Spectroscopic Quantification of the Inverted Singlet-Triplet Gap in Pentaazaphenalene.

We report the direct and accurate spectroscopic quantification of the inverted singlet-triplet gap in 1,3,4,6,9b-pentaazaphenalene. This measurement is achieved by directly probing the lowest singlet and triplet states via high-resolution cryogenic anion photoelectron spectroscopy. The assignment of the first excited singlet state is confirmed by visible absorption spectroscopy in an argon matrix at 20 K. Our measurements yield an inverted singlet-triplet gap with ΔEST= -0.047(7) eV. The accurate quantification of the singlet-triplet gap presented here allows for direct evaluation of various computational electronic structure methods and highlights the critical importance of the proper description of the double excitation character of these electronic states. Overall, this study validates the idea that despite Hund's multiplicity rule, useful organic chromophores can have inherently inverted singlet-triplet gaps.

Spectroscopy and Theoretical Modeling of Tetracene Anion Resonances.

The positions and widths of the optically allowed electronic states of the tetracene radical anion located above the detachment threshold energy (i.e anion resonances) are mapped out using total photodetachment yield spectroscopy of cryogenically cooled ions. The presence of these states is detected via the sharp increase in the photodetachment yield compared to that of the monotonic nonresonant direct photodetachment background. The resolution of the resulting spectrum is limited by the ∼5 cm-1 line width of the tunable laser and thus provides a stringent benchmark for computations of the energies and autodetachment lifetimes of these resonance states. The experimental results are compared to high-level electronic structure computations and line width modeling using the orbital stabilization method. These theoretical results are found to be in near quantitative agreement with the experimental data, highlighting their capability to accurately describe the energies and lifetimes of anion resonances for relatively large molecules.

Size-Dependent Pressure-Response of the Photoluminescence of CsPbBr3 Nanocrystals.

We report the size-dependent pressure response for CsPbBr3 perovskite nanocrystals in the size range 5.7-10.9 nm using photoluminescence spectroscopy in a diamond anvil cell. As the nanocrystal size decreases below ca. 7.5 nm, we observe a decrease in the transition pressure at which there is a change in the mode of deformation concomitant with an isostructural phase transition. We hypothesize that surface fluctuations regarding the tilt and distortion of surface PbBr6 octahedra facilitate the change in the mode of deformation and phase transition at lower pressures for smaller nanocrystals.