In order to investigate the photochromic mechanism of photochromic materials based on supramolecular host–guest systems, we designed and synthesized a unique viologen derivative (benzimidazolyl benzyl viologen, guest 1·Cl₃), which does not contain oxygen atoms. The binding interaction of guest 1³⁺ with host cucurbit[7]uril (Q[7]) was investigated by various techniques. The obtained supramolecular host–guest complex 1³⁺@Q[7] exhibits interesting fluorescence emission and reversible photochromism. The ESR and XPS experimental data suggest that the photochromic process of the complex 1³⁺@Q[7] comes from the electron transfer from the carbonyl O atoms of the host Q[7] to the bipyridinium N atoms of the guest 1³⁺.