To evaluate the substituent effect in DAAN radicals, a series of nineteen DAAN derivatives with different substituents, which are precursors for fluorescent radicals via polymer-matrix-assisted radicalization, was synthesized. Each compound was analyzed using 1 H NMR, C NMR, and FT-IR spectroscopy as well as mass spectrometry (MS). The detailed experimental procedures for the synthesis of the compounds and their spectral data are described in the Supplementary Information (Figures ). The DAAN derivatives are denoted as R 1 /R 2 , where R 1 and R 2 refer to the substituents. In addition to the nineteen synthesized DAAN compounds, a commercially available DAAN derivative without any substituents on the aromatic ring (H/H) was used for evaluating the fluorescence properties. These DAAN derivatives were color-coded into four groups according to the type of substituent: derivatives for which a sufficient captodative effect, which refers to the stabilization of radicals by a synergistic effect of an electron-withdrawing substituent and an electron-donating substituent, cannot be expected (gray), derivatives with one electron-donating group and one electronwithdrawing group on the aromatic ring (blue), derivatives with one or two electron-donating groups on the aromatic ring (red), and derivatives with substituents at positions other than the para-position (green) (Figure ). Fluorescence properties of the DAAN radicals. To investigate the fluorescence properties of the DAAN radicals, ballmilling experiments were carried out using a Retch MM400 mixer mill (10 mL stainless-steel milling jar using one stainlesssteel ball with a diameter of 5 mm). Polystyrene (Mn = 66.0 kg mol -1 ; Mw/Mn = 1.46; 100 mg) was ground in the presence of each DAAN derivative (50 µmol) via ball-milling at 30 Hz for 10 min. Then, the solid-state fluorescence spectra of the samples were measured. The maximum fluorescence wavelength (lem,max) and the appearance of each sample after grinding are summarized in Table (for the fluorescence spectra, see Fig- ). Under irradiation from UV light, fluorescence was observed for all samples except for NO2/OMe. Surprisingly, this result clearly demonstrates that the wavelength of the fluorescence originating from the radicals can be tuned from 519 nm to 635 nm only by changing the substituents. It is important to note here that the fluorescence wavelength of triarylmethyl radical derivatives is, in contrast, controlled by extending the conjugation length. This should probably be interpreted in terms of the cyano group of DAAN radicals, which allows the two aromatic rings to adopt a near-planar conformation and thus amplifies the resonance effect of the substituents. These results represent an important aspect of the design and control of the fluorescence wavelength of fluorescent radicals.