Although n-π∗ molecules can serve as electroluminescent materials because of the harvesting of singlet and triplet excitons through El-Sayed-rule-allowed reverse intersystem crossing, the weak fluorescence emissions of such molecules have prevented applications into devices. We have discovered a 1 order of magnitude enhancement of the fluorescence of a prototypical n-π∗ fluorophore, 7-phenylcoumarin (PC), upon aggregation. We performed a mechanistic study consisting of structural, photophysical, and quantum chemical investigations, and found that the aggregation positioned the fluorescent electronic state below the nonemissive triplet n-π∗ transition state. Our studies, for the first time, demonstrate intramolecular geometry and intermolecular arrangements in the solid state to be significant factors in the photoluminescence quantum yields of n-π∗ fluorophores.
Bibliographical noteFunding Information:
This research was supported by a grant from the Samsung Research Funding Center for Future Technology (SRFCMA1301-01 to Y.Y.). J.C. acknowledges the financial support from the Ministry of Science, ICT and Future Planning (DGIST R&D Program 17-BD-0403 and CGRC 2016M3D3A01913243 to J.C.). Work at Seoul National University was supported by the National Research Foundation of Korea (NRF) through a grant funded by the Korean Government (MSIP; No. 2009-0081571[RIAM0417- 20150013] to S.Y.P.).
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