Mario
Gutiérrez‡
a,
Maria Rosaria
Di Nunzio‡
a,
Elena
Caballero-Mancebo
a,
Félix
Sánchez
b,
Boiko
Cohen
*a and
Abderrazzak
Douhal
*a
aDepartamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III, S. N., 45071, Toledo, Spain. E-mail: boyko.koen@uclm.es; abderrazzak.douhal@uclm.es
bInstituto de Química Orgánica, CSIC, Juan de la Cierva, 3, 28006, Madrid, Spain
First published on 28th November 2022
Controlling the composition, as well as the number and type of defects in metal–organic frameworks (MOFs) helps in the understanding of the interactions that govern their behaviours, and in consequence, their potential employment in a number of photonic applications. In this work, we report on the spectroscopy and photodynamics of two heterolinker (naphthalene-2,6-dicarboxylic acid, NDC, and 4-amino-8-cyanonaphthalene-2,6-dicarboxylic acid, NACDC) Zr-based MOFs that differ in the modulator used during the synthesis, thus leading to possible defects in the frameworks. We observed an inter-particle energy transfer (ET) that occurs from <10 to 160 ps in competition with an ultrafast intra-particle ET event from excited NDC to NACDC. In addition, due to the heterogeneity of the system, we found two different populations of the push–pull NACDC linker emitting from the charge transfer (CT) state. Single crystal fluorescence microscopy experiments further demostrated that the photobehavior of the studied MOFs is dominated by NACDC. Nanosecond (ns)-flash photolysis also revealed different photobehaviours related to the presence of defects, while the femtosecond (fs)-dynamics is independent of the modulators used for the MOFs’ synthesis. These results may help in engineering of MOFs for their application in photon-based science and technology and may lead to the construction of optoelectronic devices and better photocatalysts based on these materials.
Luminescent and photoactive MOFs have attracted great interest for their unique photophysical properties making these materials excellent candidates in lighting, sensing, drug photodelivery/imaging, light-harvesting/energy transfer (ET) and photocatalysis applications.18–32 Their photophysical properties can be manipulated and tuned by a proper selection of photoactive organic linkers or metal clusters that may confer the MOF material with the desired spectroscopic and photodynamical properties or may induce additional photophysical pathways involving linker/linker or linker/metal interactions such as excimer/exciplex formation, ET, antennae effects and ligand-to-metal (or metal-to-ligand) charge transfer (LMCT or MLCT, respectively).33–36 Additionally, the MOFs’ structure might contain randomly generated defects (generally missing linkers or missing clusters) or they can be synthetically engineered.37–39 The defects might have a significant impact on the spectroscopic and photophysical properties of MOFs, in particular for their implementation in optoelectronic devices (light emitting diodes, LEDS) and catalysis or photocatalytic technologies.39–43 Recently, we reported on the photodynamical behaviour of a Zr-based MOF with naphthalene linkers (NDC, naphthalene-2,6-dicarboxylic acid), the Zr-NDC.36 The material showed rich photodynamics, revealing a great variety of photoreactions, such as ligand-to-cluster charge transfer (LCCT), excimers formation or intramolecular charge transfer (ICT) reactions.36 The understanding of these photoproperties was essential for developing the first LED in which a Zr-based MOF acted as the electroluminescent layer.40 In a subsequent work, we demonstrated how the addition of a secondary ligand, with an amine group added to the naphthalene core (NADC, 4-amino-naphthalene-2,6-dicarboxylic acid) enriches the photobehavior of the MOF, leading to a competition between the excimer formation and an ET reaction between the two ligands.44 Moreover, we observed that the LCCT process and the subsequent formation of long-lived charge separated states – both mechanisms being essential for photocatalysis applications45,46 – were retained in the mixed-linker MOF. In addition, we demonstrated that the functionalization of the linker and the partial doping of the MOF with this linker drives its absorption close to the visible region of the electromagnetic radiation, which is highly desired for photocatalytic applications.44
Herein, we present the synthesis, and the steady-state and time-resolved photophysical characterizations of two Zr-based MOFs composed by a mixture of NDC and a push–pull organic linker, (NACDC, 4-amino-8-cyanonaphthalene-2,6-dicarboxylic acid) with a ratio of 9:1 by weight. The use of the push–pull organic linker is expected to steer even more the absorption to the visible region and to trigger the LCCT mechanism through an ICT within NACDC. The synthesis of both MOFs was performed under the same experimental conditions but using different modulators (benzoic acid and acetic acid) aiming to get knowledge on the possible presence of defects and their impact on the MOFs photoproperties. In dichloromethane (DCM) suspensions, we observed a small spectral shift in the emission spectra of the investigated materials explained in terms of presence of defects due to the use of different modulators. Pico- to microsecond (ps–μs) time-resolved spectroscopy and single crystal fluorescence microscopy experiments revealed multi-exponential photobehaviors due to the presence of different conformations and environments of the ligands and the presence of defects. An inter-particle ET occurs at the ps-time scale, together with an efficient ultrafast intra-particle ET process from the excited NDC monomers to the NACDC chromophores. For both materials, the global emission lifetimes of inter-particle ET process are 20–150 ps, depending on the excitation wavelength and the concentration of the