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Abstract
Understanding and directing the energy transfer in nanocrystals-chromophore heterostructure is critical to improve the efficiency of their photocatalytic and optoelectronic applications. In this work, we studied the energy transfer process between inorganic-organic molecular complexes composed of cesium halide perovskite nanoplatelets (CsPbBr3 NPLs) and boron dipyrromethene (BODIPY) by photoluminescence spectroscopy (PL), time-correlated single photon-counting (TCSPC) and femtosecond transient absorption spectroscopy. The quenching of PL in CsPbBr3 NPLs occurred simultaneously with the PL enhancement of BODIPY implied the singlet energy transfer process. The rate of energy transfer has been determined by transient absorption spectrum as kET = 3.8 × 109 s-1. The efficiency of Förster energy transfer (FRET) has been quantitatively calculated up to 70%. Our work advances the understanding of the interaction between BODIPY and perovskite nanoplatelets, providing a new solution based on their optoelectronic and photocatalytic applications.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The phenomenon of energy transfer is widely observed in the fields of physics, chemistry, and biology. For example, energy transfer plays a major role in photosynthesis [1]. Research has shown that the rate of energy transfer depends upon the quantum yield of the donor, the extent of spectral overlap of the emission spectrum of the donor with the absorption spectrum of the acceptor, the distance between the donor and acceptor molecules [2]. Therefore, rational design of the donor-acceptor structure is of great significance to improve the efficiency of light energy conversion. Recently years, extensive research has emerged on the energy transfer between semiconductor nanocrystals (NCs) and dyes [3–8]. Semiconductor NCs have strong light absorption, tunable optical properties and tunable flexible surface chemical properties, making it suitable for combining with sensitizing dyes as building blocks for capturing photons over a wide range of visible and near-infrared regions [9–12].
In recent years, perovskite stands out in NCs because of its high photoluminescence quantum yield (PLQY), tunable band gap, long carrier lifetime and diffusion length [13–16]. In particular, the excited state properties of the sensitization process between lead halide perovskite (LHP) NCs and organic chromophores have become the key to the design of new hybrid materials with optoelectronics and photoelectrons as well as energy transfer photocatalysis for future applications [17]. Boron dipyrromethene (BODIPY) is a small fluorophore with superior photophysical properties, e.g. high visible light, remarkable thermo-photochemical stability and high PLQY [18]. The photophysical and photochemical properties of organic materials can be effectively improved by energy transfer or charge transfer between the donor of the light harvest and the organic chromophores acceptor [11]. These enhanced properties can be applied to light-activated diagnosis and treatment (phototherapy), i.e. photodynamic therapy (PDT), photoacoustic (PA) imaging and photothermal therapy (PTT) [19–22].
Previous studies also shown that the energy transfer between semiconductor nanocrystals (NCs) and dye. For instance, Tao Jin et al. designed a donor-acceptor system composed of CdSe quantum dots and BODIPY. They demonstrated that the competition between energy transfer and charge transfer for QD-sensitized triplet excited state generation in CdSe quantum dots-modified BODIPY complexes [23]. Panniello, A et al. and Maity, P et al. also reported a singlet energy transfer process between BODIPY pairs formed by the combination of heterojunction with nano materials [24,25]. In recent years, two-dimensional perovskite nanoplatelets with strong quantum confinement effect along the nanoscale thickness, especially cesium halide perovskite nanoplatelets (CsPbBr3 NPLs) stand out for their easy synthesis at room temperature and high absorption coefficient [26–34]. Although CsPbBr3 NPLs seem to meet the objective conditions of light harvesting components, the study of photophysical processes between two-dimensional CsPbBr3 NPLs and BODIPY molecular systems is still in its infancy.
