XIV
h
International Conference on Molecular Spectroscopy, Białka Tatrzańska 2017
317
T6: P–25
Fourier excitation spectroscopy of single molecules at ambient
conditions
Łukasz Piatkowski
1
, Esther Gellings
2
, and Niek F. van Hulst
2,3
1
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw,
Poland, e-mail: lpiatkowski@ichf.edu.pl
2
ICFO—Institut de Ciences Fotoniques, Mediterranean Technology Park, 08860 Castelldefels
(Barcelona), Spain
3
ICREA—Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain
Detection of individual molecules opens up the ensemble, providing unique information on
molecular interactions, transients and localization. Single molecule spectroscopy at ambient
conditions in practice implies emission spectroscopy, which so far has left excitation
spectroscopy elusive. Excitation spectroscopy gives access to the vibrational progression and its
dynamics in the electronic excited state, occurring on a wide range of timescales from fs to ps to
ns. The excited state gives way to a variety of decay channels, such as energy transfer, non-
radiative decay, photo-dissociation, inter-system-crossing and of course fluorescence. As such,
excited state dynamics is very sensitive to interactions with the local environment. Instead, the
well-established emission spectroscopy has its source in spontaneous decay from the final
nanosecond-lived electronic state and missing out all ultrafast dynamics. Clearly, a simple and
efficient method enabling acquisition of excitation spectra of individual molecules and nano-
ensembles is required; and, only combination of both, excitation and emission techniques, would
yield a full picture of the photophysical properties of individual molecules and their
surroundings.
We presented a novel approach based on interferometric broadband excitation combined
with confocal fluorescence detection, bringing single molecule excitation spectroscopy side-by-
side to single molecule emission spectroscopy. We demonstrated that our broadband
interferometric approach is very resilient against blinking
and bleaching. Since the entire excitation spectrum is
probed at once only the spectral resolution is decreased in
case of an incomplete measurement without any
truncation of the spectrum. Unprecedented spectral
heterogeneities of single molecules, with individual
excitation spectra shifted in wavelength by as much as
100 nm were revealed. Conventional narrowband
excitation techniques would be incapable to capture the
whole extent of the spectral distribution and would miss
out on molecules detected by the broadband scheme.
Keywords: single molecule; excitation spectrum; heterogeneity
References
[1] L. Piatkowski, E. Gellings, N.F. van Hulst, Nature Commun. 7 (2016) 10411.
[2] L. Piatkowski, E. Gellings, NF. van Hulst, Farad. Discuss. 184 (2015) 207.
XIV
h
International Conference on Molecular Spectroscopy, Białka Tatrzańska 2017
318
T6: P–26
UV-induced oxidation of bifunctional proton donor-acceptor indole
derivatives
Barbara Golec
1
, Krzysztof Nawara
2
, Randolph P. Thummel
3
, and Jacek Waluk
1,2
1
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw,
Poland, e-mail: bgolec@ichf.edu.pl
2
Faculty of Mathematics and Science, Cardinal Stefan Wyszyński University, Dewajtis 5, 01-815
Warsaw, Poland
3
Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States
Oxidation of indole and its derivatives has been a topic of a number of interesting
experimental and theoretical studies for years because this reaction leads to the formation of a
variety of products which are of biological as well as synthetic importance. Previous studies
have proved that the indole moiety and its derivatives can be bio-oxidized by many oxygenases
or different of chemical oxidizing agents. The oxidation can be also induced by UV/Vis
irradiation in the presence of photosensitizers. These processes involve excitation of the
photosensitizers to yield singlet and triplet excited states. The excited triplet states having longer
lifetimes may undergo bimolecular reactions more efficiently and may transfer energy to
molecular oxygen, forming the reactive singlet oxygen, responsible for type II photooxidation
reactions. Electron transfer involving excited triplet states and different substrates may also
initiate photooxidation reactions (type I).
Here, we seek to explain the oxidation processes and types of products formed under UV
(365 nm) irradiation of two bifunctional indole derivatives (12,13-dihydro-5H-indolo[3,2-
c]acridine, IA, and
2-(1′H-indol-2′-yl)-[1,5]naphthyridine, IN), molecules which simultaneously possess
hydrogen bond donor and acceptor groups located in separate moieties, linked by a single bond,
both in the protic and aprotic environment. It was demonstrated for this type of compounds that
their photophysical properties and photostability may be strongly affected by formation of
hydrogen bonds with proton donors and / or acceptors [1]. These molecules can absorb the UV-
A light so that no additional photosensitizer is necessary to enable the photooxidation reaction.
The results of our studies indicate that photoirradiation leads to formation only of oxidation
reaction products. No destruction of the studied compounds was observed upon irradiation of
oxygen free solutions. The rate of photooxidation of IA and IN is observed to be slower in protic
solvents than in the aprotic ones, but the reaction mechanisms seem to be similar, in all studied
solvents (n-hexane, acetonitrile, 1-propanol, and methanol). In the case of 12,13-dihydro-5H-
indolo[3,2-c]acridine, 13H-indolo[3,2-c]acridine was identified as the main oxidation product
and probably singlet oxygen is involved in the formation of this molecule. The photooxidation
of 2-(1′H-indol-2′-yl)-[1,5]naphthyridine (IN) leads mainly to the formation of 2-(1,5-
naphthyridin-2-yl)-4H-3,1-benzoxazin-4-one. For this molecule the reaction with singlet oxygen
does not occur, which suggests that an electron transfer process is probably involved in the
photooxidation of IN.
Keywords: photooxidation, UV irradiation, photosensitizers, indole
References
[1] B. Golec, M. Kijak, V. Vetokhina, A. Gorski, R. P. Thummel, J. Herbich, J. Waluk, J. Phys. Chem. B
119 (2015) 7283.
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