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Extensive Reduction in Back Electron Transfer in Twisted Intramolecular Charge‐Transfer (TICT) Coumarin‐Dye‐Sensitized TiO 2 Nanoparticles/Film: A Femtosecond Transient Absorption Study
Author(s) -
Debnath Tushar,
Maity Partha,
Lobo Hyacintha,
Singh Balvant,
Shankarling Ganapati S.,
Ghosh Hirendra N.
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201303903
Subject(s) - femtosecond , intramolecular force , photochemistry , ultrafast laser spectroscopy , electron transfer , transient (computer programming) , charge (physics) , coumarin , materials science , absorption (acoustics) , fluorescence , nanoparticle , chemistry , nanotechnology , optics , laser , organic chemistry , physics , quantum mechanics , composite material , computer science , operating system
We report the synthesis, characterization, and optical and electrochemical properties of two structurally similar coumarin dyes ( C1 and C2 ). These dyes have been deployed as sensitizers in TiO 2 nanoparticles and thin films, and the effect of molecular structure on interfacial electron‐transfer dynamics has been studied. Steady‐state optical absorption, emission, and time‐resolved emission studies on both C1 and C2 , varying the polarity of the solvent and the solution pH, suggest that both photoexcited dyes exist in a locally excited (LE) state in solvents of low polarity. In highly polar solvents, however, C1 exists in an intramolecular charge‐transfer (ICT) state, whereas C2 exists in both ICT and twisted intramolecular charge‐transfer (TICT) states, their populations depending on the degree of polarity of the solvent and the pH of the solution. We have employed femtosecond transient absorption spectroscopy to monitor the charge‐transfer dynamics in C1 ‐ and C2 ‐sensitized TiO 2 nanoparticles and thin films. Electron injection has been confirmed by direct detection of electrons in the conduction band of TiO 2 nanoparticles and of radical cations of the dyes in the visible and near‐IR regions of the transient absorption spectra. Electron injection in both the C1 /TiO 2 and C2 /TiO 2 systems has been found to be pulse‐width limited (<100 fs); however, back‐electron‐transfer (BET) dynamics has been found to be slower in the C2 /TiO 2 system than in the C1 /TiO 2 system. The involvement of TICT states in C2 is solely responsible for the higher electron injection yield as well as the slower BET process compared to those in the C1 /TiO 2 system. Further pH‐dependent experiments on C1 ‐ and C2 ‐sensitized TiO 2 thin films have corroborated the participation of the TICT state in the slower BET process in the C2 /TiO 2 system.