [Fe2(S2C3H6)(CO)6]+ (2a+), two simple models of the catalytic site of the enzyme. Two main results came out from these investigations. First, a–CO and 2a+–CO are both bound with respect to any CO dissociation with the lowest free energy barriers around 10 kcal mol−1, suggesting that at least 2a+–CO may be synthesized. Second, focusing on the cationic form, we found at least two clear excited-state channels along the PESs of 2a+–CO that are unbound with respect to equatorial CO dissociation.

Resonance-Enhanced Charge Delocalization in Carbazole-Oligoyne-Oxadiazole Conjugates.

There are notably few literature reports of electron donor-acceptor oligoynes, even though they offer unique opportunities for studying charge transport through "all-carbon" molecular bridges. In this context, the current study focuses on a series of carbazole-(C≡C)n-2,5-diphenyl-1,3,4-oxadiazoles (n = 1-4) as conjugated π-systems in general and explores their photophysical properties in particular. Contrary to the behavior of typical electron donor-acceptor systems, for these oligoynes, the rates of charge recombination after photoexcitation increase with increasing electron donor-acceptor distance. To elucidate this unusual performance, we conducted detailed photophysical and time-dependent density functional theory investigations. Significant delocalization of the molecular orbitals along the bridge indicates that the bridging states come into resonance with either the electron donor or acceptor, thereby accelerating the charge transfer. Moreover, the calculated bond lengths reveal a reduction in bond-length alternation upon photoexcitation, indicating significant cumulenic character of the bridge in the excited state. In short, strong vibronic coupling between the electron-donating N-arylcarbazoles and the electron-accepting 1,3,4-oxadiazoles accelerates the charge recombination as the oligoyne becomes longer.

A TDDFT investigation of the Photosystem II reaction center: Insights into the precursors to charge separation

Photosystem II (PS II) captures solar energy and directs charge separation (CS) across the thylakoid membrane during photosynthesis. The highly oxidizing, charge-separated state generated within its reaction center (RC) drives water oxidation. Spectroscopic studies on PS II RCs are difficult to interpret due to large spectral congestion, necessitating modeling to elucidate key spectral features. Herein, we present results from time-dependent density functional theory (TDDFT) calculations on the largest PS II RC model reported to date. This model explicitly includes six RC chromophores and both the chlorin phytol chains and the amino acid residues <6 Å from the pigments' porphyrin ring centers. Comparing our wild-type model results with calculations on mutant D1-His-198-Ala and D2-His-197-Ala RCs, our simulated absorption-difference spectra reproduce experimentally observed shifts in known chlorophyll absorption bands, demonstrating the predictive capabilities of this model. We find that inclusion of both nearby residues and phytol chains is necessary to reproduce this behavior. Our calculations provide a unique opportunity to observe the molecular orbitals that contribute to the excited states that are precursors to CS. Strikingly, we observe two high oscillator strength, low-lying states, in which molecular orbitals are delocalized over ChlD1 and PheD1 as well as one weaker oscillator strength state with molecular orbitals delocalized over the P chlorophylls. Both these configurations are a match for previously identified exciton-charge transfer states (ChlD1+PheD1-)* and (PD2+PD1-)*. Our results demonstrate the power of TDDFT as a tool, for studies of natural photosynthesis, or indeed future studies of artificial photosynthetic complexes.