The sample of conventional yellow PCN (Y-CNx) was prepared by thermal polycondensation of 30 g melamine at 530 ºC for 4h in a lid-covered crucible. The red PCN materials (R-CNx) were prepared by a heat treatment of Y-CNx in a quartz tube initially having a reduced pressure of 1.8 × 10 -2 mbar using a tube furnace operating at 900 C for 3 min. After the treatment the samples were cooled down, brought to ambient pressure and powdered.