Preferential Solvation Unveiled by Anomalous Conformational Equilibration of Porphyrin Dimers: Nucleation Growth of Solvent–Solvent Segregation

Preferential solvation was explored using ethynylene- or butadiynylene-linked porphyrin dimers bearing 3,4,5-tri­((<i>S</i>)-3,7-dimethyloctyloxy)­phenyl groups at the meso positions in binary hexafluorobenzene (C₆F₆) and cyclohexane (C₆H₁₂) mixture, expecting contrasting solvent affinity of the porphyrin core and the alkyl side chains toward the individual solvent component. Although the solvent polarity remained nearly constant along with the continuous variation of the solvent composition, the porphyrin dimer showed dramatic change in spectroscopic signatures, indicating the occurrence of preferential solvation. Because of small rotational barrier around the ethynylene and butadiynylene linkage, the torsional conformations of the porphyrin dimers varied from orthogonal to planar due to continuous variation of molar fraction of C₆H₁₂–C₆F₆ mixture. Thorough thermodynamic analyses inferred that nucleation as the enthalpic component and phase segregation as the entropic component operated preferential solvation. The porphyrin dimer nucleated the C₆H₁₂–C₆F₆ segregation, and the torsional conformation was diagnostic of the inversion of the interfacial curvature of the solvent segregation along with the continuous variation

Porphyrins Sheathed in Quadrupolar Solvation Spheres of Hexafluorobenzene: Solvation-Induced Fluorescence Enhancement and Conformational Confinement

Hexafluorobenzene (C₆F₆) strongly solvated the porphyrin ring via a quadrupolar interaction. The solvation sphere of C₆F₆ hindered the thermal fluctuations near the porphyrin ring and evoked remarkable photoelectronic properties of the porphyrins such as fluorescence enhancement and spectral sharpening due to confined torsional planarity

Highly Fluorescent Slipped-Cofacial Phthalocyanine Dimer as a Shallow Inclusion Complex with α-Cyclodextrin

Supramolecular control of the π-stacked configuration of aqueous phthalocyanine (Zn­[Pc­(SO₃)₄]) was achieved, allowing organization of a J-type slipped-cofacial dimer with per-<i>O</i>-methylated α-cyclodextrin (TMe-α-CDx) by the aid of host–guest interactions. Pristine Zn­[Pc­(SO₃)₄] forms nonfluorescent face-to-face aggregates in water. The π-stacked configuration was controlled in the slipped-cofacial dimer, which was formed as a shallow inclusion complex with TMe-α-CDx, giving remarkably enhanced fluorescence with a very small Stokes shift. Organization of the J-type slipped-cofacial dimer as a 2:2 Zn­[Pc­(SO₃)₄]–TMe-α-CDx complex was achieved through π-stacking of the unencapsulated segment of Zn­[Pc­(SO₃)₄] shallowly encapsulated by a small TMe-α-CDx cavity

Group 14 Dithienometallole-Linked Ethynylene-Conjugated Porphyrin Dimers

The considerably conjugated π systems of the group 14 dithienometallole-linked ethynylene-conjugated porphyrin dimers (<b>1M</b>s) were described based on comprehensive experimental and theoretical studies. The electronic absorption spectra of <b>1M</b> displayed a large splitting in the Soret band and a red-shifted Q-band, indicating that the dithienometallole spacer was effective in facilitating the porphyrin–porphyrin electronic coupling. Torsional planarization behaviors of <b>1M</b> were observed in the time-resolved fluorescence spectra. Density functional theory (DFT) calculations revealed that the dithienometallole spacer is an ideal partner for the ethynylene-conjugated porphyrin to produce fully delocalized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels due to their similar HOMO and LUMO levels. Finally, <b>1M</b> exhibited a strong propensity for the quinoidal–cummulenic conjugation in the dithienometallole spacer when in a photoexcited state

Metallic Lustrous Porphyrin Foil with an Exceptional Refractive Index

Metal-lustrous porphyrin foils, self-standing films of fully π-conjugated polymers composed of a glassy porphyrin unit bearing the elastic 3,4,5-tri((S)-dihydrocitronellyloxy)phenyl groups, are disclosed. A remarkable refractive index of 3.04 due to the anomalous dispersion effect of the intense resonance absorption was found for the porphyrin foil of a polymer with a meso-to-meso 1,4-diethynylphenylene-linked backbone (1). Due to the anomalous dispersion effect, the sharp contrast between the intense absorption and specular reflection at the smooth glassy porphyrin foils provides a brilliant noniridescent green metallic luster. On the other hand, the porphyrin foil of a 1,4-diethynyl-2,3,5,6-tetrafluorophenylene-linked polymer with a conformation-locked conjugated backbone via the C–F···H–C interaction between and the edge-faced porphyrin β-protons (2) results in an insignificant refractive index presumably because of the low dielectric constant of fluorine. Nevertheless, the enlarged π-conjugated domain of 2 forms a stable ground-state biradical, while the open-shell character of 1 is moderate. The study reveals that brilliant metallic luster manifests the considerably delocalized π-system in a relatively bulk and/or macroscopic context rather than the molecular-scale properties

Fully Conjugated Porphyrin Glass: Collective Light-Harvesting Antenna for Near-Infrared Fluorescence beyond 1 μm

Expanded π-systems with a narrow highest occupied molecular orbital–lowest unoccupied molecular orbital band gap encounter deactivation of excitons due to the “energy gap law” and undesired aggregation. This dilemma generally thwarts the near-infrared (NIR) luminescence of organic π-systems. A sophisticated cofacially stacked π-system is known to involve exponentially tailed disorder, which displays exceptionally red-shifted fluorescence even as only a marginal emission component. Enhancement of the tail-state fluorescence might be advantageous to achieve NIR photoluminescence with an expected collective light-harvesting antenna effect as follows: (i) efficient light-harvesting capacity due to intense electronic absorption, (ii) a long-distance exciton migration into the tail state based on a high spatial density of the chromophore site, and (iii) substantial transmission of NIR emission to circumvent the inner filter effect. Suppression of aggregation-induced quenching of fluorescence could realize collective light-harvesting antenna for NIR-luminescence materials. This study discloses an enhanced tail-state NIR fluorescence of a self-standing porphyrin film at 1138 nm with a moderate quantum efficiency based on a fully π-conjugated porphyrin that adopts an amorphous form, called “porphyrin glass”