Upconverted emission based on triplet‐triplet annihilation with bimolecular design

Three anthracene derivatives—9‐(2′‐mesitylethynyl)‐10‐phenylanthracene ( MEPA ), 4,4′‐bis(9‐(2′‐mesitylethynyl)‐10‐phenylanthracene) ( BMEPA ), and 1,4‐bis(2‐((10′‐phenylanthracen‐9′‐yl)ethynyl)mesityl)butane ( DMEPA )—were used as fluorophores to study photon energy upconversion. Rose Bengal (RB) served as an organic photosensitizer and was mixed with each fluorophore in a tetrahydrofuran/methanol solution. Upon excitation at 568 nm, upconverted fluorescence was observed at 450 nm, with an anti‐Stokes shift of 0.57 eV, corresponding to green‐to‐blue upconversion. Residual fluorescence emission from RB contributed to a combined emission spectrum, resulting in white light generation. The key mechanism driving this process is triplet‐triplet annihilation (TTA), where two triplet‐excited chromophores interact to form a singlet‐excited fluorophore. Since TTA is diffusion‐controlled, its efficiency is significantly reduced in solid‐state films. To address this, a carbon chain was used to link two chromophore units in the structures of BMEPA and DMEPA , enabling intramolecular TTA after simultaneous photoexcitation. This approach is aimed at enhancing energy upconversion efficiency, particularly in solid or low‐concentration environments. Experimental results showed the highest upconversion fluorescence quantum yields: 0.52% for MEPA , 0.43% for BMEPA , and 0.21% for DMEPA . Analysis of fluorescence intensity versus fluorophore concentration revealed a likely intramolecular TTA in the BMEPA system. However, a higher rate of quenching due to more flexible structures limited the overall performance of BMEPA and DMEPA .