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Comprehensive photophysical behaviour of ethynyl fluorenes and ethynyl anthracenes investigated by fast and ultrafast time-resolved spectroscopy. | LitMetric

Comprehensive photophysical behaviour of ethynyl fluorenes and ethynyl anthracenes investigated by fast and ultrafast time-resolved spectroscopy.

Chemphyschem

Department of Chemistry and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, Perugia, Italy.

Published: February 2012

AI Article Synopsis

  • Detailed investigations using advanced spectroscopy techniques revealed how different solvents affect the excited states of ethynyl fluorenes and anthracenes.
  • The polarity of the solvent impacts the efficiency of fluorescence and intersystem crossing, leading to a decrease in fluorescence yield but an increase in internal conversion as solvent dielectric constant increases.
  • The findings indicate a notable charge-transfer state that is stable in polar solvents, as shown by ultrafast spectroscopy identifying two distinct states, where the charge-transfer state is longer-lived and more fluorescent in such environments.

Article Abstract

Detailed investigations by time-resolved transient absorption and fluorescence spectroscopies with nano- and femtosecond time resolutions are carried out with the aim of characterising the lowest excited singlet and triplet states of three ethynyl fluorenes (1-3) and three ethynyl anthracenes (4-6) in solvents of different polarity. The solvent is found to modify the deactivation pathways of the lowest excited singlet state of compounds 1-4, thus changing their fluorescence, intersystem crossing and internal conversion efficiencies. The fluorescence and triplet yields gradually decrease, while the internal conversion quantum yield increases upon increasing the solvent dielectric constant. These experimental results, coupled with the marked fluorosolvatochromic effect, point to the involvement of an emitting state with a charge-transfer (CT) character, strongly stabilised by polar solvents. This is proved by ultrafast spectroscopic studies in which two transients, distinguished by characteristic spectral shapes assigned to locally excited (LE) and CT states, are detected, the CT state being the longer lived and fluorescent one in highly polar solvents. The intramolecular LE→CT process, operative in highly polar media, becomes particularly fast (up to ≈300 fs) in the case of the NO(2) derivative 1. No push-pull character is found for 5 and 6, which exhibit different photophysical behaviour; indeed, the solvent polarity does not modify significantly the dynamics of the lowest excited singlet states. Quantum mechanical calculations at the TDDFT level are also used to determine the state order and nature of the lowest excited singlet and triplet states and to rationalise the different photophysical behaviour of fluorine and anthracene derivatives, particularly concerning the intersystem crossing process.

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Source
http://dx.doi.org/10.1002/cphc.201100674DOI Listing

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