MOFs. In addition, two different populations of NACDC were detected, increasing the heterogeneity of the system. Femtosecond transient absorption (fs-TA) spectroscopy was also employed in order study the non-radiative ultrafast processes at short time scale. The comparable fs-TA dynamics for both MOFs indicates that the ultrafast processes are independent of the modulators used in the synthesis. An ultrafast component of 110 fs was assigned to the LCCT process. The electron–hole (e−–h+) recombination occurs from two different trap states, in the case of ZrMOF-630, or just one trap state, in the case of ZrMOF-640. The different behaviour at the ns-time scale is ascribed to the different nature/number of defects present in the studied MOFs. Single crystal fluorescence microscopy revealed that in the ensemble solids, the inter-particle ET plays a significant role in the de-activation of the excited NDC linker. These results provide key knowledge on the photodynamics of these kinds of mixed-linker MOFs, confirming their potential to be implemented as photocatalysts.
– ZrMOF-630: Zr-based MOF with 90% of NDC and 10% of NACDC ligands was prepared with acetic acid (Scheme 1).
– ZrMOF-640: Zr-based MOF with 90% of NDC and 10% of NACDC ligands was prepared with benzoic acid (Scheme 1).
Fig. S1A (ESI†) shows the FTIR spectra of ZrMOF-630 and ZrMOF-640, whose band positions are similar to those observed for the Zr-NDC system.51 Moreover, both MOFs displayed characteristic powder X-ray diffraction (PXRD) profiles of unfunctionalized Zr-NDC MOF (Fig. S1B, ESI†), indicating that the functionalization with the secondary linker does not affect the long-range periodicity of the materials. However, the PXRD of ZrMOF-640 shows an additional small peak at ∼6.2° that could be attributed to the (110) reflection of the reo topology of Zr-MOF-640. This reo phase has been previously observed for UiO-66 MOF (isostructural to the MOFs reported here) synthesized using acid modulators.52,53 In this phase, the clusters are eight-connected with the linkers, instead of the typical twelve connections, leading to the so-called “missing cluster” defects. This is an additional indication of the difference in the number and type of defects presented in these two MOFs. Fig. 1 and Fig. S2 (ESI†) display the SEM images of the synthesized ZrMOF-630 and ZrMOF-640 materials. From these micrographs, we can conclude that the morphology of these materials has the typical octahedral shape of isostructural UiO-type MOFs, and that there are no remarkable differences either in the morphology or size of both MOFs.
The emission spectrum exciting at 355 nm consists of a broad band with no-vibrational resolution and with maximum intensity located at 530 and 540 nm for ZrMOF-630 and ZrMOF-640, respectively. Since both MOFs have the same chemical composition, morphology, and crystalline structure, it is plausible to attribute this difference in the band position to the presence of different number of defects created in the framework because of the used modulator in the synthesis, as confirmed by elemental analysis (see above). Exciting at 400 nm, which corresponds to the absorption of the NACDC linker, results in emission spectra centred at 544 nm comparable to that exciting at 355 nm (Fig. S3, ESI†). Most notably, we did not see the characteristic emission arising from the excited NDC linkers. For Zr-NDC and Zr-NADC MOFs, highly vibrational-resolved emission spectra located at ∼400 nm were reported and assigned to the emission of the NDC monomer (with contribution of NDC excimers as well),36,44 and only when the proportion of the NADC linkers was 35%, the emission of NDC became almost negligible, because of an ET from the NDC to the NADC linkers.44 Hence, we suggest that in the present materials, the emission band at 530–540 nm is due to the emission of the NACDC ligands that might be populated via efficient ET (when exciting the NDC linkers at higher energies, 355 nm) or by direct excitation. We have previously demonstrated that the emission of NACDC linker arises from a CT state, due to an ultrafast ICT.47 Moreover, it should be noted that upon irradiation at 371 nm, the NACDC CT state can be populated either following direct excitation, since the linker presents a broad absorption spectrum between 300 and 500 nm,47 or through an efficient ET photoprocess from the photoexcited NDC linker to the NACDC one. It is also worth to note that the maximum of emission for both ZrMOFs in DCM suspension is significantly red-shifted (∼530–540 nm) in comparison to the emission spectrum of the pristine NACDC linker in DCM (506 nm).47 On the other hand, it is similar to the one reported for NACDC in more polar solvents, such as acetonitrile (ACN) (542 nm) and methanol (MeOH) (559 nm),47 which could be explained by two facts: (i) in the studied materials, the linker might experience local environments with polarity similar to ACN and MeOH, especially attending the Zr-oxo-hydroxo composition of the secondary building units (SBUs) and; (ii) the NACDC might undergo an efficient LCCT interaction with the metal clusters, which produces a red-shift in the emission spectrum as previously demonstrated.34,36 In order to shed light on the working mechanism, we have explored the possible correlation between the MOFs’ spectroscopic properties and the solvent polarity, by recording the emission spectra of ZrMOF-630 in diethyl ether (DE) and MeOH suspensions. Neither the absorption nor the emission spectra or QY values showed appreciable changes with the used solvent (Table S1 and Fig. S4A, ESI†). Even though this observation does not completely exclude the polarity environment as the cause of the emission red shift of the MOFs compared to the linkers, it suggests that the most probable cause is a strong LCCT phenomenon.
