Chen
Lyu
a,
Hongfei
Li
a,
Peter B.
Wyatt
b,
William P.
Gillin
*ac and
Huanqing
Ye
*ac
aMaterials Research Institute and School of Physics and Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, UK. E-mail: h.ye@qmul.ac.uk; w.gillin@qmul.ac.uk
bMaterials Research Institute and School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK
cChromosol Ltd, The Walbrook Building, 25 Walbrook, London, EC4N 8A, UK
First published on 1st July 2020
Organic prolonged luminescent materials have attracted attention with various candidates reported providing both UV and visible emission for applications in bio-imaging, light storage, security, etc. However, there is a lack of prolonged near-infrared (NIR) emitting materials, which are highly desirable for many of the proposed applications. NIR-emitting organic lanthanide(III) molecules have suitable wavelengths, but all of them are unsuitable for persistent luminescence due to weak sensitization and the restricted intrinsic ion radiative lifetimes. Herein, this paper demonstrates efficiently sensitized organic Yb(III) compounds with prolonged emission lifetimes up to ∼0.3 s at 1 μm, far exceeding the intrinsic Yb3+ emission lifetime of ∼1 ms. The dynamic equilibrium is studied to demonstrate that this prolonged emission is caused by energy transfer from long-lived organic triplet excitons. Experiment and simulation results suggest a new route to develop bright and ultralong-lived 1 μm emitting materials that could be coupled with other existing organic persistent luminescence materials to shift their emission in to the infrared.
Organic Yb(III) materials with an organic chromophore that strongly absorbs light to sensitize Yb3+ ions can give bright and sharp 1 μm emission under low-power photoexcitation. Various organic Yb(III) complexes or hybrid organic-conjugated Yb3+-doped nanoparticles have been demonstrated to provide sensitized Yb3+ luminescence.16,17 However, a general problem is that Yb3+ emission can be severely quenched by high-energy organic vibrations, e.g. C–H, N–H or O–H bonds, and this causes luminescence lifetimes as short as nanoseconds to microseconds with poor efficiencies of <0.1%.18,19 Even though there are effective protective methods to eliminate quenching, the sensitized Yb3+ emission lifetime is capped by the intrinsic radiative lifetime which is usually shorter than 2 ms and far from the demands of persistent PL.
Our approach utilizes an organic chromophore to sensitize an organic Yb(III) molecule in a molecular composite thin film. The organic Yb(III) molecule we use is a chelate, Yb(F-TPIP)3 that has a Yb3+ ion incorporated with tetrakis-(pentafluorophenyl)imidodiphosphinate, F-TPIP− ligands, and it is inert to visible light.20 The organic chromophore we employ is the zinc salt of 2-(tetrafluoro-2-hydroxyphenyl)tetrafluorobenzothiazole, Zn(F-BTZ)2, which has strong absorption of blue light.21 The chemical properties of the two individual materials have been well studied in previous work.20,21
The 405 nm light photoexcites Zn(F-BTZ)2 to give the Zn(F-BTZ)2 emission broadly spanning from ∼450 nm to 900 nm and sensitizes the sharp Yb3+ emission centred at 975 nm due to the Yb3+:2F5/2 → 2F7/2 transition, shown in Fig. 1. The excitation spectrum for the Yb3+ emission recorded at 1005 nm matches features of the Zn(F-BTZ)2 excitation spectrum at 500 nm, indicating a sensitization effect of Zn(F-BTZ)2 onto Yb3+ excitations (i.e. light is being absorbed by the Zn(F-BTZ)2 and the energy is then transferred to the Yb3+ ions from which it is emitted). Previous studies22 have shown abundant triplet excitons in Zn(F-BTZ)2 and its phosphorescence is centred around 550 nm extending to the NIR beyond 900 nm. This implies the phosphorescence band slightly overlaps the Yb3+ absorption which makes the sensitization from the triplet states possible, probably explaining the observation that higher Yb(F-TPIP)3 doping concentrations selectively quench the phosphorescence to narrow the FWHM of the Zn(F-BTZ)2 emission (Fig. 1).
Fig. 2(a) shows the time-resolved PL (TRPL) of excited Yb3+ ions recorded at 975 nm. The directly excited Yb3+ emission lifetime is 0.46 ± 0.12 ms, indicating an internal quantum efficiency of 46%, using a reported intrinsic Yb3+ radiative lifetime of ∼1 ms in Yb(F-TPIP)3.23 The sensitized Yb3+ emission lifetime becomes distinctly longer than those obtained by direct excitation into the Yb3+:2F5/2 level. Four lifetimes are obtained by fitting with their component percentages. The shortest lifetime (τ1) of 0.22 ± 0.11 ms with a component percentage of 6.8% is believed to result from Yb3+ ions that are close to residual quenching centres in the film. The longer lifetime (τ2) of 0.68 ± 0.15 ms (24.83%) corresponds to Yb3+ ions distant from quenching centres. The information about quenching centres and protective techniques is discussed in the ESI.† These demonstrate that the film contains at least two quenching environments, which lead to corresponding internal quantum efficiencies (IQEs) of 21% and 68%, respectively. Interestingly, the lifetime (τ3) of 2.22 ± 0.5 ms (26.59%) and the longest lifetime (τ4) of 10.2 ± 1.5 ms (45.7%) are considerably prolonged compared to that for direct excitation and exceed the intrinsic Yb3+ radiative lifetime (1 ms). Also, these two prolonged lifetimes are comparable to the long phosphorescence lifetimes of 2.2 ± 0.4 ms and 11.9 ± 2.7 ms for the triplet emission at 600 nm (used to reduce the singlet contribution to the decay curve) (Fig. S7 in the ESI†), which implies that energy transfer from the triplets is probably responsible.
