Triplet Photodynamic and Up-Conversion Luminescence in Donor-Acceptor Dyads with Slip-stacked vs. Co-facial Arrangement

Triplet Photodynamic and Up-Conversion Luminescence in Donor-Acceptor Dyads with Slip-stacked vs. Co-facial Arrangement Young Ju Yun†,‡, Francesca Peccati§, Gary P. Wiederrecht◊, David J. Gosztola◊, Benjamin T. Diroll◊, Gonzalo Jiménez-Osés§,ƒ and A. Jean-Luc Ayitou*,†,‡ † Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, United States. ‡ Contribution from the Department of Chemistry, Illinois Institute of Technology, Chicago, IL 60616, United States. § Center for Cooperative Research in Biosciences (CIC bioGUNE), Basque Research and Technology Alliance (BRTA), Bizkaia Technology Park, Building 800, 48160 Derio, Spain. ƒ Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain ◊ Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439, United States. * Corresponding Author e-Mail: [email protected] Abstract: The design/synthesis and characterization of organic donor-acceptor (D–A) dyads can provide precious data allowing to improve the efficiency of classical photo-induced bimolecular interactions/processes. In this report, two novel triplet D–A dyads (4 and 5) were synthesized and fully characterized. While the optical absorption and emission profiles of these new systems exhibit similar spectral structures as that of the triplet donor/sensitizer quinoidal thioamide (QDN), the transient absorption (TA) spectra of these two dyads produced new features that can be associated with triplet transients and charge transfer species. However, the kinetics of the excited-state processes/dynamics is significantly influenced by the geometrical arrangement(s) of donor/acceptor chromophores. Further analysis of the TA data suggests that the dyad with slipstack geometry (4) is less effective in undergoing both intra– and inter–dyad triplet energy transfer than the dyad with co-facial geometry (5). Subsequently, triplet sensitization of 9,10diphenylanthracene (DPA) using both dyads led to upconverted photoluminescence via triplettriplet annihilation of DPA triplet transients. But, it was found that a maximum upconversion quantum yield could be achieved at lower power density using the co-facial type dyad 5. Altogether, these results provide valuable guidance in the design of triplet donor-acceptor dyads, which could be used for light-harvesting/modulation applications. Keywords: Triplet Energy Transfer; Charge Transfer; Triplet-Triplet Annihilation Photon Upconversion ■ Introduction Organic triplet donor-acceptor (D–A) dyads are versatile scaffolds for many light-induced processes viz. photovoltaic devices,1–7 light-emitting diodes,8–10 and photocatalysis.11–14 To maximize the efficiency of the aforementioned processes, it is crucial to design/engineer the ideal D–A scaffolds such that the excited-state interaction(s) between the triplet energy donor (D) and the acceptor (A) is not constrained by intra-dyad geometrical parameters or extra/inter-dyad quenchers.15–19 In various pertinent literature, it has been reported that the distance separating the D/A chromophores,20 the overlap of the D/A wavefunctions/orbitals or dipole-dipole interactions,20,21 the optoelectronic bandgap,22–24 and the structural orientation of D/A units can affect the overall photophysics of a triplet dyad.25–27 For example, Min-Ji et al.25 have found that the geometrical features (ortho-, para-, and meta-) in triplet dyads could impact photophysical events in these systems. Alexei et al.28 also demonstrated that depending on the geometry/length of the molecular linker/spacer in a D–A dyad, photo-induced energy transfer could follow either the Dexter mechanism or the Forster-type dynamics. The investigation of geometry effect in cofacial type and slip-stacked perylene-diimide (PDI) based dyads (known to undergo singlet fission to generate triplet pairs) have also been investigated by Wasilewski et al.29,30 In this work, it was found that the co-facial type PDI dyad shows the rapid formation of excimer over the slip-stacked system. The investigation of these systems has led to the conclusion that due to slower excitons deactivation, via excimers formation, in the slip-stacked D–A dyad, this scaffold could be used to improve the performance of optoelectronic devices. Contrary to the investigation by Wasilewski et al., we wish to report two triptycene-based dyads, where the