The excitation spectra of both MOFs (Fig. S5, ESI†) provide essential clues about the spectroscopic properties of these materials. On one hand, there is an increase in the intensity of the NACDC-band (maximum at ∼440 nm) at observation wavelengths above 490 nm, confirming that the emission mainly arises from the NACDC linkers. On the other hand, and very importantly, when the excitation spectrum was collected at 570 nm, we still observed the vibrationally-resolved band at 360 nm corresponding to the NDC linkers, which do not emit above 500 nm.36 This observation is an unequivocal evidence of an efficient ET from the photoexcited NDC to NACDC and will be discussed in more details within the time-resolved emission experiments section (vide infra).
The emission quantum yields (QYs) exciting at 355 nm in DCM were very similar for the two MOFs, being 0.04 ± 0.01 and 0.05 ± 0.01 for ZrMOF-630 and ZrMOF-640, respectively (Table S1, ESI†). These values were reduced to 0.02 ± 0.01 and 0.03 ± 0.01 when the excitation was changed to 430 nm. The wavelength-dependence of the QYs is explained in terms of a more efficient LCCT interaction with the Zr clusters when the excited linker is NACDC (430 nm) instead of NDC (355 nm) one. Similar results were reported for the Zr-NDC and Zr-NADC MOFs containing different amounts (from 2 to 35%) of the NADC linker with respect to the NDC one in DCM suspensions.44 In particular, the QY for the pristine Zr-NDC MOF (100% NDC linker) was 0.30 following excitation at 355 nm and it gradually decreased to 0.012 upon increasing the NADC content (similar trend was observed for the 400 nm excitation) due to more efficient LCCT reactions.44
In the solid state, the reflectance (converted to Kubelka–Munk, K–M) spectra of the studied ZrMOFs show a vibrationally-structured band with an intensity maximum at 355 nm and a second broad one ranging from ∼400 to 500 nm corresponding to the absorption of NDC and NACDC linkers, respectively (Fig. S6, ESI†). The emission spectra of ZrMOF-630 are centred at 555 nm and do not depend on the excitation wavelength, while for ZrMOF-640 we observed a 5 nm red-shift (from 550 to 555 nm) going from λexc = 355 nm to λexc = 390/433 nm (Fig. S6, ESI†). Compared with the emission spectra recorded in DCM suspensions, those in the solid-state are red-shifted by 25 and ∼15 nm for ZrMOF-630 and ZrMOF-640, respectively, suggesting a stronger stabilization of the NACDC linkers in the powder samples. The excitation spectra recorded at 550 nm confirm that, even in the solid-state, the emission is arising predominantly from the NACDC linkers (Fig. S6, ESI†). Moreover, the excitation spectra for both materials also show behaviour that is similar to the one observed in DCM suspensions indicating that ET process is still operational in the solid state.
