Scopus İndeksli Yayınlar Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.12573/395
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Article Citation - WoS: 13Citation - Scopus: 11Understanding the Effect of Symmetry Breaking on Plasmon Coupling From TDDFT(Amer Chemical Soc, 2021-05-26) Alkan, Fahri; Aikens, Christine M.We perform a time-dependent density functional theory (TDDFT) investigation for the optical properties of nanorod assemblies for different sizes (Ag-10, Ag-59, and Ag-139), interparticle distances, and orientations with a focus on the effect of symmetry breaking via an angle on plasmon coupling. For the model systems, the angle (theta) between the particles is varied between 0 and 180 degrees, where theta = 0 degrees and theta = 180 degrees correspond to symmetric side-by-side and end-to-end orientations of the nanorods, respectively. Our analysis reveals that for a sufficiently large interparticle distance (r > 0.7 nm), where the wave-function overlap between monomers is negligible, TDDFT results agree quite well with the predictions of the dipole-dipole interaction model for the intensity of the different modes of coupled plasmons. For smaller gap distances (0.4-0.5 nm), a charge-transfer plasmon (CTP) mode occurs for the symmetry broken case of the Ag-10 dimer. For the assemblies of larger nanorods, however, the CTP mode is predicted to be less pronounced, especially for the cases where the deviation from the end-to-end geometry is larger than 30 degrees. The orbital overlap and configuration-interaction analyses show that these results are related to the fact that the relative overlap strength between monomeric energy levels is significantly reduced for symmetry-broken orientations of larger nanorods.Article Citation - WoS: 19Citation - Scopus: 19Understanding and Tailoring Excited State Properties in Solution-Processable Oligo(p-Phenyleneethynylene)s: Highly Fluorescent Hybridized Local and Charge Transfer Character via Experiment and Theory(Amer Chemical Soc, 2021-10-13) Usta, Hakan; Cosut, Bunyemin; Alkan, FahriRod-shaped oligo(p-phenyleneethynylene) (OPE) offers an attractive p-framework for the development of solution-processable highly fluorescent molecules having tunable hybridized local and charge transfer (HLCT) excited states and (reverse) intersystem crossing ((R)ISC) channels. Herein, an HLCT oligo(p-phenyleneethynylene) library was studied for the first time in the literature in detail systematically via experiment and theory. The design, synthesis, and full characterization of a new highly fluorescent (Phi(PL-solution) similar to 1) sky blue emissive 4',4 ''-((2,5-bis((2-ethylhexyl)oxy)-1,4-phenylene)bis(ethyne-2,1-diyl))bis(N,N-diphenyl-[1,1'-bi-phenyl]-4-amine) (2EHO-TPA-PE) was also reported. The new molecule consists of a D'-Ar-pi-D-pi-Ar-D' molecular architecture with an extended pi-spacer and no acceptor unit, and detailed structural, physicochemical, single-crystal, and optoelectronic characterizations were performed. A high solid-state quantum efficiency (Phi(PL-solution) similar to 0.8) was achieved as a result of suppressed exciton-phonon/vibronic couplings (no pi-pi interactions and multiple (14 per dimeric form) strong C-H center dot center dot center dot pi interactions). Strong solution-phase/solid-state dipole-dependent tunable excited state behavior (local excited (LE) -> HLCT -> charge transfer (CT)) and decay dynamics covering a wide spectral region were demonstrated, and the CT state was observed to be highly fluorescent despite extremely large Stokes shift (similar to 130 nm)/fwhm (similar to 125 nm) and significant charge separation (0.75 charge.nm). Employing the Lippert-Mataga model, along with detailed photophysical studies and TDDFT calculations, key relationships between molecular design-electronic structure-exciton characteristics were elucidated with regards to HLCT and hot exciton channel formations. The interstate coupling between CT and LE states and the interplay of this coupling with respect to medium polarity were explored. A key relationship between excited-state symmetry breaking process and