To further investigate those prolonged Yb3+ emission lifetimes, we varied the pulse-width of a 405 nm laser photoexcitation to manipulate the percentages of the four lifetime components, shown in Fig. 2(b). With a longer pulse width, the population of triplet states around each Yb(F-TPIP)3 molecule will accumulate to increase the probability to sensitize the Yb3+ ion. Consequently, the total percentage of the prolonged lifetimes of τ3 and τ4, which appear to be relevant to triplet states, increases with longer pulse widths. The increase in the total percentage makes the average lifetime for the obtained Yb3+ emission become longer with extending pulse width. We resolved a set of rate equations to understand the relationship between the prolonged Yb3+ excitations and triplet states. The construction and deduction of rate equations contain two different time regimes. More details can be found in the ESI.†
The calculation shows that the decay rate for the Yb3+:2F5/2 level is a combination of both the triplet decay rate and the intrinsic decay rate of excited Yb3+ ions. The e−(RETT+RT)×t2 term (see eqn (S17) in the ESI†) is a decay process for the Yb3+ emission that follows the decay rate of the Zn(F-BTZ)2 triplet states. This explains why those prolonged Yb3+ emission lifetimes are similar to the persistent phosphorescence lifetimes of the triplet states. As mentioned above, the presence of either Yb(F-TPIP)3 or Y(F-TPIP)3 makes no difference to the phosphorescence lifetimes within the range of the experimental error bars. Hence, it is reasonable to use the maximum value of the error bars to estimate the ET rate, which is RETT = ∼35 s−1 (see Fig. S7 in the ESI†). Moreover, the calculation can give the average Yb3+ emission lifetime as a function of the photoexcitation pulse width which is shown as the solid lines in Fig. 2(b), with the calculated time-resolved component percentage of the prolonged Yb3+ emission lifetime. The simulation is seen to match the experimental data.
We have used the spectral overlap integral between the triplet phosphorescence spectrum and the Yb3+ absorption spectrum to estimate a Förster resonant energy transfer (FRET) rate, RFRET, which is a conventional method to quantify the Förster energy transfer. The detail of the FRET rate calculation can be found in the ESI.† There are always considerable uncertainties in the parameters used in FRET calculations, not least including the donor quantum yield, orientation factor and the donor–acceptor separation. However, by taking reasonable estimations of the quantum yield as 10%, and the donor–acceptor distance from 7 Å (the radius of single Yb(F-TPIP)3 molecule) to 10 Å (the maximum working distance of FRET),20 the calculated RFRET varies from 64 s−1 to 5 s−1, which are in agreement with our RETT. It is noted that eqn (S21) and (S22) in the ESI† indicate the quantum yield of the energy donor QD would only linearly scale the value of RFRET. Hence, as limited by the maximum and minimum value of QD, the value of RFRET is consistent with RETT to within one order of magnitude.
We present two possible processes to explain the prolonged Yb3+ excitations. Firstly, since the low ET rate keeps triplet excitons long-lived, if an excited Yb3+ ion relaxes to the ground state, it is possible for a neighbouring triplet exciton to re-excite that de-excited Yb3+ ion. Also, long-lived triplet excitons can diffuse via intermolecular interactions among Zn(F-BTZ)2 molecules to reach such an Yb3+ ion. These two routes maintain the population level of excited Yb3+ ions to overcome the intrinsic Yb3+ decay rate.
We have also measured the temperature dependent sensitized-Yb3+ TRPL at 975 nm in a co-doped film of [Yb(F-TPIP)3]0.5[Zn(F-BTZ)]0.5. The contour-colour display of the spectra is shown in Fig. 3(a). It can be clearly seen that prolonged Yb3+ emission becomes dominant with components of 334.5 ± 30.1 ms (87.9%) with 74.4 ± 5.9 ms (11.3%) at 80 K. The values of the fitted lifetimes and their component percentages are shown in Table S3 (ESI†). This prolonged Yb3+ PL implies that a triplet quenching process is suppressed at low temperature. We believe this process is thermally activated reverse-intersystem-crossing (RISC). Using the simple rate model shown in the inset to Fig. 3(b) we can fit the data and obtain an energy gap of 0.17 ± 0.02 eV between the triplet state (T1) and the singlet state (S1) of Zn(F-BTZ)2. The fitted energy gap is of the order of the singlet–triplet energy separation in the chromophore. The result implies the use of a chromophore material with a larger singlet–triplet energy separation would provide a route to exceptionally prolonged room temperature Yb3+ PL lifetimes using this approach.
Fig. 3 (a) Temperature dependent sensitized Yb3+ TRPL of a co-doped film of [Yb(F-TPIP)3]0.5[Zn(F-BTZ)2]0.5 at 975 nm. (b) The measured and simulated temperature-dependent prolonged Yb3+ decay rates, which resulted from the introduction of a reverse ISC process into the analytical solution of the rate equations shown in the ESI.† |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0tc02075c |
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