TY - JOUR
T1 - The contributions of molecular vibrations and higher triplet levels to the intersystem crossing mechanism in metal-free organic emitters
AU - Huang, Rongjuan
AU - Avó, João
AU - Northey, Thomas
AU - Chaning-Pearce, E.
AU - Santos, Paloma L. dos
AU - Ward, Jonathan S.
AU - Data, Przemyslaw
AU - Etherington, Marc K.
AU - Fox, Mark A.
AU - Penfold, Thomas J.
AU - Berberan-Santos, Mário N.
AU - Lima, João C.
AU - Bryce, Martin R.
AU - Dias, Fernando B.
N1 - R.H. acknowledges a PhD grant funded by Durham University. P.L.S. thanks the CAPES Foundation, Ministry of Education of Brazil, for a PhD studentship, Proc. 12027/13-8, Science without Borders Program and thanks to networking action funded from the European Union's Horizon 2020 research and innovation programme under grant agreement No 691684. TJP acknowledges the EPSRC, Project EP/N028511/1 for funding. P.D. thanks the EU for a Marie Curie Fellowship H2020 research and innovation programme under grant agreement No. 659288. M.K.E. acknowledges EU's Horizon 2020 for funding the PHEBE project under grant no. 641725. F.B.D. acknowledges Samsung-SAIT for funding under grant number EP/L02621X/1. We thank Dr Kathryn C. Moss and Professor Igor F. Perepichka for the initial synthesis of some of the emitters.
PY - 2017/7/7
Y1 - 2017/7/7
N2 - Dual luminescence, i.e. intense, simultaneous, room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) is observed in a series of donor-acceptor-donor (D-A-D) molecules. This dual luminescence is stronger in the "angular" isomers, compared to their "linear" regioisomers, which is consistent with an enhanced intersystem crossing (ISC) in the former. Herein, we demonstrate that the small energy gap between the triplet levels, T1-Tn, below the lowest singlet state, S1, in the "angular" regioisomers, enhances the coupling between S1 and T1 states and favors ISC and reverse ISC (rISC). This is consistent with a spin-vibronic mechanism. In the absence of this "triplet ladder", due to the larger energy difference between T1 and Tn in the "linear" regioisomers, the ISC and rISC are not efficient. Remarkably, the enhancement of the ISC rate in the "angular" regioisomers is accompanied by an increase of the rate of internal conversion (IC). These results highlight the contributions of higher triplet excited states and molecular vibronic coupling to the harvest of triplet states in organic compounds, and cast the TADF and RTP mechanisms into a common conceptual framework.
AB - Dual luminescence, i.e. intense, simultaneous, room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) is observed in a series of donor-acceptor-donor (D-A-D) molecules. This dual luminescence is stronger in the "angular" isomers, compared to their "linear" regioisomers, which is consistent with an enhanced intersystem crossing (ISC) in the former. Herein, we demonstrate that the small energy gap between the triplet levels, T1-Tn, below the lowest singlet state, S1, in the "angular" regioisomers, enhances the coupling between S1 and T1 states and favors ISC and reverse ISC (rISC). This is consistent with a spin-vibronic mechanism. In the absence of this "triplet ladder", due to the larger energy difference between T1 and Tn in the "linear" regioisomers, the ISC and rISC are not efficient. Remarkably, the enhancement of the ISC rate in the "angular" regioisomers is accompanied by an increase of the rate of internal conversion (IC). These results highlight the contributions of higher triplet excited states and molecular vibronic coupling to the harvest of triplet states in organic compounds, and cast the TADF and RTP mechanisms into a common conceptual framework.
KW - Fluorescence
KW - Luminescence
KW - Isomers
UR - http://www.scopus.com/inward/record.url?scp=85021628788&partnerID=8YFLogxK
U2 - 10.1039/c7tc01958k
DO - 10.1039/c7tc01958k
M3 - Article
AN - SCOPUS:85021628788
SN - 2050-7534
VL - 5
SP - 6269
EP - 6280
JO - Journal of Materials Chemistry C
JF - Journal of Materials Chemistry C
IS - 25
ER -