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The photophysical properties of a series of the four-coordinate N-heterocyclic carbene (NHC) Cu(I) complexes are investigated. The present of the methyl substituent at the α-position of the pyridine ring in complexes effectively suppresses the structural distortion of the excited states and thus weakens the structural relaxation occurred from the T1 to the S0 states, leading to the higher PLQY.
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Seven luminescent copper(I) complexes bearing four-coordinate N-heterocyclic carbene (NHC) ligands with varying electron-withdrawing substituents including –CF3, –CN, –COCH3, and –CHO groups at the pyridine ring part of the carbene are reported in this study. P1-P4 without the methyl group show better light absorption in low-energy region compared with P5-P7 with the methyl group at the α-position of the pyridine ring. The emitting state of all the complexes is regarded as the 3MLCT/3LLCT character. P1-P4 exhibit the emission maximum in the range of 537–547 nm with the photoluminescence quantum yields (PLQY) of 12.7–21.9% in PMMA films and excited state lifetimes (τ) of 14.7–22.6 μs. The emission wavelengths of P5-P7 are blue-shifted to 505–513 nm compared with those of P1-P4, showing the higher PLQY of 38.0–57.3% and longer τ of 36.2–84.8 μs. Theoretical calculations were used to rationalize the photophysical properties.