Proton-Gated Photoisomerization of Amino-Substituted Dibenzofulvene Rotors

Significance Statement

The use of molecular devices is of current interest in areas such as nanotechnology, advanced materials, electronics, sensors and drug delivery systems. Some are able to switch their structural states when external factors such as temperature, light or pH are applied. Certain molecules that are responsive to light involve photoisomerization of a double bond, producing a large change in molecular shape. The ability to turn photoisomerization on and off by means of a second external stimulus would be useful, and employing pH for this purpose is reported here.

Researchers led by Dr. Udaya Jayasundara and Professor Thomas Bell at the University of Nevada published new research in the journal, ChemPhysChem, discussing the discovery of proton-gated photoisomerization of derivatives of 9-(2,2,2-triphenylethylidene)fluorene (TEF). The researchers capitulated on structural changes during the photoisomerization phenomenon, as the molecules contain a dibenofulvene chromophore, which acts as a rotor about the exocyclic C=C bond.

The authors investigated photoisomerization of two different 2-substituted 9-(2,2,2-triphenylethylidene)fluorene molecules, 2-amino-9-(2,2,2-triphenylethylidene)fluorene (ATEF) and 2-dimethylamino-9-(2,2,2-triphenylethylidene)fluorene (DTEF), with and without addition of trifluoroacetic or trifuoromethanesulfonic acid. In acetonitrile solution, both ATEF and DTEF have negligible photoisomerization efficiencies (quantum yields) in the free base form with the 2-position nitrogen atom unprotonated. Upon addition of acid, both compounds become photoactive and their E and Z isomers are interconverted rapidly.

The authors compared the protonation effect of trifluoromethanesulfonic acid with trifluoroacetic acid and triethylamine. It was discovered that trifuoromethanesulfonic acid produced higher quantum yields, resulting from full protonation, which wasn’t the case for other added substances. Likewise, fewer equivalents of trifuoromethanesulfonic acid were needed for full protonation and gave less photodecomposition than trifluoracetic acid.

The authors found a good correlation between the photoisomerization quantum yields and levels of protonation in acetonitrile solution and discovered that amine protonation increased efficiency by factors of 30-60 compared to unprotonated cases.

This work introduced four new polar substituents (NH2, NH3+, NMe2, and NMe2H+) at the 2-position of 9-(2,2,2-triphenylethylidene)fluorene, in addition to the previously reported four (tBu, NO2, CN and I). Listing them in order of increasing quantum yield shows that electron withdrawing groups generally increase photoisomerization efficiency, but the list does not exactly correlate with the order of substituent electronegativity. In the cases of ATEF and DTEF, protonation may turn on photoisomerization by eliminating intramolecular charge transfer, which hinders effective photoisomerization of these molecular devices.

According to the authors, “Such molecules could be used in light controlled actuators, storage registers and power sources, as well as in switching and sensing applications.”

The two derivatives of 9-(2,2,2-triphenylethylidene)fluorene investigated by the authors have excellent molecular sensing features which are of relevance to numerous fields, including nanotechnology and drug delivery systems.

Proton-Gated Photoisomerization of Amino-Substituted Dibenzofulvene Rotors-Advances in Engineering

About the author

Thomas W. Bell is Professor of Chemistry at the University of Nevada, Reno. He obtained a BS degree with honors from California Institute of Technology in 1974 then received his PhD in Organic Chemistry from University College, London in 1980, having conducted his thesis research with Franz Sondheimer (UCL) and Donald J. Cram (UCLA).  He worked with Jerrold Meinwald as an NIH Postdoctoral Fellow at Cornell University, then he joined the State University of New York at Stony Brook as an Assistant Professor in 1982.  There, he reached the rank of Professor in 1991, then moved to his current position in 1995.  He has been a Fellow of the American Association of the Advancement of Science since 1994. In 1990 and 1996 he was appointed Visiting Professor at Université Louis Pasteur in Strasbourg, France, and for the 2015-2016 academic year he was appointed Visiting Professor at the University of Leuven (KU Leuven) in Belgium.

His research interests over his entire academic career have included supramolecular chemistry, chemosensors, advanced materials, nanoscale molecular assemblies and devices, and medicinal chemistry. His research is currently focused on antiviral and immunomodulatory drugs, collaborating with microbiologists and biochemists at the Rega Institute for Medical Research in Leuven, Belgium, and on muscle relaxants, neuroprotectives and male contraceptives, working with collaborators in the School of Medicine at the University of Nevada, Reno.

He currently has more than 100 publications in scientific journals and is inventor or co-inventor on 11 patents/patent applications. His recreational interests include hiking, backpacking, rock climbing, mountain biking, skiing, and snowboarding.

About the author

Dr Udaya K Jayasundara is a Senior Lecturer in the College of Chemical Sciences at the Institute of Chemistry Ceylon, Sri Lanka. He received BSc (Hons) in 2004 from the University of Peradeniya Sri Lanka. Then he moved to the University of Nevada Reno for his doctoral studies. He trained in physical and analytical chemistry and completed his PhD in 2011. His doctoral study, under Professor Joseph Cline, explored photoisomerization efficiencies of dibenzofulvene molecular motor prototype in collaboration with Prof. Thomas Bell. His work has been presented in several National and International Conferences and he has authored several peer reviewed articles. After completing his PhD, he worked as an adjunct professor at Truckee Meadows Community College and as a scientific associate at the Charles River Laboratories in Reno NV area. He is currently working as an Editorial Board Member for the American Journal of Nanosciences.

His research interests are focused in spectroscopy to understand the chemical kinetics.  He has recently embarked on new research activities in the field of Pharmaceutical Method Development and Method Validation for commercially available drug components using High Performance Liquid Chromatography (HPLC), Ultraviolet Visible Spectroscopy (UV-Vis), and Gas Chromatography (GC‑FID) with Good Laboratory Practice (GLP) regulations.   This work involves studying various drug substances with possible application to determine the active component’s concentrations.  It is aimed to extend his research further into other drug substances using LC‑MS in future.

Other than his academic career, he is an expert in Chemical Safety and Hygiene Plans, Good Laboratory Practices (GLP), Environmental Health and Safety (EH&S), Occupational Safety and Health (OSHA standards), Laboratory Safety Procedures, and Chemical Risk Assessment.  He has delivered over 15 public speeches on these subject areas since 2015.  



Jayasundara, U.K., Kim, H.J., Sahteli, K.P., Cline, J.I., Bell, T.W. Proton-Gated Photoisomerization of Amino-Substituted Dibenzofulvene Rotors, ChemPhysChem 18 (2017) 59-63.

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