Unusual high fluorescence of two nitro-distyrylbenzene-like compounds induced by CT processes affecting the fluorescence/intersystem-crossing competition

Abstract

Two nitro-substituted 1,4-distyrylbenzene-like compounds have been investigated using stationary and time-resolved (ns/fs) spectrometric techniques as a function of solvent polarity. In the two compounds the central benzene ring is substituted with a p-nitrostyryl group at one side while, at the other side, compound 1 (asymmetric) bears a pyrid-4-ylethenyl group and compound 2 (symmetric) another p-nitrostyryl group. The solvent dependent intramolecular charge transfer (ICT) in the singlet manifold was found to strongly affect the competition among fluorescence, intersystem crossing and trans-cis photoisomerization. The presence of nitro-groups in the 1,4-distyrylbenzene skeleton causes the usual strong decrease of fluorescence in favour of intersystem crossing to a reactive triplet state. However, the favoured formation of the ICT state in polar solvents induces an unexpected important increase of the fluorescence quantum yield (three/two order of magnitude for the nitro and dinitro derivatives, respectively). The ultrafast spectral transients helped to understand the solvent effects measured by stationary techniques and gave information on the dynamics of the locally excited singlet state ((1)LE*) and the (1)ICT* state, fast produced in polar solvents. Evidence of dual fluorescence in a limited range of solvent polarity, particularly for compound 1, is also reported. The role of an upper triplet state in a non-polar solvent is discussed also based on quantum-mechanical calculations (TD-DFT method) and temperature effects on the photophysical parameters.

Ultrafast Investigation of Intramolecular Charge Transfer and Solvation Dynamics of Tetrahydro[5]-helicene-Based Imide Derivatives

“…The lower energy level of S 1 state in more polar solvent would make ISC difficult to happen, then the radiation transition is the main way for decaying back to ground state. Similar cases were reported previously 33 34 . When the solvent polarity increases such as ACE, ACN, the nonradiation processes play a dominant role in decrease the quantum yield.…”

Pyrrolo[3,2‐b]pyrroles—From Unprecedented Solvatofluorochromism to Two‐Photon Absorption

“…The difference in the fluorescence quantum yield can be explained by the fact that in polar solvents compound 7 tends to fluoresce more efficiently from the ICT state. [17] …”

“…As a result, compound 6 in cyclohexane and CCl 4 solutions emits very efficiently. Although there are examples of nitro derivatives possessing fluorescence properties, they are rather rare in the literature, [16,17,19,20] and to the best of our knowledge no nitro derivatives with almost 100% fluorescence quantum yield have been reported. When increasing the solvent polarity, the single bonds at positions 2 and 5 adopt a partial double-bond character in favor of the ICT state (Figure 4), form B), resulting in a more planar structure quenching the fluorescence as is typical for the nitro group.…”

An Ionic 1,4-Bis(styryl)benzene-Based Fluorescent Probe for Mercury(II) Detection in Water via Deprotection of the Thioacetal Group

“…Molar absorption coefficients ( ε max ) were determined to be 5.35 × 10 4 M −1 cm −1 in CH 3 CN, 5.52 × 10 4 M −1 ·cm −1 in CH 3 CN/water for M1, and 3.85 × 10 4 M −1 ·cm −1 in water for M1Q at each maximum absorption wavelength (Table 1). The ionic M1Q shows a largely increased Stokes shift of ~200 nm in water and decreased PL quantum efficiency due to enhanced nonradiative relaxations in highly polar solvent of water [38,39,40,41,42,43]. …”