Ligand-to-Ligand Charge-Transfer Transitions of Platinum(II) Complexes with Arylacetylide Ligands with Different Chain Lengths: Spectroscopic Characterization, Effect of Molecular Conformations, and Density Functional Theory Calculations

The complexes [Pt(tBu3tpy){CC(C6H4CC)n−1R}]+ (n=1: R=alkyl and aryl (Ar); n=1–3: R=phenyl (Ph) or Ph‐N(CH3)2‐4; n=1 and 2, R=Ph‐NH2‐4; tBu3tpy=4,4’,4’’‐tri‐tert‐butyl‐2,2’:6’,2’’‐terpyridine) and [Pt(Cl3tpy)(CCR)]+ (R=tert‐butyl (tBu), Ph, 9,9’‐dibutylfluorene, 9,9’‐dibutyl‐7‐dimethyl‐amine‐fluorene; Cl3tpy=4,4’,4’’‐trichloro‐2,2’:6’,2’’‐terpyridine) were prepared. The effects of substituent(s) on the terpyridine (tpy) and acetylide ligands and chain length of arylacetylide ligands on the absorption and emission spectra were examined. Resonance Raman (RR) spectra of [Pt(tBu3tpy)(CCR)]+ (R=n‐butyl, Ph, and C6H4‐OCH3‐4) obtained in acetonitrile at 298 K reveal that the structural distortion of the CC bond in the electronic excited state obtained by 502.9 nm excitation is substantially larger than that obtained by 416 nm excitation. Density functional theory (DFT) and time‐dependent DFT (TDDFT) calculations on [Pt(H3tpy)(CCR)]+ (R= n‐propyl (nPr), 2‐pyridyl (Py)), [Pt(H3tpy){CC(C6H4CC)n−1Ph}]+ (n=1–3), and [Pt(H3tpy){CC(C6H4CC)n−1C6H4‐N(CH3)2‐4}]+/+H+ (n=1–3; H3tpy=nonsubstituted terpyridine) at two different conformations were performed, namely, with the phenyl rings of the arylacetylide ligands coplanar (“cop”) with and perpendicular (“per”) to the H3tpy ligand. Combining the experimental data and calculated results, the two lowest energy absorption peak maxima, λ1 and λ2, of [Pt(Y3tpy)(CCR)]+ (Y=tBu or Cl, R=aryl) are attributed to 1[π(CCR)→π*(Y3tpy)] in the “cop” conformation and mixed 1[dπ(Pt)→π*(Y3tpy)]/1[π(CCR)→π*(Y3tpy)] transitions in the “per” conformation. The lowest energy absorption peak λ1 for [Pt(tBu3tpy){CC(C6H4CC)n−1C6H4‐H‐4}]+ (n=1–3) shows a redshift with increasing chain length. However, for [Pt(tBu3tpy){CC(C6H4CC)n−1C6H4‐N(CH3)2‐4}]+ (n=1–3), λ1 shows a blueshift with increasing chain length n, but shows a redshift after the addition of acid. The emissions of [Pt(Y3tpy)(CCR)]+ (Y=tBu or Cl) at 524–642 nm measured in dichloromethane at 298 K are assigned to the 3[π(CCAr)→π*(Y3tpy)] excited states and mixed 3[dπ(Pt)→π*(Y3tpy)]/3[π(CC)→π*(Y3tpy)] excited states for R=aryl and alkyl groups, respectively. [Pt(tBu3tpy){CC(C6H4CC)n−1C6H4‐N(CH3)2‐4}]+ (n=1 and 2) are nonemissive, and this is attributed to the small energy gap between the singlet ground state (S0) and the lowest triplet excited state (T1). Longer chain always leads to a redshift in energy? No! Platinum(II) complexes bearing terpyridine and acetylide ligands w