Density Functional Theory Studies on the Photophysical Properties of N,N-Chelate Boron Complexes†

doi:10.7503/cjcu20141054

Abstract

Abstract:

The photophysical properties of four N,N-chelate boron complexes 1, 1q, 2 and 2q, including the geometric structures of the ground and excited state, absorption and emission spectra, transition density matrices(TDM), intramolecular charge transfer, Huang-Rhys(HR) factors, Franck-Condon factors and electronic coupling, were investigated with density functional theory(DFT) and time-dependent DFT(TD-DFT) calculations to shed light on the origin of the apparent decrease in the luminescence efficiency of complexes 1q and 2q due to the tiny modification. The computed results show that the substitution of quinolyl ligand and the incorporation of N-heteroatom significantly influence the photophysical properties of these complexes. Complexes 1q and 2q show a relatively narrower HOMO-LUMO energy gap owing to the extended conjugation, thus leading to a redshift absorption wavelength. An in-depth insight into HR factors and TDM is provided to inspect the geometric distortions and the character of excited states pertaining to absorption. The computed results indicate that complexes 1q and 2q show dramatically enhanced charge transfer between fragments, and the decreased emission energy and oscillator strength lead to a smaller radiative rate constant. Furthermore, the sharp enhanced electronic coupling between the excited and ground state results in fast non-radiative decay, thus complexes 1q and 2q are not luminescent in CH₂Cl₂. In addition, complexes 1 and 2 show great potential application as emitters as they display relatively high luminescence efficiency and large stokes shift.

Key words: Photophysical property, Absorption spectrum, Luminescent efficiency, Organoboron, Density functional theory

CLC Number:

O641

TrendMD:

JIN Junling, DING Xiang, OU Lihui, ZHANG Xiangyang, SHEN Youming, GENG Yun, SU Zhongmin. Density Functional Theory Studies on the Photophysical Properties of N,N-Chelate Boron Complexes^†[J]. Chem. J. Chinese Universities, 2015, 36(5): 962.

Figures/Tables 8

Fig.1 Molecular structures for complexes 1, 1q, 2 and 2q

Fig.2 Illustration of the highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital(LUMO) plots, the orbi-tal energy levels, and the HOMO-LUMO energy gaps for complexes 1, 1q, 2 and 2q at their optimized S0 geometries in vacuum at B3LYP/6-31G(d) levelThe molecular orbital plots were obtained with an isosurface of 0.02 a.u.

Table 1 Calculated absorption wavelengths(λabs), excitation energies(Ex), oscillator strengths f, and dominant excitation character of complexes 1, 2, 1q, and 2q together with experimental results(λexp.)*

Complex	Transition	λ_abs/nm	E_x/eV	f	Composition	λ_exp./nm
1	S₀→S₁	353.9	3.503	0.367	HOMO→LUMO(98%)	378
1q	S₀→S₁	431.8	2.872	0.207	HOMO→LUMO(98%)	450
2	S₀→S₁	343.5	3.610	0.515	HOMO→LUMO(99%)	374
2q	S₀→S₁	419.8	2.954	0.237	HOMO→LUMO(99%)	446

Table 2 Calculated emission wavelengths(λem), emission energies(Em), oscillator strengths f, dominant excitation character, Stokes shift(SS), radiative rate constants(kr) and nonradiative rate constants(knr) of complexes 1, 2, 1q, and 2q together with the reported experimental results of the emission wavelength(λexp.) and quantum yield(Φexp.)a

Complex	Transition	λ_em/nm	E_m/eV	f	Composition	SS/nm	k_r/s^-1	k_nr/s^-1	Φ_exp.	λ_exp./nm
1	S₁→S₀	410.2	3.022	0.128	LUMO→HOMO(98%)	56.3	5.10×10⁷	1.02×10⁷	0.29^b	516
1q	S₁→S₀	538.5	2.302	0.060	LUMO→HOMO(99%)	96.7	1.40×10⁷	2.46×10¹⁰	N/A^c	578
2	S₁→S₀	400.1	3.099	0.185	LUMO→HOMO(99%)	56.6	7.70×10⁷	0.99×10⁷	0.61^b	476
2q	S₁→S₀	521.9	2.376	0.080	LUMO→HOMO(99%)	102.1	2.00×10⁷	2.62×10¹⁰	N/A^c	546

Fig.3 Transition density matrix of the hot excitation(S0 state) for complexes 1(A, A'), 1q(B, B'), 2(C, C') and 2q(D, D') calculated by B3LYP/6-31G(2d,p) in vacuumThe unit of color scale is a.u.2, 1 a.u.=1.6×10-19 C.

Fig.4 Calculated results for the centroids of charge(C+/C-) and the charge-transfer distance(DCT) between the barycenters of density depletion(gray) and density increment(purple) zone, and the transferred charge(Δq) computed for the first electronic transitionThe calculation level is B3LYP/6-31G(2d,p) level.

Fig.5 Calculated HR factors and the normal mode for complexes 1(A), 1q(B), 2(C) and 2q(D)

Fig.6 Calculated R2/ћ and the normal mode for complexes 1(A), 1q(B), 2(C) and 2q(D)

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