Probing and Managing Photo-Induced Electronically Excited-State Dynamics of Molecular to Nanoscale Systems
Yusong Bai, Ph.D. candidate
Michael Therien, advisor
Friday, November 17, 2017 - 1:00pm to 2:00pm
Location: French Family Science Center 3225
Avery, Meg

Light-matter interactions, spectroscopically depicted by the generation and decay of materials’ photo-excited states, are the core to majority of modern optic and optoelectronic technologies that include photovoltaics (PVs), optical power limiting (OPL), bioimaging, spintronics, photodetections, etc. Achieving high performance at device levels in these applications, thus, often requires deep understanding towards excited state dynamics of the relevant materials and capabilities to manage the fate of the corresponding photo-excited species at microscopic levels. In this regard, the current study exploits spectroscopic techniques and computational analyses to probe structure-property relationships of various molecular to nanoscale conjugated materials, and then tunes their excited-state dynamical properties based on the established relationships.

The herein investigated materials include ethyne-bridged (polypyridyl)metal(II) (M)-(porphinato)metal(II) (PM’) (M-(PM’)n-M; M = Ru, Os; M’ = Zn, Pt, Pd) and (porphinato)metal(II) (PM)-proquinoidal spacer (Sp)-(porphinato)metal(II) (PM-Sp-PM; M = Zn, Pt, Pd) supermolecular architectures, as well as semiconducting single-walled carbon nanotubes (SWNTs) wrapped by binaphthalene-based polyanionic semiconducting polymers. For majority of the work, steady-state electronic absorption, steady-state and time-resolved emission, and pump-probe transient absorption spectroscopies were employed in conjunction with time-dependent density functional theory (TD-DFT) calculations to interrogate the nature of the photo-excited states and the subsequent relaxation dynamics of various conjugated materials.

At a molecular level, this work shows that many unusual, but desirable photophysical properties can be engineered through (i) coupling distinct chromophoric oscillator photophysics via an ethyne-linkage topology; (ii) varying the nature of the initially prepared electronically excited state wavefunctions. Along this line, two new libraries of NIR-active supermolecular chromophores (i.e. M-(PM’)n-M and PM-Sp-PM) have been established, wherein exceptional red-to-NIR spectral coverage and tunable excited-state relaxation dynamics (i.e. S1→S0 radiative decay, S1→S0 internal conversion, S1→T1 intersystem crossing, and T1→S0 decay rate constants) are simultaneously attained, making M-(PM’)n-M and PM-Sp-PM well poised for a number of optoelectronic applications that include OPL, triplet-triplet annihilation photon upconversion (TTA UC), and NIR bioimaging. Moving to nanoscale systems, this work probes the dynamics of charged excitons (i.e. trions) in electronically and morphologically homogeneous SWNTs, which demonstrate: (i) trion quasiparticles exclusively derive from a precursor exciton state, (ii) exciton-to-trion conversion can approach unity under appropriate excitation and charge-doping conditions. Importantly, because trions simultaneously carry excitation energy, charge and spin, the findings here may guide design of new SWNT-based optoelectronic devices that include photovoltaics, photodetectors, and spintronics.