Sample | λ obs (nm) | τ 1 ± 20 (ps) | a 1 (%) | c 1 (%) | τ 2 ± 50 (ps) | a 2 (%) | c 2 (%) | τ 3 ± 0.2 (ns) | a 3 (%) | c 3 (%) | τ 4 ± 0.3 (ns) | a 4 (%) | c 4 (%) |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
630/DCM λexc = 340 nm | 405 | 80 | 81 | 37 | 400 | 16 | 36 | 1.6 | 2 | 18 | 6.0 | 1 | 9 |
425 | 72 | 27 | 24 | 43 | 3 | 26 | 1 | 4 | |||||
450 | 57 | 14 | 34 | 41 | 8 | 36 | 1 | 9 | |||||
475 | 30 | 3 | 43 | 21 | 23 | 44 | 4 | 32 | |||||
490 | 12 | 1 | 42 | 13 | 36 | 41 | 10 | 45 | |||||
510 | 1 | <1 | 34 | 7 | 46 | 36 | 19 | 57 | |||||
550 | −100 | −100 | 21 | 3 | 48 | 28 | 31 | 69 | |||||
600 | −100 | −100 | 13 | 2 | 49 | 25 | 38 | 73 | |||||
630/DCM λexc = 371 nm | 425 | 150 | 82 | 36 | 780 | 16 | 36 | 2.7 | 1 | 8 | 7.5 | 1 | 20 |
450 | 73 | 26 | 23 | 42 | 3 | 17 | 1 | 15 | |||||
475 | 46 | 8 | 38 | 34 | 14 | 41 | 2 | 17 | |||||
490 | 31 | 3 | 42 | 25 | 23 | 48 | 4 | 24 | |||||
510 | 13 | 1 | 42 | 16 | 35 | 47 | 10 | 36 | |||||
530 | 4 | 1 | 39 | 11 | 42 | 44 | 15 | 44 | |||||
550 | −100 | −100 | 36 | 9 | 45 | 42 | 19 | 49 | |||||
575 | −100 | −100 | 31 | 7 | 47 | 41 | 22 | 52 | |||||
600 | −100 | −100 | 29 | 7 | 49 | 41 | 22 | 52 | |||||
625 | −100 | −100 | 29 | 7 | 49 | 42 | 22 | 51 | |||||
650 | −100 | −100 | 31 | 8 | 49 | 44 | 20 | 48 | |||||
630/DCM λexc = 433 nm | 475 | 140 | 56 | 10 | 800 | 33 | 35 | 2.9 | 10 | 40 | 8.0 | 1 | 15 |
490 | 43 | 5 | 37 | 27 | 17 | 46 | 3 | 22 | |||||
520 | 33 | 3 | 33 | 15 | 25 | 41 | 9 | 41 | |||||
550 | 26 | 2 | 30 | 10 | 29 | 36 | 15 | 52 | |||||
600 | 22 | 1 | 27 | 8 | 32 | 34 | 19 | 57 | |||||
650 | 21 | 1 | 28 | 8 | 32 | 35 | 19 | 56 |
For further discussion, we focus on the origin of the 150 ps component, which is present as a decay at the blue and as a rise at the red side of the emission spectrum. Although its value is similar to the one found for the excimer formation in Zr-NDC MOF (250–840 ps), its spectral behaviour differs significantly, which suggests that it corresponds to a different process. For the Zr-NDC MOF, this component turns into a rise at 475 nm, coincident with the emission region of NDC excimers, while for the systems under study, this component becomes a rise at 550 nm (Table 1), a wavelength into the red from the emission of the NDC excimers, thus excluding the possibility that this component might be attributed to the excimer formation. Since it is decaying in the blue region and rising in the red part, it indicates the existence of an excited-state process in the MOF. Taking into account this behavior and the large spectral overlap between the emission of the NDC linker and the absorption of the NACDC one (Fig. S8, ESI†), it is clear the occurrence of an ET from the photoexcited NDC linker to the NACDC one, and thus, we attribute the first component (150 ps) to the time constant of this ET process. To better characterize this ET process, we recorded emission decays of suspensions with different ZrMOF-630 concentrations (from 1 mg/5 mL to 0.01 mg/5 mL) in DCM following excitation at 371 nm (Table S2 and Fig. S9A, B, ESI†). While the ns-decay components remained largely unaffected by the change in the concentration, the value of the time constant assigned to the ET event decreases significantly from ∼150 ps in the more concentrated sample to 30 ps for the intermediate one (0.25 mg mL−1), and <10 ps (below system resolution) in the more diluted one (0.01 mg mL−1). The speeding up of the ET process is concomitant with an increase in the relative contribution of the NDC band in the steady-state emission spectra (Fig. S9C and D, ESI†). Thus, the observed behaviour indicates the presence of an inter-particle ET taking place in tens to hundreds of picoseconds in addition to an ultrafast (few picoseconds) intra-particle ET. Additionally, we expect that the LCCT process from the excited NDC linkers that takes place on ultrafast timescales (vide infra) could also compete with the ET events.34
Next, we attribute the second component (780 ps) to the locally excited state of the photoexcited (either directly or through ET) NACDC, while the third (2.7 ns) and fourth (7.5 ns) ones to the emission of different CT populations of NACDC produced following an ultrafast ICT event (Scheme 2). These CT species experience different local environment giving rise to the observed decay components. In a recent study on the excited state dynamics of NACDC linker in solutions, a dependence on the solvent polarity was observed for the emission lifetime of CT species ranging from 8.9 ns in MeOH to 17.6 ns in DE.47 The significant decrease in the lifetime of the CT state for NACDC linker in ZrMOF-630 is explained in terms of efficient LCCT process. Note that we cannot gauge whether the LCCT occurs preferentially from a NACDC population in a specific local environment or that both contribute equally.
