  Cancer is still among the leading health issues today, considering the cost,
effectiveness, complexity of detection/treatment modalities and survival rates.
One of the most important criteria for higher survival rates is the early and
sensitive diagnosis of the disease that can direct the treatment modalities
effectively. Fluorescence imaging agents emerged as an important alternative to
the current state of the art due to their spatial and temporal resolution, high
sensitivity and selectivity, ease of modification towards generating
activatable agents, ease of operation, and low cost. In addition to imaging,
light-based treatment modality, photodynamic therapy (PDT), attained remarkable
attention, as it is minimally invasive and has fewer side effects compared to
the current standard of care treatments. Even though fluorescence imaging and
PDT have these significant advantages, light that needs to excite the agent has
limited penetration in tissues, hindering widespread utilization. Hybrid
xanthene dyes, particularly ones bearing silicon or phosphine oxide as the
bridging unit of xanthene moiety, gained significant interest not only due to
their excellent photochemical properties in aqueous media, high fluorescence
quantum yield, photostability but also their proper absorption and emission
maxima that allow for deep tissue imaging and therapy. Here, the design and
synthesis strategies, key photophysical properties, their application in
fluorescence imaging applications, and surprisingly limited utilization as PDT
agents of hybrid xanthene dyes that emerged in the last decade have been
reviewed in detail.
