An ultrafast spectroscopic and quantum mechanical investigation of multiple emissions in push–pull pyridinium derivatives bearing different electron donors

Abstract

A joint experimental and theoretical approach, involving state-of-the-art femtosecond fluorescence up-conversion measurements and quantum mechanical computations including vibronic effects, was employed to get a deep insight into the excited state dynamics of two cationic dipolar chromophores (Donor-π-Acceptor(+)) where the electron deficient portion is a N-methyl pyridinium and the electron donor a trimethoxyphenyl or a pyrene, respectively. The ultrafast spectroscopic investigation, and the time resolved area normalised emission spectra in particular, revealed a peculiar multiple emissive behaviour and allowed the distinct emitting states to be remarkably distinguished from solvation dynamics, occurring in water in a similar timescale. The two and three emissions experimentally detected for the trimethoxyphenyl and pyrene derivatives, respectively, were associated with specific local emissive minima in the potential energy surface of S1 on the ground of quantum-mechanical calculations. A low polar and planar Locally Excited (LE) state together with a highly polar and Twisted Intramolecular Charge Transfer (TICT) state is identified to be responsible for the dual emission of the trimethoxyphenyl compound. Interestingly, the more complex photobehaviour of the pyrenyl derivative was explained considering the contribution to the fluorescence coming not only from the LE and TICT states but also from a nearly Planar Intramolecular Charge Transfer (PICT) state, with both the TICT and the PICT generated from LE by progressive torsion around the quasi-single bond between the methylpyridinium and the ethene bridge. These findings point to an interconversion between rotamers for the pyrene compound taking place in its excited state against the Non-equilibrated Excited Rotamers (NEER) principle.

Tetramethoxy‐Aminorhodamine (TMARh): A Bichromophore, an Improved Fluorophore, and a pH Switch

“…Eur.J.2016, 22,7046 -7049 www.chemeurj.org ac onformation locked at the ground state equilibrium twist angle (band 2, 838), and emission from S 1 in ap opulation of dyes relaxed to the excited state equilibrium twist angle (band 3, 908). This behavior is as expected from an arylpyrylium dye, [13] and lends credence to the model representedi nt he Jablonksi diagram in Figure 4. In summary,anew rhodamine dye, tetramethoxy-aminorhodamine (TMARh)w as synthesized using as eries of nucleophilic aromatic substitutions, commonly used to make triangulenium dyes.…”

Highly Reversible Electrofluorochromism from Electrochemically Decoupled but Electronically Coupled Molecular Dyads

“…As shown in Figure b, the dyads displayed broad and structureless emission bands. The dyads containing the PQ unit (i.e., AnPQ and BpPQ) exhibited dual emission that likely originated from the emergence of ICT and TICT fluorescence . The emission peak wavelengths (λ em s) depended strongly on the identity of the A and D units: AnPQ, 650 nm; BpPQ, 636 nm; AnBQ, 554 nm; BpBQ, 480 nm.…”

Long-living optical gain induced by solvent viscosity in a push–pull molecule

“…7 From an electronic point of view, the TICT conformation involves a large charge separation because the mesomeric interaction between D and A is blocked owing to their reciprocal twisting, while the PICT model postulates an ICT quinoidal structure with partial positive charges on the D and A groups. The TICT/PICT mechanisms in push-pull systems have been extensively investigated in various polar and non-polar environments [8][9][10][11][12][13] as well as in environments with different viscosities. [14][15][16][17][18][19] While the controversy on TICT and PICT mechanisms in push-pull systems is still the subject of debate at both experimental and theoretical levels, 14,[20][21][22][23] the ICT process of photoinduced charge separation is of fundamental and practical significance.…”