To further study the origin of the time component observed for ZrMOF-630 in DCM suspensions, the system was also interrogated upon excitation at 340 and 433 nm (Fig. S7A and C, ESI†). At 340 nm excitation wavelength, the photodynamics is still characterized by four-exponential decays with time components: τ1 = 80 ps, τ2 = 400 ps, τ3 = 1.6 ns, and τ4 = 6.0 ns (Table 1). The trend is similar to the one upon excitation at 371 nm, with the 80 ps lifetime decaying in the blue side and rising starting from 550 nm. These results further indicate the presence of an efficient ET from the NDC to NACDC linkers, considering that at 340 nm we are exciting almost exclusively the NDC linkers (Fig. S4B, ESI†). The shortening of τ1 from 150 to 80 ps is due to a faster relaxation after pumping the system with an excess energy at 340 nm. By tuning the excitation wavelength to 433 nm we predominantly excite the NACDC ligands. Fig. 2C shows a comparison of the emission decays for ZrMOF-630 in DCM suspensions upon excitation at 371 and 433 nm and observation at two different emission wavelengths, while Table 1 collects the obtained parameters from the multi-exponential fit. The decays were also fitted by a model of four components giving: τ1 = 140 ps, τ2 = 800 ps, τ3 = 2.9 ns, and τ4 = 8.0 ns. Most notably, here we did not observe a rising component, which further demonstrates that the rising component following excitation of the system at 340 nm and 371 nm has its origin in the NDC linker (ET from NDC to NACDC). Although in the emission transients upon excitation at 433 nm we got a time component (∼140 ps) similar to the one assigned to the inter-particle ET, it behaves as a decay through the whole observation spectral range. This behavior suggests that this component has a different origin from the one obtained following excitation at 340 and 371 nm. In similarity with the assignments for the decays following excitation at 371 nm, we expect to see the contribution of two different NACDC populations having CT character produced as a result of an ultrafast ICT in the directly excited the NACDC linker as a consequence of different local environments with different polarities (Scheme 2). Thus, we assign τ1 and τ2 (maxima contributions at the blue side of the emission spectrum) to the lifetimes of the LE states of the two NACDC species, while τ3 and τ4 (maxima contributions at 490 and 600 nm, in that order) correspond to the emission lifetime of the NACDC species with CT character.
To get more insight in the ps-dynamics and further assess of our assignments, we recoded ps time-resolved emission spectra (TRES) of ZrMOF-630 in DCM suspensions upon excitation at 371 (Fig. 2D) and 433 nm (Fig. S10, ESI†). To begin with 371 nm-excitation TRES (Fig. 2D), the observed red shift from 430 to 530 nm, on increasing the gating time, supports the explanation of the occurrence of the ET process between the NDC and NACDC linkers, along with the emission of NACDC from the LE state (∼500 nm) and the longer-lived emission from the CT state/s. Upon excitation at 433 nm (Fig. S10, ESI†), remarkably no spectral shift is observed in the recorded spectra, further confirming the presence of ET upon excitation of NDC linkers, and not from NACDC ones.
Next, we also studied the photodynamics of the ZrMOF-640 in DCM suspensions upon excitations at 340, 371, and 433 nm (Fig. S11, ESI†). The decays were also fitted using a four-exponential model giving time constants: τ1 = 160 ps, τ2 = 910 ps, τ3 = 3.2 ns, and τ4 = 8.6 ns. The observed components behave as those of ZrMOF-630; thus, we assign τ1 to the NDC-NACDC ET event, τ2 to the emission of the NACDC linker from the LE state, τ3 and τ4 to NACDC emission lifetimes from the CT states after experiencing an ultrafast ICT. The differences in the lifetimes between ZrMOF-630 and ZrMOF-640 are within the experimental and fit errors, which indicates that the modulator used during the synthesis does not affect significantly the photodynamics of the resulting materials at this timescale. This is further supported by the similar behaviour of both materials upon excitation at 340 and 433 nm (Tables 1 and 2).