In this study, we performed the steady-state photoluminescence spectroscopy (PL), time-correlated single photon-counting (TCSPC) and femtosecond transient absorption spectroscopy of BODIPY (BDP), CsPbBr3 NPLs (NPLs) and their complexes (NPLs\BDP). Through steady-state spectroscopy analysis, we found that the singlet energy transfer between CsPbBr3 NPLs and BODIPY. The reabsorption as a transfer mechanism has been excluded by Time-resolved photoluminescence (TRPL) spectra. The ultrafast excited state dynamics of energy transfer from CsPbBr3 NPLs to BODIPY was studied by transient absorption (TA) spectroscopy, and the rate of energy transfer has been determined to be kET = 3.8 × 109 s-1. The femtosecond transient absorption spectra and the effective spectral overlap between them indicate that an effective singlet energy transfer mediated by the FRET mechanism occur between CsPbBr3 NPLs and BODIPY. The efficiency of energy transfer between CsPbBr3 NPLs and BODIPY could be up to 70%, suggesting that the assemblies has a great potential for capturing photons energy conversion.
2. Results and discussion
2.1 Steady-state absorption and PL spectroscopy
The CsPbBr3 NPLs showing blue PL were synthesized according to the recent literature (see Section 1 in Supplement 1) [32]. Figure S1 shows the transmission electron microscopy (TEM) images of highly monodispersed CsPbBr3 NPLs. The lateral size of CsPbBr3 NPLs is approximately 2.73 nm, in agreement with previous reports [31,32]. The CsPbBr3 NPLs used in this paper are all four layers (one layer represents a [PbBr6]4- octahedron). We first measured the UV-vis absorption spectra and PL spectra of BODIPY dispersed in toluene and four-layer CsPbBr3 NPLs dispersed in n-hexane (Fig. 1(a), (b)). The illustrations in Figs. 1(a) and 1(b) show the structures of the CsPbBr3 NPLs and BODIPY, respectively. The obvious absorption peak at 454 nm in the absorption spectrum of CsPbBr3 NPLs is attributed to the first exciton transition. The sharp PL peak at 464 nm corresponds to the band-edge emission [35]. The Stokes shift and full width at half maximum (FWHM) of the CsPbBr3 NPLs spectra are smaller and narrower, due to the high exciton binding energy of perovskite NPLs in these closed nanostructures [26,30]. BODIPY shows a sharp S1 peak at 505 nm corresponding to the S0-S1 transition with a large extinction coefficient, which is due to its vertically allowed Frank-Conton transition [36]. The shoulder peak at 478 nm corresponds to the S0 to S2 transition (π-π*) [37]. In the PL spectra of BODIPY, the signals between 400 ∼ 500 nm do not come from BODIPY itself, but from the solvent. The detailed analysis can be found in Supplement 1, Fig. S2. Figure S3 compares the UV-vis absorption spectra of CsPbBr3 NPLs with and without BODIPY molecules, which reveal the additive spectral characteristics of the single exciton band of CsPbBr3 NPLs and BODIPY molecules. This spectral signature indicates that there is no ground state interaction between CsPbBr3 NPLs and BODIPY molecules. The weak red shift of the absorption peak of CsPbBr3 NPLs could be assign to the solvent effect [38]. Figure S4 compares the emission spectra of the BODIPY molecules and the absorption spectrum of CsPbBr3 NPLs, from which we observed an obvious spectral overlap. This is a primary prerequisite for the energy transfer between the donor-acceptor system. As present in Fig. 3(c), the PLQY of BODIPY increase from 12.55% to 34.50% after adding CsPbBr3 NPLs. First excitonic peak (λAbs), PL emission peak (λPL) and PLQY of CsPbBr3 NPLs and BODIPY molecules are summarized in Table 1.There are two possible reasons for the increase of PLQY in BODIYP after the addition of perovskite. One is the suppression of non-radiative recombination process in BODIPY. After the addition of perovskite, the decrease of solvent polarity in BODIPY leads to the decrease of additional deactivation channels in the excited state, resulting in an increase in BODIPY's PLQY [39]. Another reason is the increase of the radiative recombination in BODIPY, which is caused by the energy transfer from the perovskite NPLs. By TCSPC experiment, we found that the photoluminescence lifetime of the BODIPY after adding the perovskite NPLs is one times longer than that of the single BODIPY. (Please see Supplement 1 Table S2). Since solvent polarity have little effect on the fluorescence lifetime of the BODIPY, the increase of PLQY is mainly attributed to the second case. The detailed TCSPC fitting processes are described in the next section.
