The ability of the doublet excited state of perylene diimide anion radical 2(PDI–•)* to reduce aromatic electron acceptors was probed by picosecond time-resolved transient absorption (TA) spectroscopy. Excitation of PDI–• produces visible TA due to 2(PDI–•)* that decays with τ = 160 ps. Aromatic electron acceptors with varying reduction potential quench 2(PDI–•)* and, in some cases, give a new visible region absorption that is attributed to the products of bimolecular photoinduced electron transfer, 2(PDI–•)* + Ar–X → PDI + Ar–X–•. Stern–Volmer quenching of 2(PDI–•)* accomplished with a series of acceptors provides bimolecular quenching rate constants as a function of acceptor reduction potential. Rehm–Weller analysis of the electron transfer quenching data affords the potential for the (*PDI–•/PDI) electrochemical half-reaction as −1.87 V vs SCE.

Fluorophores with high quantum yields, extended maximum emission wavelengths, and long photoluminescence (PL) lifetimes are still lacking for sensing and imaging applications in the second near‐infrared window (NIR‐II). In this work, a series of rod‐shaped icosahedral nanoclusters with bright NIR‐II PL, quantum yields up to ≈8%, and a peak emission wavelength of 1520 nm are reported. It is found that the bright NIR‐II emission arises from a previously ignored state with near‐zero oscillator strength in the ground‐state geometry and the central Au atom in the nanoclusters suppresses the non‐radiative transitions and enhances the overall PL efficiency. In addition, a framework is developed to analyze and relate the underlying transitions for the absorptions and the NIR‐II emissions in the Au nanoclusters based on the experimentally defined absorption coefficient. Overall, this work not only shows good performance of the rod‐shaped icosahedral series of Au nanoclusters as NIR‐II fluorophores, but also unravels the fundamental electronic transitions and atomic‐level structure‐property relations for the enhancement of the NIR‐II PL in gold nanoclusters. The framework developed here also provides a simple method to analyze the underlying electronic transitions in metal nanoclusters.

A series of 11 complexes of the type trans-(NHC) 2 Pt(CC-Ar) 2 (where NHC = N-heterocyclic carbene) have been synthesized and their photophysics characterized. The complexes display moderately efficient deep blue to green phosphorescence from a triplet excited state that is localized mainly in the aryl acetylide ligand (CC-Ar). The emission energy varies with the substituent on CC-Ar, with the highest energy emission for Ar = 4-pyridyl. The emission quantum efficiency and lifetime for the series decreases with increasing emission energy (E em ), and the effect is identified as arising from an increase in the nonradiative decay rate (k nr ) with E em . Temperature-dependent emission lifetime studies for three complexes give activation energies for the nonradiative decay process ∼1000 cm −1 , and the thermally activated decay process is attributed to crossing to a nonemissive metal-centered (d−d) excited state. At a low temperature, two different emission progressions are observed. Density functional theory calculations suggest that the triplet energy varies with the torsion of the aryl acetylide rings relative to the plane defined by the PtC 4 unit (where C = the carbon atoms bonded to Pt). The multiple emission is ascribed to emission from complexes differing with respect to the aryl acetylide ring torsion. Ultrafast transient absorption spectroscopy reveals a fast relaxation (∼5 ps) that may also be due to aryl acetylide ring torsional relaxation in the triplet excited state.

A series of linear thiophene oligomers containing 4, 6, 8, 10, and 12 thienylene units were synthesized and end-capped with naphthalene diimide (NDI) acceptors with the objective to study the effect of oligomer length on the dynamics of photoinduced electron transfer and charge recombination. The synthetic work afforded a series of nonacceptor-substituted thiophene oligomers, T, and corresponding NDI end-capped series, TNDI (where n is the number of thienylene repeat units). This paper reports a complete photophysical characterization study of the T and TNDI series by using steady-state absorption, fluorescence, singlet oxygen sensitized emission, two-photon absorption, and nanosecond-microsecond transient absorption spectroscopy. The thermodynamics of photoinduced electron transfer and charge recombination in the TNDI oligomers were determined by analysis of photophysical and electrochemical data. Excitation of the T oligomers gives rise to efficient fluorescence and intersystem crossing to a triplet excited state that is easily observed by nanosecond transient absorption spectroscopy. Bimolecular photoinduced electron transfer from the triplet states, T*, to N,N-dimethylviologen (MV) occurs, and by using microsecond transient absorption it is possible to assign the visible region absorption spectra for the one electron oxidized (polaron) states, T. The fluorescence of the TNDI oligomers is quenched nearly quantitatively, and no long-lived transients are observed by nanosecond transient absorption. These findings suggest that rapid photoinduced electron transfer and charge recombination occurs, NDI-(T)*-NDI → NDI-(T)-NDI → NDI-T-NDI. Preliminary femtosecond-picosecond transient absorption studies on TNDI reveal that both forward electron transfer and charge recombination occur with k > 10 s, consistent with both reactions being nearly activationless. Analysis with semiclassical electron transfer theory suggests that both reactions occur at near the optimum driving force where -ΔG ∼ λ.

