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

Abstract

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)