Structural and Luminescence Studies on π•••π and Pt•••Pt Interactions in Mixed Chloro-lsocyanide Cyclometalated Platinum(II) Complexes

[Pt(bzq)CI(CNR)] [bzq = benzoquinolinate; R = tert-butyl (Bu-1 1), 2-6-dimethylphenyl (Xyl 2), 2-naphthyl (2-Np 3)] complexes have been synthesized and structurally and photophysically characterized. 1 was found to co-crystallize in two distinct pseudopolymorphs: a red form, which exhibits an infinite 1D-chain ([1](infinity)) and a yellow form, which contains discrete dimers ([1](2)), both stabilized by interplanar pi center dot center dot center dot pi (bzq) and short Pt center dot center dot center dot Pt bonding interactions. Complex 3, generated through the unexpected garnet-red double salt isomer [Pt(bzq)(CN-2-Np)(2)][Pt(bzq)Cl-2] 4, crystallizes as yellow Pt center dot center dot center dot Pt dimers ([3](2)), while 2 only forms pi center dot center dot center dot pi (bzq) contacting dimers, Their electronic absorption and luminescence behaviors have been investigated. According to Time-Dependent Density Functional Theory (TD-DFT) calculations, the lowest-lying absorption (CH2Cl2) has been attributed to combined (ILCT)-I-1 and (MLCT)-M-1/(ML)-M-1'CT (L = bzq, L' = CNR) transitions, the latter increasing from 1 to 3. In solid state, while the yellow form [1](2) exhibits a green (MLCT)-M-3 unstructured emission only at 77 K, the 1-D form [1](infinity) displays a characteristic low-energy red emission (672 nm, 298 K; 744 nm, 77 K) attributed to a mixed 3MMCT [d(sigma*) p(sigma)]/(MMLCT)-M-3 [d sigma*(M-2)->sigma(pi*)(bzq)] excited state. However, upon exposure to standard atmospheric conditions, Ii],.. shows an irreversible change to an orangeochre solid, whose emissive properties are similar to those of the crude 1. Complexes 2 and 3 (77 K) exhibit a structured emission from discrete fragments ((LC)-L-3/(MLCT)-M-3), whereas the luminescence of the garnet-red salt 4 is dominated by a low energy emission (680 nm, 298 K; 730 nm, 77 K) arising from a (MMLCT)-M-3 excited state. Solvent (CH2Cl2, toluene, 2-MeTHF and CH3CN) and concentration-dependent emission studies at 298 K and at 77 K are also reported for 1-3. In CH2Cl2 solution, the low phosphorescent emission band is ascribed to bzq intraligand charge transfer (ILCT)-I-3 mixed with metal-to-ligand (L = bzq, L' = CNR) charge transfer (MLCT)-M-3/(ML)-M-3'CT character with the Pt to CNR contribution increasing from 1 to 3, according to computational studies.