Fig. 1. (a), (b) UV and PL spectra of CsPbBr3 NPLs and BODIPY. (c) PLQY cubic plots for BODIPY (green) alone and PLQY cubic plots for BODIPY in the mixture (blue) dispersed in toluene and four-layer CsPbBr3 NPLs dispersed in n-hexane (Fig. 1(a), (b)).
Table 1. Optical properties of synthesized CsPbBr3 NPLs and BODIPY
In order to probe the excited state interaction between CsPbBr3 NPLs and BODIPY, the emission spectra of CsPbBr3 NPLs, BODIPY and their complexes are recorded in Figs. 2(a) and (b). According to Lambert-Beer's law [40], the absorbance is proportional to the concentration at low concentrations of solution. In the experiment, we use the absorbance of BODIPY in toluene to represent the concentration. Besides, when the concentration of BODIPY is 0.0435 µmol / ml, the measured absorbance is 0.6. It should be noticed that BODIPY molecules have negligible absorption at 420 nm compared to the CsPbBr3 NPLs (Supplement 1, Fig. S2). Therefore, the excitation light was set at 420 nm to selectively excite of CsPbBr3 NPLs when the photoluminescence spectra and transient absorption spectra of the mixture of the two are measured. The fluorescence of BODIPY is very weak (red line), while the fluorescence of CsPbBr3 NPLs is much stronger (blue line). However, in the spectra of the complex composed of CsPbBr3 NPLs and BODIPY (green line), the intensity of the two are exchanged. We observed that the fluorescence of CsPbBr3 NPLs was quenched and the fluorescence of BODIPY was significantly enhanced. The quenching of CsPbBr3 NPLs emission indicates that the excited state is inactivated, and the increase of BODIPY emission imply that there may be singlet energy transfer between them [41]. The quenching of the emission of the donor (CsPbBr3 NPLs) only establishes the deactivation of the excited state of the nanocrystals, rather than the mechanism details of the deactivation pathway. In order to further confirm the energy transfer between CsPbBr3 NPLs and BODIPY, we measured the PL spectra of mixture of the two at different concentrations of BODIPY (Fig. 2(b)). The fact that the emission intensity of CsPbBr3 NPLs gradually decreases while the emission intensity of BODIPY gradually increases with the increase of BODIPY concentration means that energy transfer between the two. The energy transfer efficiency (ΦET) of CsPbBr3 NPLs-BODIPY interaction can be estimated by the relative emission of CsPbBr3 NPLs. The relative emission of ΦET and CsPbBr3 NPLs can be connected by the following expression:
where, Φobs is the fluorescence observed for the interaction of CsPbBr3 NPLs with BODIPY, while Φ0 is the fluorescence of the CsPbBr3 NPLs alone [42]. For BODIPY with absorbance of 0.4, the estimated energy transfer efficiency is 70%. Then, we performed Photoluminescence excitation (PLE) spectrum to trace the origin of fluorescence emission. The monitoring wavelength was set at 550 nm (BODIPY emission, CsPbBr3 NPLs non-emission), and the scanning excitation wavelength was set from 375 to 540 nm. When BODIPY is detected alone, the maximum response of BODIPY absorbance is observed at 505 nm (see Supplement 1, Fig. S5), and there is no obvious response at the absorption peak of CsPbBr3 NPLs. However, when we repeated the experiment on the mixture of CsPbBr3 NPLs and BODIPY, two visible response signals appeared, corresponding to the absorption peaks of CsPbBr3 NPLs and BODIPY, respectively (Fig. 2(c)). The significant spectral feature at 460 nm, corresponding to the CsPbBr3 NPLs exciton band, gives strong evidence that the fluorescence emission of BODIPY come from excited CsPbBr3 NPLs. Besides, we measured the PLE of perovskite nanoplatelets under 550 nm detection and compared it with the PLE spectra of CsPbBr3 NPLs-BODIPY as Fig. S6. To summarize, the origin of singlet excited (S0-S1) emission of BODIPY observed in CsPbBr3 NPLs-BODIPY sample is determined by recording the PLE spectra and comparing it to the absorption spectra of the two emitting species. There are two processes result in the formation of singlet excited BODIPY, one is the direct excitation of BODIPY and the other is energy transfer from CsPbBr3 NPLs.Fig. 2. (a) PL spectra of BODIPY, CsPbBr3 NPLs and their complexes under excitation at 420 nm. (b) PL spectra of adding different BODIPY concentrations’ complexes under excitation at 420 nm. (c) PLE spectra of CsPbBr3 NPLs-BODIPY and comparing to the absorption spectra of the CsPbBr3 NPLs and BODIPY.