the formation of HLCT state was discussed for TPA-ended rod-shaped OPE p-systems. (R)ISC-related delayed fluorescence (tau similar to 2-6 ns) processes were evident following the prompt decays (similar to 0.4-0.9 ns) both in the solution and in the solid-state. As a unique observation, the delayed fluorescence could be tuned and facilitated via small dielectric changes in the medium. Our results and the molecular engineering perspectives presented in this study may provide unique insights into the structural and electronic factors governing tunable excited state and hot-exciton channel formations in OPEs for (un)conventional solution-processed luminescence applications.Article Citation - WoS: 3Citation - Scopus: 3Theoretical Investigation of Substituent Effects on the Relative Stabilities and Electronic Structure of [BnXn]2- Clusters(Springer, 2021-11-29) Tahaoglu, Duygu; Alkan, Fahri; Durandurdu, MuratIn 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.Article Citation - WoS: 14Citation - Scopus: 16TD-DFTB Study of Optical Properties of Silver Nanoparticle Homodimers and Heterodimers(Amer Inst Physics, 2020-10-14) Liu, Zhen; Alkan, Fahri; Aikens, Christine M.The absorption spectra for face-centered cubic nanoparticle dimers at various interparticle distances are investigated using time-dependent density functional tight binding. Both homodimers and heterodimers are investigated in this work. By studying nanoparticles at various interparticle distances and analyzing their vertical excitations, we found that as the interparticle distance decreases, a red shift arises from contributions of the transition dipole moment that are aligned along the z-axis with nondegenerate features; blue shifts occur for peaks that originate from transition dipole moment components in the x and y directions with double degeneracy. When the nanoparticles are similar in size, the features in the absorption spectra become more sensitive to the interparticle distances. The best-fit curves from vertical excitation energy in the form of AR(-b) for Delta E-redshift/Delta E-blueshift vs R are determined. In this way, we determined trends for absorption peak shifts and how these depend on the interparticle distance.Article Citation - WoS: 63Citation - Scopus: 63Highly Efficient Deep-Blue Electroluminescence Based on a Solution-Processable A-Π Oligo(p-Phenyleneethynylene) Small Molecule(Amer Chemical Soc, 2019-10-14) Usta, Hakan; Alimli, Dilek; Ozdemir, Resul; Dabak, Salih; Zorlu, Yunus; Alkan, Fahri; Can, AyseThe development of solution-processable fluorescent small molecules with highly efficient deep-blue electroluminescence is of growing interest for organic light-emitting diode (OLED) applications. However, high-performance deep-blue fluorescent emitters with external quantum efficiencies (EQEs) over 5% are still scarce in OLEDs. Herein, a novel highly soluble oligo(p-phenyleneethynylene)-based small molecule, 1,4-bis((2-cyanophenyl)ethynyl)-2,5-bis(2-ethylhexyloxy)benzene (2EHO-CNPE), is designed, synthesized, and fully characterized as a wide band gap (2.98 eV) and highly fluorescent (Phi(PL) = 0.90 (solution) and 0.51 (solid-state)) deep-blue emitter. The new molecule is functionalized with cyano (-CN)/2-ethylhexyloxy (-OCH2CH(C2H5)C4H9) electron-withdrawing/-donating substituents, and ethynylene is used as a pi-spacer to form an acceptor (A)-pi-donor (D)-pi-acceptor (A) molecular architecture with hybridized local and charge transfer (HLCT) excited states. Physicochemical and optoelectronic characterizations of the new emitter were performed in detail, and the single-crystal structure was determined. The new molecule adopts a nearly coplanar pi-conjugated framework packed via intermolecular "C-H center dot center dot center dot pi" and "C-H center dot center dot center dot N" hydrogen bonding interactions without any pi-pi stacking. The OLED device based on 2EHO-CNPE shows an EQE(max) of 7.06% (EQE = 6.30% at 200 cd/m(2)) and a maximum current efficiency (CEmax) of 5.91 cd/A (CE = 5.34 cd/A at 200 cd/m(2)) with a deep-blue emission at CIE of (0.15, 0.09). The electroluminescence performances