Bi- <i>ortho</i> -Carborane Unit-Riveted Perylene Monoimides: Structure-Tuned Optical Switches for Electron Transfer and Robust Thin Film-Based Fluorescence Sensors

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES4 Dec 2023Bi-ortho-Carborane Unit-Riveted Perylene Monoimides: Structure-Tuned Optical Switches for Electron Transfer and Robust Thin Film-Based Fluorescence Sensors Nannan Ding†, Yu-Chan Liao†, Gang Wang, Kai-Hsin Chang, Zhaolong Wang, Ke Liu, Jiani Ma, Pi-Tai Chou and Yu Fang Nannan Ding† Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi'an 710019 , Yu-Chan Liao† Department of Chemistry, Taiwan University, Taipei 10617 , Gang Wang Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi'an 710019 , Kai-Hsin Chang Department of Chemistry, Taiwan University, Taipei 10617 , Zhaolong Wang Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi'an 710019 , Ke Liu Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi'an 710019 , Jiani Ma *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi'an 710019 , Pi-Tai Chou *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Taiwan University, Taipei 10617 and Yu Fang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Shaanxi, Xi'an 710019 https://doi.org/10.31635/ccschem.023.202202664 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Innovative design of sensing fluorophores possessing superior photophysical properties, porosity, and packing-resistance structures is pivotal for high performance film-based fluorescent sensors. Herein, PDCB, a perylene monoimide (PMI) derivative incorporating large spatial phenyl-carborane was synthesized and found to exhibit unexpected photophysical properties. The structurally bent PDCB exhibits not only PMI-like emission but also a red-shifted emission. In sharp contrast, PMI-CBH, a linear PMI derivative, exhibits only PMI-like emission. Furthermore, upon local excitation, PDCB undergoes a photoinduced electron transfer (PET) between PMI and phenyl-carborane, resulting in a charge-transfer state. Two other PMI derivatives, PCB and PDCBP, showed a similar phenomenon. The PET rate is in the order of PCB (48 ps−1) > PDCB (163 ps−1) > PDCBP (815 ps−1) in toluene, which decreases with increasing steric hindrance, inferring structure reorganization prior to the PET process. As expected, a fabricated PDCB-based sensor showed excellent performance in acetone sensing. Download figure Download PowerPoint Introduction Extraordinary development in the Internet of Things has resulted in a qualitative leap of sensors.1–6 While target molecules are becoming increasingly complex, fluorescent film sensors (FFSs) have emerged as a new kind of powerful sensory technology as they not only meet the needs of fast, sensitive, selective, on-site, and real-time detection of harmful chemicals, but are also much more versatile.7–12 A key issue in the development of FFSs is to attain high sensitivity where innovative design of sensing fluorophores plays a central role. Meanwhile, optimization of the adlayer structures of the fluorescent films is also crucial for selectivity. In other words, practically, the fluorophore employed in a sensing film determines the sensing possibility, and the adlayer structure determines the sensing practicality.13 Unlike inorganic materials-based sensors, where the sensing materials function as a whole, the organic fluorophores in FFSs depict their sensing properties individually, suggesting that the percentage of the availability of the fluorescent molecules within the adlayer will have a great impact upon the film sensing performance. This is because the fluorescent molecules, to which the analytes are not reachable, will contribute to background, greatly reducing the signal-to-noise ratio. Therefore, to be in high-performance applications, a fluorescent film must be porous at the molecular level. For similar reasons, various fluorescent materials, including low molecular weight fluorophores, polymers, quantum dots, carbon dots, metal complexes, and coinage metal clusters, etc., have been used as sensing fluorophores.14–19 Compared with others, low molecular weight fluorophores are extremely attractive due to their great designability and excellent processability.20,21 We have long focused on the study of FFSs, and developed a variety of new sensing fluorophores and film fabrication strategies. Recent progress is credited to the implementation of a new strategy, where capillary condensation originating from the porous adlayer structures, adsorption and desorption kinetics, and the microenvironment effect of sensing fluorophores were integrated to construct new fluorescent-sensing films.22–24 In these approaches, low molecular weight compound-based fluorophores with non-planar structures exhibit advantages in sensing as there are may "molecular channels" within the fluorescent adlayer.25,26 