Interfacial Charge Transfer in Photoexcited QD-Molecule Composite of Tetrahedral CdSe Quantum Dot Coupled with Carbazole.

Photoexcited charge transfer dynamics in CdSe quantum dots (QDs) coupled with carbazole were explored to model QD-molecule systems for light-harvesting applications. The absorption spectra of QDs with different sizes, i.e., Cd35Se20X30L30 (T1), Cd56Se35X42L42 (T2), and Cd84Se56X56L56 (T3) were simulated with quantum dynamical methods, which qualitatively match the reported experimental spectra. The carbazole is attached with a 3-amino group at the apex position of T1 (namely T1-3A-Cz), establishing proper electronic communication between T1 and carbazole. The spectra of T1-3A-Cz is 0.22 eV red-shifted compared to T1. A time-dependent perturbation was applied in tune with the lowest energy peak (3.63 eV) of T1-3A-Cz to investigate the charge transfer dynamics, which revealed an ultrafast charge separation within the femtosecond time scale. The electronic structure showed a favorable energy alignment between T1 and carbazole in T1-3A-Cz. The LUMO of carbazole was situated below the conduction band of the QD, while the HOMO of carbazole mixed perfectly with the top of the valence band of the QD, developing the interfacial charge transfer states. These states promoted the photoexcited electron transfer directly from the CdSe core to carbazole. A rapid and enhanced charge separation occurred with the laser field strength increasing from 0.001 to 0.005 V/Å. However, T1 connected to the other positions of carbazole did not show charge separation effectively. The photoinduced charge transfer is negligible in the case of T2-carbazole systems due to poor electronic coupling, and it is not observed in T3-carbazole systems. So, the T1-3A-Cz model acts as a perfect donor-acceptor QD-molecule nanocomposite that can harvest photon energy efficiently. Further enhancement of charge transfer can be achieved by coupling more carbazoles to the T1 QD (e.g., T1-3A-Cz2) due to the extension of hole delocalization between T1 and the carbazoles.

A First-Principles Investigation of Structure-Luminescence Activity of Donor-Acceptor-Donor Organic Triad.

Structure-property relationships of different conformers of an organic D-A-D triad are explored to rationalize the structural motif toward photoluminescence activity. In a recent experiment ( Chem. Sci. 2017, 8, 2677-2686), Takeda and co-workers revealed that the PTZ-DBPHZ-PTZ (D-A-D) triad exhibited multicolor luminescence properties and thermally activated delayed fluorescence (TADF) emission. We computationally studied the photophysical properties of the conformers of that D-A-D triad to provide a detailed description of the luminescence activity. Our analysis confirms that the twisting of the axial phenothiazine (PTZ) unit to an equatorial position altered the nature of the S1 state from local to a charge transfer state and was responsible for the large red shift in emission (S1) energy. Calculated fluorescence and intersystem crossing (ISC) rate constants suggest that the prompt fluorescence is turned on for axial-axial conformers while it is turned off for others. Fast reverse intersystem crossing (RISC) from triplet CT to the S1 state (3CT1 → 1CT1), close spacing and effective crossing between 3LE1A, 3CT1 and 1CT1 states cause efficient harvesting of triplet excitons to S1 state, thus enabling TADF emission for equatorial-equatorial conformer.

Luminescent 1H-1,3-benzazaphospholes.

2-R-1H-1,3-Benzazaphospholes (R-BAPs) are an interesting class of σ2P heterocycles containing P[double bond, length as m-dash]C bonds. While closely related 2-R-1,3-benzoxaphospholes (R-BOPs) have been shown to be highly photoluminescent materials depending on specific R substituents, photoluminescence of R-BAPs has been previously limited to an example having a fused carbazole ring system. Here we detail the synthesis and structural characterization of a new R-BAP (3c, R = 2,2'-dithiophene), and compare its photoluminescence against two previously reported R-BAPs (3a, R, R' = Me and 3b, R = 2-thiophene). The significant fluorescence displayed by the thiophene derivatives 3b (φ = 0.53) and 3c (φ = 0.12) stands in contrast to the weakly emissive methyl substituted analogue 3a (φ = 0.08). Comparative computational investigations of 3a-c offer insights into the interplay between structure-function relationships affecting excited state relaxation processes.

Controlling the Emissive Activity in Heterocyclic Systems Bearing C═P Bonds.

The photophysical properties of a series of heteroatom substituted indoles are explored to identify chemical means to control their emissive activity. In particular, we consider impacts of changes in the conjugated backbone, where the C═N bonds of benzoxazoles are replaced by C═P bonds (benzoxaphospholes). The effects of extending the π-conjugation, incorporating various secondary heteroatoms (X-C═P), and enforcing planar rigidity are also examined. Our computational analysis explains the higher fluorescence efficiency observed with extended π-conjugation and highlights the importance of maintaining molecular planarity at both ground- and emissive-state geometries.

Enhancing charge mobilities in organic semiconductors by selective fluorination: a design approach based on a quantum mechanical perspective.