Sample | λ obs (nm) | τ 1 ± 20 (ps) | a 1 (%) | c 1 (%) | τ 2 ± 50 (ps) | a 2 (%) | c 2 (%) | τ 3 ± 0.2 (ns) | a 3 (%) | c 3 (%) | τ 4 ± 0.3 (ns) | a 4 (%) | c 4 (%) |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
640/DCM λexc = 340 nm | 405 | 70 | 69 | 4 | 410 | 9 | 3 | 1.7 | 3 | 3 | 6.4 | 19 | 90 |
425 | 72 | 6 | 14 | 6 | 3 | 5 | 11 | 83 | |||||
450 | 59 | 6 | 28 | 17 | 7 | 18 | 6 | 59 | |||||
475 | 23 | 1 | 42 | 15 | 27 | 39 | 8 | 45 | |||||
490 | 8 | 1 | 40 | 9 | 38 | 37 | 14 | 53 | |||||
510 | −100 | −100 | 30 | 5 | 45 | 30 | 25 | 65 | |||||
550 | −100 | −100 | 15 | 2 | 44 | 21 | 41 | 77 | |||||
600 | −100 | −100 | 5 | 1 | 43 | 17 | 52 | 82 | |||||
640/DCM λexc = 371 nm | 425 | 160 | 60 | 4 | 910 | 6 | 2 | 3.2 | 15 | 22 | 8.6 | 19 | 72 |
450 | 61 | 7 | 23 | 16 | 5 | 11 | 11 | 66 | |||||
475 | 41 | 5 | 40 | 28 | 15 | 38 | 4 | 29 | |||||
490 | 28 | 3 | 43 | 24 | 23 | 44 | 6 | 29 | |||||
510 | 15 | 1 | 40 | 15 | 34 | 44 | 11 | 40 | |||||
530 | 7 | 1 | 35 | 10 | 40 | 41 | 18 | 48 | |||||
550 | −100 | −100 | 32 | 8 | 45 | 39 | 23 | 53 | |||||
575 | −100 | −100 | 24 | 5 | 46 | 35 | 30 | 60 | |||||
600 | −100 | −100 | 21 | 4 | 48 | 35 | 31 | 61 | |||||
625 | −100 | −100 | 21 | 5 | 49 | 36 | 30 | 59 | |||||
650 | −100 | −100 | 25 | 6 | 48 | 37 | 27 | 57 | |||||
640/DCM λexc = 433 nm | 475 | 220 | 54 | 10 | 1100 | 33 | 32 | 3.4 | 9 | 25 | 8.8 | 4 | 33 |
490 | 44 | 7 | 35 | 26 | 16 | 39 | 5 | 28 | |||||
520 | 32 | 3 | 32 | 16 | 25 | 39 | 11 | 42 | |||||
550 | 25 | 2 | 28 | 11 | 30 | 36 | 17 | 51 | |||||
600 | 21 | 1 | 24 | 8 | 33 | 34 | 22 | 57 | |||||
650 | 21 | 1 | 26 | 9 | 34 | 36 | 19 | 54 |
Excitation of solid ZrMOF-630 and ZrMOF-640 at 371 and 433 nm gives rise to emission decays that were fitted to four decaying components at all the observation wavelengths. Representative transients are shown in Fig. S12 (ESI†), while the respective fit parameters are reported in Table S3 (ESI†). For the ZrMOF-630 powder irradiated at 371 nm, we obtained: τ1 = 110 ps and τ2 = 600 ps, with maximum contribution at the blue side of the emission spectrum 450–500 nm; τ3 = 2.5 ns and τ4 = 8.6 ns, with maximum contribution at wavelengths > 550 nm (Table S3, ESI†). By exciting the sample at 433 nm, the analysis gives: τ1 = 170 ps, τ2 = 890 ps, and τ3 = 3.0 ns, with c% max at 500 nm; τ4 = 8.7 ns, with c% max at 600 nm (Table S3, ESI†). It is worth to note that we did not observe a rising component following excitation at 371 nm.
