Ashwath
Kudlu‡
a,
Dhritismita
Sarma‡
b,
Deep
Kumar Das
a,
Alisha Basheer
Shamla
a,
Rangarajan
Bakthavatsalam
c,
Venkatesha R.
Hathwar
d,
Arup
Mahata
*b and
Janardan
Kundu
*a
aIndian Institute of Science Education and Research (IISER) Tirupati, Tirupati, Andhra Pradesh 517507, India. E-mail: janardan@iisertirupati.ac.in
bIndian Institute of Technology Hyderabad Sangareddy, Kandi, Telangana 502284, India. E-mail: arup@chy.iith.ac.in
cB.M.S College of Engineering, Bull Temple Road, Bengaluru, Karnataka 560019, India
dSchool of Physical and Applied Sciences, Goa University, Panaji, Goa 403206, India
First published on 8th November 2024
Luminescent zero dimensional (0D) antimony halide (Sb–X) hybrids showcase emissive properties (emission peak position; photoluminescence quantum yield – PLQY) that are strongly dependent on the local metal halide geometry/site asymmetry. However, controlling the local metal halide geometry has been synthetically challenging due to the diverse coordination geometries adopted by the Sb–X units. Consequently, efforts ascertaining a clear structure–luminescence relation in 0D Sb–X hybrids have met with limited success. Reported here is an attempt to draw a structure–luminescence relationship by controlling the Sb–X geometry utilizing 2D cadmium halide hybrids as the host that serves as a framework for incorporating emissive Sb3+ dopants. The choice of a series of organic cations tunes the local metal halide geometry/distortion in the host hybrids that controllably alters the luminescent properties of the emissive dopants in 2D Sb3+ doped hybrids. A clear structure–luminescence relationship is observed: red-shifted emission peak positions and enhanced PLQYs as the extent of the local metal halide distortion increases. DFT calculations of the doped compounds, characterizing ground and excited state structural and electronic properties, reveal the operative luminescence mechanism and the origin of different efficiency of luminescence (PLQY). This work provides deeper insight into the luminescence mechanism highlighting the importance of ground and excited state structural distortions in Sb3+ doped 2D cadmium halide hybrids. The experimental–computational insights gained here are beneficial for establishing the structure–luminescence relationship for 0D Sb halide hybrids targeting their rational synthesis.
In order to reliably control the Sb halide geometry/asymmetry and unravel the operative structure–luminescence correlation, a synthetic strategy of Sb3+ doping31–36 into 2D metal halide host hybrids incorporating different organic ammonium cations is exploited here. High band gap 2D octahedral cadmium chloride host hybrids, having different Cd–halide local geometry/site asymmetry, undergo heterovalent substitutional doping with Sb3+ dopant ions (Sb3+ ionic radii ∼ 0.76 Å; Cd2+ ionic radii ∼ 0.95 Å)37 allowing control over the local metal halide geometry around the dopant centre. Here, Sb3+-doped 2D octahedral cadmium halide hybrids with different metal halide geometry/site asymmetry mimic the 0D Sb halide hybrids with different site symmetry and provide a systematic route to correlate their structure with their photo-physical properties. Specifically, a series of Sb3+ doped octahedral cadmium chloride 2D DJ hybrids have been crystallized utilizing different organic amines [1,5-diaminopentane (denoted as PDA); 1,4-diaminobutane (denoted as DAB); and diethylenetriamine (denoted as DETA); Scheme 1], which allows controlled tuning of the local metal halide octahedral geometry (around metal/Sb3+ dopant center). Single crystal structural analysis (SCXRD) of the compounds reveals that the synthesized 2D doped hybrids have emissive Sb3+ dopants with octahedral metal chloride geometry showcasing different extents of distortions/site asymmetry as quantitated by the distortion indices (bond length distortion, bond angle variance). Correlated with such differences in the ground state distortions, PL peak positions of the compounds are observed to redshift from 520 nm to 560 nm as the extent of distortion increases. Concomitantly, these compounds show extreme values of PLQY (ranging from 60% to ∼96%) with the observed strong correlation of higher PLQY for compounds manifesting higher structural distortion. Such differences in the photo-physical properties of the compounds, bearing correlation with ground and excited state energetic and electronic behavior, reveals the origin of the extent of better luminescence in the Sb-doped structures. Through systematic synthetic efforts and DFT calculations, this work successfully unravels the underlying structure–luminescence relationship in 2D Sb3+-doped cadmium halide hybrids that provides deeper insight into the observed photo-physical properties (PL peak position; PLQYs) highlighting the importance of ground and excited state structural distortions. Such insight is beneficial for establishing the structure–luminescence relationship for luminescent 0D Sb halide hybrids enabling their rational design and targeted synthesis for lighting applications.