Selective fluorination of organic semiconducting molecules is proposed as a means to achieving enhanced hole mobility. Naphthalene is examined here as a root molecular system with fluorination performed at various sites. Our quantum chemical calculations show that selective fluorination can enhance attractive intermolecular interactions while reducing charge trapping. Those observations suggest a design principle whereby fluorination is utilized for achieving high charge mobilities in the crystalline form. The utility of this design principle is demonstrated through an application to perylene, which is an important building block of organic semiconducting materials. We also show that a quantum mechanical perspective of nuclear degrees of freedom is crucial for a reliable description of charge transport.

The electronic and optical properties of MoS(2(1-x))Se(2x) and MoS(2(1-x))Te(2x) monolayers.

In a very recent article, atomically thin two-dimensional MoS2xSe2(1-x) nanosheets have been synthesized with complete composition tunability using a temperature gradient assisted chemical vapor deposition technique [J. Am. Chem. Soc., 2014, 136, 3756]. To have a better understanding of the composition dependent tunability of the properties of this class of materials we here perform first principles calculations on the detailed electronic structure of single layered transition metal dichalcogenides MoS2(1-x)Se2x and MoS2(1-x)Te2x. The positive value of mixing energy of both MoS2(1-x)Se2x and MoS2(1-x)Te2x 2D sheets at various composition confirms their formation at some energy cost. The analysis of the composition dependent band structure and the density of states of these 2D sheets reveals certain interesting features. The band gap variation of the MoS2(1-x)Se2x nanosheets is almost linear with composition while that of MoS2(1-x)Te2x deviates slightly from linearity. We have also calculated the optical absorption spectrum of these nanosheets as a function of composition and found that the optical transitions are mainly metal d-d type spin forbidden transitions.

Theoretical prediction of a new two-dimensional carbon allotrope and NDR behaviour of its one-dimensional derivatives.

By using state of the art theoretical methods we have predicted a new two-dimensional (2-D) carbon allotrope. This new planar carbon framework is made of hexagons, octagons and pentagons and hence named as HOP graphene (HOPG). The possibility of existence of HOPG is evident from its dynamical stability as confirmed by phonon-mode analysis and also from an energetic point of view since it is energetically more favorable than recently synthesized graphdiyne. The band structure shows the metallic behaviour of this new form of carbon allotrope. We also explored the electronic structure and transport properties of a 1-D derivative (nanoribbon) of HOPG. Most of the nanoribbons exhibit multiple negative differential resistance (NDR) behaviour with high peak to valley ratio.

Energetics and electronic structure of encapsulated graphene nanoribbons in carbon nanotube.

We report results of our total energy electronic structure calculation of encapsulation of graphene nanoribbon (GNR) in the carbon nanotube (CNT). The encapsulation of both coronene and perylene based graphene nanoribbons in zigzag (n,0) carbon nanotubes (where n ranges from 14 to 18 for perylene based nanoribbon and from 16 to 20 for coronene based nanoribbons) is an exothermic process. Our study shows that in certain cases arm-chair GNR (aGNR) encapsulated CNT results in type II band alignment and may be useful in the application in solar cells. We have also studied the potential of this composites for hydrogen storage. We found that the encapsulated GNR composite systems have higher hydrogen adsorption energies than the individual components of either GNR and CNT. The hydrogen molecules oriented perpendicular to GNR are found to be more stable as compared to hydrogen molecules parallel to GNR.

Explicit spectral response of the geometrical isomers of a bio-active pyrazoline derivative encapsulated in ß-cyclodextrin nanocavity: a photophysical and quantum chemical analysis.

The existence of two geometrical isomers (cis- and trans-) of a biologically significant pyrazoline derivative [5-(-1'-(4-bromo-phenyl)-3a',7a'-hexahydro-1'H-indazol-3'-yl)-3-methyl-1-phenyl-1H-pyrazole-4-carbonitrile] (PZ) has been established using a combined theoretical and experimental investigation. Solvatochromic analysis of PZ revealed the existence of said cis- and trans- isomers. The unique solvatochromic response of the PZ isomers and their preferential encapsulation within ß-cyclodextrin (ß-CD) nanocavity clearly shows the difference in the behavioral nature of the isomers of PZ in homogeneous and heterogeneous medium. Solvent polarity, time-resolved study, and anisotropy results also reinforce in favor of the existence of the isomers. To evaluate the actual orientation of cis and trans-PZ, the ground and excited state geometry of these isomers were optimized by the DFT/LanL2DZ and CIS/LanL2DZ methods, respectively. The experimentally observed results and the theoretically calculated results are found to be in close agreement.

Self-Consistent-Charge Density-Functional Tight-Binding Parameters for Cd-X (X = S, Se, Te) Compounds and Their Interaction with H, O, C, and N.

Parameters for CdX, SeX, and TeX (X = H, C, N, O, S, Se, Te, and Cd) have been generated within the self-consistent-charge density-functional tight-binding (SCC-DFTB) framework. The approach has been tested against ab initio density-functional theory calculations for the relevant bulk phases, surfaces, nanowires, and small molecular systems. The SCC-DFTB approach reproduces structural, electronic, and energetic properties very well, demonstrating that the developed parameters are fully transferable among different chemical environments.