No abstract

Photochemical isomerization in sterically crowded chiral alkenes is the driving force for molecular rotary motors in nanoscale machines. Here the excited-state dynamics and structural evolution of the prototypical light-driven rotary motor are followed on the ultrafast time scale by femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption (TA). TA reveals a sub-100-fs blue shift and decay of the Franck-Condon bright state arising from relaxation along the reactive potential energy surface. The decay is accompanied by coherently excited vibrational dynamics which survive the excited-state structural evolution. The ultrafast Franck-Condon bright state relaxes to a dark excited state, which FSRS reveals to have a rich spectrum compared to the electronic ground state, with the most intense Raman-active modes shifted to significantly lower wavenumber. This is discussed in terms of a reduced bond order of the central bridging bond and overall weakening of bonds in the dark state, which is supported by electronic structure calculations. The observed evolution in the FSRS spectrum is assigned to vibrational cooling accompanied by partitioning of the dark state between the product isomer and the original ground state. Formation of the product isomer is observed in real time by FSRS. It is formed vibrationally hot and cools over several picoseconds, completing the characterization of the light-driven half of the photocycle.

Mixed organolead halide perovskites (MOHPs), CH 3 NH 3 Pb(Br x I 1Àx ) 3 ,h ave been shown to undergo phase segregation into iodide-richd omains under illumination, which presents am ajor challenge to their development for photovoltaic and light-emitting devices.Recent work suggested that phase-segregated domains are localized at crystal boundaries,driving investigations into the role of edge structure and the growth of larger crystals with reduced surface area. Herein, am ethod for growing large (30 30 1 mm 3 )m onocrystalline MAPb(Br x I 1Àx ) 3 single crystals is presented. The direct visualization of the growth of nanocluster-like I-richd omains throughout the entire crystal revealed that grain boundaries are not required for this transformation. Narrowband fluorescence imaging and time-resolved spectroscopyp rovided new insight into the nature of the phase-segregated domains and the collective impact on the optoelectronic properties.