Dehydrated Sb:DABSbCl hybrid was obtained by heating the freshly synthesized Sb:DABCdCl hybrid in a 100 ml Schlenk flask to remove the water. The dehydration at 150 °C for three hours is done using a Schlenk line with continuous vacuum of the argon gas environment in the flask. It is then allowed to cool down to room temperature naturally in an argon environment and stored in a glovebox for further analysis.
Single-crystal X-ray diffraction (ScXRD) analysis revealed that the prepared compounds (host and doped hybrids) crystallize in a Dion–Jacobson (DJ)-type corner shared 2D structure with alternating metal halide and organic layers. Fig. 1(a)–(c) shows the asymmetric unit and crystal packing diagram of the Sb-doped cadmium chloride hybrids. The anionic [(Cd/Sb)Cl6]2− octahedral units are charge balanced by the chosen protonated organic cations. As evident from the ICP data (Table S1; ESI†), Sb3+ ions (ionic radius ∼ 0.76 Å) undergo substitutional doping at the Cd2+ sites (ionic radius ∼ 0.95 Å) of the host 2D framework in low amounts due to the differences in their ionic sizes.37 The crystal structure of the doped hybrid halides is found to be similar to that of the corresponding host structure (Fig. S4; ESI†). The ScXRD details of the host and doped hybrids are provided in the ESI† (Tables S3 and S4) while Tables S5–S13 (ESI†) provide a summary of ScXRD structural analysis and tables of bond lengths, bond angles, and dihedral angles for all the compounds.
Briefly, the Sb:DETACdCl system crystallizes in the orthorhombic crystal system with Pnma space group, Sb:PDACdCl crystallizes in the orthorhombic crystal system with Imma space group, and Sb:DABCdCl crystallizes in the monoclinic system with P21/c space group with water molecules present in the structure. This is in accordance with the structure of the corresponding host systems (Table S4, ESI†). The experimental powder X-ray diffraction (PXRD) patterns match well with the simulated one (Fig. S5 and S6, ESI†) confirming the phase purity of the synthesized host and doped hybrids. To note, the control hybrids have different structures as compared to the host and doped systems. DETASbCl crystallizes in the triclinic system (P space group) forming a non-networked 0D assembly of highly distorted octahedral Sb–halide units dispersed in the cation matrix. PDASbCl forms a 1D structure with an orthorhombic crystal system (P212121 space group) wherein a linear chain of corner shared Sb–halide octahedral units are charge balanced by the organic cation. DABSbCl crystallizes in the monoclinic system (P21/c space group) with 0D rendition of the square pyramidal Sb–halide units. The simulated and experimental PXRD of the control systems are in agreement with each other indicating the phase purity of the control systems (Fig. S7–S9, ESI†).
