Near-infrared Charge-transfer Research Articles

Modulation of Deep-Red to Near-Infrared Room-Temperature Charge-Transfer Phosphorescence of Crystalline "Pyrene Box" Cages by Coupled Ion/Guest Structural Self-Assembly.

Organic cocrystals obtained from multicomponent self-assembly have garnered considerable attention due to their distinct phosphorescence properties and broad applications. Yet, there have been limited reports on cocrystal systems that showcase efficient deep-red to near-infrared (NIR) charge-transfer (CT) phosphorescence. Furthermore, effective strategies to modulate the emission pathways of both fluorescence and phosphorescence remain underexplored. In this work, we dedicated our work to four distinct self-assembled cocrystals called "pyrene box" cages using 1,3,6,8-pyrenetetrasulfonate anions (PTS4-), 4-iodoaniline (1), guanidinium (G+), diaminoguanidinium (A2G+), and hydrated K+ countercations. The binding of such cations to PTS4- platforms adaptively modulates their supramolecular stacking self-assembly with guest molecules 1, allowing to steer the fluorescence and phosphorescence pathways. Notably, the confinement of guest molecule 1 within "pyrene box" PTSK{1} and PTSG{1} cages leads to an efficient deep-red to NIR CT phosphorescence emission. The addition of fuming gases like triethylamine and HCl allows reversible pH modulations of guest binding, which in turn induce a reversible transition of the "pyrene box" cage between fluorescence and phosphorescence states. This capability was further illustrated through a proof-of-concept demonstration in shrimp freshness detection. Our findings not only lay a foundation for future supramolecular designs leveraging weak intermolecular host-guest interactions to engineer excited states in interacting chromophores but also broaden the prospective applications of room-temperature phosphorescence materials in food safety detection.

Understanding Charge-Transfer Characteristics in Crystalline Nanosheets of Fullerene/(Metallo)porphyrin Cocrystals.

Cocrystals in the form of crystalline nanosheets comprised of C70 and (metallo)porphyrins were prepared by using the liquid-liquid interfacial precipitation (LLIP) method where full control over the morphologies in the C70/(metallo)porphyrins nanosheets has been accomplished by changing the solvent and the relative molar ratio of fullerene to (metallo)porphyrin. Importantly, the synergy of integrating C70 and (metallo)porphyrins as electron acceptors and donors, respectively, into nanosheets is substantiated in the form of a near-infrared charge-transfer absorption. The presence of the latter, as reflection of ground-state electron donor-acceptor interactions in the nanosheets, in which a sizable redistribution of charge density from the electron-donating (metallo)porphyrins to the electron-accepting C70 occurs, leads to a quantitative quenching of the localized (metallo)porphyrin fluorescence. Going beyond the ground-state characterization, excited-state electron donor-acceptor interactions are the preclusion to a full charge transfer featuring formation of a radical ion pair state, that is, the one-electron reduced fullerene and the one-electron oxidized (metallo)porphyrin.

Charge-transfer emission of compact porphyrin–fullerene dyad analyzed by Marcus theory of electron-transfer

A porphyrin–fullerene dyad, which is characterized by a close proximity of the porphyrin donor and the fullerene acceptor, was found to undergo a photoinduced electron transfer both in solutions and in solid films. Near-infrared charge-transfer (CT) emission was observed and analyzed in frame of the semi-classical Marcus electron-transfer theory yielding values for the reaction free energy, −Δ G°=1.75 eV, the internal reorganization energy, λ v=0.05 eV, and the donor–acceptor vibrational energy, hv v=0.14 eV, both in solution and in solid film. The influence of the environment on the CT properties of the dyad is described by a single parameter, the outer-sphere reorganization energy, λ s, which varies from 0.05 eV in non-polar solvents and films to 0.13 eV in solvents of moderate polarity. At low temperatures ( T<200 K), the CT emission consists of distinct bands shifted from each other by value hv v. This is the first direct observation of the vibrational frequencies of a porphyrin–fullerene donor–acceptor system.

Optical Properties of a Supramolecular Assembly Containing Polydiacetylene

We report polarized optical measurements of a new type of single-crystal supramolecular assembly containing isolated chains of polydiacetylene (PDA): poly(1,6-bis(nictonyloxy)-2,4-hexadiyne). Room-temperature spectra reveal a large anisotropy, confirming the low-dimensional nature of the material. In particular, the characteristic optical signature of PDA correlates well with the chain orientation from the X-ray crystal structure. A near-infrared charge-transfer band is also observed, which we attribute to low-energy charge transfer from the polymer backbone to the pyridine rings. Temperature-dependent spectra reveal a weak doublet splitting of the optical bands, indicating a slight distortion of the side chains which contain the pyridine ring at low temperature.

