Fig. 4.

Experimental resonance Raman spectra of Agp2-PAiRFP2, measured from frozen solution and single crystals at 90 K. a Pfr, crystal; b Pfr, frozen solution; c Lumi-F, frozen solution obtained by irradiation at ca. 130 K; d Meta-F obtained by irradiation at ca. 240 K, frozen solution; e Meta-F obtained by irradiation at ca. 240 K, crystal. The RR spectra of Pfr measured from the crystal (trace a) and from frozen solution (trace b) are nearly identical. Upon irradiation of Pfr at 130 K, Lumi-F is obtained (trace c), whereas the subsequent intermediate Meta-F (trace d, frozen solution) was trapped upon irradiation at ca. 240 K. The spectrum of the irradiated crystals displays far-reaching similarities with that of Agp2-PAiRFP2 in frozen solution and irradiated under the same conditions and thus allows the unambiguous assignment of the Meta-F state (trace e). Note that, as for the solution spectra, there are essentially no spectral differences if the crystals are irradiated at 240 or 273 K. Only minor differences exist between the RR spectra of the Meta-F states in frozen solution and in the crystalline form (see also Supplementary Fig. 9)