Structural properties and distortions of host hybrids are presented in Table S2, ESI† while Table 1 provides the structural properties of the doped systems. The doped hybrids, all having a 2D DJ structure, show different extents of metal halide distortions/site symmetry due to their interactions with the organic cations. The metal halide distortions around the dopant centre are expected to have notable impact on their photophysical properties. Hence, we have calculated the following parameters for characterization of the structural distortions in Sb-doped 2D hybrids: (i) bond length distortions (octahedral elongation); (ii) bond angle variance; (iii) metal–halide–metal bond angles (M–Cl–M); and (iv) angle of tilt (ϕtilt(deg)). A summary of the distortion parameters along with the photophysical properties is provided in Table 1 (doped hybrids) and Table S2 (ESI†) (host hybrids). The octahedral distortion is calculated based on the equation33
Compound | Space group | Octahedral distortion (d) | Bond angle variance (deg2) | M–Cl–M (deg) | ϕ tilt (deg) | PL Max (nm) | PLE peak positions (nm) | PLQY (%) |
---|---|---|---|---|---|---|---|---|
Sb:DETACdCl C4H16N3Cd Cl5 | Orthorhombic Pnma | 9.04 × 10−5 | 2.2432 | 153.54 | 11.26 | 520 | 271, 290, 330 | 60 |
Sb:PDACdCl C5H16N2Cd Cl4 | Orthorhombic Imma | 1.5 × 10−4 | 2.2300 | 160.76 | 12.45 | 540 | 281, 293, 333 | 61 |
Sb:DABCdCl 2(C4H14N2) Cd2Cl8O | Monoclinic P21/c | 1.4 × 10−3 | 0.6409 | 165.91 | 13.02 | 560 | 275, 291, 333 | 96 |
Sb:DETACdCl has lower distortion parameters: bond length distortion (Δd = 9.04 × 10−5), M–Cl–M (153.54°), bond angle variance (σ2 = 2.2432) and tilt angle (ϕtilt = 11.26°). The Sb:PDACdCl hybrid has distortion parameters: bond length distortion (Δd = 1.5 × 10−4), M–Cl–M (160.76°), bond angle variance (σ2 = 2.2300) and tilt angle (ϕtilt = 12.45°) showcasing comparatively higher distortions than the Sb:DETACdCl system. Furthermore, the Sb:DABCdCl system shows the highest degree of distortion: bond length distortion (Δd = 1.4 × 10−3), M–Cl–M (165.91°), bond angle variance (σ2 = 0.6409) and tilt angle (ϕtilt = 13.02°) likely due to the presence of the water molecules in the structure that can show strong hydrogen bonding interactions.44 The structural distortion analysis clearly reveals the following trend in the extent of distortion- Sb:DABCdCl > Sb:PDACdCl > Sb:DETACdCl highlighting the strong impact of the choice of the organic amine on the local structure/distortion around the dopant centre in the doped 2D metal halide hybrids. A pictorial representation of the distortion parameters is shown in Fig. S10–S12; ESI.† Furthermore, by adjusting the feed percentage of the Sb precursor (relative to cadmium precursor) during the synthesis of the doped hybrids, the amount of incorporated dopants was varied systematically. The resulting percentage of Sb in the crystals, quantified via ICP analysis, is found to rise as the Sb feed percentage is increased (Table S1; ESI†). Furthermore, Fig. S13; ESI† shows a comparison between the PXRD patterns of the host and various percentages of Sb-doped samples. Notably, across all the doped systems, altering the dopant percentage did not induce any significant changes in the crystal structure as supported by the absence of peak shifts or the appearance of new peaks in the PXRD patterns of the doped samples. Encouragingly, variation of the Sb feed percentage for a given hybrid allowed maximizing the photoluminescence intensity. This results in the successful synthesis of Sb doped hybrids containing different organic cations with the highest emission strength. Hereafter, Sb-doped samples (with different organic cations) with the brightest emission were further characterized and analyzed for drawing structure–photophysical property correlations.
