Left: Chemical structures of the polymers studied. Right: Spatial structure of the L,D,L-PIAAA -sheet helix.17 The backbone is shown in green. For clarity, the octyl esters at the end of the side chains have been omitted.
Rotation-free transient-absorption change in L,D-IAA at different pump-probe delays (connected points), together with the conventional absorption spectrum (thin curve). Inset: Delay dependence at two probing frequencies, showing the decay due to excited-state population relaxation of the NH-stretch mode. The curves are least squares of single-exponential decays with time constants of and .
(a) Transient-absorption change in L,D-PIAA at different pump-probe delays. The inset shows transient-absorption spectrum at 0.8 ps for parallel and perpendicular polarizations of the pump and probe pulses. (b) Delay dependence at three probing frequencies. The curves represent least-squares fitted exponential decays, with time constants of , , and .
(a) Structure of the CO–NH chain in PIAA beta-sheet polymer. (b) Structure of the chain in the O:NH 1:15 random copolymer, in which the CO–NH units are isolated from each other. (c) Steady-state absorption spectrum and transient-absorption change in the NH:O 1:15 mixed copolymer L,D-PIAA. For comparison, the absorption change in L,D-PIAA is also shown.
Transient-absorption changes in L,D,L-PIAAA for two different pump frequencies. In contrast to the low-frequency NH-stretch mode of the chain with short hydrogen bonds, the high-frequency NH-stretch mode of the chain with stretched hydrogen bonds exhibits no self-trapping.
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