Due to the multiple reactive sites, superior photochemical stability, and great fluorescent properties of perylene bisimide (PBI) and perylene monoimide (PMI), their derivatives have been widely used in fluorescence sensing, photothermal therapy, photodynamic therapy, etc.27–33 The polyaromatic feature, however, makes PBI/PMI derivatives easier to form H-aggregates, a less- or even non-emissive state, which is generally unfavorable for sensing. In yet another approach, o-carborane (1,2-dicarba-closo-decaborane) is an "element block" with a regular icosahedral structure, and a variety of its aromatic derivatives function as aggregation-induced emission luminogens (AIEgens).34–36 Therefore, tethering PBI/PMI moieties to o-carborane would result in fluorophores with inherent non-planar structures, laying foundations for use as sensing fluorophores in FFSs.33,37,38 Two typical examples are our recently reported PMI derivatives of o-carborane, which showed fluorescent properties of the PMI unit and AIEgens, allowing film-based fluorescent detection of benzene, toluene, xylene, and methamphetamine.37,38 With this aim, in this study, we strategically designed and synthesized a unique o-carborane derivative of PMI, PDCB, where the presence of the neighboring o-carborane-phenyl group in a V-shape configuration significantly hindered the rotational motion of the PMI. As a result, PDCB exhibits unexpected photophysical properties, where it exhibits not only PMI-like locally excited emission but also an additional solvent-polarity dependent emission in the red and near-infrared regions. In sharp contrast, the reference compound, PMI-CBH, which is in a linear configuration, only exhibits a PMI-like and solvent-independent emission profile. Comprehensive time-resolved fluorescence and transient absorption (TA) studies were carried out for PDCB and PMI-CBH to probe their distinct mechanism regarding excited-state relaxation/reaction. Moreover, supplementary support is chemically provided by synthesizing two other PDCB derivatives, PCB and PDCBP (Scheme 1), where the former truncates a carborane moiety and the latter adds an ortho-phenyl ring from the parent PDCB to probe the structure-reaction relationship. The V-shaped structure with a unique porous site makes possible the use of the film-based PDCB as a highly sensitive and selective sensor with fast response time. Details of the results and discussion are elaborated in the following sections. Scheme 1 | The molecular structures and synthetic routes of PMI-CBH, PDCB, PCB, and PDCBP. Download figure Download PowerPoint Experimental Methods Scheme 1 depicts the molecular structures and brief synthetic routes of the studied riveted PMI derivatives. Details of the synthesis and structure characterization are elaborated in the Supporting Information. In brief, a facile bromination was carried out in PMI, forming PMI-Br, which then underwent Sonogashira–Hagihara reactions and decaborane insertion reactions under an argon atmosphere to give PMI-CBH.37 Synthesis of PDCB were done by anchoring one more decaborane to an acetylene precursor PMI-CBH via an alkyne insertion reaction with the aid of N,N-dimethylaniline as a Lewis base (Scheme 1). One carborane-containing compound PCB and two benzene rings-bearing compound PDCBP were attained by two similarly controlled syntheses via PMI-Ph and PMI-CBP precursors, respectively (see Scheme 1). All studied compounds were strictly characterized using NMR and mass spectrometry, and the relative data including Supporting Information Figures S13–S19 and synthetic details are provided in the Supporting Information. Results and Discussion Single crystal structure To demonstrate how the nonplanar structure of a fluorophore can result in porous structures in the molecular packing state, a single crystal of PDCB was cultivated in CH2Cl2/n-hexane. The crystal structure and corresponding crystallographic data are presented in Figure 1a and Supporting Information Table S1, respectively. As expected, the PMI unit in the molecule takes a face-to-face position with the neighboring o-carborane phenyl unit, forming an interlocked rigid structure. Examination of the single crystal structure of PDCB shows that the packing unit is in a dimeric arrangement with no effective π–π stacking between the two PMI units (∼6.880 Å). Instead, connection of the two molecules is realized through two hydrogen bonds, which are Ccage-H···O (∼2.234 Å) and Ar-H···O (2.536 Å), respectively (Figure 1a).39,40 Synergy of the π–π stacking between the PMI moieties of neighboring packing units with the alternative electrostatic association of relevant packing units may have resulted in the formation of a single crystal (Figure 1a). Figure 1 | (a) The molecular configuration and packing mode of PDCB in the crystal state. (b, c) and (d, e) Steady-state