The impact of the extent of the structural distortions present in the Sb-doped metal halide hybrids is observable in their strikingly different luminescence properties, as summarized in Table 1. Unlike the non-luminescent host hybrids, the Sb doped hybrids show bright green/yellowish green/yellowish orange emission when viewed under UV excitation (365 nm), as shown in the insets of Fig. 2. The optical properties (absorbance, photoluminescence-PL, photoluminescence excitation-PLE) of the doped systems are shown in Fig. 2 while the peak positions of the PL and PLE spectra along with the photoluminescence quantum yield (PLQY) for the doped hybrids are listed in Table 1. The Sb:DETACdCl system shows a strong emission centered at 520 nm when excited at 272 nm with PLQY ∼ 60%. The PLE collected at 520 nm shows sharp onset around 375 nm characteristics of Sb doped hybrids confirming the successful incorporation of Sb in the hybrids. The PL and PLE profiles remain unchanged across the excitation and emission range, respectively (Fig. S14; ESI†). The Sb:PDACdCl system shows an emission peak at 540 nm when it is excited at 293 nm with a PLQY ∼ 61%. The PLE feature is comparable with that of the Sb:DETACdCl system. The PL and PLE profiles remain unaltered throughout the excitation and emission ranges (Fig. S15; ESI†). The Sb:DABCdCl system has an emission at 560 nm when excited at 274 nm with high PLQY ∼ 96%. The PLE features remain similar for all the systems with respect to the corresponding emissions (Fig. S16; ESI†). The absence of any additional excitation dependent emissive features for the Sb-doped hybrids strongly suggests that the observed emissions originate from unique emissive Sb3+ dopant species. Sb3+ dopant ions, with ns2 configuration, have ground state 1S0 and excited state 1P1 and 3Pn (n = 0,1,2). The transitions 1S0 → 3P0 and 1S0 → 3P2 are forbidden while 1S0 → 1P1 and 1S0 → 3P1 are partially allowed due to spin–orbit coupling (C and A band, respectively). The observed PLE features of the doped hybrids in the ∼280–290 nm region and ∼340–350 nm region correspond to C and A bands, respectively (Fig. 2). PL lifetime decay analysis of these Sb-doped hybrids provides long lived emissions with average lifetime values of 2.0 μs, 2.6 μs and 3.1 μs for Sb:DETACdCl, Sb:PDACdCl and Sb:DABCdCl hybrids, respectively (Fig. 2(d)). The observed long lifetimes with characteristic broad emission and PLE features for the Sb doped hybrids are in accordance with the generally accepted self-trapped excitonic (STE) emissive nature of the strongly luminescent Sb dopants in the doped hybrids. Optical characterization of the host and control systems, discussed below, further support the assignment of Sb3+ as the emissive species in the doped hybrids.
The host hybrids reported here are weakly emissive with low PLQY. The DETACdCl hybrid has a weak emission peak centered at 490 nm (PLQY ∼ 3.5%) with PLE peaks in the 270–290 nm and 330–350 nm regions (Fig. S17; ESI†). The PDACdCl hybrid shows a weak emission peak at 480 nm with PLQY ∼ 0.5%. The PLE collected at 480 nm has a narrow feature in the 400 nm region (Fig. S17; ESI†). The DABCdCl hybrid has PL peaks at 440 nm and 465 nm (PLQY ∼ 3.6%). The PLE shows features similar to that of the DETACdCl hybrid (Fig. S17; ESI†). The average lifetime values, estimated at 80 K, are 7.8 ns, 5.4 ns and 7.4 ns for the DETACdCl, PDACdCl, and DABCdCl hybrid hosts, respectively (Fig. S17; ESI†). Noteworthy here, the measured lifetimes of the doped systems are in the microsecond timescale, much longer than the host lifetimes (nanoseconds). The weak blue emission observed in the host hybrids likely arises due to emission contribution from both the organic cations and cadmium halide framework (Fig. S18; ESI†). Next, the optical properties of the control systems are presented in Fig. S19; ESI.† The DETASbCl hybrid emits weakly at 560 nm (PLQY ∼ 2%) when excited at 360 nm. The PLE exhibits characteristic Sb3+ absorption features with a sharp onset at 400 nm. The PDASbCl hybrid has a weak emission at 570 nm (PLQY ∼ 0.1%) while the DABSbCl hybrid has a PL peak centred at 560 nm (PLQY ∼ 1.8%) with PLE features comparable to that of the DETASbCl hybrids. The lifetime of the weak emissions of the control systems, measured at 80 K, were estimated to be in the microsecond timescales (DETASbCl ∼ 2.9 μs, PDASbCl ∼ 3.7 μs, and DABSbCl ∼ 3.9 μs). Notable here is the similarity of the lifetime values of the control and doped systems. Cumulatively, the comparative analysis of the optical properties of the doped, host, and control systems suggests that the luminescence in the doped 2D hybrids originates from the substitutional Sb3+ dopant centres.