Self-assembly of small molecules through noncovalent interactions into nanoscale architectures has been extensively studied in supramolecular chemistry. However, it is still challenging to develop a biologically inspired self-assembly system that functions in water with complex structure and dynamics by analogy with those found in nature. Here, we report a new water-soluble cationic porphyrin that undergoes adenosine triphosphate (ATP)-templated self-assembly into right-handed double-helical supramolecular structures. Direct observation of the porphyrin–ATP assembly by transmission electron microscopy has been accomplished. The assemblies consist of superhelical fibers with length greater than 1 μm and width ∼46 nm. The chiral superhelical fibers show reversible disassembly to monomers upon hydrolysis of ATP catalyzed by alkaline phosphatase (ALP), and the nanofibers can be re-formed with subsequent addition of ATP. Moreover, transient self-assembly of a chiral double helix is formed when ALP is present to consume ATP.

A trans-N-heterocyclic carbene (NHC) platinum(II) acetylide complex bearing phenyl acetylene ligands (NPtPE1) has been synthesized via the Hagihara reaction in 64% yield. The complex features spectrally narrow deep blue emission with a phosphorescence quantum yield (0.30) and lifetime (∼10 μs) in the solid state. The modest quantum yield and lifetime make NPtPE1 a candidate for incorporation into an organic light emitting diode (OLED). Prototype devices exhibited a maximum EQE of 8% with CIE (0.20,0.20). To the best of our knowledge, this is the first example of a platinum(II) acetylide bearing NHC ligands to be incorporated into an OLED.

Gold nanoclusters with near-infrared (NIR) photoluminescence (PL) have great potential as sensing and imaging materials in biomedical and bioimaging applications. In this work, Au21(S-Adm)15 and Au38S2(S-Adm)20 are used to unravel the underlying mechanisms for the improved quantum yields (QY), large Stokes shifts, and long PL lifetimes in gold nanoclusters. Both nanoclusters show decent PL QY. In particular, the Au38S2(S-Adm)20 nanocluster shows a bright NIR PL at 900 nm with QY up to 15% in normal solvents (such as toluene) at ambient conditions. The relatively lower QY for Au21(S-Adm)15 (4%) compared to that of Au38S2(S-Adm)20 is attributed to the lowest-lying excited state being symmetry-disallowed, as evidenced by the pressure-dependent antispectral shift of the absorption spectra compared to PL, yet Au21(S-Adm)15 maintains some emissive properties due to a nearby symmetry-allowed excited state. Furthermore, our results show that suppression of nonradiative decay due to the surface “lock rings”, which encircle the Au kernel and the surface “lock atoms” which bridge the fundamental Au kernel units (e.g., tetrahedra, icosahedra, etc.), is the key to obtaining high QYs in gold nanoclusters. The complicated excited-state processes and the small absorption coefficient of the band-edge transition lead to the large Stokes shifts and the long PL lifetimes that are widely observed in gold nanoclusters.

This study explores the effect of substitution of selenium (Se) for sulfur (S) on the photophysical properties of a series of π-conjugated donor-acceptor-donor chromophores based on 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT). The effect of Se substitution is studied systematically, where the substitution is in the thiophene donors only, the benzothiadiazole acceptor only, and in all of the positions. The fluorescence quantum yield decreases with an increase in Se substitution. Nanosecond-microsecond transient absorption and singlet oxygen sensitization experiments show that the effect of Se is due to an increase in the rate and efficiency of intersystem crossing with increased Se substitution. The relationship between intersystem crossing efficiency and heteroatom substitution pattern shows that the effects are largest when the heavy atom Se is in the acceptor benzothiadiazole unit. DFT calculations support the hypothesis that the effect arises because the LUMO is concentrated in the acceptor moiety, enhancing the spin-orbit coupling effect imparted by the Se atom.