Abstract
Quantum interference is a well-known phenomenon that dictates charge transport properties of single molecule junctions. However, reports on quantum interference in donor-bridge-acceptor molecules are scarce. This might be due to the difficulties in meeting the conditions for the presence of quantum interference in a donor-bridge-acceptor system. The electronic coupling between the donor, bridge, and acceptor moieties must be weak in order to ensure localised initial and final states for charge transfer. Yet, it must be strong enough to allow all bridge orbitals to mediate charge transfer. We present the computational route to the design of a donor-bridge-acceptor molecule that features the right balance between these contradicting requirements and exhibits pronounced interference effects.
Highlights
By contrast to PDIim the HOMOÀ1 of PDIbay is delocalised over PDI and biphenyl as depicted in Fig. 3(b): the highest occupied fragment orbital (HOFO) of PDI contributes to the HOMO–1 with 78%
We have shown a computational route to the design of a linearly and a cross-conjugated DBA molecule containing a biphenyl bridge with hole transfer characteristics dominated by quantum interference effects
The screening of several hole donor and hole acceptor candidates demonstrated how difficult it is to find the right balance between a too weak and too strong coupling between the donor, bridge, and acceptor moieties in order to assure localised initial and final states while still allowing for all bridge orbitals to provide hole transfer pathways. This condition needs to be especially met in quantum interference based DBA systems, but should generally be examined in the interpretation of experimental results on electron or hole transfer in all DBA systems
Summary
Photoinduced charge transfer is at the heart of numerous biological processes and technological applications, such as (artificial) photosynthesis,[1,2,3] DNA damage and repair,[4,5,6,7,8] organic solar cells,[9,10] molecular computation and biosensing.[11,12,13] Synthetic donor-bridge-acceptor (DBA) systems have been used extensively for systematic studies of charge (electron or hole) transfer.[14,15,16,17,18,19] These DBA systems consist of three covalently bound moieties: the charge donor where the transferring charge is generated by absorption of light, the bridge through which the charge passes, and the acceptor where the charge arrives. Energy barrier, and driving force have been identified as the most important ones.[20,21,22] Recently, other parameters like bridge conjugation, and the position at which donor, bridge, and acceptor are connected to each other received attention.[23,24,25]. Experimental demonstrations of the occurrence of quantum interference in DBA systems are scarce.[23,47,48] This could be due to the difficulty in designing suitable DBA systems. The design rules are evident considering that quantum interference originates. By using the same biphenyl bridges as in pp and mp and screening for an appropriate hole donor and acceptor, we emphasise how substantial the thorough choice of initial and final states is. The screening follows the two design rules by examining the initial and final state and comparing the effective electronic coupling for the linearly and the cross-conjugated bridge
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