| 1. |
- Pander, Piotr, et al.
(author)
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Thermally activated delayed fluorescence in a deep red dinuclear iridium(iii) complex: a hidden mechanism for short luminescence lifetimes
- 2023
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In: Chemical Science. - : ROYAL SOC CHEMISTRY. - 2041-6520 .- 2041-6539. ; 14:47, s. 13934-13943
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Journal article (peer-reviewed)abstract
- The high luminescence efficiency of cyclometallated iridium(III) complexes, including those widely used in OLEDs, is typically attributed solely to the formally spin-forbidden phosphorescence process being facilitated by spin-orbit coupling with the Ir(III) centre. In this work, we provide unequivocal evidence that an additional mechanism can also participate, namely a thermally activated delayed fluorescence (TADF) pathway. TADF is well-established in other materials, including in purely organic compounds, but has never been observed in iridium complexes. Our findings may transform the design of iridium(III) complexes by including an additional, faster fluorescent radiative decay pathway. We discover it here in a new dinuclear complex, 1, of the form [Ir(N<^>C)(2)](2)(mu-L), where N<^>C represents a conventional N<^>C-cyclometallating ligand, and L is a bis-N<^>O-chelating bridging ligand derived from 4,6-bis(2-hydroxyphenyl)-pyrimidine. Complex 1 forms selectively as the rac diastereoisomer upon reaction of [Ir(N<^>C)(2)(mu-Cl)](2) with H2L under mild conditions, with none of the alternative meso isomer being separated. Its structure is confirmed by X-ray diffraction. Complex 1 displays deep-red luminescence in solution or in polystyrene film at room temperature (lambda(em) = 643 nm). Variable-temperature emission spectroscopy uncovers the TADF pathway, involving the thermally activated re-population of S-1 from T-1. At room temperature, TADF reduces the photoluminescence lifetime in film by a factor of around 2, to 1 mu s. The TADF pathway is associated with a small S-1-T-1 energy gap Delta E-ST of approximately 50 meV. Calculations that take into account the splitting of the T-1 sublevels through spin-orbit coupling perfectly reproduce the experimentally observed temperature-dependence of the lifetime over the range 20-300K. A solution-processed OLED comprising 1 doped into the emitting layer at 5 wt% displays red electroluminescence, lambda(EL) = 625 nm, with an EQE of 5.5% and maximum luminance of 6300 cd m(-2).
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| 2. |
- Sil, Milan, et al.
(author)
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Chemical Complexity of Phosphorous-bearing Species in Various Regions of the Interstellar Medium
- 2021
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In: Astronomical Journal. - : American Astronomical Society. - 1538-3881 .- 0004-6256. ; 162:3
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Journal article (peer-reviewed)abstract
- Phosphorus-related species are not known to be as omnipresent in space as hydrogen, carbon, nitrogen, oxygen, and sulfur-bearing species. Astronomers spotted very few P-bearing molecules in the interstellar medium and circumstellar envelopes. Limited discovery of the P-bearing species imposes severe constraints in modeling the P-chemistry. In this paper, we carry out extensive chemical models to follow the fate of P-bearing species in diffuse clouds, photon-dominated or photodissociation regions (PDRs), and hot cores/corinos. We notice a curious correlation between the abundances of PO and PN and atomic nitrogen. Since N atoms are more abundant in diffuse clouds and PDRs than in the hot core/corino region, PO/PN reflects <1 in diffuse clouds, MUCH LESS-THAN1 in PDRs, and >1 in the late warm-up evolutionary stage of the hot core/corino regions. During the end of the post-warm-up stage, we obtain PO/PN > 1 for hot core and <1 for its low-mass analog. We employ a radiative transfer model to investigate the transitions of some of the P-bearing species in diffuse cloud and hot core regions and estimate the line profiles. Our study estimates the required integration time to observe these transitions with ground-based and space-based telescopes. We also carry out quantum chemical computation of the infrared features of PH3, along with various impurities. We notice that SO2 overlaps with the PH3 bending-scissoring modes around similar to 1000-1100 cm(-1). We also find that the presence of CO2 can strongly influence the intensity of the stretching modes around similar to 2400 cm(-1) of PH3.
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| 3. |
- Srivastav, Satyam, et al.
(author)
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Astrochemical model to study the abundances of branched carbon-chain molecules in a hot molecular core with realistic binding energies
- 2022
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In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 515:3, s. 3524-3538
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Journal article (peer-reviewed)abstract
- Straight-chain (normal-propyl cyanide, n - C3H7CN) and branched-chain (iso-propyl cyanide, i - C3H7CN) alkyl cyanides are recently identified in the massive star-forming regions (Sgr B2(N) and Orion). These branched-chain molecules indicate that the key amino acids (side-chain structures) may also be present in a similar region. The process by which this branching could propagate towards the higher order (butyl cyanide, C4H9CN) is an active field of research. Since the grain catalysis process could have formed a major portion of these species, considering a realistic set of binding energies are indeed essential. We employ quantum chemical calculations to estimate the binding energy of these species considering water as a substrate because water is the principal constituent of this interstellar ice. We find significantly lower binding energy values for these species than were previously used. It is noticed that the use of realistic binding energy values can significantly change the abundance of these species. The branching is more favourable for the higher order alkyl cyanides with the new binding energies. With the inclusion of our new binding energy values and one essential destruction reaction (i - C3H7CN + H -> CH3C(CH3)CN + H-2 , having an activation barrier of 947 K), abundances of t - C4H9CN dramatically increased.
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