We suggest that due to the close packing between the MOF particles, the time constant related to the inter-particle ET becomes comparable to the one associated with the intra-particle ET. Based on these considerations and the similarity of the behaviour of the transients collected in solid and in DCM suspensions, we assign τ1 and τ2, following excitation at both 371 nm and 433 nm, to the LE lifetimes of NACDC. The notable exception is that the value of τ1 following excitation at 371 nm in solid state is most probably affected by the ultrafast inter- and intra-particle ET. Finally, τ3 and τ4 correspond to emission lifetimes of different CT-character NACDC populations either directly excited or coming from the ET event (the latter following excitation at 371 nm), after having experienced an ICT reaction. The study of ZrMOF-640 led to comparable results and interpretation, as it can be seen from Fig. S12 and Table S3 (ESI†).
Scheme 2 resumes the photophysical processes in the investigated compounds in DCM suspensions following excitation at 340 nm, 371 nm and 433 nm.
To further characterize the behaviour of the single ZrMOF crystals, we applied spectral filtering and recorded the emission decays at two discrete spectral regions, corresponding to the emission of the NDC (450–480 nm) and NACDC (580–620 nm) linkers. Between 40 and 50 decays were collected for each sample and analysed separately. Fig. 3C and D show representative decays on a logarithmic scale for both MOFs along with the respective multiexponential fits at the selected observation regions, while Table 3 gives the corresponding fit parameters. When the emission was monitored at the blue emission band, we obtained two-time components of 0.44 ns (90%) and 2.15 ns (10%) for ZrMOF-630; and 0.45 ns (86%) and 1.92 ns (14%) for ZrMOF-640. The emission at the reddest region decays tri-exponentially with values of the obtained time constants of 0.70 ns (61%), 2.78 ns (36%) and 7.31 ns (3%) for ZrMOF-630; and 0.83 ns (60%), 2.85 ns (33%) and 6.78 ns (7%) for the ZrMOF-640. In agreement with the ensemble experiments and previous reports, the two components for the blue- and red-emission decays arise from populations of NDC and NACDC linkers, respectively, having different intra-MOF environments.34,44 We expect that the MOF structure and the presence of defects should allow for different conformations of the linkers and environments, giving rise to the observed multiexponential and wavelength-dependent photobehavior. Since the time constant associated with the intra-particle ET process is below the time resolution of the experimental system (∼50 ps) we could not observe evidence for ET from the excited NDC to NACDC in the single crystals. However, the time components at the blue side of the emission spectra that correspond to the NDC monomer (0.45 and ∼2 ns) are significantly shorter than the previously reported ones for pristine NDC ZrMOF (2.5–4.2 ns). Thus, we explain these results as a result of the presence of efficient intra-particle ET from NDC to NACDC within the ZrMOF structure, which decreases the value of the fluorescence lifetime. On the other hand, the fit of the decays collected at the red side of the emission spectra (NACDC emission) shows comparable behaviour to the one observed for the ensemble average time-resolved measurements. Thus, we assign the longer components (∼2.8 ns and ∼7.5 ns) to the emission of NACDC (excited directly or through ET) CT state in local environments with different polarity, while the shorter component of ∼0.8 ns arises from the LE state prior to the ICT reaction. Notably, we did not observe the 0.14–0.16 ns component observed in the ensemble measurements, which we explain in terms of the lower resolution (∼0.20 ns) of the experimental set-up.
System | 450–480 nm | 580–620 nm | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
τ 1 ± 0.05 (ns) | a 1 (%) | τ 2 ± 0.30 (ns) | a 2 (%) | τ 1 ± 0.05 (ns) | a 1 (%) | τ 2 ± 0.30 (ns) | a 2 (%) | τ 3 ± 0.50 (ns) | a 3 (%) | |
ZrMOF-630 | 0.44 | 90 | 2.15 | 10 | 0.70 | 61 | 2.78 | 36 | 7.31 | 3 |
ZrMOF-640 | 0.45 | 86 | 1.92 | 14 | 0.83 | 60 | 2.85 | 33 | 6.78 | 7 |
The transients were fitted using multi-exponential functions with two ultrafast components of 110 ± 40 fs and 2.48 ± 0.25 ps along with a longer one of ∼55 ± 6 ps for ZrMOF-630 at 360 nm excitation, and with two components of 3.17 ± 0.35 and 30 ± 4 ps, following 420 nm excitation (Table 4). We associate the fs-component that appears as a rise following excitation at 360 nm with the formation of the NIR band, which reflects an ultrafast LCCT process, generating CSSs in the MOFs. Similar time constant for the LCCT has been found for the NIR transients of Zr-NADC MOF in DCM.34 Note that due to the significantly lower transient signal following excitation at 420 nm, we observe a considerable contribution from the solvated electron signal in the first 200 fs, which does not allow us to adequately estimate the contribution of this component to the collected signals.
