Unexpected transition metal (TM)-π interaction enhanced fluorescence in Cu-π-diborene complexes is a novel phenomenon compared with other metal enhanced fluorescence.In order to discover the mechanism,theoretical investigation was carried out for Cu-π-diborene as well as diborene.Simulation results show the main decay method in diborene and Cu-π-diborene are intal conversion (IC) and fluorescence (FL),respectively.TM-π interaction leads to larger HOMO-LUMO gap of Cu-π-diborene than that of the free diborene,which results in lower IC rates and makes them smaller than the FL rates.At the same time,ISC rates are always smaller than IC and FL rates,which cause enhanced fluorescence of Cu-π-diborene.More interestingly,even though Cu-π-diborene shows enhanced fluorescence,intersystem crossing (ISC) in Cu-π-diborene is enhanced from diborene.The theoretical analysis shows the competition among IC,FL and ISC is the key factor for TM-π interactions enhanced fluorescence,which also shows that cation-π complexes have potential to be used as luminescent probes.