co-facial D/A interaction(s) is preferred and could lead to efficient intra– and inter-dyad TEnT. The present investigation complements a recent work22 (from our group), where a triptycene-based dyad 3 (Fig. 1)–synthesized from a quinoidal naphthylthioamide (QDN) triplet energy donor and 3-ethynyl perylene (Per)–underwent essentially photo-induced charge transfer (CT)31–34 in place of the desired QDN→Per triplet energy flow (TnET) (Supporting Information, Fig. S13). Fig. 1. Chemical structures of the triplet energy donor/sensitizer (QDN), acceptors (Per, A1, and A2), and the corresponding dyads 3, 4, and 5. With dyad 3, it was further rationalized that the CT dynamics stemmed from a change in the electronic bandgap of the acceptor Per. The introduction of the acetylenic spacer has contributed to reducing the HOMO/HOMO-1 levels of the acceptor unit, albeit the triplet energy level of the acceptor (ET ≈ 1.53 eV )35 remains unchanged. Concurrent to the report on dyad 3, we also N N S S 4 N N S S N N S S A1A2 5 7 77 Per N N S S 3 QDN 7 Previous work This work reported36 that QDN (with ET ≈ 1.77 eV in DCM at 77 K and 1.67 eV in PEG200) could be used to perform endothermic triplet sensitization of 9,10-diphenyl anthracene (DPA) (ET ≈ 1.78 eV)37 leading to DPA triplet-triplet annihilation photon upconversion (TTA-UC) with a quantum yield (ΦUC) of 2% (4% after correcting for inner filter effect). By replacing Per (in dyad 3) with DPA, it is expected that the QDN→DPA TEnT process will not be affected but the issue with mismatched electronic bandgap would be “corrected” based on the relative energies of the frontier molecular orbitals (See Fig. 4). Furthermore, the new molecular dyad(s) exhibits slip-stack and/or co-facial geometrical features, which may influence the kinetics of the QDN→DPA TEnT process. Herein, we report the synthesis and photophysical characterization of two triptycene-based dyads 4 and 5 using anthracene/DPA derivatives A1 and A2 (Fig. 1). In the present work, we investigated the kinetics of intra-dyad TEnT as well as intermolecular TEnT in the presence of free DPA. While the mechanism of the intermolecular triplet sensitization (via either QDN or A1/A2) cannot be fully elucidated in the present work, spectroscopy tools such as time-resolved pumpprobe setup were used to establish that formation of CT species during intra-dyad TEnT could affect the efficiency of the TTA-UC process and/or its underlying photophysical steps. ■ General Methods The synthetic procedures for all precursors for sensitizer QDN and acceptors A1 & A2 are reported in the Supporting Information (Schemes S1–S7). All spectroscopy measurements were performed using spectroscopy-grade solvents. All NMR characterizations were carried out on a Bruker 300 MHz spectrometer at 298 K. High-resolution mass spectrum data were recorded on a Bruker microTOF II or Shimadzu IT-TOF spectrometer in positive (ESI+) ion mode. All spectroscopy measurements were performed using spectroscopy-grade solvents. UV-vis absorption spectra were recorded on an Ocean Optics spectrometer (DH-MINI UV−vis−NIR light source and QE-Pro detector). Emission spectra were recorded on an Edinburgh Instrument FLS980 spectrometer. Time-resolved pump-probe spectroscopy was performed using an amplified Ti:sapphire laser system (Spectra Physics Spitfire) equipped with an optical parametric amplifier (OPA, Light Conversion, TOPAS). This system produces 130 fs pulses at 5 kHz centered at 800 nm. 95% of the output from the amplifier is directed to the OPA to generate tunable pump pulses in the visible and near-infrared spectral regions. For operation with 130 fs temporal resolution, the pump pulse and the remaining 5% of the output from the amplifier are directed to a transient absorption spectrometer (Helios from Ultrafast Systems), where the 5% output is used to generate a continuum probe pulse extending from 450 nm to 1,400 nm by focusing into a thin sapphire window. The pump pulse is chopped at half the repetition rate to measure a difference spectrum for the transient absorption measurement. The incident pump pulse for these experiments at 510 nm had energy on the sample of 300 nJ per pulse, focused to a 200-μm-diameter spot. The transmitted probe light was collected, and fiber optically coupled to a spectrograph that used a visible (Si) array detector. Data were collected for continuum wavelengths from 450 nm to 750 nm as