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Controlling the excited-state dynamics of low band gap, near-infrared absorbers via proquinoidal unit electronic structural modulation.

ABSTRACT: While the influence of proquinoidal character upon the linear absorption spectrum of low optical bandgap ?-conjugated polymers and molecules is well understood, its impact upon excited-state relaxation pathways and dynamics remains obscure. We report the syntheses, electronic structural properties, and excited-state dynamics of a series of model highly conjugated near-infrared (NIR)-absorbing chromophores based on a (porphinato)metal(ii)-proquinoidal spacer-(porphinato)metal(ii) (PM-Sp-PM) structural motif. A combination of excited-state dynamical studies and time-dependent density functional theory calculations: (i) points to the cardinal role that excited-state configuration interaction (CI) plays in determining the magnitudes of S1 ? S0 radiative (kr), S1 ? T1 intersystem crossing (kISC), and S1 ? S0 internal conversion (kIC) rate constants in these PM-Sp-PM chromophores, and (ii) suggests that a primary determinant of CI magnitude derives from the energetic alignment of the PM and Sp fragment LUMOs (?EL). These insights not only enable steering of excited-state relaxation dynamics of high oscillator strength NIR absorbers to realize either substantial fluorescence or long-lived triplets (?T1 > ?s) generated at unit quantum yield (?ISC = 100%), but also crafting of those having counter-intuitive properties: for example, while (porphinato)platinum compounds are well known to generate non-emissive triplet states (?ISC = 100%) upon optical excitation at ambient temperature, diminishing the extent of excited-state CI in these systems realizes long-wavelength absorbing heavy-metal fluorophores. This work highlights approaches to: (i) modulate low-lying singlet excited-state lifetime over the picosecond-to-nanosecond time domain, (ii) achieve NIR fluorescence with quantum yields up to 25%, (iii) tune the magnitude of S1-T1 ISC rate constant from 109 to 1012 s-1 and (iv) realize T1-state lifetimes that range from ?0.1 to several ?s, for these model PM-Sp-PM chromophores, and renders new insights to evolve bespoke photophysical properties for low optical bandgap ?-conjugated polymers and molecules based on proquinoidal conjugation motifs.

PROVIDER: S-EPMC5619129 | BioStudies |

REPOSITORIES: biostudies

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