Tunable Dual Fluorescence of 3‐(2,2′‐Bipyridyl)‐Substituted Iminocoumarin

Abstract

3-(2,2'-Bipyridyl)-substituted iminocoumarin molecules (compounds 1 and 2) exhibit dual fluorescence. Each molecule has one electron donor and two electron acceptors that are in conjugation, which leads to fluorescence from two independent charge transfer (CT) states. To account for the dual fluorescence, we subscribe to a kinetic model in which both CT states form after rapid decays from the directly accessed S(1) and S(2) excited states. Due to the slow internal conversion from S(2) to S(1), or more likely the slow interconversion between the two subsequently formed CT states, dual emission is allowed to occur. This hypothesis is supported by the following evidence: 1) the emission at short and long ends of the spectrum originates from two different excitation spectra, which eliminates the possibility that dual emission occurs after an adiabatic reaction at the S(1) level. 2) The fluorescence quantum yield of compound 2 grows with increasing excitation wavelength, which indicates that the high-energy excitation elevates the molecule to a weakly emissive state that does not internally convert to the low-energy, highly emissive state. The intensity of the two emission bands of 1 is tunable through the specific interactions between either of the two electron acceptors with another species, such as Zn(2+) in the current demonstration. Therefore, the development of ratiometric fluorescent indicators based on the dual-emitting iminocoumarin system is conceivable. Further fundamental studies on this series of compounds using time-resolved spectroscopic techniques, and explorations of their applications will be carried out in the near future.

5-Arylvinylene-2,2′-bipyridyls: Bright “push–pull” dyes as components in fluorescent indicators for zinc ions

“…At a lower energy the charge transfer from the methoxy to the bipy moiety could instead become dominant (BIPY state). This hypothesis is based on the calculations on the frontier molecular orbital arrangements of 38 , which is consistent with the experimental observations [115]. Either the COUM or the BIPY transitions might be favored in two populations of ground state conformers that differ in the dihedral angle between the coumarin and bipy moieties.…”

“…Compound 38 consists of an iminocoumarin moiety and a bipy substituent [115]. The excitation of 38 may lead to the formation of two different CT excited states depending on the applied excitation energy.…”

“…Either the COUM or the BIPY transitions might be favored in two populations of ground state conformers that differ in the dihedral angle between the coumarin and bipy moieties. After the fluorescence properties of compound 38 was reported [115], we acquired the single crystal structure of the analogous compound 39 (Fig. 16a), which has a dihedral angle (C1–C2–C3–C4) of 45 − , close to the 40 − of the most stable conformer predicted from the conformational analysis [115].…”

Zn(ii)-coordination modulated ligand photophysical processes – the development of fluorescent indicators for imaging biological Zn(ii) ions

“…Compound 34 (Scheme 13) offers another example. 240 This compound has a single fluorophore which contains one electron donor – methoxy – and two electron accepters – bipy moiety and imino nitrogen. Therefore, two charge transfer (CT) excited states may form to move the electron density from methoxy to either electron-accepting site.…”

Hydrogen-bonding dynamics of photoexcited coumarin 138 and 339 in protic methanol solution: Time-dependent density functional theory study

“…This intermolecular interaction can assemble multiple molecules into a cluster, which is formed in the ground state and influence a series of photophysical processes, such as internal conversion (IC), photoinduced proton transfer (PPT), and photoinduced electron transfer (PET) [8][9][10][11][12][13][14]. The numerous researchers and engineers are making efforts to understand the hydrogen bonding dynamics with theoretical and experimental methods, exploring the applications in photoelectronics, such as fluorescence probe, optical sensor and light emitting devices [6,[15][16][17][18][19][20][21][22].…”