Issue 6, 2016

Triplet excited state properties in variable gap π-conjugated donor–acceptor–donor chromophores

Abstract

A series of variable band-gap donor–acceptor–donor (DAD) chromophores capped with platinum(II) acetylide units has been synthesized and fully characterized by electrochemical and photophysical methods, with particular emphasis placed on probing triplet excited state properties. A counter-intuitive trend of increasing fluorescence quantum efficiency and lifetime with decreasing excited state energy (optical gap) is observed across the series of DAD chromophores. Careful study of the excited state dynamics, including triplet yields (as inferred from singlet oxygen sensitization), reveals that the underlying origin of the unusual trend in the fluorescence parameters is that the singlet–triplet intersystem crossing rate and yield decrease with decreasing optical gap. It is concluded that the rate of intersystem crossing decreases as the LUMO is increasingly localized on the acceptor unit in the DAD chromophore, and this result is interpreted as arising because the extent of spin–orbit coupling induced by the platinum heavy metal centers decreases as the LUMO is more localized on the acceptor. In addition to the trend in intersystem crossing, the results show that the triplet decay rates follow the Energy Gap Law correlation over a 1.8 eV range of triplet energy and 1000-fold range of triplet decay rates. Finally, femtosecond transient absorption studies for the DAD chromophores reveals a strong absorption in the near-infrared region which is attributed to the singlet excited state. This spectral band appears to be general for DAD chromophores, and may be a signature of the charge transfer (CT) singlet excited state.

Graphical abstract: Triplet excited state properties in variable gap π-conjugated donor–acceptor–donor chromophores

Supplementary files

Article information

Article type
Edge Article
Submitted
27 Nov 2015
Accepted
12 Feb 2016
First published
12 Feb 2016
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2016,7, 3621-3631

Author version available

Triplet excited state properties in variable gap π-conjugated donor–acceptor–donor chromophores

S. Cekli, R. W. Winkel, E. Alarousu, O. F. Mohammed and K. S. Schanze, Chem. Sci., 2016, 7, 3621 DOI: 10.1039/C5SC04578A

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