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A series of 11 complexes of the type trans-(NHC)2Pt(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 (Eem), and the effect is identified as arising from an increase in the nonradiative decay rate (knr) with Eem. 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 PtC4 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.

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.

iClick reactions between Au(I) acetylides PPh3Au-C≡CR, where R = nitrophenyl (PhNO2), phenyl (Ph), thiophene (Th), bithiophene (biTh), and dimethyl aniline (PhNMe2), and Au(I)-azide PPh3AuN3 provide digold complexes of the general formula R-1,5-bis-triphenylphosphinegold(I) 1,2,3-triazolate (Au2-R). Within the digold triazolate complexes the Au(I) atoms are held in close proximity but beyond the distance typically observed for aurophilic bonding. Though no bond exists in the ground state, time-dependent density functional theory interrogation of the complexes reveals excited states with significant aurophilic bonding. The series of complexes allows for tuning of the excited-state "turn-on" of aurophilicity, where ligand to metal charge transfer (LMCT) induces the aurophilic bonding. Complexes containing ligand-localized excited states, however, do not exhibit aurophilicity in the excited state. As a control experiment, a monogold complex was synthesized. The computed excited state of the monogold species exhibited LMCT to the gold ion as in the dinuclear cases, but without a partnering gold ion only a distinct N-Au-P bending occurs, revealing a potential mechanism for the excited-state turn-on of aurophilic bonding. Analysis of the steady-state electronic spectra indicates that LMCT states are achievable for compounds with sufficiently strong electron-donating ligands, and in digold complexes this is associated with enhanced fluorescence, suggestive of an aurophilic interaction.

This report describes the iClick synthesis and characterization of a metallopolymer that contains contiguous conjugation across the polymer repeat units and Pt(II) ions. A bifunctional A–Pt-B monomer, where A = azido and B = acetylide, was synthesized. Using a Cu(I) source, the A–Pt-B monomer was polymerized via azide–alkyne cycloaddition to give a low-molecular-weight metallopolymer. Photophysical interrogation and computation modeling of the metallopolymer indicates limited conjugation and the spectra are dominated by ligand-centered transitions and emission.

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.

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.

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, Tn, and corresponding NDI end-capped series, TnNDI2 (where n is the number of thienylene repeat units). This paper reports a complete photophysical characterization study of the Tn and TnNDI2 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 TnNDI2 oligomers were determined by analysis of photophysical and electrochemical data. Excitation of the Tn oligomers gives rise to efficient fluorescence and intersystem crossing to a tr...

The interactions of two anionic poly(phenylene ethynylene) sulfonate-conjugated polyelectrolytes (mPPESO3– and pPPESO3–) with two alkynylplatinum(II) terpyridine complexes (Pt2+ and Pt3+) were studied. The Pt(II) complexes interact with helical mPPESO3– by intercalation within the polymer helix to form a “guest–host” ensemble. Titration of Pt(II) complexes into an aqueous solution of mPPESO3– gives rise to efficient quenching of the polymer’s fluorescence; meanwhile, triplet metal–metal-to-ligand charge transfer (3MMLCT) state emission from the intercalated Pt(II) complexes appears when the ensembles are excited into the polymer’s absorption band. The 3MMLCT state emission implies that the Pt(II) complexes aggregate or dimerize on the mPPESO3– scaffold. The responses of the mPPESO3– and Pt(II) complex ensembles to various proteins were examined by monitoring the mPPESO3– fluorescence change. Negatively charged proteins recover the mPPESO3– fluorescence more than the positively charged proteins under physi...

Photoinduced charge transfer is a key step in the mechanism of charge generation in organic solar cells. Charge transfer typically occurs from a photoexcited conjugated polymer donor to an electron acceptor. In an effort to better understand the primary events in solar cells, we have investigated photoinduced charge transfer in model donor-acceptor systems consisting of pi-conjugated oligomer donors that are covalently linked to diimide electron acceptors. These studies utilized oligo(thiophene), oligo(phenylene ethynylene) and oligo(fluorene) pi-conjugated systems with lengths varying from 4 to 12 repeat units linked to naphthalene diimide electron acceptors. Excitation with 100 femtosecond pulses at wavelengths correspoinding to the conjugated oligomer absorption band(s) leads to rapid photoinduced charge transfer to produce a charge separated state, (oligomer+)-(NDI-), which subsequently decays on timescales ranging from 100 ps to 5 ns. The dynamics of the forward and reverse electron transfer reactions depend strongly on the structure and length of the pi-conjugated oligomers, with the fastest rates occurring for oligo(thiophene)s, and considerably slower rates for oligo(phenylene ethynylene)s. The talk will discuss the structure-property relationships and energetic correlations that control the dynamics of charge separation and recombination.