Large Hyperpolarizabilities at Telecommunication-Relevant Wavelengths in Donor–Acceptor–Donor Nonlinear Optical Chromophores

Abstract

Octopolar D2-symmetric chromophores, based on the MPZnM supermolecular motif in which (porphinato)zinc(II) (PZn) and ruthenium(II) polypyridyl (M) structural units are connected via ethyne linkages, were synthesized. These structures take advantage of electron-rich meso-arylporphyrin or electron-poor meso-(perfluoroalkyl)porphyrin macrocycles, unsubstituted terpyridyl and 4′-pyrrolidinyl-2,2′;6′,2″-terpyridyl ligands, and modulation of metal(II) polypyridyl-to-(porphinato)zinc connectivity, to probe how electronic and geometric factors impact the measured hyperpolarizability. Transient absorption spectra obtained at early time delays (tdelay < 400 fs) demonstrate fast excited-state relaxation, and formation of a highly polarized T1 excited state; the T1 states of these chromophores display expansive, intense T1 → Tn absorption manifolds that dominate the 800–1200 nm region of the NIR, long (μs) triplet-state lifetimes, and unusually large NIR excited absorptive extinction coefficients [ε(T1 → Tn) ∼ 105 M–1 cm–1]. Dynamic hyperpolarizability (βλ) values were determined from hyper-Rayleigh light scattering (HRS) measurements, carried out at multiple incident irradiation wavelengths spanning the 800–1500 nm spectral domain. The measured βHRS value (4600 ± 1200 × 10–30 esu) for one of these complexes, RuPZnRu, is the largest yet reported for any chromophore at a 1500 nm irradiation wavelength, highlighting that appropriate engineering of strong electronic coupling between multiple charge-transfer oscillators provides a critical design strategy to realize octopolar NLO chromophores exhibiting large βHRS values at telecom-relevant wavelengths. Generalized Thomas–Kuhn sum (TKS) rules were utilized to compute the effective excited-state-to-excited-state transition dipole moments from experimental linear-absorption spectra; these data were then utilized to compute hyperpolarizabilities as a function of frequency, that include two- and three-state contributions for both βzzz and βxzx tensor components to the RuPZnRu hyperpolarizability spectrum. This analysis predicts that the βzzz and βxzx tensor contributions to the RuPZnRu hyperpolarizability spectrum maximize near 1550 nm, in agreement with experimental data. The TKS analysis suggests that relative to analogous dipolar chromophores, octopolar supermolecules will be likely characterized by more intricate dependences of the measured hyperpolarizability upon irradiation wavelength due to the interactions among multiple different β tensor components.

Engineering opposite electronic polarization of singlet and triplet states increases the yield of high-energy photoproducts

“…1B). Two-and three-level models for β λ (6,(9)(10)(11)(12), along with energetic considerations of the CT transitions (6), support these assignments (SI Appendix). Indeed, the dipole magnitudes of S 1 , S 2 , and S 3 are substantial, calculated to span a 13-43-D range (12).…”

“…The unusual frequency dispersion of the RuPZn molecular hyperpolarizability, highlighted in part by the large dynamic hyperpolarizability (β λ ) at 1,300-nm incident irradiation ( Fig. 1B), and the small β λ at 800 nm, indicate that S 1 has an electric dipole moment that is antiparallel to that of S 2 and S 3 (6,(9)(10)(11)(12). S 2 and S 3 feature electron density pushed from Ru toward PZn, while S 1 features electron density oppositely polarized from PZn toward Ru (Fig.…”

Excitation energy-dependent photocurrent switching in a single-molecule photodiode

“…1 differ markedly from those characteristics of individual (porphyrinato)zinc(II) (abbreviated as Zn) and bis(terpyridyl)ruthenium(II) (abbreviated as Ru 2+ ) complexes. In (PhZnRu) 2+ and (RuZn) 2+ , extensive mixing of PZn B-and Q-electronic states with ruthenium terpyridyl metalligand CT states gives rise to supramolecular chromophores that manifest high oscillator strength absorptive manifolds that provide an unusual degree of spectral coverage of the 250-to 750-nm wavelength domain (19)(20)(21)(22)(23)(24). Note that (PhZnRu) 2+ is anchored…”

“…S21. The precursors, ZnE′ (21), TerpyBr (22), ZnTerpy (20), ZnRuCl 3 (33), and terpyPO 3 H 2 (33), were prepared based on the reported procedures (20)(21)(22)33). ZnRuCl 3 (138.9 mg, 0.1 mmol), terpyPO 3 H 2 (31.3 mg, 0.1 mmol), triethylamine (4 mL), and ethanol (5 mL) were added in N-methylpyrrolidone (30 mL).…”

“…1B are ethyne-bridged (porphyrinato)zinc(II) and bis(terpyridyl)ruthenium(II) complexes with carboxylic or phosphonic acid groups for surface binding to nanoITO electrodes. This connectivity aligns the bis(terpyridyl) ruthenium(II) and (porphyrinato)zinc(II) transition dipoles in a head-to-tail arrangement, enforcing extensive excited-state electronic communication between these units and driving significant charge-transfer (CT) character in the chromophore's three lowenergy, singlet excited-state manifolds (19)(20)(21)(22)(23)(24). The two chromophores on nanoITO exhibit frequency-dependent, excited-state CT characters at their excited states.…”