Excited Charge Transfer Promoted Electron Transfer in all Perylenediimide Derived, Wide-Band Capturing Conjugates: A Mimicry of the Early Events of Natural Photosynthesis

Fundamental discoveries in electron transfer advance scientific and technological advancements. It is suggested that in plant and bacterial photosynthesis, the primary donor, a chlorophyll or bacteriochlorophyll dimer, forms an initial excited symmetry-breaking charge transfer state (1CT*) upon photoexcitation that subsequently promotes sequential electron transfer (ET) events. This is unlike monomeric photosensitizer-bearing donor-acceptor dyads where ET occurs from the excited donor or acceptor (1D* or 1A*). In the present study, we successfully demonstrated the former photochemical event using an excited charge transfer molecule as a donor. Electron-deficient perylenediimide (PDI) is functionalized with three electron-rich piperidine entities at the bay positions, resulting in a far-red emitting CT molecule (DCT). Further, this molecule is covalently linked to another PDI (APDI) carrying no substituents at the bay positions, resulting in wide-band capturing DCT-APDI conjugates. Selective excitation of the CT band of DCT in these conjugates leads to an initial 1DCT* that undergoes subsequent ET involving APDI, resulting in DCT +-APDI - charge separation product (kCS~109 s-1). Conversely, when APDI was directly excited, ultrafast energy transfer (ENT) from 1APDI* to DCT (kENT~1011 s-1) followed by ET from 1DCT* to PDI is witnessed. While increasing solvent polarity improved kCS rates, for a given solvent, the magnitude of the kCS values was almost the same, irrespective of the excitation wavelengths. The present findings demonstrate ET from an initial CT state to an acceptor is key to understanding the intricate ET events in complex natural and bacterial photosynthetic systems possessing multiple redox- and photoactive entities.

Panchromatic Light-Capturing Bis-styryl BODIPY-Perylenediimide Donor-Acceptor Constructs: Occurrence of Sequential Energy Transfer Followed by Electron Transfer

Two wide-band-capturing donor-acceptor conjugates featuring bis-styrylBODIPY and perylenediimide (PDI) have been newly synthesized, and the occurrence of ultrafast excitation transfer from the 1 PDI* to BODIPY, and a subsequent electron transfer from the 1 BODIPY* to PDI have been demonstrated. Optical absorption studies revealed panchromatic light capture but offered no evidence of ground-state interactions between the donor and acceptor entities. Steady-state fluorescence and excitation spectral recordings provided evidence of singlet-singlet energy transfer in these dyads, and quenched fluorescence of bis-styrylBODIPY emission in the dyads suggested additional photo-events. The facile oxidation of bis-styrylBODIPY and facile reduction of PDI, establishing their relative roles of electron donor and acceptor, were borne out by electrochemical studies. The electrostatic potential surfaces of the S1 and S2 states, derived from time-dependent DFT calculations, supported excited charge transfer in these dyads. Spectro-electrochemical studies on one-electron-oxidized and one-electron-reduced dyads and the monomeric precursor compounds were also performed in a thin-layer optical cell under corresponding applied potentials. From this study, both bis-styrylBODIPY⋅+ and PDI⋅- could be spectrally characterizes and were subsequently used in characterizing the electron-transfer products. Finally, pump-probe spectral studies were performed in dichlorobenzene under selective PDI and bis-styrylBODIPY excitation to secure energy and electron-transfer evidence. The measured rate constants for energy transfer, kENT , were in the range of 1011  s-1 , while the electron transfer rate constants, kET , were in the range of 1010  s-1 , thus highlighting their potential use in solar energy harvesting and optoelectronic applications.

Preparation of zinc phthalocyanine-loaded amphiphilic phosphonium chitosan nanomicelles for enhancement of photodynamic therapy efficacy

To increase the solubility and the encapsulation of zinc phthalocyanine (ZnPc) photosensitizer for photodynamic therapy (PDT), a positively charged amphiphilic phosphonium chitosan nanomicelle with multi-benzene structure was developed, and its application to PDT was explored. N-acetyl-l-phenylalanine-(4-carboxybutyl) triphenylphosphonium bromide chitosan (CTPB-CS-NAP), a chitosan derivative with tunable amphiphilicity, was synthesized first. ZnPc was encapsulated in CTPB-CS-NAP at the critical micelle concentration (CMC) of 4.898 mg/L by a hydrophobic self-assembly method to form ZnPc-loaded nanomicelles (ZnPc@CTPB-CS-NAP). The method gives the highest encapsulation efficiency and drug loading of 89.4 % and 22.3 %, respectively. ZnPc@CTPB-CS-NAP is stably dispersed in aqueous solution and shows the average particle size of 103±5 nm. PDT experiments suggest the phototoxicity of ZnPc@CTPB-CS-NAP is much higher than that of ZnPc, but no obvious dark cytotoxicity is observed. Our study has provided a new strategy for improving the photodynamic therapy efficacy of hydrophobic photosensitizer by the encapsulation with chitosan derivative carriers.