2.2 Time-resolved PL spectroscopy
Having proved the energy transfer between CsPbBr3 NPLs and BODIPY, we further quantitatively analyzed the lifetime of the energy transfer process using TCSPC. It is worth noting that it is crucial to ensure that only PL emission from the pure donor or acceptor reaches the detector. We selected the positions where CsPbBr3 NPLs and BODIPY have strong fluorescence emission and almost no spectral overlap to prevent the effects of the mixture signal. Figure 3(a) shows that compared with CsPbBr3 NPLs alone, the photoluminescence lifetime at 465 nm corresponding to CsPbBr3 NPLs donor in CsPbBr3 NPLs-BODIPY composites gradually decreases with the increase of BODIPY concentration. (The average lifetime decreased from 1.55 ns to 1.18-0.89 ns). The fact that the emission lifetime of the donor decays faster means that there must be an additional decay pathway. It also excludes reabsorption as a transfer mechanism, because the simple reabsorption of photons by the acceptor does not change the lifetime of the donor [43]. At the same time, this also indicates the interaction between the excited CsPbBr3 NPLs and BODIPY, confirming that the energy transfer occurs between the donor of CsPbBr3 NPLs and the acceptor of BODIPY. Figure 3(b) shows that the photoluminescence lifetime of BODIPY increased significantly after the addition of CsPbBr3 NPLs donor, and the photoluminescence lifetime of BODIPY in the complex increased with the increase of the concentration of CsPbBr3 NPLs donor. (The average lifetime increased from 2.58 ns to 5.12-6.22 ns). This indicates that the excited state lifetime of BODIPY is increased by the singlet energy transfer from CsPbBr3 NPLs to BODIPY. We have fitted the fluorescence lifetimes of CsPbBr3 and BODIPY alone and in the complexes, respectively. The fitting trajectory of CsPbBr3 NPLs alone or BODIPY alone conforms to the double exponential decay equation:
Fig. 3. The photoluminescence lifetime decays detected at (a) 465 nm corresponding to CsPbBr3 NPLs donor emission and (b) 530 nm corresponding to BODIPY acceptor emission.
The fitting trajectory of the complexes conforms to the three-exponential decay equation:
The decay parameters τ1, τ2, τ3, A1, A2, and A3 are the lifetimes and their corresponding amplitudes. The best-fitting parameters are listed in Supplement 1, Table S1 and Table S2.