a function of delay track position for the continuum probe relative to the undelayed pump pulse. The temporal chirp of the data was experimentally determined and corrected before analysis. For longer time scale processes, the probe light comes from a continuum light source (EOS from Ultrafast Systems). In this case, the system operates at 1 kHz and has a time resolution of 200ps/point. Decay times of several hundred microseconds can be measured. Computational Methods: Full geometry optimizations were carried out with Gaussian 1638 using the CAM-B3LYP hybrid functional39 and 6-31+G(d,p) basis set40 with ultrafine integration grids. Bulk solvent effects in dichloromethane and ethanol were considered implicitly through the IEF- PCM polarizable continuum model.41 The possibility of different conformations was taken into account for all structures. All stationary points were characterized by a frequency analysis performed at the same level used in the geometry optimizations. Potential energies (ΔE) were used for the discussion on the relative stabilities of the considered structures. The quasiharmonic approximation reported by Truhlar et al. was used to replace the harmonic oscillator approximation for the calculation of the vibrational contribution to enthalpy and entropy.42 Vertical excitations were computed at the same level of theory with TD-DFT (Tamm-Dancoff approximation43) with a linear response non-equilibrium treatment of the solvent.44 In this regime, only the fast degrees of freedom of the solvent are equilibrated with the excited state electronic redistribution. Computation of solvent accessible surface areas and orbital representation were performed with UCSF Chimera.45 TTA-UP Procedure: The upconversion study was performed using a 532 nm continuous wave laser beam (Nd:YAG with varying power from 20 µW to 2500 µW), which was focused onto the samples in a 2 mm cuvette. The laser beam spot size had a diameter of 118 µm. Samples of dyads 4 and 5 were prepared freshly in inhibitor-free THF with 0.1 O.D. at 532 nm, then an equal amount of DPA (saturated in THF) was added to each sample followed by 3 cycles of freeze-pump-thaw for deaeration. A 532 band stop filter was used to block the incident light from the detector. Upconverted photoluminescence emissions were collected and processed with Igor Pro v8 software. ■ Syntheses of dyads 4 and 5 Scheme 1. Syntheses of triptycene-based D−A dyads 4 and 5. Dyad 4 and 5 were synthesized following a convergent synthetic strategy, as shown in Scheme 1. The synthetic procedures for all precursors of dyads 4 and 5 are comprehensively described in the Supporting information (Scheme S1-S7). Triplet sensitizer QDN (and Br–QDN) was previously reported by our group.22,46 Dyad 4 and 5 were synthesized using Suzuki coupling reaction conditions, where Trip–1 was reacted with Br–QDN to afford Trip–2 (Supporting from dichloro-triptycene then borylation Cl Bpin + N N S S Br Trip–2 7 THF/Toluene/H2O (1:3:1) 110 °C, 48 h Pd(PPh3)4 K2CO3 Cl N N S S Trip–1 N N S S 7 Bpin 4 N N S S 7 Bpin THF/Dioxane/H2O (1:4:1) 120 °C, 48 h Pd(PPh3)4 K2CO3 A1 A2 5 THF/Dioxane/H2O (1:4:1) 120 °C, 48 h Pd(PPh3)4 K2CO3 7 Br–QDN Information, Scheme S5). Then, Trip–2 was reacted with anthracene A1 or A2 under Suzuki coupling conditions to afford dyads 4 and 5, respectively (Supporting Information, Schemes S6, S7). Both 4 and 5 were fully characterized by 1H and 13C NMR (Supporting Information, Fig. S1S12). ■ Results and Discussion UV-vis and Emission Spectroscopy Fig. 2. UV-vis absorption and emission spectra: A) QDN, A1, and A2; B) dyads 4 and 5. Samples O.D. = 0.2 at λExc = 470 nm for QDN, O.D. = 0.1 at λExc = 400 nm for A1 and A2, and O.D. = 0.1 at λExc = 510 nm for 4 and 5. Fig. 6. (A) Optoelectronic bandgap of QDN a, A1, and A2. (B) singlet/triplet energy levels of QDN, A1, and A2. Note: QDN a represents QDN with one N-bromo-phenyl substituent. From the TA data analyses, we demonstrated that dyads 4 and 5 can still perform both TEnT and CT processes; but the degree of (CT)* formation is significantly reduced in these scaffolds than what was previously observed for dyad 3 (Supporting Information, Fig. S14). In our analysis of the energies of the frontier molecular orbitals and triplet state for the donor and all acceptor units (Fig. 6), we found that while the sensitizer can perform both endothermic and exothermic TEnT to Per, A1, and A2; the