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Cu(I) carbene complexes are common emitters of current emerging interest for next-generation OLED technology based on their encountered thermally activated delayed fluorescence (TADF) properties. However, general molecular design principles for successfully observable TADF properties of such complexes usually rely on π-acidic N-heterocyclic carbene (NHC) ligands, while Cu(I) carbene complexes with purely σ-donating NHC ligands are still scarce. Based on a simple synthetic approach reported by us earlier that offers large-scale access to this class of Cu(I) carbene complexes, we aim to elucidate design principles to enhance their TADF properties. In this work, we combined temperature-dependent time-resolved luminescence spectroscopy on different time scales (ns, μs, ms) with high-level DFT/MRCI calculations on a number of representative Cu(I) NHC complexes with the anionic carbene An6DAC and additional pyridine-derived ligands. Good agreement between experiment and theory related to the electronic nature of the optical transitions, the energy levels of the electronic states and the kinetics of interconversion controlled by the respective energy gaps was found. We conclude that both the electron-rich or electron-poor nature of the pyridine ligands and the coordination geometry (linear, trigonal planar) play a significant role in the outcome of the optical properties of these complexes. While electron-rich pyridine moieties induce decoupled prompt fluorescence and phosphorescence with connected low photoluminescence quantum yields, electron-poor pyridine moieties enhance a "push–pull" effect mediated by Cu(I) that favours TADF properties so that the luminescence intensity is increased by 1–2 orders of magnitude if the complex is trigonal. In the trigonal formyl pyridine complex, theory predicts high MLCT contributions to the electronic wavefunctions, so that the T1 states should exhibit large spin–orbit coupling matrix elements (SOCMEs) with several excited singlet states. Indeed, in temperature dependent luminescence measurements, the electronic states behave as an intimately coupled ensemble. At 270 K, 56% of its luminescence can be attributed to TADF. However, while the luminescence of the trigonal complexes is brighter, their chemical Cu–pyridine bond is more labile. Overall, this study provides a detailed overview of the possibilities of targeted molecular design to selectively address desirable optical properties in organometallic compounds for potential applications in lighting. [ABSTRACT FROM AUTHOR]

The technology of lanthanide-based luminescent thermometers enabling remote optical temperature measurements is intensively developed. Sensors that allow the most precise, remote temperature readouts can be successfully used in biological, electronic, engineering and industrial processes. This work analyses the often-underestimated effect of excitation and detection geometry, as well as the sample thickness on the spectroscopic properties of the material and performance of the resulting luminescent thermometers. In the case of thick-layer samples, a significant change in the luminescence intensity ratios was observed compared to the thin-layer counterparts. This resulted in varying values of absolute and relative sensitivities for different sample thicknesses. The observed changes are assigned to a reabsorption effect, which is particularly significant for the thick-layer samples examined in the transmission (back-illuminated) geometry. The results presented in this work have very important implications on the accuracy of optical temperature monitoring via luminescence thermometry with Er3+ ions. [ABSTRACT FROM AUTHOR]

The photoluminescence properties of the compounds 3,3′-(phenazine-5,10-diyl)dibenzonitrile (mBN) and 4′,4′′′-(phenazine-5,10-diyl)bis(([1,1′-biphenyl]-4-carbonitrile)) (BPN) are presented and compared to those of the known fluorophore 4,4′-(phenazine-5,10-diyl)dibenzonitrile (pBN), which has been reported to show thermally activated delayed fluorescence (TADF). In the solid state, pBN shows clear TADF properties. In contrast, TADF is only weakly pronounced in mBN, and BPN is a conventional fluorescent emitter. This is discussed in terms of the provided through-space overlap between donating phenazinediyl and accepting benzonitrile units in these three molecules, which is only effective in pBN. These compounds are only weakly luminescent in toluene, tetrahydrofuran (THF) or dichloromethane (CH2Cl2) solution, most intensively in toluene, with yellow (pBN and mBN) to orange (BPN) colors of the emission. The emission maxima λem,max in toluene differ slightly between pBN (562 nm) and mBN (572 nm) and overlap in THF (634 nm) and CH2Cl2 (∼660 nm), respectively. Their emission is broad-banded and strongly solvent-dependent and thus indicates a CT-type nature of the excited state. BPN shows weak solvent-dependent photoluminescence (604 nm in toluene, 589 nm in THF, and 587 nm in CH2Cl2) and together with an observable vibronic structure in the low temperature spectra of the powder, it can be concluded that emission in BPN occurs from a localized electronic (LE) state non-beneficial for TADF properties. This demonstrates that even tiny modifications in the molecular templating structure of the phenazines can significantly affect their TADF properties. [ABSTRACT FROM AUTHOR]

Correction for 'How to tune luminescent Cu(I) complexes with strong donor carbenes towards TADF?' by Jasper Guhl et al., J. Mater. Chem. C, 2024, https://doi.org/10.1039/D4TC01487A. [ABSTRACT FROM AUTHOR]

Recent technological developments require knowledge of temperature down to the micro- or even nano-scale. Lanthanide-doped nanoparticles became a popular tool to achieve this. Their temperature sensitive luminescence enables their application as remote thermometers and for mapping temperature profiles with high spatial resolution. Applicability of luminescence thermometry is, however, often limited at high temperatures. In nanoelectronics or chemical reactors, high temperatures above 500 K are common and new approaches for accurate high temperature sensing need to be developed. In this work, we report three different shapes of upconverting LiLuF4:2% Er3+,18% Yb3+ nanocrystals both with and without shells and study the influence of the shell on the thermometric properties. We observed peculiar behavior of the core–shell particles suggesting the presence of the dopants within the protective and 'undoped' shells. Coating the nanoparticles with a silica layer extends the operational temperature range. In an upconversion (UC) Yb3+–Er3+ system temperature sensing relies on thermal coupling between the 4S3/2 and 2H11/2 energy levels. At sufficiently high temperatures (>550 K), we observe additional thermal coupling involving the higher 4F7/2 energy levels. The larger energy gap allows to increase the relative sensitivity at elevated temperatures and to sustain a high temperature precision over a wider temperature range than for a two-level Boltzmann thermometer. The thermal coupling between the 4S3/2 and 2H11/2 energy levels is used for lower temperature sensing (<550 K) and the 4F7/2 energy level is crucial for higher temperature sensing (>550 K). [ABSTRACT FROM AUTHOR]

The photoluminescence of Eu2+ ions doped in iodidoperovskites, CsMI3 (M = Mg, Ca, Sr), was studied. Eu2+ occupies the alkaline earth site in each of these compounds. The emission spectra of the doped perovskites are characterized by 4f65d1→ 4f7 transitions of Eu2+ located in the yellow range for CsMgI3 and the blue range for CsCaI3 and CsSrI3, respectively. All excitation spectra provide a well-resolved fine structure arising from different 7FJ (J = 0,…,6) levels of the 4f6 core of the excited state configuration, indicating a weak exchange interaction with the 5d electron. The decay times and temperature dependence of the luminescence were also analyzed. Both CsMgI3:Eu2+ and CsSrI3:Eu2+ show a redshifted emission with respect to CsCaI3:Eu2+. This is explained by trigonal distortions of the [EuI6]4− octahedra in the former two compounds. The energetic order of the resulting crystal field states is justified in terms of the angular overlap model of ligand field theory. This series of Eu2+ doped earth alkaline iodidoperovskites may serve as a textbook example for the detailed understanding of the structure–luminescence relationships of Eu2+ ions, which is very important for tailoring functional materials for respective applications. [ABSTRACT FROM AUTHOR]