absorption and fluorescence emission spectra of PMI-CBH and PDCB, respectively. Note: the spectra were recorded at a concentration of 5 × 10−6 mol.L−1 in various solvents and at room temperature. The insets in panels (c) and (e) are the chemical structures of PMI-CBH and PDCB, respectively. Download figure Download PowerPoint Quantitative analysis by Multiwfn41 of the crystal structure demonstrates larger than 38.72% (free volume/whole volume in 1 × 1 × 1 cell) porosity, and ∼2.066 Å average radius of the pore size, allowing efficient mass transfer of analytes of suitable sizes, a pre-requirement for fast and reversible sensing. PDCB exhibits broadened absorption and emission spectra in powder (see Supporting Information Figure S1) with a fluorescence quantum yield as high as ∼25.4%, further indicating the significance of the molecular design. On the other hand, as shown in Supporting Information Figure S2, the reference compound PMI-CBH having a linear structure shows a smaller void volume of 30.32%, and ∼1.680 Å average radius of the pore size.37 Accordingly, PDCB shows powerful film-based fluorescent sensing advantages in both sensitivity and selectivity (vide infra). Steady-state photophysical properties Figure 1b–e reveals the UV–vis absorption and fluorescence emission spectra of PMI-CBH and PDCB recorded in dilute solution, where the ET (30) values of the solvents are varied from 30.9 to 43.2.42 As seen, all the absorption spectra of the two PMI derivatives showed clear 0-0 and 0-1 vibronic bands and comparable Franck–Condon ratios (A0-0/A0-1)43 in parallel with a small shoulder at shorter wavelengths, indicative of fully dissolved PDCB with monomeric PMI behavior in the ground state ( Supporting Information Table S2). Further examination revealed that both PMI-CBH and PDCB show increasingly broadened, red-shifted, and less-resolved UV–vis absorption changes with increasing solvent polarity, probably owing to certain charge-transfer (CT) characteristics of the absorption (Figure 1b,d).44 In stark contrast to PMI-CBH which only exhibits an emission band in various solvents (Figure 1c), the fluorescence behavior of PDCB is solvent dependent (Figure 1e). In nonpolar solvents such as cyclohexane and carbon tetrachloride, the emission of PDCB is dominated by the locally excited PMI monomer fluorescence, but contributions from other emission centers seems to be non-negligible, as indicated by the increase of the emission intensity in the region of the long-wavelength tail compared with that of PMI-CBH in cyclohexane and carbon tetrachloride (cf. Figure 1c,e). Upon increasing the solvent polarity from, e.g., toluene to dimethylformamide (DMF), shown in Figure 1e, the PMI monomer emission (herein, defined as the F1 band) diminishes gradually and nearly disappears in DMF, accompanied by the appearance of a broad emission band at > 600 nm (defined as the F2 band), which is red shifted with increasing solvent polarity. The result indicates that the nature of the F2 band is subject to changes in the dipole moment versus that of the ground state, the relative stabilization of which depends on the environment polarization.45 To explore if the observed F2 band of PDCB is associated with the aggregation effect, its absorption and emission spectra were recorded at concentrations from 5.0 × 10−7 to 1.0 × 10−5 mol.L−1 in both toluene and dichloromethane (DCM) at 298 K. The results are shown in Supporting Information Figures S3. The linear Lambert–Beer's plots46 indicate the monomeric nature of PDCB in both tested solutions. Also, the molar extinction coefficients at the 0-0 vibronic band of PDCB in both toluene and DCM solutions are close to 3.72 × 104 L.mol−1.cm−1 and 3.91 × 104 L.mol−1.cm−1, respectively, which further confirm non-aggregation of PDCB in both solvents. Likewise, as shown in Supporting Information Figures S3 and S4, the insensitive emission spectral profile to concentrations also draws the conclusion that the anomalous dual emission does not originate from the aggregation associated phenomena such as dimer, oligomer, or excimer, etc. Finally, Supporting Information Figure S5 shows excitation spectra of PDCB as a function of the monitored emission wavelength in DCM. Clearly, independent of monitored wavelength at either the F1 or F2 band, the excitation spectrum is identical, which is also the same as the absorption spectrum in both the spectral profile and peak wavelength. The combination of the above results unambiguously concludes that the F1 and F2 bands of PDCB originate from the same ground-state species, which, upon Franck–Condon excitation, giving a PMI monomer associated localized excited (LE) state and a CT-like state, result in F1 and F2 emissions, respectively. To gain further insight into the relationship between the LE and CT states of PDCB, i.e., the F1 and F2 emissions