2.3 Transient absorption spectroscopy analysis
The ultrafast excited state dynamics processes of CsPbBr3 NPLs, BODIPY and CsPbBr3 NPLs - BODIPY complexes have been distinguished by femtosecond TA spectroscopy, and the energy transfer process from CsPbBr3 NPLs to BODIPY was directly monitored. All TA experiments were performed at room temperature with 420 nm excitation to achieve selective excitation of CsPbBr3 NPLs. The pump energy density was maintained to be 20 µJ/cm2. Figure S6 in Supplement 1 shows the TA spectra of BODIPY excited by 420 nm wavelength and there is no bleaching signal of BODIPY in the TA spectra, which proved that only CsPbBr3 NPLs is selectively pumped in the mixture with under 420 nm excitation. Figures 4(a) and (d) show the TA and evolution correlation difference spectra of single CsPbBr3 NPLs. The strong bleaching signal at 450 nm is attributed to the band edge filling effect of CsPbBr3 NPLs [40,44,45]. The TA and evolution correlation difference spectra of BODIPY are shown in Fig. S7. The bleaching signal at 505 nm can be attributed to the singlet excited state of BODIPY [23]. Upon excited, the singlet excited state signal of BODIPY appears rapidly on a time scale of 1ps. The TA and evolution correlation difference spectra of CsPbBr3 NPLs-BODIPY are shown in Figs. 4(b) and (e). Compared with the TA and evolution correlation difference spectra of CsPbBr3 NPLs excited alone (Figs. 4(a) and (d)), the signal attenuation of the CsPbBr3 NPLs is significantly faster and the signal intensity is also decreased at the same pump density. In addition to the CsPbBr3 NPLs bleaching signal, there is another obvious bleaching signal at 505 nm, which could be attributed to the energy transfer from CsPbBr3 NPLs. The spectra in the 490-530 nm region have been magnified to highlight the absorption changes of BODIPY in Fig. 4(c) and (f). It is worth noting that the bleach gradually increases at 505 nm over time (0.5-500 ps). This increase matches the recovery of the bleaching signal of CsPbBr3 NPLs at 450 nm (Fig. 4(e)), suggesting the energy transfer between CsPbBr3 NPLs and BODIPY.
Fig. 4. (a),(b) TA spectra recorded after 420 nm laser pulse excitation for CsPbBr3 NPLs and CsPbBr3 NPLs - BODIPY and (c) TA spectra enlarged to the region where BODIPY absorbs (490−550 nm). (d),(e) Transient difference absorbance spectra recorded after 420 nm laser pulse excitation for CsPbBr3 NPLs and CsPbBr3 NPLs - BODIPY and (f) transient difference absorbance spectra enlarged to the region where BODIPY absorbs (490−550 nm). The laser fluence is 20 µJ/cm2.
In order to visualize the energy transfer process between CsPbBr3 NPLs and BODIPY on an ultrafast time scale, we compared the dynamics curves of CsPbBr3 NPLs, BODIPY and their complex respectively. First, we fitted the dynamics for the purpose of quantifying the decay process of dynamics. The general CsPbBr3 nanoplatelets and BODIPY data can be fitted into a fast generation process and two decay processes, but the best fitting of the BODIPY growth process after the addition of CsPbBr3 nanoplatelets is a double exponential fitting. Therefore, it is necessary to use different exponential fitting to achieve the best fitting when fitting different processes respectively. The fitting parameter trajectories of transient absorption data are either applicable to the double exponential growth / decay equation:
or a monoexponential growth / decay equation: where A1, A2, and A stand for amplitudes τ1, τ2 and τ denote growth / decay time constants [46]. The fitting parameters of CsPbBr3 NPLs bleach recovery traces are shown in Supplement 1, Table S3. The bleaching dynamics of CsPbBr3 NPLs includes three time constants: femtosecond rise time, picosecond decay time and nanosecond decay time. The two decay components are usually associated with the faster surface trapping process and the longer radiative electron-hole recombination process, respectively [44,47]. The pump fluence-dependent experiment are performed to exclude the effect of Auger recombination. It has been added in the modified manuscript.In the presence of BODIPY, the recovery rate of bleach of CsPbBr3 NPLs almost an order of magnitude faster (Fig. 5(a)), indicating that there might be additional non-radiative deactivation channels (energy transfer and electron transfer). This result is also consistent with the TCSPC spectra. The rate constant (kET) can be estimated from the lifetime at CsPbBr3 NPLs bleaching to quantify the energy transfer process by taking the reciprocal of the time constant as Eq. (6) :Fig. 5. (a) Normalized dynamics of the bleach decay corresponding of the single CsPbBr3 NPLs (Black line) and the addition of BODIPY (Red line). (b) Normalized dynamics of the bleach decay of the single BODIPY by itself (Black line) and with CsPbBr3 NPLs (Red line). (c) Normalized dynamics of the transient bleach decay for the CsPbBr3 NPLs-BODIPY system monitored at 457 nm (signal from CsPbBr3 NPLs) and 505 nm (signal from BODIPY).
