The photochemical properties and fluorescence emission of organic fluorescent dyes are influenced by several factors, which can be broadly categorized into molecular factors and environmental factors.1. Molecular factors:a Molecular structure: The molecular structure of the dye, including the presence of chromophores, auxochromes, and conjugated systems, plays a significant role in determining the absorption and emission spectra, quantum yield, and photostability of the dye.b Conjugation length: The extent of conjugation in the dye molecule affects the energy levels of the dye and, consequently, its absorption and emission properties. Generally, an increase in conjugation length leads to a redshift in the absorption and emission spectra.c Excited-state properties: The nature of the excited states singlet or triplet and the energy gap between them can influence the fluorescence quantum yield and lifetime. A smaller energy gap between the singlet and triplet states can lead to increased intersystem crossing and decreased fluorescence.d Intramolecular charge transfer ICT : The presence of donor and acceptor groups within the dye molecule can lead to ICT, which can affect the fluorescence properties, such as the emission wavelength and quantum yield.2. Environmental factors:a Solvent polarity: The polarity of the solvent can have a significant impact on the fluorescence properties of a dye. In general, a more polar solvent can lead to a redshift in the emission spectrum and a decrease in quantum yield due to solvation effects.b Solvent viscosity: The viscosity of the solvent can affect the fluorescence lifetime and quantum yield by influencing the rate of non-radiative processes, such as internal conversion and intersystem crossing.c pH: The protonation state of the dye can affect its absorption and emission properties. Some dyes are sensitive to pH changes, leading to shifts in their fluorescence spectra and quantum yields.d Concentration: At high concentrations, dye molecules can undergo self-quenching or aggregation, leading to a decrease in fluorescence intensity and quantum yield.e Temperature: Temperature can affect the fluorescence properties of a dye by influencing the rate of non-radiative processes and the solvation dynamics of the solvent.f Presence of quenchers: The presence of quenching agents, such as oxygen or other molecules that can interact with the excited state of the dye, can lead to a decrease in fluorescence intensity and quantum yield.g Interactions with other molecules: The fluorescence properties of a dye can be affected by its interactions with other molecules, such as binding to proteins or other biomolecules, which can lead to changes in the local environment around the dye and, consequently, its fluorescence properties.