Fig. 5. The diffuse-reflectance spectra of (a) 1-ox (dotted red line), (b) 2 (solid blue line) and 2-ox (dotted blue line), (c) 3 (solid green line) and 3-ox (dotted green line), and (d) 4 (solid pink line). Insets: the NIR region (1200–2400 nm) featuring the characteristic TTFTC/TTFTC˙⁺ IVCT peaks.