Push–Pull Porphyrin-Containing Polymers: Materials Exhibiting Ultrafast Near-IR Photophysics

Four push–pull polymers of structure (CC–<b>[Zn]</b>–CC–<b>A</b>)_<i>n</i> (<b>A</b> = isoindigo (<b>P1</b>), bis­(α-methylamino-1,4-benzene)­quinone (<b>P2</b>), 2-(<i>N</i>-methylamino-1,4-benzene)-<i>N</i>-1,4-benzene-maleimide (<b>P3</b>), and 2,2′-anthraquinone (<b>P4</b>); <b>[Zn]</b> = [bis­(<i>meso</i>-aryl)­porphyrin]­zinc­(II) = donor) and models <b>M1</b> and <b>M2</b> (<b>A</b>′–CC–<b>[Zn]</b>–CC–<b>A</b>′; <b>A</b>′ = respectively naphtoquinone and 2-anthraquinone) were prepared and characterized (¹H and ¹³C NMR, elemental analysis, GPC, TGA, cyclic voltammetry, steady state and ultrafast time-resolved UV–vis and emission spectroscopy) and studied by density functional theory (DFT) and time-dependent DFT (TDDFT) in order to address the nature of the low-lying singlet and triplet excited states. <b>P1</b> (fully conjugated polymer), <b>P2</b> (formally nonconjugated but exhibit strong electronic communication accross the chain) and <b>P4</b> (formally nonconjugated but local conjugation between the donor and acceptor) are near-IR emitters (λ_max > 750 nm). <b>M1</b> and <b>M2</b> are mono-CC–<b>[Zn]</b>–CC species, and <b>P3</b> exhibits a very modest CT contribution (as maleimide is a weak acceptor) and are not near-IR emitters. The nature of the S₁ and T₁ excited states are CT processes donor* → acceptor. In <b>P1</b>–<b>P4</b>, a dual fluorescence (7.7 < τ_F < 770 ps; except one value at 2.5 ns; <b>P3</b>) is depicted, which are assigned to fluorescences arising from the terminal and central units of the polymers identified from the comparison with <b>M1</b> and <b>M2</b>. The high and low energy fluorescences are respectively short (77 < τ_F < 166 ps) and long-lived (688 < τ_F < 765 ps) suggesting S₁ energy transfers with rates, k_ET, of 7.1 (<b>P1</b>), 12 (<b>P2</b>) and 4.5 (ns)⁻¹ (<b>P4</b>). The fs transient absorption spectra exhibit particularly very short triplet lifetimes (2.3 < τ_T1 < 87 ns) explaining the absence of phosphorescence. Also ultrafast lifetimes (85 < τ < 1290 fs) for species excited in the 0–0 peak of the Q-band (650 nm; i.e., ππ* porphyrin level) indicating its rather efficient nonradiative deactivation (S_<i>n</i> ∼ > S₁ and S_<i>n</i> ∼ > <i>T</i>_m). When cooling takes place or the solution concentration is increased, new red-shifted fluorescence bands appear, evidencing aggregate formation. Both fluorescence and transient absorption lifetimes of <b>P1</b>–<b>P4</b> become shorter and their band intensity lower. Finally, the position of the optically silent phosphorescence has been predicted to be in the 1300 (<b>P1</b>, <b>P2</b>) and 1000 nm (<b>P3</b>, <b>P4</b>) zones (DFT)

Ultrafast Electron Transfers in Organometallic Supramolecular Assemblies Built with a NIR-Fluorescent Tetrabenzoporphyrine Dye and the Unsaturated Cluster Pd₃(dppm)₃(CO)²⁺

