Isoelectronic Pt(ii) complexes of cyclometalating C boolean AND N boolean AND N ligands with phenyl/(benzo)thiophenyl and pyridyl/(benzo)thiazolyl moieties

A series of cyclometalated Pt(ii) complexes [Pt(C<^>N<^>N)X] (X = Cl, C CPh, C CC6F5) was synthesised from the protoligands HC<^>N<^>N containing either phenyl (ph), naphthyl (na) or (benzo)thiophenyl (b(th)) C aryl functions and either pyridyl (py) or (benzo)thiazolyl ((b)tz) peripheral N units, alongside the central 4-phenyl-pyridyl (ppy) or tBu(2)-phenyl-pyridyl (tbppy) N group. Depending on the combination of the peripheral N or C aryl building blocks, these square planar complexes reveal very different electrochemical, UV-vis absorption and emission behaviour. The reversible reductions shift anodically along the series th/py < ph/tz approximate to th/tz < ph/btz while the irreversible oxidations shift cathodically along the series Cl approximate to C CC6F5 < C CPh. Similar trends were observed for the long-wavelength UV-vis absorption and photoluminescence properties. The emission maxima range from 605 to 675 nm at 298 K in CH2Cl2 solution and from 555 to 655 nm at 77 K in glassy frozen CH2Cl2/MeOH matrices. Large differences in amplitude-weighted average lifetimes tau(av) (up to 0.9 mu s at 298 K, up to 12 mu s at 77 K) and photoluminescence quantum yields phi(L) (up to 0.15 at 298 K and up to 0.82 at 77 K) were found. TD-DFT calculations showed that the decomposition of the triplet excited states into LC (pi-pi*, centred in the individual parts of the C<^>N<^>N ligand) and LLCT (pi-pi*, between the individual parts of the C<^>N<^>N ligand + X-pi* from coligand to C<^>N<^>N) contributions for the ligand-centred states as well as MLCT (d(Pt)-to-pi*(C<^>N<^>N)) and LMCT (p(Cl) or pi(CCR)-to-d(Pt)) character for the charge-transfer states involving the metal is beneficial to assess the participation of the individual heteroaryl groups of the C<^>N<^>N ligands. In view of the modular synthesis of these ligands, this will allow the realisation of tailor-made Pt(ii) triplet emitters in future work.