and their relaxation dynamics, time-resolved emission and TA measurements were carried out and elaborated below. Time-resolved spectroscopy measurements Figure 2 shows the time-resolved fluorescence trace for PMI-CBH and PDCB based on time-correlated single photon counting (TCSPC) measurements. In this study, two representative emission wavelengths, 540 and 700 nm, were selected to probe the respective relaxation dynamics. As for compound PMI-CBH, the emission relaxation dynamics acquired by TCSPC consists of a single exponential decay component. The lifetime is fitted to be 4.02 and 3.95 ns in toluene (Figure 2a) and DCM (see Supporting Information Figure S6), respectively. Since the steady-state emission of PMI-CBH exhibits only one band, the result of single fluorescence with a single exponential spontaneous decay in toluene and DCM is straightforward. Figure 2 | (a) and (b) Kinetic traces recorded by ps-TCSPC monitoring at 540 nm (black) and 700 nm (gray) for (a) PMI-CBH and (b) PDCB in toluene at 298 K. The blue/orange and red solid lines represent the best fitting and instrument response function, respectively. λex = 440 nm. (c) and (d) fs-TA data of PMI-CBH (λex = 530 nm) and PDCB (λex = 500 nm) in toluene at 298 K. In panel (c), along the arrow bar indicates elongation of the pump-probe delay time. (e) The early relaxation dynamics of PCB in toluene acquired by fluorescence up-conversion monitored at F1 (540 nm) and F2 bands (700 nm). Inset: The population decay. (f) The relaxation dynamics of PDCBP in toluene monitored at F1 (540 nm) and F2 (700 nm) bands. λex = 440 nm. Download figure Download PowerPoint For PDCB in toluene where the dual emission bands (F1 and F2 bands, vide supra) are observed in the steady-state measurements (Figure 1e), upon monitoring at the 540 nm (the F1 band), the emission clearly consists of a short decay component of 97 ps, followed by a long population decay of 3.48 ns (Figure 2b). When monitoring at the F2 band of 700 nm, as shown in Figure 2b, a fast rise component having the same value of 97 ps (but negative pre-exponential factor) as the corresponding fast decay of the F1 band can be resolved, followed by a long population lifetime of 3.48 ns that is identical to the population decay of the F1 band. The same population decay time between F1 and F2 bands and identical fast decay (the F1 band) and rise (the F2 band) kinetics led us to conclude a precursor (F1)–successor (F2) type of reaction kinetics followed by an equilibrium between F1- and F2-emitting species for PDCB in toluene. Supplementary support of the TCSPC result is given by the femtosecond (fs) TA measurement. Figure 2c,d shows fs-TA data of PMI-CBH and PDCB in toluene, respectively. For clarity, steady-state absorption spectra of PMI-CBH and PDCB in toluene are also drawn and shaded by the cyan color. In comparison, one can promptly perceive the distinct difference between PMI-CBH and PDCB, where TA of PMI-CBH in toluene is straightforward, showing a typical PMI associated S1→Sn absorption maximized at 750 nm, followed by a ∼4 ns population decay. This assignment is consistent with a previous ultrafast TA study on perylene.47 Furthermore, the negative absorption bands maximized at ∼500 and ∼600 nm are ascribed to the ground-state bleaching (GSB) and stimulated emission (SE). The observation of an isosbestic point at ∼648 nm indicates only two TA components, i.e., the SE and S1-Sn absorption at this region, supporting the singlet excited-state species S1 for PMI-CBH. Similar results of TA spectra and single exponential decay dynamics are observed for PMI-CBH in DCM (see Supporting Information Figures S6 and S7). Therefore, independent of the solvent, PMI-CBH only shows PMI moiety-associated S1-Sn TA, consistent with only LE emission in the steady-state measurement. In comparison, the TA of PDCB is much more complicated. In addition to negative GSB and SE bands, shown in Figure 2d, the TA of LE PMI S1→Sn absorption (750 nm in PMI-CBH, Figure 2c) is interfered by a new and broad positive TA from 550 to 700 nm, resulting in a blue-shifted signal of PMI TA maximized at 690 nm. In sharp contrast to PMI-CBH, the relaxation dynamics of TA of PDCB in toluene, shown in the inset of Figure 2d, is rather complicated at every selected wavelength. This is mainly due to the combination of GSB, LE, CT TA, and their associated the decay trace at the selected wavelength e.g., and nm, which is mainly ascribed to TA of the LE state, with a rise of the CT component monitored at 550 and both a ps time that is consistent with the 97 ps by TCSPC (see Figure 2b). The results support a precursor type of photoinduced electron transfer that all TA long population decay of a (see inset of Figure the equilibrium type of PET in the TCSPC As for TA of PDCB in toluene, we the SE band of the CT state that is to be