# Bridging light and sound: A spironaphtopyran-rhodamine dyad with high-contrast photoswitching between fluorescence and photoacoustic signal

## TL;DR

This paper introduces a new photoswitchable probe that can switch between fluorescence and photoacoustic imaging, enabling dual-modality imaging with high contrast.

## Contribution

The novel contribution is a spironaphtopyran-rhodamine dyad that enables reversible switching between fluorescence and photoacoustic signal with high contrast.

## Key findings

- The spironaphtopyran photoswitches undergo reversible photoisomerization with distinct absorbing and non-absorbing states.
- The FRET dyad with rhodamine shows robust and reversible switching between fluorescent and photoacoustic states in vitro.
- Structure-property relationships were established, providing insights for future multimodal imaging probe development.

## Abstract

Fluorescence and photoacoustic imaging are complementary modalities that provide distinct advantages for biological imaging: fluorescence microscopy offers high sensitivity and resolution, while photoacoustic imaging enables deeper penetration in complex tissue. Leveraging the strengths of both modalities through optically switchable contrast agents can offer enhanced imaging contrast and facilitate dual-modality imaging. Here, we report a photoswitchable probe capable of toggling between high fluorescence and high photoacoustic signal upon illumination, exploiting Förster Resonance Energy Transfer (FRET). We engineer novel spironaphtopyran photoswitches which undergo reversible photoisomerization between absorbing and non-absorbing states. Their photoswitching properties were systematically characterized, establishing structure-properties relationships, and providing the first photoacoustic investigation into this class of compounds. The best-performing switch was incorporated into a FRET dyad with a rhodamine fluorophore, which exhibits robust, reversible switching between fluorescent and photoacoustic-dominant states with excellent contrast in vitro, establishing a foundation for multimodal imaging probes with promising potential for dynamic correlative imaging.