achieved here are among the highest reported to date for a solution-processed deep-blue fluorescent small molecule, and, to the best of our knowledge, it is the first time that a deep-blue OLED is reported based on the oligo(p-phenyleneethynylene) pi-framework. TDDFT calculations point to facile reverse intersystem crossing (RISC) processes in 2EHO-CNPE from high-lying triplet states to the first singlet excited state (T-2/T-3 -> S-1) (hot-exciton channels) that enable a high radiative exciton yield (eta(r) similar to 69%) breaking the theoretical limit of 25% in conventional fluorescent OLEDs. These results demonstrate that properly designed fluorescent oligo(p-phenyleneethynylenes) can be a key player in high-performance deep-blue OLEDs.Article Citation - WoS: 53Citation - Scopus: 55Dicarboxylic Acids Induced Tandem Transformation of Silver Nanocluster(Amer Chemical Soc, 2023-08-30) Wang, Zhi; Gupta, Rakesh Kumar; Alkan, Fahri; Han, Bao-Liang; Feng, Lei; Huang, Xian-Qiang; Sun, DiStructural transformation of metal nanoclusters (NCs) is of great ongoing interest regarding their synthesis, stability, and reactivity. Although sporadic examples of cluster transformations have been reported, neither the underlying transformation mechanism nor the intermediates are unambiguous. Herein, we have synthesized a flexible 54-nuclei silver cluster (Ag54) by combining soft ((BuC)-Bu-t=C-) and hard ((PrCOO-)-Pr-n) ligands. The existence of weakly coordinated nPrCOO(-) enhances the reactivity of Ag54, thus facilitating the dicarboxylic acid to induce structural transformation. X-ray structural analyses reveal that Ag54 transforms to Ag-28 cluster-based 2D networks (Ag28a and Ag28b) induced by H(2)suc (succinic acid) and H(2)glu (glutaric acid), whereas with H(2)pda (2,2'-(1,2-phenylene)diacetic acid), a discrete Ag-28 cluster (Ag28c) is isolated. The key intermediate Ag17 that emerges during the self-dissociation of Ag54 was isolated by using cryogenic recrystallization and characterized by X-ray crystallography. The "tandem transformation" mechanism for the structure evolution from Ag54 to Ag28a is established by time-dependent electrospray ionization mass spectrometry (ESIMS) and UV-vis spectroscopy. In addition, the catalytic activity in the 4-nitrophenol reduction follows the sequence Ag28c > Ag28b > Ag28a > Ag54 due to more bare silver sites on the surface of the Ag-28 cluster unit. Our findings not only open new avenues to the synthesis of silver NCs but also shed light on a better understanding of the structural transformation mechanism from one cluster to another or cluster-based metal-organic networks induced by dicarboxylates.Article Citation - WoS: 70Citation - Scopus: 71An Ultrastable 155-Nuclei Silver Nanocluster Protected by Thiacalix[4]Arene and Cyclohexanethiol for Photothermal Conversion(Wiley-VCH Verlag GmbH, 2022-06-15) Wang, Zhi; Alkan, Fahri; Aikens, Christine M.; Kurmoo, Mohamedally; Zhang, Zhen-Yi; Song, Ke-Peng; Sun, DiThiacalix[4]arenes have emerged as a family of macrocyclic ligands to protect metal nanoparticles, but it remains a great challenge to solve the mystery of their structures at the atomic level, especially for those larger than 2 nm. Here, we report the largest known mixed-valence silver nanocluster [Ag-155(CyS)(40)(TC4A)(5)Cl-2] (Ag155) protected by deprotonated cyclohexanethiol (CySH) and macrocyclic ligand p-tert-butylthiacalix[4]arene (H(4)TC4A). Its single-crystal structure consists of a metallic core of four concentric shells, Ag-13@Ag-42@Ag-30@Ag-70, lined with a organic skin of 40CyS(-) and 5TC4A(4-) and 2Cl(-). Ag155 manifests an unusual pseudo-5-fold symmetry dictated by the intrinsic metal atom packing and the regioselective distribution of mixed protective ligands. This work not only reveals a macrocyclic ligand effect on the formation of a large silver nanocluster, but also provides a new structural archetype for comprehensively perceiving their interface and metal kernel structures.Book Part Advances in the Computation of NMR Parameters for Inorganic Nuclides(Elsevier, 2023) Holmes, Sean T.; Alkan, Fahri; Dybowski, Cecil R.In