Nonparallel Stacks of Donor and Acceptor Chromophores Evade Geminate Charge Recombination

We report a nonparallel stacked arrangement of donor–acceptor (D–A) pairs for prolonging the lifetime of photoinduced charge-separated states. Hydrogen–hydrogen steric repulsion in naphthalimide-naphthalene (NIN) dyad destabilizes the planar geometry between the constituent units in solution/ground state. Sterically imposed nonplanar geometry of the dyad allows the access of nonparallel arrangement of the donor and acceptor stacks having triclinic space group in the crystalline state. Antiparallel trajectory of excitons in nonparallel D–A stacks can result in lower probability of geminate charge recombination, upon photoexcitation, thereby resulting in a long-lived charge-separated state. Upon photoexcitation of the NIN dyad, electron transfer from naphthalene to the singlet excited state of naphthalimide moiety results in radical ion pair intermediates that survive >10,000-fold longer in the aggregated state (τ_cr^a > 1.2 ns) as compared to that of monomeric dyad (τ_cr^m < 110 fs), monitored using femtosecond transient absorption spectroscopy

S···π, π–π, and C–H···π Contacts Regulate Solid State Fluorescence in Regioisomeric Bisthiazolylpyrenes

The formation of well-defined packing motifs and control of π–π stacking by rationalizing the effects of molecular substitution in π-conjugated organic materials represent a significant challenge in supramolecular design. To explore the influence of the thiazolyl group in the solid state packing and photophysical properties of pyrene, a new class of three regioisomeric bisthiazolylpyrenes (2-2″TP) were designed and synthesized. Single crystal X-ray structure and quantum theory of atoms in molecules analysis revealed the presence of S···π interactions in 2′TP. Hirshfeld analysis showed the γ and β packing motif in 2TP and 2″TP, respectively, due to extensive π–π interactions, observed S···π interactions led to herringbone packing in 2′TP. The diverse packing arrangement of 2-2″TP led to a notable difference in the fluorescence behavior in the crystalline state. 2′TP possessing S···π interaction showed remarkable enhanced emission in the crystalline state compared to the amorphous state, in contrast to 2TP and 2″TP. Our results provide new insight for the rational design and regulation of intermolecular interactions of conjugated heterocyclic aromatics to achieve a diverse degree of orbital overlap between neighboring chromophore units and thereby favorable optoelectronic properties

<i>ansa</i>-Ferrocene-Incorporated Calixpyrroles and Calixphyrins: Syntheses and Spectral/Structural Characterization

The syntheses and spectral/structural characterization of <i>ansa</i>-ferrocene-incorporated <i>normal</i> calixphyrins and <i>core-modified</i> calixpyrroles and calixphyrins are reported. Acid-promoted dehydrative condensation of 1,1′-bis­(dimethylpyrrolylmethyl)­ferrocene and 2,5-bis­(dimethylhydroxymethyl)­thiophene/furan yielded <i>ansa</i>-ferrocene-based <i>core-modified</i> calixpyrroles, while acid-catalyzed dehydrative condensation of 1,1′-bis­(diphenylpyrrolylmethyl)­ferrocene with the aryl aldehydes and 2,5-bis­(phenylhydroxymethyl)­thiophene followed by DDQ oxidation resulted in the formation of <i>ansa</i>-ferrocene-appended <i>normal</i> and <i>core-modified</i> calixphyrins, respectively. The newly synthesized macrocycles were characterized by FAB-MS, NMR, and UV–vis spectral analyses and finally confirmed by single-crystal X-ray structural analysis. All these studies clearly revealed the introduction of ferrocene in the main framework of the corresponding macrocycles in an <i>ansa</i>-type way. The <i>core-modified</i> calixpyrroles adopt a 1,3-alternate conformation, while the corresponding calixphyrins maintained partial planarity along the tripyrrin plane due to the presence of <i>meso</i> sp² carbon and generated curved staircase conformation. In addition to the intramolecular hydrogen-bonding interactions, calixphyrins generate self-assembled dimers, one- and two-dimensional supramolecular assemblies through intermolecular hydrogen bonding in the solid state