Browsing by Author "Pahapill, Juri"

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Two-photon absorption reveals low-energy excited states of a 2,5,8-triamino-heptazine chromophore
(SPIE, 2024-06) Stark, Charles W.; Arak, Johanna; Trummal, Aleksander; Uudsemaa, Merle; Sildoja, Meelis-Mait; Pahapill, Juri; Rebane, Aleks
Triamino-heptazines (TAH's) comprise the fundamental building blocks of graphitic carbon nitride, an alluring material with promising applications in optoelectronics. However, the core D3h molecular symmetry enforces a forbidden lowest-energy excited singlet state, making it a challenge to characterize via conventional spectroscopy. Here, we measure oneand two-photon absorption spectra of an acidic form of triamino-heptazine, 3H-TAH, and use reversible acid/base titration to further probe the symmetry of the low-energy transitions in aqueous solution, which suggests the molecular base structure is dimelem. Two-photon absorption reveals two distinct low-energy transitions in acidic conditions, both of which are one-photon forbidden. The lowest energy state additionally becomes one-photon allowed in basic conditions. Spectroscopic changes can be described according to chromophore symmetry switching, with C3h, D3h, or Cs point group symmetry in respective acidic, neutral, or basic environments.
Two-photon absorption spectra of fluorescent isomorphic DNA base analogs
(2018-01) Mikhaylov, Alexander E.; Reguardati, Sophie de; Pahapill, Juri; Callis, Patrik R.; Kohler, Bern; Rebane, Aleksander
Fluorescent DNA base analogs and intrinsic fluorophores are gaining importance for multiphoton microscopy and imaging, however, their quantitative nonlinear excitation properties have been poorly documented. Here we present the two-photon absorption (2PA) spectra of 2-aminopurine (2AP), 7-methyl guanosine (7MG), isoxanthopterin (IXP), 6-methyl isoxanthopterin (6MI), as well as L-tryptophan (L-trp) and 3-methylindole (3MI) in aqueous solution and some organic solvents measured in the wavelength range 550 - 810 nm using femtosecond two-photon excited fluorescence (2PEF) and nonlinear transmission (NLT) methods. The peak 2PA cross section values range from 0.1 GM (1 GM = 10-50 cm4 s photon-1) for 2AP to 2.0 GM for IXP and 7MG. Assuming typical excitation conditions for a scanning 2PEF microscope, we estimate a maximum image frame rate of ~175 frames per second (FPS).