Experimental and Computational Study on Photophysical Properties of Mesoionic Chalcogenones

Abstract

N‐Heterocyclic carbene adducts with main group elements (NHC=E) have aroused great interest and have been widely investigated in coordination chemistry. Among them, N‐heterocyclic carbene adducts with chalcogens (NHC=Ch) have been known for a long time. Their investigations mostly focused on synthesis, coordination chemistry and electrochemistry. Their photophysical properties still remain unexplored. In this work, the photophysical properties of mesoionic carbene adducts with sulfur and selenium have been investigated both in solution and solid state. These compounds showed blue fluorescence in dichloromethane. While in solid state, orange to red room‐temperature phosphorescence can be observed, and dual emission was found in mesoionic thiones. Furthermore, time‐dependent density functional theory (TD‐DFT) calculations were used to obtain insights into the luminescent mechanism.

Synthesis, Characterization and Photophysical Properties of Mesoionic N‐Heterocyclic Imines

“…Recently our group investigated the photophysical properties of MICderived chalcogenones (MIC=Ch). [20] We found that MIC=Ch exhibited very low QYs in DCM solution, which is between 0.13% and 4.0%. The totally difference in QYs between 4-6 and MIC=Ch can be ascribed to the various transition mode from S0 Scheme 2.…”

“…The length of C2−N1 is 1.296 Å, which is identical with that of its analogs, imidazolin‐2‐imine (1.296 Å), [12] and it is in the range of length of partial double bonds to length of single bond (1.279 Å to 1.465 Å), [13] according with the calculated Wiberg bond order (1.78). The bond length of C1−C2 is 1.430 Å, which is longer than other MICs and their derivatives, such as triazolium (1.357 Å to 1.387 Å), [14–17] N ‐heterocyclic olefins (mNHOs) (1.417 Å to 1.418 Å) [18–19] and mesoionic chalcogenones (MIC=Ch) (1.398 Å to 1.412 Å) [20] . This is due to the strong electronegativity of exocyclic nitrogen atom as well as the π lone pair electrons on of the exocyclic nitrogen atom, which results in a more delocalized π system.…”

“…These emission bands were all associated to the corresponding S1 to S0 transition. Different from MIC=Ch (Ch=S and Se), [20] where there was only very small oscillator strength value of the transition from S1 to S0, there was considerable oscillator strength value for 4 – 6 , that was 0.262 for 4 , 0.295 for 5 and 0.299 for 6 .The reason was that the substituted group at C4 can strongly couple with the triazole ring as well as the exocyclic nitrogen atom and thus formed an enhanced delocalized π system in 4 – 6 . While that was absent in the MIC=Ch systems.…”

“…The bond length of C1À C2 is 1.430 Å, which is longer than other MICs and their derivatives, such as triazolium (1.357 Å to 1.387 Å), [14][15][16][17] N-heterocyclic olefins (mNHOs) (1.417 Å to 1.418 Å) [18][19] and mesoionic chalcogenones (MIC=Ch) (1.398 Å to 1.412 Å). [20] This is due to the strong electronegativity of exocyclic nitrogen atom as well as the π lone pair electrons on of the exocyclic nitrogen atom, which results in a more delocalized π system. And also the phenyl group at C4 position makes contribution to the more delocalized π system since the dihedral of C2À C1À C3À C4 is 171.3°.…”

“…Note that 4 – 6 exhibited high quantum yields in DCM (QY, 29.0% for 4 , 43.8% for 5 and 29.4% for 6 , respectively), while N ‐methyl mNHI 7 , which was prepared by methylation of 4 , displayed green fluorescence with the quantum yields of only 2.6% (Figure S1‐S2). Recently our group investigated the photophysical properties of MIC‐derived chalcogenones (MIC=Ch) [20] . We found that MIC=Ch exhibited very low QYs in DCM solution, which is between 0.13% and 4.0%.…”