The sodium 9,18,27,36-tetra-(4-carboxyphenyl­ethynyl)­tetrabenzo­porphyrinatozinc­(II) (<b>TCPEBP</b>) and sodium 5,10,15,20-tetra-(4-carboxy­phenyl­ethynyl)­porphyrinatozinc­(II) (<b>TCPEP</b>, for comparison purposes) salts were prepared to investigate the ionic driven host–guest assemblies made with the unsaturated redox-active cluster Pd₃(dppm)₃(CO)²⁺ (<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>, dppm = Ph₂PCH₂PPh₂ as a PF₆^– salt). Nonemissive dye···<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i></b>} assemblies (<i>x</i> = 1–4) are formed in methanol with <i>K</i>_1<i>x</i> (binding constants) values of 83 200 (<b>TCPEBP</b>) and 70 400 M^–1 (<b>TCPEP</b>; average values extracted from graphical methods (Benesi–Hildebrand, Scott, and Scatchard), matching those obtained from fluorescence quenching experiments (static model)). These values are consistent with the more electron rich <b>TCPEBP</b> dye. This conclusion is corroborated by electrochemical data, which indicate a lower oxidation potential of the <b>TCPEBP</b> dye (+0.46 V) vs <b>TCPEP</b> (+0.70 V vs SCE) and by shorter calculated average Pd···O distances (DFT (B3LYP): 3.259 vs 3.438 Å, respectively). Using the position of the 0–0 component of the Q-bands and the electrochemical data, the excited-state driving forces for dye*···<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i></b>} <b> → </b> dye^<b>+•</b>···<b>[Pd</b>_<b>3</b>^<b>+•</b><b>]­[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i>–1</b>} are estimated for <b>TCPEBP</b> (+1.22 V vs SCE) and <b>TCPEP</b> (1.08 V vs SCE). The time scale for this process occurs within the laser pulse (fwhm <75–110 fs) during the measurements of the femtosecond transient absorption spectra. Conversely, the back electron transfers (dye^<b>+•</b>···<b>[Pd</b>_<b>3</b>^<b>+•</b><b>]­[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i>–1</b>} <b> → </b> dye···<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i></b>}) occur well within 1 ps (respectively 650 and 170 fs for <b>TCPEBP</b> and <b>TCPEP</b>). Arguments are provided that the reorganization energy governs this difference

Electron-Transfer Kinetics within Supramolecular Assemblies of Donor Tetrapyrrolytic Dyes and an Acceptor Palladium Cluster

9,18,27,36-Tetrakis­[<i>meso</i>-(4-carboxyphenyl)]­tetrabenzoporphyrinatozinc­(II) (TCPBP, as a sodium salt) was prepared in order to compare its photoinduced electron-transfer behavior toward unsaturated cluster Pd₃(dppm)₃(CO)²⁺ ([Pd₃²⁺]; dppm = Ph₂PCH₂PPh₂ as a PF₆^– salt) with that of 5,10,15,20-tetrakis­[<i>meso</i>-(4-carboxyphenyl)]­porphyrinatozinc­(II) (TCPP) in nonluminescent assemblies of the type dye···[Pd₃²⁺]_<i>x</i> (<i>x</i> = 0–4; dye = TCPP and TCPBP) using femtosecond transient absorption spectroscopy. Binding constants extracted from UV–vis titration methods are the same as those extracted from fluorescence quenching measurements (static model), and both indicate that the TCPBP···[Pd₃²⁺]_<i>x</i> assemblies (<i>K</i>₁₄ = 36000 M^–1) are slightly more stable than those for TCPP···[Pd₃²⁺]_{<b><i>x</i></b>} (<i>K</i>₁₄ = 27000 M^–1). Density functional theory computations (B3LYP) corroborate this finding because the average ionic Pd···O distance is shorter in the TCPBP···[Pd₃²⁺] assembly compared to that for TCPP···[Pd₃²⁺]. Despite the difference in the binding constants and excited-state driving forces for the photoinduced electron transfer in dye*···[Pd₃²⁺] → dye^•+···[Pd₃^•+], the time scale for this process is ultrafast in both cases (<85 fs). The time scales for the back electron transfers (dye^•+···[Pd₃^•+] → dye···[Pd₃²⁺]) occurring in the various observed species (dye···[Pd₃²⁺]_<i>x</i>; <i>x</i> = 0–4) are the same for both series of assemblies. It is concluded that the structural modification on going from porphyrin to tetrabenzoporphyrin does not greatly affect the kinetic behavior in these processes