Low temperature PL/PLE measurements were performed in the range of 80–300 K to get more insight towards the emission mechanism in the doped hybrids. From the measured temperature dependent PL (excited at 330 nm; Fig. 3(a)–(c)), it is observed that the doped systems show a monotonic trend of decreasing intensity of emission as the temperature is increased from 80 K to 300 K. This is due to the opening up of non-radiative channels as the temperature is raised thereby lowering the PL intensity. Fig. S20; ESI† shows the temperature dependent PL for other excitations and PLE for the corresponding emissions. Fig. 3(d)–(f) shows the Huang–Rhys factor (S) for the doped systems. The S value usually indicates the strength of electron–phonon coupling in the soft/flexible metal halide hybrids. The Huang–Rhys factor (S) can be estimated as per the equation33
Comparison of the room temperature photophysical properties of the doped hybrids incorporating different organic cations clearly reveals differences in the emission peak positions and PLQY values (Table 1). The CIE coordinates for the doped samples are given in Fig. 4(c), which depicts the change in emission color for the doped samples as the local metal halide geometry asymmetry/distortion changes (Table 1). Local metal halide asymmetry along with other factors (crystallographic defects, impurities, surface) may be responsible for the observed changes in the luminescence behavior of the hybrids. However, the synthesis method is optimized to incorporate Sb3+ dopants that maximize PLQY without changing the photo-physical properties. Furthermore, the luminescence profiles do not vary from single crystals to powders for a given doped system (Fig. S21; ESI†). These observations suggest that local metal halide asymmetry may have a strong impact on the photo-physical properties of the hybrids. It is evident that the least distorted (octahedral elongation and bond angle distortion) doped system (Sb:DETACdCl) shows blue shifted emission (520 nm) while the highly distorted doped system (Sb:DABCdCl) shows red shifted emission (560 nm). The moderately distorted doped system (Sb:PDACdCl) has emission (540 nm) lying in between the Sb:DETACdCl and Sb:DABCdCl hybrids. Interestingly, the doped hybrids show different PLQY ranging from 60% to ∼96% correlated with the extent of distortion. The strongly distorted Sb:DABCdCl doped hybrid (incorporating water molecules in the crystal structure) showcases the highest PLQY when compared with the less distorted doped hybrids (Sb:PDACdCl; Sb:DETACdCl; Table 1). Overall, it is observed that the ground state distortion of the doped hybrids is correlated with the PL emission shifted to longer wavelengths and associated with higher PLQY.
![]() | ||
Fig. 4 Comparison of the (a) PL spectra; (b) PLE spectra; and (c) CIE chromaticity coordinates (CIE 1931) of Sb:DETACdCl, Sb:PDACdCl and Sb:DABCdCl hybrids. |
Excitingly, it is found that the brightly emitting Sb:DABCdCl hybrid, incorporating water molecules in the crystal structure, shows a highly quenched emission when dehydrated, as shown in Fig. 5(a). The room temperature PLQY of the hydrated hybrid decreases from 96% to 39% when dehydrated. Heating the as prepared Sb:DABCdCl sample releases water molecules and shows an accompanying decrease in PL intensity (Fig. 5(b)) albeit maintaining similar PL/PLE/lifetime profiles (Fig. S22; ESI†). Such dehydrated Sb:DABCdCl hybrid was characterized using SCXRD and thermogravimetric analysis (TGA) confirming the absence of water molecules in the structure (Fig. S23; ESI†). Detailed distortion analysis of the dehydrated system reveals a comparable extent of distortion with that of the hydrated Sb:DABCdCl hybrid (Table S14; ESI†). However, the strong effect of the water molecule on the quenching of the luminescence intensity is clearly observed in the Sb:DABCdCl hybrid. A detailed understanding of the structural distortion parameters (modulated by the water molecule) and the luminescence properties is thus required to unravel the operative structure–luminescence relationship.
