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

[Pt(bzq)Cl(CNR)] [bzq = benzoquinolinate; R = tert-butyl (tBu 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]∞) and a yellow form, which contains discrete dimers ([1]2), both stabilized by interplanar π···π (bzq) and short Pt···Pt bonding interactions. Complex 3, generated through the unexpected garnet-red double salt isomer [Pt(bzq)(CN-2-Np)2][Pt(bzq)Cl2] 4, crystallizes as yellow Pt···Pt dimers ([3]2), while 2 only forms π···π (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 1ILCT and 1MLCT/1ML′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 3MLCT unstructured emission only at 77 K, the 1-D form [1]∞ displays a characteristic low-energy red emission (672 nm, 298 K; 744 nm, 77 K) attributed to a mixed 3MMCT [d(σ*)→p(σ)]/3MMLCT [dσ*(M2)→σ(π*)(bzq)] excited state. However, upon exposure to standard atmospheric conditions, [1]∞ shows an irreversible change to an orange-ochre 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 (3LC/3MLCT), 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 3MMLCT 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 3ILCT mixed with metal-to-ligand (L = bzq, L′ = CNR) charge transfer 3MLCT/3ML′CT character with the Pt to CNR contribution increasing from 1 to 3, according to computational studies.