O2-induced emission color changing/sensing via the combination of red-emitting COF and blue-emitting halogen-bridging Cu(I) phosphor: In-situ synthesis and performance

Covalent organic frameworks (COFs) have shown virtues of structural diversity and porous diversity, due to their versatile organic building blocks. These advantages make COFs a promising host for O2 gas sensors. In this work, a porous COF (denoted as COF-X, X = Br or Cl) having halogen ions (Br or Cl ions) in its micropores was firstly synthesized as host, then Cu clusters [Cu2(PPh3)3X2] were in-situ synthesized. The obtained composite structure was discussed in detail focusing on its photophysical parameters and optical oxygen sensing performance. Probe doping ratio was determined as 2:1 (C162H120N18O12X4@C54H45X2Cu2P3, X = Br or Cl). Host COF-X showed red emission around 628 nm, while probe Cu2(PPh3)3X2 exhibited blue emission around 436 nm with lifetime of 5.21 μs and quantum yield of 0.41 under pure N2 atmosphere. Upon pure O2 atmosphere, probe Cu2(PPh3)3Br2 lifetime and quantum yield were decreased to 1.16 μs and 0.09. Probe emission intensity followed a linear calibration curve with maximum sensitivity of 20.25 (a.u.) and response/recovery time of 9/41 s. This sensitivity was by far the highest one among phosphorescent Cu(I)-based sensors. The relative emission intensity variation between dopant blue emission and host red emission led to an emission color change between blue (under pure N2 atmosphere) and red (under pure O2 atmosphere).