In addition, we also found that the dynamics of the ground state bleaching signal at 505 nm of the BODIPY and 505 nm of the CsPbBr3 NPLs-BODIPY are also significantly different. Figure 5(b) shows the bleach recovery dynamics of individual BODIPY under the excitation of 510 nm and the bleach recovery dynamics of BODIPY in the complex under the selective excitation of 420 nm. The fitting parameters of BODIPY bleach recovery traces are shown in Supplement 1, Table S4. The bleaching signal of BODIPY alone appears almost instantaneously (855 fs), showing a rapidly bleaching signal corresponding to the formation of the singlet excited state (black line in Fig. 5(b)). When BODIPY is excited together with CsPbBr3 NPLs, we noticed that the bleach signal of BODIPY gradually increase over 150 ps (red line in Fig. 5(b)). Such a large difference between the two cases in time scales can be attributed to different relaxation processes. In the absence of CsPbBr3 NPLs, BODIPY singlet bleach appears almost instantaneously (< 1 ps) since it is directly excited. By contrast, energy transfer could account for the slowly appearance of BODIPY bleaching in the presence of CsPbBr3 NPLs. And the best fitting of the BODIPY growth process after the addition of nanosheets is a double exponential fitting. The additional growth process can be attributed to the energy transfer between them and only one decay process corresponds to the literature [48]. In Fig. 5(c), we normalized the transient bleaching dynamics of CsPbBr3 NPLs (457 nm) and BODIPY (505 nm) in the mixture. It is found that the decay of dynamics of CsPbBr3 NPLs (457 nm) corresponds exactly with the rise of the dynamics of BODIPY (505 nm). Besides, the two dynamics are almost symmetric, implying the rate constant to be same. Therefore, when the slow growth of BODIPY ground state bleach is attributed to energy transfer, the rate constant of energy transfer can be estimated by taking the reciprocal of the time constant as Eq. (7):
The dynamics analysis of the fast rising component of BODIPY shows that the rate constant of energy transfer is 3.7 × 109 s-1. The average value between the estimated kET and the average value in Supplement 1, Table S3 is 3.8 × 109 s-1. This dynamics analysis provides direct evidence of the time scale of the energy transfer process. Besides, it also gives us sufficient evidence that there is indeed a singlet energy transfer.
Finally, we further analyzed the mechanism of energy transfer between CsPbBr3 NPLs (457 nm) and BODIPY. In general, there are two kind of energy transfers, Förster resonance energy transfer (FRET) and Dexter energy transfer. FRET enables long range energy transfer through the dipole transition while Dexter energy transfer (DET) achieve short-range energy transfer through electron exchange. Combined with the large spectral overlap between the CsPbBr3 NPLs donor and the BODIPY acceptor, we can conclude that the energy transfer between them is mediated by the Foster resonance energy transfer, which realized by the resonant dipole-dipole coupling [43,49].
3. Conclusion
In summary, we have successfully elucidated the FRET process between CsPbBr3 NPLs and BODIPY for the first time. First, by fabricating the mixed solution of BODIPY and CsPbBr3 NPLs at different concentrations, we observed the concentration dependent fluorescence quenching, which could be attributed to energy transfer between them. Then, we proved the singlet energy transfer between BODIPY and CsPbBr3 NPLs by PLE spectrum. And we further quantitatively analyzed the lifetime of the energy transfer process using TCSPC. Finally, the excited state dynamics between BODIPY and CsPbBr3 NPLs is analyzed by femtosecond transient absorption spectroscopy, and the time scale of energy transfer is obtained to be 3.8 × 109 s-1. The energy transfer efficiency of up to 70% between CsPbBr3 NPLs and BODIPY effectively improves the fluorescence quantum yield of BODIPY. Our research is helpful to understand the interaction between two-dimensional perovskite and chromophore and provides a solution in this field of optoelectronic and photocatalytic applications based on NCs and dyes.
Funding
Young Backbone Teacher Training Program in Higher Education of Henan Province (No. 2019GGJS065); National Key Research and Development Program of China (2022YFA1604302); Natural Science Foundation of Henan Province (No. 222300420057); National Natural Science Foundation of China (12074104, 12174090).
Disclosures
The authors declare no conflicts of interest
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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