Expanding the Family of Fluorescent Coumarin[4]arenes: Improved Synthesis, π‐Extension, and Characterization

Abstract

A novel series of bicyclic fluorescent heterocalixarenes – C‐alkyl coumarin[4]arenes – with built‐in chirality has been synthesized successfully in excellent yields under well‐optimized, improved reaction conditions. Further, structural elaboration (including π‐extension) of coumarin[4]arenes has been demonstrated by using Pd‐catalyzed Suzuki and Sonogashira cross‐coupling methods. All of these macrocycles are shown to possess averaged C4‐symmetric conformation in solution as opposed to the coumarin[4]arene tetratriflate derivative of C2 symmetry in the solid state. Finally, the macrocyclic coumarin[4]arenes reveal UV/near‐UV absorption and visible blue emission in solution. Such a visible blue luminescence of this macrocycle highlights its potential for sensing, bioimaging, and light‐emissive applications.

Leveraging Torsional and Steric Strains: A Pre‐macrocyclization Strategy Enables Conformation‐Specific Fullerene Binding in m‐Cyclophanes

“…[4a, 8d, 23] The variable low-temperature (up to À 90 °C) 1 Can the α-branched C-alkyl substituents induce conformation locking in the hydrogen-bonding-free arm-extended m-cyclophanes as well? To answer this, we synthesized dissymmetric α-branched CÀ iPr 5 e and CÀ Cy 5 f coumarin- [4]arenes (CAs as racemates up to 93 % yield, Scheme 1 and Pages S6-S7) from the corresponding TMRs 2 e-f. We followed our optimized two-step method developed for 5 b-d. [9] Delightfully, the 1 H NMR signals of CAs 5 e and 5 f did not split up to À 90 °C (Figures 2B and S30) attesting to their conformational rigidity. On the contrary, 5 b-c (Figures S27-S28) and 5 d [9a] were all conformationally flexible.…”

“…We first scrutinized the conformational behavior of hydrogen-bonded TMRs in solution. The CÀ Me (2 a), [20] CÀ Et (2 b), [21] CÀ Pr (2 c), [21] and CÀ iBu (2 d) [9,22] TMRs displayed a C 4 -symmetric conformation at room temperature (RT). The mass spectral analysis in conjunction with S4) also revealed a same scenario.…”

“…[3a, 7] When resorcinarenes are devoid of intramolecular hydrogen bonding (Figure 1C), they become fluxional (conformationally dynamic) like m-cyclophanes. [4a, 8] For example, octamethoxyresorcinarene (OMR, 4) [4a] and coumarin [4]arene (CA, 5) [9] exhibit a boat conformation in the solid state and a fast boat-to-boat conformation interconversion in solution.…”

Fluorescent cyclophanes and their applications

“…In 2018, Kumar and Venkatakrishnan 109 introduced a new kind of chalice-like macrocycle whose structure is similar to resorcinarenes. These cyclophanes are based on coumarin moieties linked by methylene bridges and are intrinsically luminescent because of the fluorescent units present in their cores.…”

Six-membered ring systems: with O and/or S atoms

“…A series of C-alkyl coumarin[4]arenes and various derivatives have been synthesized and their fluorescent properties studied (19EJO7787). The importance of the solvent and charge of the xanthene moiety on their photophysical properties was studied in pyridyl and N-methylpyridinium analogues of rosamines (19CEJ15073).…”