Molecular engineering of intensely near-infrared absorbing excited states in highly conjugated oligo(porphinato)zinc-(polypyridyl)metal(II) supermolecules

A new series of chromophores, MPZnn, which combine ethyne-bridged bis(terpyridyl)metal(II)-(porphinato)zinc(II) (MPZn) and oligomeric, ethyne-bridged (porphinato)zinc(II) (PZnn) architectures, have been synthesized and characterized, along with a series of derivatives bearing pyrrolidinyl electron-releasing groups on the ancillary terpyridine units (Pyr(m)MPZn(n)). Cyclic voltammetric studies, as well as NMR, electronic absorption, fluorescence, and femtosecond pump-probe transient absorption spectroscopies, have been employed to study the ground- and excited-state properties of these unusual chromophores. All of these species possess intensely absorbing excited states having large spectral bandwidth that penetrate deep in the near-infrared (NIR) energy regime. Electronic structural variation of the molecular framework shows that the excited-state absorption maximum can be extensively modulated [lambda(max)(T-1 -> T-n)] (880 nm < lambda(max) < 1126 nm), while concomitantly maintaining impressively large T-1 -> T-n absorption manifold spectral bandwidth (full width at half-maximum, fwhm, similar to 2000-2500 cm(-1)). Furthermore, these studies enable correlation of supermolecular electronic structure with the magnitude of the excited-state lifetime (tau(es)) and demonstrate that this parameter can be modulated over 4 orders of magnitude (similar to 1 ns < tau(es) < 45 mu s). Terpyridyl pyrrolidinyl substituents can be utilized to destabilize terpyridyl ligand pi* energy levels and diminish the E-1/2 (M3+/2+) value of the bis(terpyridyl)metal(II) center: such perturbations determine the relative energies of the PZnn-derived (1)pi-pi* and bis(terpyridyl)metal(II) charge-transfer states and establish whether the T-1-state wave functions of MPZnn and Pyr(m)MPZn(n) species manifest the extensive electronic delocalization and charge-separated (CS) features characteristic of long-lived triplet states that absorb strongly in the NIR.