this article, we discuss practical aspects of the computation of NMR parameters of inorganic nuclides, as well as insights afforded by such calculations into the characterization of molecular-level structure and dynamics and the validation of theoretical models. An emphasis is placed on calculation of the magnetic shielding tensors of solids using cluster-based models that account for intermolecular interactions. In particular, the use of valence modification of terminal atoms using bond valence theory (VMTA/BV), which reduces net charges on clusters through terminal pseudoatoms with nonstandard nuclear charges, is demonstrated to be a robust technique for calculations on nuclei in network solids. Cluster-based calculations, including those that employ the VMTA/BV method, afford a unique opportunity to calculate magnetic shielding tensors for nuclei in solids by using density functional theory approximations beyond the generalized gradient approximation and by incorporating relativistic effects at the spin-orbit level. These developments are spurred by use of the zeroth-order regular approximation (ZORA), which provides a robust method of accounting for relativistic effects (up to the spin-orbit level) experienced by valence electrons. Calculations of NMR parameters are discussed for fluorine, cadmium, tin, tellurium, mercury, lead, and platinum, all of which have seen significant advances in recent years. These examples highlight the importance of such factors as coordination geometry, oxidation state, relativistic effects, and density functional approximations on computed magnetic shielding tensors. © 2023 Elsevier B.V., All rights reserved.Article Citation - WoS: 33Citation - Scopus: 34A Hybridized Local and Charge Transfer Excited State for Solution-Processed Non-Doped Green Electroluminescence Based on Oligo(p-Phenyleneethynylene)(Royal Soc Chemistry, 2020) Usta, Hakan; Alimli, Dilek; Ozdemir, Resul; Tekin, Emine; Alkan, Fahri; Kacar, Rifat; Can, AyseWe herein report a new highly efficient green emissive hot-exciton molecule, 1,4-bis((4'-diphenylamino3-cyano-[1,1'-biphenyl]-4-yl)ethynyl)-2,5-bis(2-ethylhexyloxy)benzene (2EHO-TPA-CNPE) that consists of an extended D'-pi'-A-pi-D-pi-A-pi'-D' molecular p-system with diphenylamino end units (D') and ethynylene/phenylene spacers (pi/pi'). The new molecule exhibits high photoluminescence (PL) quantum efficiencies (Phi(PL) = 0.95 (solution) and 0.45 (spin-coated neat thin-film)), and a strong PL solvatochromic behavior revealing significant changes in excited state energies/characteristics (locally excited (LE) -> hybridized local and charge transfer (HLCT) - charge-transfer (CT)) depending on solvent polarity. Highly efficient (radiative exciton yield (eta(r)) = 50-59% >> 25%) green-emitting OLEDs were fabricated in a conventional device architecture by employing (non-)doped thin-films reaching a maximum current efficiency (CEmax) of 12.0 cd A(-1) and a maximum external quantum efficiency (EQE(max)) of 5.5%. The emission profile of the non-doped OLED has CIE 1976 (u', v') chromaticity coordinates of (0.10, 0.55) corresponding to a night vision imaging system (NVIS) compatible Green A region. 2EHO-TPA-CNPE-based OLED devices of industrial relevance were also fabricated by ink-jet printing the emissive layer and by fabricating an inverted architecture, which possessed respectable device performances of 2.4-6.1 cd A(-1). The solid-state solvation effect in OLED devices yields HLCT electronic behavior resulting in high Zr's, which is confirmed by TDDFT to originate from energetically/spatially favorable reverse intersystem crossings (RISCs) (T-2/3 -> S-1). As a unique observation, delayed fluorescence due to this RISC was evident in the PL decay lifetime measurement with a ns-scale lifetime of similar to 10 ns. These results clearly allow a better understanding of the structure-photophysical property-electroluminescence relationships in this new class of oligo(p-phenyleneethynylene)-based hot-exciton molecules, and it could open up new opportunities for high-performance solution-processed optoelectronic/sensing applications.