posted on 2015-09-03, 00:00authored byTumpa Sadhukhan, Anindya Datta, Sambhu N. Datta
Low-spin
ground states and low-lying excited states of higher spin
were investigated for four pyrene oxoverdazyl monoradicals 1–4 and eight pyrene dioxoverdazyl diradicals 5–12. The ground states for quartet and
quintet spin symmetries that are in reality excited states were found
in the region of 565–775 nm above the respective electronic
ground states. We calculated the “adiabatic” magnetic
exchange coupling constant in the electronic ground state of each
isolated biradical (5–12) by unrestricted
density functional theory. A number of hybrid functionals such as
B3LYP, PBE0, M06, and M06-2X were used. We also used range-separated
functionals such as LC-ωPBE and ωB97XD to compare their
effects on the coupling constant and the relative energy of the high-spin
state. Molecular geometries were optimized for the doublet and quartet
spin states of every monoradical (1–4), and the broken symmetry and triplet solutions were optimized for
every biradical (5–12), by systematically
using 6-311G, 6-311G(d,p), and 6-311++G(d,p) basis sets with each
functional. The geometry of each quintet diradical (5–12) was optimized using 6-311G basis set. B3LYP
produced the best spin values. The excited state (quartet or quintet)–ground
state energy difference (ΔE) increases in the
presence of para-phenylene connectors. These energy differences were
predicted here. The nature of spin coupling and consequently the ground
state spin agree with spin alternation rule and the calculated atomic
spin population. The adiabatic coupling constants were predicted for
the biradicals (5–12) in their electronic
ground states. Electron paramagnetic resonance parameters were determined
at 6-311++G** level for the ground state and the quartet state of 1 and compared with the available experimental data. Low-lying
excited states were found for the radical center (oxoverdazyl), pyrene,
molecule 1, and diradical 5 by time-dependent
density functional theory (TDDFT) method using B3LYP hybrid, 6-311++G(d,p)
basis set, and the molecular geometry in the electronic ground state. Data
from these calculations were used to discuss possible mechanisms for
the achievement of the high-spin (excited) states in 1 and 5 and to predict a similar outcome for radicals 2–4 and 6–12 upon excitation. A comprehensive mechanism for the first excitation
is proposed here. In particular, we show that the initial excitation
of 1 involves large contributions from mixed transitions
between pyrene and oxoverdazyl moieties, whereas the initial excitation
of 5 is basically that of only the pyrene fragment. Subsequent
internal conversion and intersystem crossing are likely to lead to
the high-spin states of lower energy. Sample spin-flip TDDFT calculations
were also done to confirm the energetic location and composition of
the quartet state of 1 and the quintet state of 5.