Dual emission behavior of phenyleneethynylene gold(I) complexes dictated by intersystem crossing: A theoretical perspective

• Calculations for model molecules prove to be reliable by comparison with experimental values.
• The nature of absorption and dual emission spectra is analyzed.
• The consecutive decrease of kISC in experiment is correlated with ΔEST and length of PE ligand.
• The nonadiabatic decay pathway after photoexcitation is described in detail.

In commonly studied gold(I) complexes with oligo (o-, p-, or m-phenyleneethynylene) (PE) ligands, an intriguing photophysical behavior is dual emission composed of fluorescence from S1 and phosphorescence from T1 which is dictated by effective intersystem crossing (ISC) process. In order to explore the salient photodynamics of such oligo-PE gold(I) complexes effectively, we have deliberately chosen three model complexes, namely, PhCCAu(PMe3) (1a′) and PhCC(1,m)C6H4CCAu(PMe3) (m = 4, 2a′; m = 3, 3a′) in place of the real system. Firstly, electronic structure methods based on DFT and TD-DFT are utilized to perform optimization calculations for the ground- and lowest-lying excited states, respectively. Next, basic photophysical properties including absorption and emission spectra are investigated by TD-DFT under the optimized geometries. Besides, on the basis of the electronic spectra herein, we succeed in searching for surface intersections as the minima on the seam of singlet–triplet surface crossings (SCs) at the CASSCF level of theory. By integration of the results available, the process of delayed fluorescence of triplet–triplet annihilation (TTA) and phosphorescence was displayed in detail with SCs playing the lead in monitoring the ISC.

Presentation of the photo-induced decay mechanism for (a) 1a′ and (b) 2a′ suggested by the present quantum-chemical calculations. (Abs: Absorption; PF: Prompt fluorescence; DF: Delayed fluorescence; Ph: Phosphorescence; VR: Vibrational relaxation; ISC: Intersystem crossing; IC: Internal conversation; S/T-SC: Surface intersection on the crossing seam of singlet-triplet excited PES).Figure optionsDownload as PowerPoint slide