The use of near-infrared charge-transfer transitions of low-spin ferric chlorins in axial ligand assignment

The near-infrared magnetic circular dichroism spectra of some low-spin derivatives of ferric-octaethylchlorin substituted myoglobin have been recorded at cryogenic temperatures. The spectra, which include some of the lowest energy charge-transfer transitions ever observed for hemes, are clearly dependent upon the nature of the axial ligands present. While the results indicate that such spectra may have some practical utility in axial ligand assignment, as is now quite common practice for iron-porphyrin systems, there are some severe practical limitations to this protocol documented in the case of iron-chlorins.

Magnetic-circular-dichroism studies of Escherichia coli cytochrome bo. Identification of high-spin ferric, low-spin ferric and ferryl [Fe(IV)] forms of heme o.

Room-temperature (295 K) magnetic-circular-dichroism spectra at 280-2500 nm have been recorded for Escherichia coli cytochrome bo in its fast form (which has a g = 3.7 EPR signal and reacts rapidly with cyanide) and for its formate, fluoride, cyanide and hydrogen-peroxide derivatives. The spectra of all forms are dominated by signals from low-spin ferric heme b. These include a porphyrin-to-ferric ion charge-transfer transition in the near-infrared region (the near-infrared charge-transfer band) at 1610 nm. High-spin ferric heme o gives rise to a negative magnetic-circular-dichroism feature at 635, 642 and 625 nm (corresponding to a shoulder observed in the electronic absorption spectra) and a derivative charge-transfer feature at 1100, 1180 and 940 nm for the fast, formate and fluoride forms, respectively. The energies of these bands confirm that fluoride and formate are ligands to heme o. The energies of the analogous bands in the spectrum of fast cytochrome bo are typical for high-spin ferric hemes with histidine and water axial ligands. Addition of cyanide ion to fast cytochrome bo causes a red shift in the position of the Soret absorption peak, from 406.5 nm to 413 nm, and results in the loss of the 635-nm feature from the magnetic-circular-dichroism spectrum and of the corresponding shoulder in the electronic absorption spectrum. In the magnetic-circular-dichroism spectrum, the intensities of the Soret and alpha, beta bands are significantly increased. New near-infrared charge-transfer intensity is observed at 1000-2300 nm with a peak near 2050 nm. These changes are interpreted as resulting from a high-spin to low-spin transition at ferric heme o brought about by the binding of cyanide ion. The energy of the near-infrared charge-transfer band suggests that the cyanide ion is bridged to the CuB of the binuclear site. Treatment of fast cytochrome bo with hydrogen peroxide also causes a red shift in the position of the Soret absorbance, to 412 nm, and a loss of the 625-nm absorption shoulder. Changes in the magnetic-circular-dichroism spectrum at 450-600 nm are observed, but there is no significant increase in the intensity of the magnetic-circular-dichroism Soret band and no new near-infrared charge-transfer bands are detected, ruling out a similar high-spin to low-spin transition at heme o.(ABSTRACT TRUNCATED AT 400 WORDS)

The Crystal Spectra of Molecular Compounds of 1,6-Diaminopyrene

Abstract Polarized absorption spectra were observed on the single crystals of molecular compounds which involve 1,6-diaminopyrene as electron donor and p-chloranil, p-bromanil, p-iodanil, and tetracyano-p-quinodimethane as electron acceptor. It is shown that, in spite of the low ionization potential of diaminopyrene 6.56 eV, its solid molecular compounds with chloranil, bromanil, and iodanil are of non-ionic type, and their crystal spectra exhibit the general features characteristic of a typical charge-transfer molecular compound composed of neutral molecules. The diaminopyrene-TCNQ, compound was found to be of ionic type. Although the bromanil and iodanil compounds of diaminopyrene exhibit two charge-transfer bands, in the region below 10 kK and at about 20 kK respectively, the chloranil compound does not show the near-infrared charge-transfer band corresponding to the charge transfer from the highest occupied orbital of the donor to the lowest vacant orbital of the acceptor. A discussion is given for this phenomenon from the overlap between the donor and acceptor orbitals.