Taking into account the general interest in photoswitchable nanomagnets, we present an original synthetic route to well-defined photoswitchable heterotrimetallic complexes. Our strategy demonstrates the feasibility of using a “molybdenum−copper” switchable complex with terminal cyanide ligands as relevant synthons for the rational design and synthesis of heterotrimetallic photoswitchable molecular architectures. Starting from a pentanuclear compound based on an octacyanometalated precursor, [Mo^IV(CN)₄(CNCu^II(Me2en)₂)₄]⁴⁺, and a [Ni^II(cyclam)]²⁺ assembling building block, we succeeded in synthesizing a photoswitchable heterotrimetallic chain, denoted as {NiMo₂Cu₇}_<i>n</i>. The compound has been characterized by single-crystal X-ray diffraction, UV−vis and IR spectroscopies, and powder magnetic and photomagnetic susceptibility measurements. Before irradiation, the compound behaves as a paramagnet with eight independent spin carriers (Cu^II d⁹, <i>S</i> = ¹/₂, and Ni^II d⁸, <i>S</i> = 1). After irradiation, a long-lived metastable state persists until relatively high temperature (120 K) and is thermally reversible.Read less <