λ exc (nm) | System | τ 1 (ps) | a 1 (%) | τ 2 (ps) | a 2 (%) | τ 3 (ps) | a 3 (%) | τ 4 (ps) | a 4 (%) |
---|---|---|---|---|---|---|---|---|---|
360 | ZrMOF-630 | 0.11 | (−)100 | 2.48 | 36 | 55 | 39 | >200 | 25 |
ZrMOF-640 | 0.10 | (−)100 | 2.60 | 30 | 57 | 44 | >200 | 26 | |
420 | ZrMOF-630 | — | — | 3.17 | 59 | 30 | 20 | >200 | 21 |
ZrMOF-640 | — | — | 3.05 | 56 | 32 | 25 | >200 | 19 |
The longer components of the TA signals are present as decays in the whole recorded spectral region. These two components have also been observed in the transients of the Zr-NADC MOF in DCM suspensions.34 Thus, we assign the 2.48 ps (3.17 ps) one to the vibrational relaxation (VR, cooling) of the photoexcited linkers, while the 55 ps (30 ps) component is attributed to a fast non-radiative relaxation of the LCCT state to trap states. A notable difference is evidenced in the value of the longer ps-component, which has a lower value (30 ps) following excitation at 420 nm. Since at this wavelength we predominantly excite the NACDC linker, we suggest that the presence of the electron withdrawing CN group can affect the electron orbital density around both the Zr node and the linker moieties, and thus their electronic coupling, which in turn can speed up the relaxation event to the trap states.
The TAS appear as a continuous negative band in the 400–700 nm region, with intensity maximum at the shortest wavelengths. Similar TAS have been observed earlier for Zr-NDC and Zr-NADC (65% NDC, 35% NADC) materials in DMF suspensions, and they have been ascribed to long-lived CSSs photoproduced after a LCCT process.34,44
No difference was observed for the decays registered under different atmospheric (air, O2, and N2) conditions, thus indicating that no triplet states are involved in the relaxation of the excited samples (Fig. S16C and D, ESI†). For ZrMOF-630 in DCM, the fits of the decays give two components (τ1 = 1.0 μs and τ2 = 3.7 μs) in the blue region, which become slightly longer (τ1 = 1.5 μs and τ2 = 5.4 μs) in the red region. On the other hand, for the ZrMOF-640 material the fits give only one component of 3.2 μs. Previous reports have suggested the existence of different trap states within the metal cluster part of the MOF.54–58 The origin of the observed multi-exponential decays has been associated to electron–hole (e−–h+) recombination from differently located trap states such as surface defects or linker vacancies. In addition, metal-based nanoparticles possessing d-orbitals have electron trap states, which can further contribute to the multi-exponential behavior.54,55,57,58 On the basis of these considerations, we suggest that the observed long-lived components of the investigated MOFs are due to e−–h+ recombination from two different trap states, in the case of ZrMOF-630, or just one trap state, in the case of ZrMOF-640. The different behaviour reflects the different nature/number of defects present in each MOF. It is worth noting that a recent report demonstrated that a more efficient LCCT will produce higher population of CSSs, and as a result, the photocatalysis activity of the MOFs is enhanced.45 Hence, these results indicate that ZrMOF-630 and ZrMOF-640 are potential candidates for photocatalysis applications.
To summarize, Scheme 2 illustrates the main findings and discussion of photoprocesses and deactivation pathways observed in ZrMOF-630 and ZrMOF-640 upon exciting predominantly at NDC (340–370 nm) and NACDC (400–430 nm) linkers.
System | QY | LCCT (fs) | ICT (fs) | ET (ps) | Excimer formation (ps) | ||
---|---|---|---|---|---|---|---|
Excit. @ 350 or 355 nm | Excit. @ 400 or 430 nm | Intra-particle | Inter-particle | ||||
a From ref. 36. b From ref. 34. c From ref. 44. | |||||||
Zr-NDC | 0.30 ± 0.05a | — | <100b | — | — | — | 840a |
Zr-NADC (10%) | 0.07 ± 0.03 | 0.018 ± 0.008 | <100c | Ultrafastc | — | — | 200c |
Zr-NADC (35%) | 0.012 ± 0.008c | 0.003 ± 0.002c | <100c | Ultrafastc | 1.2c | — | — |
ZrMOF-630 | 0.04 ± 0.01 | 0.02 ± 0.01 | 110 | <100 | Ultrafast | <10–150 | — |
ZrMOF-640 | 0.05 ± 0.01 | 0.03 ± 0.01 | 100 | <100 | Ultrafast | <10–160 | — |
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2tc04460a |
‡ Equal contributions. |
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