Reduced and Oxidized Forms of the Pt-Organometallic Version of Polyaniline

This work represents an effort to synthesize all four forms of polyaniline (PANI) in its organometallic versions. Polymers containing substituted 1,4-benzoquinone diimine or 1,4-diaminobenzene units in the backbone exhibiting the general structure (CCC₆H₄–NC₆X₄N–C₆H₄CC-PtL₂)_<i>n</i> and (CCC₆H₄NH-C₆X₄-NHC₆H₄CC-PtL₂)_<i>n</i> along with the corresponding model compounds (CCC₆H₄–NC₆X₄N–C₆H₄CC)­(PtL₂Cl)₂ and (CCC₆H₄NH-C₆X₄-NHC₆H₄CC)­(PtL₂Cl)₂ (L = PBu₃; X = H, F, Cl) were synthesized. The polymers and corresponding model compounds were characterized (including ¹H and ³¹P NMR, IR, mass spectra, elemental analysis, and X-ray structure determinations) and investigated for their redox properties in the absence and in the presence of acid. Their optical properties, including ns transient spectroscopy were also investigated. These properties were interpreted through density functional theory (DFT) and time-dependent DFT (TDDFT) computations. These materials are found to be oligomers (GPC) with thermal stability (TGA) reaching 350 °C. The greatest stabilities were found in the cases with X = F. Using a data bank of 8 X-ray structures of diimine derivatives, a relationship between the CN bond distance and the dihedral angle between the benzoquinone ring and the flanking phenyl planes is noted. As the size of the substituent X on the benzoquinone center increases, the degree of conjugation decreases as demonstrated by the CN bond length. The largest dihedral angles are noted for X = Cl. These polymers exhibit drastic chemical differences when X is varied (X = H, F, Cl). The completely reduced polymer (CCC₆H₄NH-C₆H₄–NHC₆H₄CC-PtL₂)_<i>n</i> (i.e., X = H) was not chemically accessible whereas in the cases of X = F, Cl, these materials were obtained and represent the first examples of fully reduced organometallic versions of PANI (i.e., leucoemaraldine). For the (CCC₆H₄–NC₆X₄N–C₆H₄CC-PtL₂)_<i>n</i> polymers, the completely oxidized form for X = H was isolated (pernigraniline), but for X = F and Cl, only the largely reduced mixed-valence form (i.e., emaraldine) was obtained via chemical routes. In acidic solutions, the chemically accessible polymer for X = H, (CCC₆H₄–NC₆H₄N–C₆H₄CC-PtL₂)_<i>n</i>, exhibits two chemically reversible waves indicating that the reduced form (CCC₆H₄NH-C₆H₄–NHC₆H₄CC-PtL₂)_<i>n</i> can be generated. The absorption spectra of the highly colored diimine-containing species exhibit a broad charge transfer band (assigned based on DFT calculations (B3LYP); C₆H₄CC-PtL₂-CCC₆H₄ → NC₆X₄N) in the 450–800 nm window red shifting according X = H → Cl → F, consistent with their relative inductive effect. The largest absorptivity is measured for X = H because this polymer is fully oxidized whereas for the cases where X = F and Cl, these polymers exists in the mixed valence form. The ns transient absorption spectra of two polymers (X = F; reduced and mixed-valence polymers) were measured. The triplet excited state in the mixed-valence polymer is dominated by the reduced diamine residue and the T₁-T_<i>n</i> absorption of the diimine is entirely quenched

Profound Substituent Effect on the Structural, Optical, Photophysical, and Electrochemical Properties of <i>N</i>,<i>N</i>′‑Anthraquinone Diimine-Containing Platinum–Organometallic Polymers

A series of conjugated organometallic-substituted derivatives of anthraquinone diimine-(<i>trans</i>-diethynylbis­(tri-<i>n</i>-butylphosphine)­platinum­(II)-containing polymers along with the corresponding mixed-valence analogues bis­(4-phenylamino)­anthracene and anthraquinone diimine and model compounds (substituent = H, Br, 2Br, NH₂, 2NH₂) were investigated by electrochemistry, UV–visible and emission spectroscopy, photophysics, and DFT and TDDFT computations, as a means to shine light on the electronic communication across the chain of the seemingly unconjugated polyaniline in its emeraldine form and to examine the substituent effect