Browsing by Author "Tahaoglu, Duygu"

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Citation - WoS: 23
Citation - Scopus: 27
Revisiting the Role of Charge Transfer in the Emission Properties of Carborane-Fluorophore Systems: A TDDFT Investigation
(Amer Chemical Soc, 2022) Tahaoglu, Duygu; Usta, Hakan; Alkan, Fahri
In this study, we performed a detailed investigation of the S-1 potential energy surface (PES) of o-carborane-anthracene (o-CB-Ant) with respect to the C-C bond length on o-CB and the dihedral angle between o-CB and Ant moieties. The effects of different substituents (F, Cl, CN, and OH) on carbon- or boron-substituted o-CB, along with a pi-extended acene-based fluorophore, pentacene, on the nature and energetics of S-1 -> S-0 transitions are evaluated. Our results show the presence of a non-emissive S-1 state with an almost pure charge transfer (CT) character for all systems as a result of significant C-C bond elongation (C-C = 2.50-2.56 angstrom) on o-CB. In the case of unsubstituted o-CB-Ant, the adiabatic energy of this CT state corresponds to the global minimum on the S-1 PES, which suggests that the CT state could be involved in emission quenching. Despite large deformations on the o-CB geometry, predicted energy barriers are quite reasonable (0.3-0.4 eV), and the C-C bond elongation can even occur without a noticeable energy penalty for certain conformations. With substitution, it is shown that the dark CT state becomes even more energetically favorable when the substituent shows -M effects (e.g., -CN), whereas substituents showing +M effects (e.g., -OH) can result in an energy increase for the CT state, especially for partially stretched C-C bond lengths. It is also shown that the relative energy of the CT state on the PES depends strongly on the LUMO level of the fluorophore as this state is found to be energetically less favorable compared to other conformations when anthracene is replaced with pi-extended pentacene. To our knowledge, this study shows a unique example of a detailed theoretical analysis on the PES of the S-1 state in o-CB-fluorophore systems with respect to substituents or fluorophore energy levels. Our findings could guide future experimental work in emissive o-CB-fluorophore systems and their sensing/optoelectronic applications.
Citation - WoS: 1
Citation - Scopus: 1
Solute Aggregation in Ca72Zn28 Metallic Glass
(Elsevier Science Bv, 2018) Tahaoglu, Duygu; Durandurdu, Murat
Solidification of the Ca72Zn28 melt is achieved by using both the thermal quenching and rapid pressurizing techniques in ab initio molecular dynamics simulations within a generalized gradient approximation. A chemical segregation process is perceived in the Ca72Zn28 system and hence the resulting configurations show nanosized glassy domains with different compositions. The structural and mechanical properties of both Ca72Zn28 metallic glasses have been probed by using various analyzing methods. Although the mean coordination number of the both models is found to be fairly close to each other, a careful investigation exposes that they have a different short-range order around Zn atoms. It appears that pressurizing significantly affects the environment of Zn atom, suppresses the occurrence of Zn-centered ideal icosahedral polyhedrons and retains the Zn centered tri-capped trigonal prism like configurations. On the other hand, the impact of pressure on the environment of Ca atoms is found to be not too drastic. The computer-generated models represent slightly different mechanical properties. The model obtained using the rapid pressurizing technique is stiffer than the one produced using the thermal quenching technique.
Citation - WoS: 4
Citation - Scopus: 4
Permanent Densification of Amorphous Zinc Oxide Under Pressure: A First Principles Study
(Elsevier Science Bv, 2018) Tahaoglu, Duygu; Durandurdu, Murat
Ab initio simulations within a generalized gradient approximation are carried out to investigate the densification mechanism of amorphous zinc oxide (a-ZnO) under hydrostatic pressure. In contrast to the crystalline ZnO, the densification of a-ZnO is found to proceed gradually and is associated with a structural modification from a low density amorphous state to a high density amorphous state. Accompanied by the phase transformation, the mean coordination number increases from similar to 4.0 to similar to 5.5. The high-density amorphous form of ZnO has a local structure, partially comparable with that of the rocksalt type ZnO crystal and presents a semiconducting behavior. The phase change is irreversible because upon pressure release, an amorphous model largely consisting of fivefold coordination is recovered. The decompressed model can be, therefore, classified as an intermediate phase between the wurtzite-like and the rocksalt-like amorphous configurations.
Citation - WoS: 3
Citation - Scopus: 3
Theoretical Investigation of Substituent Effects on the Relative Stabilities and Electronic Structure of [BnXn]2- Clusters
(Springer, 2021) Tahaoglu, Duygu; Alkan, Fahri; Durandurdu, Murat
In this study, we provide a theoretical evaluation of relative stabilities and electronic structure for [BnXn](2-) clusters (n = 10, 12, 13, 14, 15, 16). Structural and electronic characteristics of [BnXn](2-) clusters are examined by comparison with the [B12X12](2-) counterparts with a focus on the substituent effects (X = H, F, Cl, Br, CN, BO, OH, NH2) on the electronic structure, electron detachment energies, formation enthalpies, and charge distributions. For the electronic structure and electron detachment energies, substituent effects on boron clusters are shown to follow a very similar trend to the mesomeric and inductive effects (+/- M and +/- I) of pi-conjugated systems, and the most stable derivatives in terms of HOMO/LUMO and electron detachment energies are calculated for CN and BO substituents due to strong -M effects. In the case of formation enthalpies for larger boron clusters (n >= 13), the icosahedral barrier is shown to increase with the halogen and CN substitution, whereas it is possible to reduce the icosahedral barrier for the cases of X = OH and NH2. It is shown that this reduction results from destabilizing the [B12X12](2-) cluster with electronic (+ M) and symmetry effects induced by OH and NH2 ligands.