To better understand the electronic structure of the doped systems, density functional theory (DFT) calculations have been carried out. In all three systems (Sb:DETACdCl, Sb:PDACdCl, and Sb:DABCdCl), 12.5% Sb-doping concentration has been considered, mimicking the 10% doped concentration observed experimentally from the ICP analysis. The projected density of states (DOS), as indicated in Fig. 6 shows that Sb:DABCdCl has the highest electronic band gap of 3.84 eV, whereas the Sb:DETACdCl and Sb:PDACdCl structures have the band gap of 3.62 and 3.19 eV, respectively. These band gap values align well with the experimental band gap trend of 3.43, 3.39, and 3.30 eV for the Sb:DABCdCl, Sb:DETACdCl and Sb:PDACdCl systems, respectively (shown in Fig. S24; ESI†). The higher absolute values of the band gap may be due to the lack of inclusion of the excitonic effect in the structures, which may significantly reduce the values in lower dimensions. However, the use of the hybrid function in conjunction with the spin–orbit coupling reliably captures trends in the band gap and accurately describes the positions of the band edges, as has been routinely done by some of us.45,46 Analysis of the DOS (Fig. 6) and the band edge charge density localization (Fig. S25; ESI†) of all three structures suggest that the Sb-center, along with the Cd-center, is populated at both the VBM and CBM, which indicates that the Sb-center can act as the luminescence source. Furthermore, in the case of Sb:DABCdCl, the H2O molecule in the crystal structure also appears in the VBM and CBM indicating that it may play a role in the luminescence behavior. To understand the bandgap reduction from Sb:DABCdCl to Sb:PDACdCl, we have aligned the DOS of the three systems with respect to the deepest energy level of α-carbon.47,48 Our results find that, while moving from Sb:DABCdCl to Sb:DETACdCl, both the VBM and CBM downshifted in energy by 0.17 and 0.39 eV, respectively with an overall band gap reduction of 0.22 eV. On the other hand, the VBM is upshifted by 0.22 eV and the CBM is downshifted by 0.43 eV, thus revealing a significant bandgap reduction of 0.65 eV in Sb:PDACdCl, compared to that of Sb:DABCdCl. Furthermore, the calculated band structure (PBE + SOC level of theory; Fig. S26; ESI†) shows the indirect nature of the bands, thus indicating higher carrier lifetime of the doped systems.
Experimentally, it has been observed that the formation of STE in the doped (Sb3+) center results in dopant-based Sb emission, resembling the 0D Sb halide hybrids, with minimal contribution from the host emission across all structures. However, the PLQY of the studied structures ranges from approximately 60% to 96%, indicating a unique structure–luminescence relationship. To understand the STE formation and its role in the PLQY, we have investigated the excited state behavior by modelling the triplet configuration of all three structures. The excited state DOS (Fig. 7(a)–(c)) reveals that the Sb3+ center is predominantly concentrated at the band edges, along with some contribution from Cd and the organic ligands in all the three structures. However, in the case of Sb:DABCdCl, the H2O molecule of the system appears at the band edges, thus acting as an efficient emission channel, resulting in an increased luminescence with a higher PLQY compared to that of Sb:PDACdCl and Sb:DETACdCl. For further quantification, the excited state electron and hole localization have been plotted (Fig. 7(f)–(h)) and the spatial overlap between the electron and hole wave functions has been considered as the extent of STE confinement. We have defined the electron and hole spatial overlap by calculating the percentage of volume occupied by the STE charge density with respect to the total volume of charge density; thus, a higher overlap percentage indicates a more confined STE. As can be seen from Fig. 7 and Table 2, the STE is localized at the inorganic (Sb3+, Cd2+) center and disperses into the organic moiety in the case of Sb:DETACdCl and Sb:PDACdCl with an electron/hole spatial overlap of 26.4 and 26.49%, respectively. Interestingly, in the case of Sb:DABCdCl, the electron and hole charge density are localized at the inorganic unit and water molecule with the electron/hole overlap of 34.04%, thus indicating the more confined STE compared to that of Sb:DETACdCl and Sb:PDACdCl. Therefore, the more localized STE leads to a higher PLQY in the case of Sb:DABCdCl.
