Ultrafast Electron Transfer from Upper Excited State of Encapsulated Azulenes to Acceptors across an Organic Molecular Wall

• Published in: Journal of physical chemistry. C Vol. 121; no. 37; pp. 20205 - 20216
• Main Authors: Raj, A. Mohan, Porel, Mintu, Mukherjee, Puspal, Ma, Xiuyuan, Choudhury, Rajib, Galoppini, Elena, Sen, Pratik, Ramamurthy, V
• Format: Journal Article
• Language: English
• Published: American Chemical Society 21.09.2017
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.7b07260

In the context of generating reactive organic radical cations within a confined capsule and exploring photoinduced electron transfer from encapsulated organic molecules to organic and inorganic acceptors through an organic molecular wall, we have investigated electron transfer from the upper excited state (S2) of azulene (Az) and guaiazulene (GAz) enclosed within an octa acid (OA) capsule to water-soluble 4,4′-dimethyl viologen2+ (MV2+) and pyridinium+ (Py+) salts or colloidal TiO2. S2 fluorescence of OA encapsulated Az and GAz was quenched by electron acceptors such as MV2+ and Py+ salts. That electron transfer is responsible for S2 fluorescence quenching was established by recording the transient absorption spectrum of the MV●+ in the femtosecond time regime. Femtosecond time-resolved fluorescence experiments suggested that the time constant for the forward and reverse electron transfer from encapsulated Az and GAz to MV2+ is 4 and 3.6 ps, and 55.7 and 36.9 ps, respectively. The observed S2 fluorescence quenching by colloidal TiO2 in aqueous buffer solution is attributed to electron transfer from encapsulated Az and GAz to TiO2. Lack of quenching by the wider band gap material ZrO2 supported the above conclusion. FT-IR-ATR experiments confirmed that OA capsules containing Az and GAz can be adsorbed on TiO2 films, and excitation of these resulted in S2 fluorescence quenching. The observations presented here are important in the context of establishing the value of OA type cavitands where charge separation and donor shielding are critical.