in the region of nm to the steady-state emission. This is mainly due to the of a positive TA the LE from the CT SE in the same of this is given by the TA of PDCB in where the SE of CT at nm in addition to the SE of the LE state at 600 nm (see Supporting Information Figure The rise time of SE of the CT state monitored at nm is to be ps, consistent with the fitted value of the PET time of ps in DCM based on TCSPC measurements (see Supporting Information Figure The above TCSPC and fs-TA with the steady-state dual emission and corresponding discussion for PDCB, an equilibrium type of excited-state where PDCB undergoes a PET from the precursor (the F1 band) to a CT state. In both LE and CT states for PDCB are more or subject to solvent polarity The peak wavelength of the F2 band is dependent on the solvent polarity but from to nm an difference of a similar of red of is observed for the F1 band. This the equilibrium type of PET in both toluene and DCM. the reference compound PMI-CBH with linear between PMI and phenyl-carborane does not exhibit PET dynamics in all studied it is to perceive that PDCB undergoes a electron transfer between PMI and phenyl-carborane moieties in a Moreover, PDCB the PET rate as long as and solvent relaxation is only a ps or for toluene and DCM. The results us to conclude that solvent polarity is not a key the excited-state reaction dynamics for PDCB but rather the optimization of the structure between PMI and phenyl-carborane moieties prior to For the of the following a mechanism is in Figure where equilibrium having and of and respectively, is in both toluene and DCM. The long population decay of a ns is for the of a > and As shown in the in the Supporting under the for the pre-exponential value of the fast decay component versus that of the population decay upon monitoring at the LE band is to which is to the equilibrium between LE and CT Accordingly, is to be and in toluene and respectively, which further values of and at room temperature. and PET rate (the observed fast decay of the F1 band), and can be which is × and × in toluene and × and × in DCM. Figure | (a) Scheme of the excited-state reaction mechanism for compound PDCB in toluene and DCM. (b) The and for the and states of PDCB at the level. (c) The associated with for the and of Download figure Download PowerPoint We then one further to probe the PET To attain this we chemically synthesized two PDCB derivatives, PCB and PDCBP (see Scheme 1), where the former truncates a carborane moiety and the latter adds an ortho-phenyl ring from the parent The of PET is by the observation of steady-state dual emissions for both PCB and PDCBP in the studied solvents (see Supporting Information Figure Therefore, the and steric for PCB and PDCBP, respectively, distinct PET dynamics. on the it is to that the rate of PET for PCB be than that of To the time of TCSPC time of we the fluorescence up-conversion with to the rate of PET of Figure shows the relaxation dynamics of PCB in toluene where an ps decay and rise component for F1 (540 nm) and F2 bands (700 respectively, is of these identical population of ns of Figure supporting the equilibrium type Figure reveals the TCSPC of PDCBP where F1 and F2 bands long decay and rise of ps, respectively, followed by an identical population decay of As a result, the rate of PET is in the order of PCB (48 ps−1) > PDCB (163 ps−1) > PDCBP (815 ps−1) in toluene, which decreases with increasing steric hindrance, supporting the of structure reorganization prior to the PET we then all for PCB and PDCBP, which, with for PDCB, are in Supporting Information Table S3. the rate of PET the same in the order of PCB ps−1) > PDCB ps−1) > PDCBP ps−1) in further the structure relationship. Further insight into the structure PET is by the where the and molecular of PDCB were in the by at the (Figure We the ground state structure of PDCB, as the state the of the structures (vide we also both and structure from also Figure with the of by the The the and are by linear in and the results are in Supporting Information Figure The relative and excitation of and compared to that of are in Figure The from the molecular to the molecular contribute for for and respectively ( Supporting Information Table and the are used to depict the excitation of the corresponding As a result, both the and of are localized on the PMI consistent with the PMI-like absorption profile for PDCB While the state also exhibits a similar of with to that of the state, the CT of the state is for its the of Due to the of PDCB, the results of the both the and emission (cf. Figure 1). the difference of between and emissions is within the same as for PDCB in toluene, where we the 0-0 of the F1 and F2 bands to be 540 and nm, respectively (Figure 1). the difference between the and states is

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