Gating Charge Recombination Rates through Dynamic Bridges in Tetrathiafulvalene-Fullerene Architectures

Theoretical Investigation of Switch Effect on the Efficiency and Adaptivity of Molecular Optoelectronic Conversion Devices

Molecular photoswitch can effectively regulate charge separation (CS) and charge recombination (CR) in donor-acceptor (D-A) systems. However, deformation of the donor-switch-acceptor (D-S-A) systems caused by the switch isomerization will destroy the geometrical stability of the battery. Here we take the planar platinum(II) terpyridyl complex of [Pt(^t Bu₃ tpy)(-C≡C-Ph)_n ]⁺ as the typical D-A model, designed six D-S-A systems using different photoswitches (dimethyldihydropyrene, fulgimide, arylazopyrazole, N-salicylideneaniline, spiropyran, and dithienylethene, denoted as D-S-A 1-6 hereafter). Our investigations show that the D-S-A 1-6 can absorb visible light of 799 nm, 673 nm, 527 nm, 568 nm, 616 nm, and 629 nm, facilitating electrons transfer from the donor and the switch to the acceptor through the Switch-on channel. Then cationic character of the photoswitch can undergo much more rapid isomerization than the neutral form due to the lower energy barrier. The Switch-off isomer breaks the conjugation of the D-S-A system, effectively turning off the CT channel and forming the CS state. Based on the evaluated conjugated backbone twist (CBT) angle, we found that D-S-A 1, 2, 4, 6 exhibit little configurational change and can be good candidates as the organic solar cell. The proposed D-S-A design controlled by the molecular switch may help to develop a solution for solar-harvesting practical applications.

Designing Self-Adaptive Donor-Switch-Acceptor for Molecular Opto-Electronic Conversion Based on Dimethyldihydropyrene/Cyclophanediene

Introduction of self-adaptive molecular switch is an appealing strategy to achieve complete charge separation (CS) in donor-acceptor (D-A) systems. Here we designed donor-switch-acceptor (D-S-A) systems using the platinum(II) terpyridyl complex as the acceptor, the dimethyldihydropyrene /cyclophanediene (DHP/CPD) as the bridge, and the methoxybenzene, thieno[3,​2-​ b ]thiophene, 2,2'-bifuran, and 4,8-dimethoxybenzo[1,2-b:4,5-b']difuran as the donors, respectively. We then systematically studied the whole opto-electronic conversion process of the donor-DHP/CPD-acceptor (D-DHP/CPD-A) systems based on time-dependent density functional theory, time-dependent ultrafast electron evolution, and electron transport property calculations. We first found that the substitution of -CH 3 by -H and -CN groups in DHP/CPD can enlarge the range of the adsorption wavenumber in opto-electric conversion. Then the light absorption induces the cationization of DHP switch, largely accelerating the forth-isomerization to CPD form. Once the D-CPD-A molecule is formed, the poor conjugation can realize the complete CS state by inhibiting the radiative and nonradiative charge recombinations. Finally, the repeatable and complete CS can be achieved through the automatic back-isomerization of CPD to DHP. The present work provides valuable insights into design of D-S-A systems for practical utilization of molecule-based solar harvesting.

Energy versus Electron Transfer: Controlling the Excitation Transfer in Molecular Triads

The photochemistry of Ru^II coordination compounds is generally discussed to originate from the lowest lying triplet metal-to-ligand charge-transfer state (³ MLCT). However, when heteroleptic complexes are considered, for example, in the design of molecular triads for efficient photoinduced charge separation, a complex structure of ¹ MLCT states, which can be populated in a rather narrow spectral window (typically around 450 nm) is to be considered. In this contribution we show that the localization of MLCT excited states on different ligands can affect the following ps to ns decay pathways to an extent that by tuning the excitation wavelength, intermolecular energy transfer from a Ru^II -terpyridine unit to a fullerene acceptor can be favored over electron transfer within the molecular triad. These results might have important implications for the design of molecular dyads, triads, pentads and so forth with respect to a specifically targeted response of these complexes to photoexcitation.

Reversible Photomodulation of Electronic Communication in a π-Conjugated Photoswitch-Fluorophore Molecular Dyad

The extent of electronic coupling between a boron dipyrromethene (BODIPY) fluorophore and a diarylethene (DAE) photoswitch has been modulated in a covalently linked molecular dyad by irradiation with either UV or visible light. In the open isomer, both moieties can be regarded as individual chromophores, while in the closed form the lowest electronic (S0 →S1 ) transition of the dyad is slightly shifted, enabling photomodulation of its fluorescence. Transient spectroscopy confirms that the dyad behaves dramatically different in the two switching states: while in the open isomer it resembles an undisturbed BODIPY fluorophore, in the closed isomer no fluorescence occurs and instead a red-shifted DAE behavior prevails.