Bane of Hydrogen-Bond Formation on the Photoinduced Charge-Transfer Process in Donor–Acceptor Systems

Abstract

Controlling the ultrafast dynamical process of photoinduced charge transfer at donor–acceptor interfaces remains a major challenge for physical chemistry and solar cell communities. The process is complicated by the involvement of other complex dynamical processes, including hydrogen bond formation, energy transfer, and solvation dynamics occurring on similar time scales. In this study, we explore the remarkable impact of hydrogen-bond formation on the interfacial charge transfer between a negatively charged electron donating anionic porphyrin and a positively charged electron accepting π-conjugated polymer, as a model system in solvents with different polarities and capabilities for hydrogen bonding using femtosecond transient absorption spectroscopy. Unlike the conventional understanding of the key role of hydrogen bonding in promoting the charge-transfer process, our steady-state and time-resolved results reveal that the intervening hydrogen-bonding environment and, consequently, the probable longer spacing between the donor and acceptor molecules significantly hinders the charge-transfer process between them. These results show that site-specific hydrogen bonding and geometric considerations between donor and acceptor can be exploited to control both the charge-transfer dynamics and its efficiency not only at donor–acceptor interfaces but also in complex biological systems.

Comparative Investigation of Ultrafast Excited-State Electron Transfer in Both Polyfluorene-Graphene Carboxylate and Polyfluorene-DCB Interfaces

“…It can be considered that PET events are facilitated by the energylevel alignment between the PFN and GC to induce favorable energetics for the charge transfer process. However it is important to note that although the reduction potential of GC (−1.02 V vs. SCE) [20] is lower than that of DCB (−1.64 V vs. SCE) [2], relating to the smaller driving force from PFN to GC, the PET from PFN to GC is much faster than that from PFN to DCB, and this is due to the opposite charge on GC to achieve strong electrostatic interactions, enhancing the electronic coupling and the rate of the PET process between PFN and GC because of the close distance of the electron donor and acceptor [16,20]. Via the electrostatic interactions, one thus can control the rate of the ultrafast PET in the non-covalent associations of the cationic polyfluorene.…”

“…In addition, their high emission intensities can be also one of their important features to be used for fluorescence resonance energy transfer [13,14]. In particular, the key issues of CPEs for solar cell applications are their flexibility, along with their simple, large scale, and low-cost fabrication devices [15,16]. Another advantage is that the functional groups of the side chains can be ionic or polar moieties, which makes it easy to modify not only solubility of the CPEs in water and other polar solvents [17] but also the redox potentials, intermolecular interactions, and energy level, which determine electronic coupling [18] and the rate of electron transfer at the donor-acceptor interface [19][20][21].…”

Investigating the Comparting between the Ultrafast Excited-State Electron Transfer in both Polyfluorene-Graphene and Polyfluorene- DCB Interfaces

“…[11][12][13] In particular, the key issues of the CPEs for the solar cell applications are their flexibility along with its simple, large scale, and low cost fabrication devices. [7,14] Other advantage is that functional groups of the side chains can be ionic or polar moieties, which make it easy to modify not only solubility of the CPEs in water and other polar solvents [15] but also redox potentials, intermolecular interactions, energy level, which determine electronic coupling [16] and the rate of electron transfer at the donor-acceptor interface. [7,13,17] Disclaimer/Publisher's Note: The statements, opinions, and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s).…”