Pyrazolo[1,5-a]pyrimidines-based fluorophores: a comprehensive theoretical-experimental study

Area of Science:

• Organic Chemistry
• Materials Science
• Photophysics

Background:

• Fluorescent molecules are vital for visualizing intracellular processes, developing chemosensors, and monitoring organic materials.
• Existing fluorescent probes often involve complex synthesis and limited tunability.
• Pyrazolo[1,5-a]pyrimidines (PPs) represent a class of heterocyclic compounds with potential for optical applications.

Purpose of the Study:

• To synthesize and characterize a new family of pyrazolo[1,5-a]pyrimidines (PPs) for optical applications.
• To investigate the impact of structural modifications, particularly electron-donating groups (EDGs), on the photophysical properties of PPs.
• To evaluate the potential of PPs as solid-state emitters and compare their performance to commercial fluorescent probes.

Main Methods:

• Green synthesis of pyrazolo[1,5-a]pyrimidines (PPs) 4a-g with reported reaction mass efficiency (RME).
• Photophysical characterization including absorption (ε) and fluorescence quantum yield (ϕF) measurements.
• Solid-state emission intensity (QYSS) evaluation and comparison with commercial probes (coumarin-153, prodan, rhodamine 6G).
• Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations for electronic structure analysis.

Main Results:

• A series of PPs (4a-g) were synthesized via a simpler and greener methodology (RME: 40-53%) compared to BODIPYs (RME: 1.31-17.9%).
• Tunable photophysical properties were observed, with absorption coefficients ranging from ε = 3320 to 20,593 M⁻¹ cm⁻¹ and fluorescence quantum yields from ϕF = 0.01 to 0.97.
• Electron-donating groups (EDGs) at position 7 significantly enhanced absorption and emission behaviors.
• PPs with aryl substituents exhibited good solid-state emission intensities (QYSS = 0.18 to 0.63), enabling the design of solid-state emitters.
• The stability and properties of the synthesized PPs were comparable to commercial fluorescent probes.
• DFT/TD-DFT calculations confirmed that EDGs at position 7 promote strong absorption/emission intensities due to intramolecular charge transfer (ICT), while electron-withdrawing groups (EWGs) resulted in lower intensities.

Conclusions:

• Pyrazolo[1,5-a]pyrimidines (PPs) are promising candidates for optical applications due to their efficient and green synthesis.
• Structural modification, specifically the incorporation of EDGs at position 7, allows for fine-tuning of photophysical properties and enhanced emission.
• The developed PPs demonstrate potential as effective solid-state emitters and offer a viable alternative to existing commercial probes.