title: Structural control of the photodynamics of boron-dipyrrin complexes.
abstract: Boron-dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-tert-butylphenyl) group show low fluorescence yields (approximately 0.06) and short excited-singlet-state lifetimes (approximately 500 ps), and decay primarily (>90%) by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (approximately 0.9) and long excited-state lifetimes (approximately 6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of approximately 60 degrees with respect to the dipyrrin framework whereas the angle is approximately 80 degrees for mesityl or o-tolyl groups. The calculated potential energy surface for the phenyl-substituted complex indicates that the excited state has a second, lower energy minimum in which the nonhindered aryl ring rotates closer to the mean plane of the dipyrrin, which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a complex bearing the sterically hindered o-tolyl or mesityl group at the 5-position, leading to a high radiative probability involving conformations at or near the initial Franck-Condon form of the excited state. These combined results demonstrate the critical role of aryl-ring rotation in governing the excited-state dynamics of this class of widely used dyes.
authors.affiliation: Department of Chemistry, Washington University, St. Louis, MO 63130-4889, USA.
authors.forename: Hooi Ling
authors.initials: HL
authors.lastname: Kee
authors.forename: Christine
authors.initials: C
authors.lastname: Kirmaier
authors.forename: Lianhe
authors.initials: L
authors.lastname: Yu
authors.forename: Patchanita
authors.initials: P
authors.lastname: Thamyongkit
authors.forename: W Justin
authors.initials: WJ
authors.lastname: Youngblood
authors.forename: Matthew E
authors.initials: ME
authors.lastname: Calder
authors.forename: Lavoisier
authors.initials: L
authors.lastname: Ramos
authors.forename: Bruce C
authors.initials: BC
authors.lastname: Noll
authors.forename: David F
authors.initials: DF
authors.lastname: Bocian
authors.forename: W Robert
authors.initials: WR
authors.lastname: Scheidt
authors.forename: Robert R
authors.initials: RR
authors.lastname: Birge
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Lindsey
authors.forename: Dewey
authors.initials: D
authors.lastname: Holten
mesh_terms: D001895:Boron / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D010777:Photochemistry / Q000379:methods*; D011758:Pyrroles / Q000737:chemistry*; D012996:Solutions; D013050:Spectrometry, Fluorescence
chemical_list: D011758:Pyrroles; D012996:Solutions; D001895:Boron