Fluoranthenes are found not only in incomplete combustion products as pollutants, but also in nature as biologically active compounds. The fluoranthene core is also utilised as a chromophore in materials science. Fluoranthene derivatives have been developed for use in fluorescent sensors, 9 optoelectronic devices, 10 and dye-sensitised solar cells. Several efforts for the syntheses of fluoranthene derivatives have been reported, and recently we have also achieved the synthesis of -extended fluoranthenes by a domino reaction (Scheme 1). Thus, a reaction of biaryl compound 1a bearing acyl and naphthylalkenyl moieties with KHMDS afforded 1-hydroxydibenzo[j,l]fluoranthenes 2a via (2+2) cycloaddition-SNAr-O,C migration-retro (2+2) cycloaddition sequence. In the course of our study, to clarify the properties of the synthetic compounds, we found that the UV-vis absorbance of 2a changed reversibly depending on the pH of the solution (halochromism). This colour change may have resulted from the various possible resonance structures owing to the cross-conjugation of the two aromatic systems of the fluoranthene skeleton. We envisaged that the introduction of an auxochrome, such as a hydroxy group, at the appropriate position in the chromophore would induce characteristic chromic properties depending on the resulting resonance hybrid. We designed two types of hydroxy-substituted dibenzo[j,l]fluoranthenes, with substitutions at positions 1 and 9, viz. 2a-c and 3a-c, respectively. We expected that the resonance contribution could be controlled by adjusting the position of the hydroxy substituent. Thus, the contribution of the fluorenyl-type structure increased in the anion of 2, whereas that of the quinoid-type resonance increased in the anion of 3 (Figure ). Herein, we report the syntheses and photophysical properties of dibenzo[j,l]fluoranthenes bearing a hydroxy group, which exhibit halochromism in their UV-vis absorption and fluorescence spectra based on changes in their -conjugation states.