Dissociative nature of C(sp2)-N(sp3) bonds of carbazole based materials via conical intersection: simple method to predict the exciton stability of host materials for blue OLEDs: a computational study.

In this work, the origin of the singlet and triplet exciton-induced degradation of host materials with C(sp2)-N(sp3) bonds around nitrogen (carbazoles, acridines, etc.), connecting donor and acceptor units, was unravelled using DFT and CASSCF methods. The results reveal that molecules (employed in OLEDs) with basic units containing C(sp2)-N(sp3) bonds (nitrogen connected to carbon in a triangular fashion) have a natural tendency to fragment at the C-N bond through an S1/S0 conical intersection (CI). The calculation of barrier heights, to reach a dissociation point, indicates that degradation via triplet states is kinetically less feasible (ΔGT1-TS* > 25 kcal mol-1) compared to that via the first singlet excited state (ΔGS1-TS* ∼7-30 kcal mol-1). However, the long lifetime of triplets (as compared to singlets) aids in the reverse intersystem crossing from triplet to singlet state for subsequent degradation. From the results and inference, ΔGS1-TS* and ΔES1-T1 are proposed to be the controlling factors for exciton-induced degradation of host materials with C(sp2)-N(sp3) bonds. Furthermore, multiple functionalization of carbazole moieties reveals that polycyclic aromatic systems employed as acceptor units of host materials are best suited for PhOLEDs as they will increase their lifetime due to the larger ΔGS1-TS* and ΔES1-T1. For TADF-based devices, materials with fused ring systems (with N(sp3) at the centre) in the donor unit are the most recommended ones based on the findings of this work, as they avoid the dissociative channel altogether. A negative linear correlation between ΔGS1-TS* and HOMO-LUMO gap is observed, which provides an indirect way to predict the kinetic stability of these materials in excitonic states. These initial results are promising for the future development of the QSAR-type approach for the smart design of host materials for long-life blue OLEDs.

Excited state C-N bond dissociation and cyclization of tri-aryl amine-based OLED materials: a theoretical investigation.

Tri-aryl amines (TAA) such as triphenylamine (TPA) are widely used for designing chromophores for dye-sensitized solar cells (DSSC) and organic light-emitting diodes (OLED). These materials degrade over time and hence result in reduced performance. Therefore, exploring the associated mechanistic pathways and factors controlling the degradation is necessary for future development of durable TAA-based devices. Hence, in this study, the complete active space self-consistent-field (CASSCF) method coupled with second order N-electron valence perturbation theory (NEVPT2) calculations was carried out to understand the excited state phenomena occurring in TAA using TPA and N,N-diphenyl-2-naphthylamine (DPNA) as model systems. The results indicated the presence of a conical intersection between ground and first excited singlet states with C-N bond dissociation, which acts as a channel for the excited molecules to dissociate and form radical fragments (phenyl/naphthyl). This occurrence is unusual for non-saturated bonds with delocalization. The resulting radical fragments formed intra-molecular products and subsequently yielded five- and six-membered cyclized products depending on the type of aryl groups. The significant findings from this study throw light on the photostability of TAA-based OLED devices as well as on the possible route to synthesize cyclic amines such as carbazoles.

Probing the effect of different graphitic nitrogen sites on the aerobic oxidation of thiols to disulfides: a DFT study.

Functionalized graphene materials are well known for their application in catalyzing the aerobic oxidation of alcohols, hydrocarbons, etc. in an aqueous medium. Despite the fact that a few catalysts are known to oxidize thiols to disulfides, their selectivity is poor and requires oxidants that are not suitable in terms of the principles of green chemistry. Therefore, in this context, an attempt has been made to investigate the possibility of utilizing nitrogen doped graphene for the aerobic oxidation of thiols to disulfides using density functional theory (DFT). Our previous study (V. S. Jeyaraj, M. Kamaraj and V. Subramanian, J. Phys. Chem. C, 2015, 119, 26438-26450) has shed light on the activation of dioxygen to form activated oxygen species (AOS) at different graphitic nitrogen sites. Hence the same has been used to study the two-electron oxidation of thiophenol and methanethiol. The AOS are of three kinds: (1) peroxide type at the edges, (2) superoxide type at the center and (3) ketonic type at edges. The findings from this study indicate that the peroxide type AOS leads to selective formation of diphenyl disulfide, whereas the superoxide type at the center facilitates the formation of hydrogen peroxide which could lead to over-oxidation of disulfide. The oxidation of aromatic thiols (thiophenol) by the ketonic type of AOS is nearly a barrier-less reaction (0.67 kcal mol-1). Similarly, AOS at the edges with the peroxide form can oxidize aliphatic thiols (methanethiol) with a less barrier of 1.55 kcal mol-1, which can be a spontaneous reaction. The mechanism of oxidation is completely different from the oxidative pathway of alcohols by the same AOS. The formation of S-OH species is strictly avoided by the strong stabilization of thiyl radicals over the π-surface of graphene.

Interaction of collagen like peptides with gold nanosurfaces: a molecular dynamics investigation.

In this study, an attempt has been made to understand the interaction between collagen like peptides (CPs) with a gold nanosurface (AuNS) using a classical molecular dynamics simulation. Results reveal that the adsorption of CPs onto the gold surface depends on the amino acid composition of the collagen like peptides. It is evident from the findings that the Hyp residue of collagen interacts favorably with the AuNS. It is interesting to note that the model CP without a Hyp residue does not adsorb well on the surface. Results indicate that gold nanosurfaces or gold nanoparticles can be exploited to detect breast cancer due to the increased content of Hyp residues in the Gly-XAA-YAA triplet of collagen in breast cancer tissues. These results provide useful information for designing collagen based scaffolds for tissues engineering applications.

Novel chemistry for the selective oxidation of benzyl alcohol by graphene oxide and N-doped graphene.

A novel mechanism for the selective activation of benzyl alcohol by graphene oxide and N-doped graphene has been proposed using density functional theory based calculations. Interestingly, the proposed mechanism opens new avenues for graphene and its derivative-based catalysis.