ΔQ | Δd (Å) | Electron hole overlap (%) | ||
---|---|---|---|---|
All atoms (Å AMU1/2) | All inorganic unit (Å AMU1/2) | |||
Sb:DETACdCl | 3.95 | 3.92 | 6.35 | 26.42 |
Sb:PDACdCl | 3.48 | 3.46 | 1.21 | 26.49 |
Sb:DABCdCl (hydrated) | 8.51 | 8.00 | 11.28 | 34.04 |
Sb:DABCdCl (dehydrated) | 5.30 | 4.96 | 6.66 | 25.41 |
Upon photoexcitation, the contraction of the SbCl and CdCl octahedra facilitates the stabilization of the trapped holes by forming self-trapped charged states. Therefore, a photo-generated hole tends to be self-trapped in an octahedron, eventually distorting the surrounding electron cloud via coulombic attraction to form the STE.44,49 To quantify the octahedral contraction phenomena, we have calculated the Sb–Cl bond length contraction upon photoexcitation. In the case of Sb:DETACdCl and Sb:PDACdCl, the average Sb–Cl bond length contractions are 3.85 and 4.15%, respectively, which are lower compared to that of 5.98% in the case of Sb:DABCdCl, thus indicating the most confined STE formation in Sb:DABCdCl, matching with our charge density localization trend. Enhanced lattice distortion at the excited state leads to strong electron–phonon (e–ph) interaction50 in the system, leading to more STE formation. To quantify such lattice distortion, we have calculated the ΔQ (mass-weighted displacement) between the atomic geometries of the ground state (GS) and the excited state (ES), which has a linear correlation with the electron–phonon coupling constant as:
Furthermore, we have plotted the configuration coordinate diagram (CCD) of the ground and excited state of all three systems as presented in Fig. 7(c)–(e), where points 1 and 3 define the singlet ground state geometry and triplet excited state geometry (STE state), respectively. As described in the configuration coordinate diagram, ΔQ is the mass-weighted displacement as shown above, and Δd is the cross-over distance, i.e., the nuclear coordinate shift from the STE state to the singlet/triplet crossover point. The values are shown in Table 2. As can be seen from Fig. 7, the Δd values are 6.35 and 1.21 Å for Sb:PDACdCl and Sb:DETACdCl, respectively whereas, it is the highest with the value of 11.28 Å in the case of Sb:DABCdCl. A higher Δd indicates a greater nuclear coordinate shift from the STE state to the crossover point, which suggests that more electron–phonon interaction is required to meet the crossover point, thereby reducing the possibility of crossover. Consequently, such a low crossover possibility can reduce the non-radiative loss, eventually leading to a higher PLQY in the case of Sb:DABCdCl.
To check the role of H2O molecules as an emission channel, we have done the excited state calculation for the crystal structure of the dehydrated Sb:DABCdCl hybrid (Fig. 8). Interestingly, we have found that the ΔQ value significantly decreases from 8.00 Å in the hydrated structure to 5.30 Å in the dehydrated one, indicating a substantial reduction in structural distortion upon photoexcitation. Additionally, the octahedral contraction also decreases from 5.98 to 3.90%. The spatial overlap between electrons and holes decreases from 34.04% to 25.41%, suggesting a notable reduction in STE confinement. The CCD diagram (Fig. 8) for the dehydrated sample also shows a decrease in the Δd value to 6.66 Å compared to that of the 11.28 Å in the hydrated sample. The reduction in structural distortion, octahedral contraction, electron–hole overlap, and Δd value suggests that the absence of water molecules in the crystal plays a significant role in reducing the extent of STE formation and hence in the reduction of PLQY, thus nicely correlating with the observed experimental trend.
The theoretical insights gained herein allow us to conclude that the higher distortion indices (octahedral contraction, ΔQ, Δd) and confined STE formation lead to a higher PLQY value in the case of Sb:DABCdCl compared to that of Sb:PDACdCl and the Sb:DETACdCl hybrid in line with the experimental observations.
Footnotes |
† Electronic supplementary information (ESI) available: EDS data, PL/PLE characterization, PXRD data, SCXRD data of bond lengths and bond angles; cif and Checkcif files for host and doped hybrids. CCDC 2376166–2376174 and 2376179. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc03543g |
‡ Equal contribution. |
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