2 research outputs found
Mechanisms of Aggregation-Induced Emission and Photo/Thermal <i>E</i>/<i>Z</i> Isomerization of a Cyanostilbene Derivative: Theoretical Insights
In this study, aggregation-induced
emission (AIE) of a cyanostilbene
derivative, called 1-cyano-1,2-bis-(4′-methylbiphenyl)Âethylene
(CN-MBE), was investigated using molecular simulations. The <i>E</i>-form of CN-MBE has been known to exhibit AIE, which is
non-emissive in dilute solutions but becomes highly emissive in aggregated
states, whereas its <i>Z</i>-form is non-emissive even in
its crystalline form. In addition, upon UV light irradiation, the <i>Z</i>-form exhibits a <i>Z</i>-to-<i>E</i> isomerization, whereas the <i>E</i>-form keeps its conformation
at room temperature and undergoes a nonradiative <i>E</i>-to-<i>Z</i> isomerization only at a high temperature.
The results from the electronic structure calculations employed in
this work showed that the potential energies of CN-MBE for the electronic
ground (S<sub>0</sub>) and first excited (S<sub>1</sub>) states were
degenerate at a twisted conformation around the ethylenic Cî—»C
Ï€-bond, which led to fluorescence quenching of the molecule.
Molecular dynamics simulations and free-energy analyses revealed that
the <i>E</i>-form molecules assembled closely, with the
Cî—»C bond rotation markedly restricted. This, in turn, prevented
fluorescence quenching via the S<sub>0</sub>/S<sub>1</sub> conical
intersection. In contrast, the <i>Z</i>-form molecules aggregated
relatively sparsely, allowing for the nonradiative <i>Z</i>-to-<i>E</i> isomerization to proceed. All in all, the
theoretical insights presented herein give a clear picture on the
AIE and photo/thermal isomerization mechanisms of CN-MBE
External Electric Field Effects on Excited-State Intramolecular Proton Transfer in 4′‑<i>N</i>,<i>N</i>‑Dimethylamino-3-hydroxyflavone in Poly(methyl methacrylate) Films
The
external electric field effects on the steady-state electronic
spectra and excited-state dynamics were investigated for 4′-<i>N</i>,<i>N</i>-(dimethylamino)-3-hydroxyflavone (DMHF)
in a polyÂ(methyl methacrylate) (PMMA) film. In the steady-state spectrum,
dual emission was observed from the excited states of the normal (N*)
and tautomer (T*) forms. Application of an external electric field
of 1.0 MV·cm<sup>–1</sup> enhanced the N* emission and
reduced the T* emission, indicating that the external electric field
suppressed the excited-state intramolecular proton transfer (ESIPT).
The fluorescence decay profiles were measured for the N* and T* forms.
The change in the emission intensity ratio N*/T* induced by the external
electric field is dominated by ESIPT from the Franck–Condon
excited state of the N* form and vibrational cooling in potential
wells of the N* and T* forms occurring within tens of picoseconds.
Three manifolds of fluorescent states were identified for both the
N* and T* forms. The excited-state dynamics of DMHF in PMMA films
has been found to be very different from that in solution due to intermolecular
interactions in a rigid environment