Photophysics of Push–Pull Distyrylfurans, Thiophenes and Pyridines by Fast and Ultrafast Techniques

Abstract

Time-resolved transient absorption and fluorescence spectroscopy with nano- and femtosecond time resolution were used to investigate the deactivation pathways of the excited states of distyrylfuran, thiophene and pyridine derivatives in several organic solvents of different polarity in detail. The rate constant of the main decay processes (fluorescence, singlet-triplet intersystem crossing, isomerisation and internal conversion) are strongly affected by the nature [locally excited (LE) or charge transfer (CT)] and selective position of the lowest excited singlet states. In particular, the heteroaromatic central ring significantly enhances the intramolecular charge-transfer process, which is operative even in a non-polar solvent. Both the thiophene and pyridine moieties enhance the S1 →T1 rate with respect to the furan one. This is due to the heavy-atom effect (thiophene compounds) and to the (1) (π,π)*→(3) (n,π)* transition (pyridine compounds), which enhance the spin-orbit coupling. Moreover, the solvent polarity also plays a significant role in the photophysical properties of these push-pull compounds: in fact, a particularly fast (1) LE*→(1) CT* process was found for dimethylamino derivatives in the most polar solvents (time constant, τ≤400 fs), while it takes place in tens of picoseconds in non-polar solvents. It was also shown that the CT character of the lowest excited singlet state decreased by replacing the dimethylamino side group with a methoxy one. The latter causes a decrease in the emissive decay and an enhancement of triplet-state formation. The photoisomerisation mechanism (singlet/triplet) is also discussed.

Effect of the π Bridge and Acceptor on Intramolecular Charge Transfer in Push–Pull Cationic Chromophores: An Ultrafast Spectroscopic and TD‐DFT Computational Study

“…Our research has been recently devoted to the investigation of the solvent effect on the spectra, the photophysics and the excited‐state dynamics of push–pull molecules, both neutral12–16 and positively charged. Relevant results have been obtained for the latter,17, 18 which have shown a peculiar negative solvatochromic behavior and an interesting competition between the occurrence of solvation and photoinduced ICT during their excited‐state deactivation.…”

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

“…[3][4][5][6][7][8][9] It has been observed that the extent of the NLO response and the two photon absorption cross-sections are strongly dependent upon the molecular structure of the pushpull molecules and are particularly affected by the strengths of the electron donor/acceptor groups. [14][15][16][17][18] This effect was observed not only in widely studied neutral donor-acceptor systems, but also in cationic chromophores bearing a methyl pyridinium or a methyl quinolinium as an electron deficient moiety, [19][20][21][22][23][24] whose photobehaviour has been less investigated so far and, therefore, not yet deeply understood. [14][15][16][17][18] This effect was observed not only in widely studied neutral donor-acceptor systems, but also in cationic chromophores bearing a methyl pyridinium or a methyl quinolinium as an electron deficient moiety, [19][20][21][22][23][24] whose photobehaviour has been less investigated so far and, therefore, not yet deeply understood.…”

New Styryl Phenanthroline Derivatives as Model D−π−A−π−D Materials for Non‐Linear Optics

“…One of the most successful approaches to achieve high coefficients in NLO properties rely on the use of dipolar‐type molecules with a donor−bridge−acceptor (D−π−A) motif and quadrupolar‐type molecules with a D−π−A−π−D structure . The optical properties of these class of compounds depend not only on the optimal choice of the electron donor/acceptor pair but also on the nature of the π‐conjugated spacer; moreover, heterocyclic fragments have been considered to be promising for organic chromophores both as D/A moieties and as elements for the π conjugation ,…”

“…[11][12][13][14][15][16][17][18] The optical properties of these class of compounds depend not only on the optimal choice of the electron donor/acceptor pair but also on the nature of the p-conjugated spacer; [19][20][21][22] moreover, heterocyclic fragments have been considered to be promising for organic chromophores both as D/A moieties and as elements for the p conjugation. [23,24] However, the optimization of NLO organic materials remains an extremely challenging task to solve, as organic chromo-phores often require multistage syntheses and product purification after each stage. Additional limitations, in practical terms, comprise low yields, low solubility in common solvents or aggregation phenomena that can further complicate the process of separation and purification of the final product, leading to a significant loss of material and an increase in its cost.…”