Ultrafast Photoinduced Electron Transfer and Charge Stabilization in Donor-Acceptor Dyads Capable of Harvesting Near-Infrared Light

• Published in: Chemistry : a European journal 2015-08, Vol.21 (32), p.11483-11494
• Main Authors: Bandi, Venugopal, Gobeze, Habtom B., D'Souza, Francis
• Format: Article
• Language: English
• Subjects: Absorption; Atomic energy levels; Charge; Charge transfer; Chemistry; donor-acceptor systems; Electrochemistry; Electron transfer; Electrons; Energy; energy conversion; Energy harvesting; Energy levels; Excitation; Femtosecond; Fluorescence; Fullerenes; Infrared radiation; Nanostructure; Optimization; Optoelectronic devices; Radiation; Recombination; sensitizers; Solar radiation; Stabilization; Studies; Triplet state
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201500728

To harvest energy from the near‐infrared (near‐IR) and infrared (IR) regions of the electromagnetic spectrum, which constitutes nearly 70 % of the solar radiation, there is a great demand for near‐IR and IR light‐absorbing sensitizers that are capable of undergoing ultrafast photoinduced electron transfer when connected to a suitable electron acceptor. Towards achieving this goal, in the present study, we report multistep syntheses of dyads derived from structurally modified BF2‐chelated azadipyrromethene (ADP; to extend absorption and emission into the near‐IR region) and fullerene as electron‐donor and electron‐acceptor entities, respectively. The newly synthesized dyads were fully characterized based on optical absorbance, fluorescence, geometry optimization, and electrochemical studies. The established energy level diagram revealed the possibility of electron transfer either from the singlet excited near‐IR sensitizer or singlet excited fullerene. Femtosecond and nanosecond transient absorption studies were performed to gather evidence of excited state electron transfer and to evaluate the kinetics of charge separation and charge recombination processes. These studies revealed the occurrence of ultrafast photoinduced electron transfer leading to charge stabilization in the dyads, and populating the triplet states of ADP, benzanulated‐ADP and benzanulated thiophene‐ADP in the respective dyads, and triplet state of C60 in the case of BF2‐chelated dipyrromethene derived dyad during charge recombination. The present findings reveal that these sensitizers are suitable for harvesting light energy from the near‐IR region of the solar spectrum and for building fast‐responding optoelectronic devices operating under near‐IR radiation input. Light harvesting: Donor–acceptor dyads using near‐IR sensitizers based on modified azaBODIPY as donor and fullerene as acceptor have been synthesized (see figure). The occurrence of photoinduced electron transfer leading to charge separated states is demonstrated by using femtosecond and nanosecond transient spectroscopic techniques.