Photoinduced Triplet-Triplet Energy Transfer in a 2-Ureido-4(1H)-Pyrimidinone-Bridged, Quadruply Hydrogen-Bonded Ferrocene-Fullerene Assembly

Radical-Enhanced Intersystem Crossing in Perylene-Oxoverdazyl Radical Dyads

Linking stable radicals to organic chromophores is an effective method to enhance the intersystem crossing (ISC) of chromophores. Herein we prepared perylene-oxoverdazyl dyads either by directly connecting the two units or using an intervening phenyl spacer. We investigated the effect of the radical on the photophysical properties of perylene and observed strong fluorescence quenching due to radical enhanced intersystem crossing (REISC). Compared with a previously reported perylene fused nitroxide radical compound (triplet lifetime = 0.1 µs), these new adducts show a longer-lived triplet excited state (9.5 µs). Based on the singlet oxygen quantum yield (7%), we propose that the radical enhanced internal conversion also plays a role in the relaxation of the excited state. Femtosecond fluorescence up-conversion indicates a fast decay of the excited state (<1.0 ps), suggesting a strong spin-spin exchange interaction between the two units. Femtosecond transient absorption (fs-TA) spectra confirmed direct triplet state population (within 0.5 ps). Interestingly, by fs-TA, we observed the interconversion of the two states (D1/Q1) at ~80 ps time scale. Time-resolved electron paramagnetic resonance (TREPR) spectral study confirmed the formation of the quartet sate ,  we observed triplet and quartet states simultaneously with weights of 0.7 and 0.3, respectively. DFT computations showed that the interaction between radical and chromophore is ferromagnetic ( J >0, 0.05~0.10 eV).

Ultrafast Vibrational Energy Transfer through the Covalent Bond and Intra- and Intermolecular Hydrogen Bonds in a Supramolecular Dimer by Two-Dimensional Infrared Spectroscopy

In this work, the structural fluctuations and vibrational energy transfer dynamics in a supramolecular homodimer model, which is composed of 2-(9-anthracene)ureido-6-(1-undecyl)-4[1H]-pyrimidinone (UPAn) with quadruple intermolecular and single intramolecular hydrogen bonds (HBs), have been examined using ultrafast two-dimensional infrared (2D IR) and steady-state IR spectroscopies. A less structurally fluctuating intermolecular HB is found between the pyrimidinone C═O and ureido N-H groups. However, a larger structurally fluctuating intramolecular HB is suggested by the equilibrium and dynamical line-shape measurements of the ureido C═O stretch. Further, dynamical time-dependent 2D IR diagonal and off-diagonal signals show that intra- and intermolecular vibrational energy transfer processes occur on the picosecond timescale, where the latter is more efficient due to intermolecular hydrogen bonding interaction and through-space interaction.

The unquenched triplet excited state of the fluorescent OFF/ON Bodipy-derived molecular probe based on photo-induced electron transfer

Different from the singlet excited state (fluorescence), the triplet state of the probes is not quenched by photo-induced electron transfer.

Bodipy-C60 triple hydrogen bonding assemblies as heavy atom-free triplet photosensitizers: preparation and study of the singlet/triplet energy transfer

Supramolecular triplet photosensitizers based on hydrogen bonding-mediated molecular assemblies were prepared. Three thymine-containing visible light-harvesting Bodipy derivatives (B-1, B-2 and B-3, which show absorption at 505 nm, 630 nm and 593 nm, respectively) were used as H-bonding modules, and 1,6-diaminopyridine-appended C60 was used as the complementary hydrogen bonding module (C-1), in which the C60 part acts as a spin converter for triplet formation. Visible light-harvesting antennae with methylated thymine were prepared as references (B-1-Me, B-2-Me and B-3-Me), which are unable to form strong H-bonds with C-1. Triple H-bonds are formed between each Bodipy antenna (B-1, B-2 and B-3) and the C60 module (C-1). The photophysical properties of the H-bonding assemblies and the reference non-hydrogen bond-forming mixtures were studied using steady state UV/vis absorption spectroscopy, fluorescence emission spectroscopy, electrochemical characterization, and nanosecond transient absorption spectroscopy. Singlet energy transfer from the Bodipy antenna to the C60 module was confirmed by fluorescence quenching studies. The intersystem crossing of the latter produced the triplet excited state. The nanosecond transient absorption spectroscopy showed that the triplet state is either localized on the C60 module (for assembly B-1·C-1), or on the styryl-Bodipy antenna (for assemblies B-2·C-1 and B-3·C-1). Intra-assembly forward-backward (ping-pong) singlet/triplet energy transfer was proposed. In contrast to the H-bonding assemblies, slow triplet energy transfer was observed for the non-hydrogen bonding mixtures. As a proof of concept, these supramolecular assemblies were used as triplet photosensitizers for triplet-triplet annihilation upconversion.

Polysaccharide–porphyrin–fullerene supramolecular conjugates as photo-driven DNA cleavage reagents

Water-soluble polysaccharide–porphyrin–fullerene supramolecular conjugates were constructed, showing the capability of completely cleaving closed supercoiled DNA to nicked DNA.

Maximizing the thiol-activated photodynamic and fluorescence imaging functionalities of theranostic reagents by modularization of Bodipy-based dyad triplet photosensitizers

The fluorescence imaging and singlet oxygen production ability of thiol-cleavable triplet photosensitizers was maximized by disintegration of the functionalities.

Switching of the photophysical properties of Bodipy-derived trans bis(tributylphosphine) Pt(ii) bisacetylide complexes with rhodamine as the acid-activatable unit

A rhodamine moiety was used in two Bodipy-derived trans bis(tributylphosphine) Pt(ii) bisacetylide complexes for switching the photophysical properties of the complexes.

Switching of the triplet excited state of rhodamine/naphthaleneimide dyads: an experimental and theoretical study

Rhodamine-bromonaphthaleneimide (RB-NI) and rhodamine-bromonaphthalenediimide (RB-NDI) dyads were prepared for switching of the triplet excited states. Bromo-NI or bromo-NDI parts in the dyads are the spin converters, i.e., the triplet state producing modules, whereas the RB unit is the acid-activatable electron donor/energy acceptor. NI and NDI absorb at 359 and 541 nm, and the T1 state energy levels are 2.25 and 1.64 eV, respectively. RB undertakes the reversible spirolactam (RB-c) ↔ opened amide (RB-o) transformation. RB-c shows no visible light absorption, and the triplet-state energy level is ET1 = 3.36 eV. Conversely RB-o shows strong absorption at 557 nm, and ET1 is 1.73 eV. Thus, the acid-activated fluorescence-resonance-energy-transfer (FRET) competes with the ISC of NI or NDI. No triplet state was observed for the dyads with nanosecond time-resolved transient absorption spectroscopy. Upon addition of acid, strong fluorescence and long-living triplet excited states were observed. Thus, the producing of triplet state is acid-activatable. The triplet state of RB-NI is localized on RB-o part, whereas in RB-NDI the triplet state is delocalized on both the NDI and RB-o units. The ISC of spin converter was not outcompeted by RET. These studies are useful for switching of triplet excited state.