Conformational switching via an intramolecular H-bond modulates the fluorescence lifetime in a novel coumarin–imidazole conjugate

Abstract

Achieving synthetic control over light-driven molecular dynamics is essential for designing complex molecule-based devices. Here we design a novel coumarin-imidazole conjugate (1) whose excited state structural dynamics are primarily controlled by a distant intramolecular H-bonding interaction within the backbone. The coumarin conjugate is based on a 1,2,4,5-aryl substituted imidazole framework (aryl = -Ph and -PhOH) covalently connected to the coumarin moiety via a C-N bond. A carefully positioned OH group in the aryl part of the imidazole fragment resulted in achieving two dissimilar O-HN and O-HO distal intramolecular hydrogen bonding interactions. NMR studies in conjunction with density functional theory (DFT) at the B3LYP/6-311G(d,p) level of theory show the existence of two ground state conformers with a rotational barrier of 6.12 kcal mol. Due to the presence of conformational isomers of 1, the local excited state dynamics of the parent coumarin get biased towards a long-lived fluorescence state with diminished non-radiative decay channels. Time-resolved emission studies show an ∼4-5 times increase in the excited state lifetime in 1 when compared to coumarin-imidazole conjugates, 2 and 3, without the OH group. Solvent dependent studies show that solvent polarity, the H-bond donating ability and viscosity dictate the conformational distribution in the ground state and the dynamical evolution to the final emissive state. Our studies highlight the importance of rotamerism around the C1-C4 single bond, which leads to rigidification along the coumarin-imidazole backbone through a combination of distal H-bonding and solvent interactions. The concept of new emission signaling pathways caused by conformational switching between two states offers a new paradigm to introduce functional allostery in macromolecular backbones.

Room-Temperature Orange-Red Phosphorescence by Way of Intermolecular Charge Transfer in Single-Component Phenoxazine–Quinoline Conjugates and Chemical Sensing

“…In the absence of molecular oxygen, a similar red-shifted emission band was observed at RT, indicating a triplet state that is formed via I 2 CT transition. Moreover, positive solvatochromism 25,52 effects and broad emission feature (Figure S23) suggest that the conjugates show RTORP via CT triplet state. Furthermore, the observation of a single well-defined isosbestic point at 505, 503, and 516 nm for PQ1, PQ2, and PQ3, respectively, indicates the presence of a thermodynamic equilibrium between two species that can be clearly ascribed to the respective monomer and dimer.…”

Biluminescence via Fluorescence and Persistent Phosphorescence in Amorphous Organic Donor(D4)–Acceptor(A) Conjugates and Application in Data Security Protection

“…The absorption characteristics in dilute toluene solution (10 –5 M) are very similar for both POP and TOP , with absorption at 280 and 320 nm, which could be assigned to the π–π* and ICT transitions, respectively (Figure S3). Photoluminescence (PL) spectra of both compounds (λ ex = 320 nm) in the same solvent show weakly emitting (PL quantum efficiency, PLQY = ∼1%) single broad peaks at 383 and 401 nm, respectively.…”

“…Further increase in solvent polarity leads to a decrease in emission intensity of the λ 468 , suggesting intramolecular exciplex formation − due to the absence of excimer formation seen for POP at the same concentration under identical conditions. In the case of TOP , a bathochromic shift with increasing solvent polarity indicates CT character of the excited state­(Figure S4b).…”

Dual Emission through Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence, and Their Thermal Enhancement via Solid-State Structural Change in a Carbazole-Quinoline Conjugate

“…The emergence of a hypsochromically shifted absorption band of λ 375 with increasing polarity of the solvents confirms the charge transfer (CT) nature of the transition (Figure a). Likewise, steady-state emission shows positive solvatochromism effects and a broad emission feature, suggesting that CQ is less polar in the Franck–Condon (FC) region compared to the S 1 minimum from which the emission occurs (Figure a). Moreover, the biexponential decay of the emission band in toluene (λ em = 441 nm), showing prompt (τ PF = 6.8 ns) and delayed components (τ DF = 1.5 μs) in the absence of molecular oxygen at RT and monoexponential decay of the λ 441 at 77 K in solution confirms a typical TADF dye − (Figure b, Figure S8c).…”