Bidirectional Singlet and Triplet Energy Transfer via the 2-Ureido-4[1H]-pyrimidinone Quadruple Hydrogen-Bonded Module

Abstract

The bidirectional singlet–singlet and triplet–triplet energy transfer processes were observed for the first time within a noncovalent hydrogen-bonded module. For this purpose, 2-ureido-4[1H]-pyrimidinone quadruple hydrogen-bonds (UPy) functionalized BF2-chelated dipyrromethene (BODIPY-UPy) and iodinated BODIPY (I-BODIPY-UPy) were synthesized. These molecules formed energy donor–acceptor assembly through quadruple self-complementary hydrogen bonds, which demonstrated interesting photophysical properties. Spectroscopic studies in toluene solution revealed that selective excitation of BODIPY-UPy in the assembly resulted in efficient intra-assembly singlet energy transfer from excited BODIPY-UPy to I-BODIPY-UPy. This was followed by intersystem crossing of I-BODIPY-UPy from S1 to T1 and the backward triplet energy transfer from the I-BODIPY to BODIPY part. Time-resolved transient absorption spectroscopy confirmed that the triplet excited-state energy was distributed on both chromophores in the assembly. Density functional theory (DFT) calculations further validated the feasibility of energy transfer as well as two degenerate triplet states in the assembly. The present studies revealed that the bidirectional singlet–singlet and triplet–triplet energy transfer processes could be manipulated through the noncovalent quadruple hydrogen bonds. Studies on such a process in supramolecular assembly are of considerable importance in the elucidation of their possible function in natural photosystems and inspire applications in various fields such as luminescent materials, optoelectronic devices, photocatalysis, and photodynamic therapy.