Excited-State Dynamics of 2,2′-Bipyridine Moieties Embedded in the Framework of Periodic Mesoporous Organosilica

Abstract

We investigate the excited-state dynamics of 2,2′-bipyridine (BPy) moieties in a framework of BPy-bridged periodic mesoporous organosilica (BPy-PMO), which is a unique solid chelating ligand that has densely and regularly arranged BPy units that are exposed on the pore surfaces. The steady-state absorption spectrum of BPy-PMO shows broad absorption bands and a prolonged absorption tail compared to its precursor molecule, which suggests that a part of the BPy moieties form dimers and/or aggregates in the ground state. Transient absorption spectra of BPy-PMO after excitation with 295 nm light show mainly intersystem crossing from the lowest singlet excited state (S1) to the lowest triplet excited state (T1) of BPy moieties with a lifetime of 52 ps. Energy transfer (7.2 ps) from the S1 of a BPy monomer to BPy dimers and/or aggregates is also suggested. The lifetime of the excited BPy dimers and/or aggregates is estimated by selective excitation (355 nm) to be 3.4 ns. The lifetime for the T1 of the BPy monomer is estimated to be 700 ns from transient absorption spectra of BPy-PMO after nanosecond 266 nm pulse irradiation. The characteristics of the excitation-relaxation processes of BPy-PMO are discussed.