The Roles of Molecular Structure and Effective Optical Symmetry in Evolving Dipolar Chromophoric Building Blocks to Potent Octopolar Nonlinear Optical Chromophores

Abstract

A series of mono-, bis-, tris-, and tetrakis-(porphinato)zinc(II) (PZn)-elaborated ruthenium(II) bis(terpyridine) (Ru) complexes has been synthesized in which an ethyne unit connects the macrocycle meso carbon atom to terpyridyl (tpy) 4-, 4′-, and 4″-positions. These supermolecular chromophores, based on the ruthenium(II) [5-(4′-ethynyl-(2,2′;6′,2″-terpyridinyl))-10,20-bis(2′,6′-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2′;6′,2″-terpyridine) 2+ bishexafluorophosphate (RuPZn) archetype, evince strong mixing of the PZn-based oscillator strength with ruthenium terpyridyl charge resonance bands. Potentiometric and linear absorption spectroscopic data indicate that for structures in which multiple PZn moieties are linked via ethynes to a [Ru(tpy) 2 ] 2+ core, little electronic coupling is manifest between PZn units, regardless of whether they are located on the same or opposite tpy ligand. Congruent with these experiments, pump-probe transient absorption studies suggest that the individual RuPZn fragments of these structures exhibit, at best, only modest excited-state electronic interactions that derive from factors other than the dipole-dipole interactions of these strong oscillators; this approximate independent character of the component RuPZn oscillators enables fabrication of NLO multipoles with extraordinary hyperpolarizabilities.Dynamic hyperpolarizability (β λ ) values and depolarization ratios (ρ) were determined from hyper-Rayleigh light scattering (HRS) measurements carried out at an incident irradiation wavelength (λ inc ) of 1300 nm. The depolarization ratio data provide an experimental measure of chromophore optical symmetry; appropriate coupling of multiple charge-transfer oscillators produces structures having enormous averaged hyperpolarizabilities (β HRS values), while evolving (PF 6 ) 2 possessing a β HRS value at 1300 nm more than a factor of three larger than that determined for any chromophore having octopolar symmetry examined to date. Because NLO octopoles possess omnidirectional NLO responses while circumventing the electrostatic interactions that drive bulk-phase centrosymmetry for NLO dipoles at high chromophore concentrations, the advent of octopolar NLO chromophores having vastly superior β HRS values at technologically important wavelengths will motivate new experimental approaches to achieve acentric order in both bulk-phase and thin film structures.

Near IR nonlinear absorption of an organic supermolecule [Invited]

“…1. By molecular design, the rigid ethynyl π-linkers drive extensive mixing of the (porphinato)zinc-based π-π* and bis(terpyridyl)osmium charge-resonance states, giving rise to a variety of low-energy electronic transitions that feature significant absorption oscillator strengths [12,13,15,20,23], as seen in Fig. 2(a).…”

“…The derived ESA spectrum has peaks near 1000 and 1150 nm, with large cross sections exceeding 10 −16 cm 2 throughout much of the range. This ESA arises from an extensively delocalized triplet state that features substantial charge-separated character [12,13,15,20,23] and has been shown to exhibit long-lived decay kinetics (τ > 900 ns). The ultrafast rise time is shown in Fig.…”

“…Molecular engineering of chromophores has been one of the major pathways that has been exploited to achieve stronger nonlinear absorption [1,9,[11][12][13][14][15][16][17][18]. Recently we have shown that nonlinear absorption can also be enhanced via incorporation of nonlinear materials into multimode waveguides [2,19].…”

“…Among the many known strong nonlinear absorbers, several possess strong two-photon absorption (TPA) in the near IR. If the direct excitation to a two-photon allowed state is followed by intersystem crossing to a long-lived charge-transfer, excited singlet and/or triplet state that absorbs in similar near IR wavelength regions, a strong nonlinear absorption is produced [4,12,13,15,18,[20][21][22]. The simultaneous presence of TPA and excited state absorption (ESA) leads to strong nonlinear optical response in the near IR.…”

Watching energy transfer in metalloporphyrin heterodimers using stimulated X-ray Raman spectroscopy

“…Because of their stability and interesting structural, electronic, and optical properties, porphyrin compounds have a wide range of uses as chemical sensors (5), photosensitizers in photodynamic therapy for cancer (6), nonlinear optical materials (7)(8)(9), and molecular electronic (10)(11)(12) and spintronic devices (13,14).…”