Probing the electronic structure and photophysics of thiophene-diketopyrrolopyrrole derivatives in solution.

Diketopyrrolopyrroles are a popular class of electron-withdrawing unit in optoelectronic materials. When combined with electron donating side-chain functional groups such as thiophenes, they form a very broad class of donor-acceptor molecules: thiophene-diketopyrrolopyrroles (TDPPs). Despite their widescale use in biosensors and photovoltaic materials, studies have yet to establish the important link between the electronic structure of the specific TDPP and the critical optical properties. To bridge this gap, ultrafast transient absorption with 22 fs time resolution has been used to explore the photophysics of three prototypical TDPP molecules: a monomer, dimer and polymer in solution. Interpretation of experimental data was assisted by a recent high-level theoretical study, and additional density functional theory calculations. These studies show that the photophysics of these molecular prototypes under visible photoexcitation are determined by just two excited electronic states, having very different electronic characters (one is optically bright, the other dark), their relative energetic ordering and the timescales for internal conversion from one to the other and/or to the ground state. The underlying difference in electronic structure alters the branching between these excited states and their associated dynamics. In turn, these factors dictate the fluorescence quantum yields, which are shown to vary by ∼1-2 orders of magnitude across the TDPP prototypes investigated here. The fast non-radiative transfer of molecules from the bright to dark states is mediated by conical intersections. Remarkably, wavepacket signals in the measured transient absorption data carry signatures of the nuclear motions that enable mixing of the electronic-nuclear wavefunction and facilitate non-adiabatic coupling between the bright and dark states.

High-Efficiency Excitation Energy Transfer in Biohybrid Quantum Dot-Bacterial Reaction Center Nanoconjugates.

Reaction centers (RCs) are the pivotal component of natural photosystems, converting solar energy into the potential difference between separated electrons and holes that is used to power much of biology. RCs from anoxygenic purple photosynthetic bacteria such as Rhodobacter sphaeroides only weakly absorb much of the visible region of the solar spectrum, which limits their overall light-harvesting capacity. For in vitro applications such as biohybrid photodevices, this deficiency can be addressed by effectively coupling RCs with synthetic light-harvesting materials. Here, we studied the time scale and efficiency of Förster resonance energy transfer (FRET) in a nanoconjugate assembled from a synthetic quantum dot (QD) antenna and a tailored RC engineered to be fluorescent. Time-correlated single-photon counting spectroscopy of biohybrid conjugates enabled the direct determination of FRET from QDs to attached RCs on a time scale of 26.6 ± 0.1 ns and with a high efficiency of 0.75 ± 0.01.

Singlet and Triplet Contributions to the Excited-State Activities of Dihydrophenazine, Phenoxazine, and Phenothiazine Organocatalysts Used in Atom Transfer Radical Polymerization.

The photochemical dynamics of three classes of organic photoredox catalysts employed in organocatalyzed atom-transfer radical polymerization (O-ATRP) are studied using time-resolved optical transient absorption and fluorescence spectroscopy. The nine catalysts selected for study are examples of N-aryl and core-substituted dihydrophenazine, phenoxazine and phenothiazine compounds with varying propensities for control of polymerization outcomes. Excited singlet-state lifetimes extracted from the spectroscopic measurements are reported in N,N-dimethylformamide (DMF), dichloromethane (DCM), and toluene. Ultrafast (<200 fs to 3 ps) electronic relaxation of the photocatalysts after photoexcitation at near-UV wavelengths (318-390 nm) populates the first singlet excited state (S1). The S1-state lifetimes range from 130 ps to 40 ns with a considerable dependence on the photocatalyst structure and the solvent. The competition between ground electronic state recovery and intersystem crossing controls triplet state populations and is a minor pathway in the dihydrophenazine derivatives but is of greater importance for phenoxazine and phenothiazine catalysts. A comparison of our results with previously reported O-ATRP performances of the various photoredox catalysts shows that high triplet-state quantum yields are not a prerequisite for controlling polymer dispersity. For example, the photocatalyst 5,10-bis(4-cyanophenyl)-5,10-dihydrophenazine, shown previously to exert good polymerization control, possesses the shortest S1-state lifetime (135 ps in DMF and 180 ps in N,N-dimethylacetamide) among the nine examples reported here and a negligible triplet-state quantum yield. The results call for a re-evaluation of the excited-state properties of most significance in governing the photocatalytic behavior of organic photoredox catalysts in O-ATRP reactions.

Following Bimolecular Excited-State Proton Transfer between Hydroxycoumarin and Imidazole Derivatives.

The ultrafast dynamics of a bimolecular excited-state proton transfer (ESPT) reaction between the photoacid 7-hydroxy-4-(trifluoromethyl)-1-coumarin (CouOH) and 1-methylimidazole (MI) base in aprotic chloroform- d1 solution were investigated using ultrafast transient infrared (TRIR) and transient absorption (TA) spectroscopies. The excited-state lifetime of the photoacid in solution is relatively short (52 ps), which at the millimolar photoacid and base concentrations used in our study precludes any diffusion-controlled bimolecular ESPT reactions. This allows the prompt ESPT reaction between hydrogen-bonded CouOH and MI molecules to be studied in isolation and the "contact" ESPT dynamics to be unambiguously determined. Our time-resolved studies reveal that ultrafast ESPT from the CouOH moiety to hydrogen-bonded MI molecules occurs within ∼1 ps, tracked by unequivocal spectroscopic signatures of CouO-* photoproducts that are formed in tandem with HMI+. Some of the ESPT photoproducts subsequently π-stack to form exciplexes on a ∼35 ps time scale, minimizing the attractive Coulombic forces between the oppositely charged aromatic molecules. For the concentrations of CouOH and MI used in our study (up to 8 mM), we saw no evidence for excited-state tautomerization of coumarin anions.

Deciphering hot- and multi-exciton dynamics in core-shell QDs by 2D electronic spectroscopies.

Although the harnessing of multiple and hot excitons is a prerequisite for many of the groundbreaking applications of semiconductor quantum dots (QDs), the characterization of their dynamics through conventional spectroscopic techniques is cumbersome. Here, we show how a careful analysis of 2DES maps acquired in different configurations (BOXCARS and pump-probe geometry) allows the tracking and visualization of intraband Auger relaxation mechanisms, driving the hot carrier cooling, and interband bi- and tri-exciton recombination dynamics. The results obtained on archetypal core-shell CdSe/ZnS QDs suggest that, given the global analysis of the resulting datasets, 2D electronic spectroscopy techniques can successfully and efficiently dispel the intertwined dynamics of fast and ultrafast recombination processes in nanomaterials. Hence, we propose this analysis scheme to be used in future research on novel quantum confined systems.