HOMO-1 level for Per (compared to A1 and A2) is lower in energy than that of the sensitizer QDN. Hence, the CT process will be the dominant photophysical process in dyad 3, whereas in dyads 4 and 5 the TEnT process should be the most favored, but at different degrees. Compared to dyad 3, as the dominant process with dyads 4 and 5 is the TEnT, our first instinct was to explore intermolecular (inter-dyad) triplet sensitization of free DPA in the solution. With this in mind, the fs-TA and ns-TA experiments were performed using samples of dyad 4/5 in the presence of free DPA (1:3 molar ratio) so that we can rationales effective DPA/dyad interaction(s) in the excited-state (Fig. 7). In Fig. 7A,B & 7D,E, the TA results show a broadening of the fs-TA spectra (compared to spectra of the dyads alone). Additionally, these spectra feature different excited-state absorption structures at 670 nm (Fig. 7 & 8), suggesting some interaction between transients of the dyads and DPA molecules (DPA•••D–A or D–A•••DPA). In this scenario, energy transfer to free DPA should be expected, and the changes seen in the absorption bands can point to the formation of DPA transient(s). Fig. 7. Time-resolved transient absorption 2D intensity map/spectra for dyads 4 and 5 in the presence of free DPA in oxygen-free THF. (A & B) fs-TA map/spectra for 4 + DPA and (D &E) fs-TA map/spectra for 5 + DPA. Sample O.D. = 0.4 at λExc = 510 nm. Pump power = 0.3 μJ/pulse and 2.5 kHz repetition rate for fs-TA and 1 μJ/pulse and 1 kHz repetition rate for ns-TA. The molar ratio of Dyad:DPA is 1:3. �600 = 2.8 µs �545 = 2.9 µs -30 0 30 m∆OD 720600 Wavelength (nm) 480 -10 0 10 m∆OD 720600 Wavelength (nm) 480 Log (time) 720600 Wavelength (nm) 480 -10 0 10 m∆OD Log (time) 720600 Wavelength (nm) 480 -10 0 10 m∆OD -10 0 10 m∆OD 720600 Wavelength(nm) 480 -30 0 30 m∆OD 720600 Wavelength (nm) 480 20 10 0 m∆OD 40200 Pump delay (µs) 20 10 0 m∆OD 40200 Time delay (µs) femtosecond TA �670 = 41.4 ± 1.1 ps �550 = 29.7 ± 1.2 ps Pump @ 510 nm (CT)* T–T (ISC) T–T Abs Pump @ 510nm Pump delay 2.1 ns • • • 1.1 µs • • • 19.5 µs femtosecond TA T–T Abs Pump @ 510nm Pump delay 2.3 ns • • • 1.1 ns • • • 24.5 µs femto-second TA nano-second TA C)B)A) F)E)D) (CT)* T–T (ISC) �670 = 44.3 ± 1.1 ps �550 = 28.7 ± 1.0 ps Pump @ 510 nm 4 + D P A 5 + D P A Pump delay -220 fs • • • 20 ps 470 ps 3.5 ps • • • • • • Pump delay -200 fs • • • 26.5 ps 180 ps 415 fs • • • • • • �600 = 1.1 ± 1.2 ps �600 = 6.3 ± 3.0 ps 1(CT)* residual 3(D)* residual 3(D)* 1(CT)* �600 = 3.1 µs �545 = 3.2 µs = 40.1 ± 4.2 ps = 32.3 ± 1.7 ps 3(CT)* 3(Dyad)* 3(CT)* 3(Dyad)* 3(Dyad)* (CT)* 3(Dyad)* & 3(CT)* 3(Dyad)* (CT)* 3(Dyad)* & 3(CT)* Fig. 8. Differential fs-TA absorption spectra of the dyads (A) 4 and (B) 5 at time 1 ps after pump/pulse: (Red) without free DPA and (Blue) in the presence of free DPA. As illustrated in Fig. 8, the comparison of the fs-TA spectra (in the absence and presence of free DPA) indicates a decrease in intensity of the absorption band at 640–730 nm, suggesting an inter-dyad TEnT process. Although the direction of the energy transfer could not be established in this work, the analysis of the decay traces for the band centered at 670 nm with free DPA produced a longer time constant value (41.4 ps) than the time constant for 1(CT)* from dyad 4 alone (34.7 ps). On the other hand, the kinetic trace of 5 + free DPA remained unchanged (ca. 44 ps). This difference in the kinetics of fs-TA data can be explained using steric arguments: the diphenylanthracene moiety in dyad 5 provides better steric shielding than the phenylanthracene in dyad 4 (Fig. 4 & Table 1). The variation in the kinetic of the triplet excited-state for the two samples (Fig. 7C & 7F) suggests that dyad 5 is a better chromophore for intermolecular/inter-dyad TEnT to free DPA than dyad 4. The changes in triplet lifetime went from 4 and 6 µs (for 4 and 5 alone, respectively) to ca. 3 µs for the two dyads indicating that the (4)* is dominated by CT dynamics whereas ISC is preferred in dyad 5, and the later system can perform both intraand inter-dyad TEnT more efficiently due to its larger, more extended electronic density. 10 0 m∆OD 700630560 Wavelength (nm) 10 0 m∆OD 700630560 Wavelength (nm) 4 4 with free DPA 5 5 with free DPA @ 1 ps @ 1 ps A) B) Triplet-Triplet Annihilation Photon Up-Conversion Samples of dyads 4 and 5 alone did not show any TTA-UC photoluminescence upon excitation at 532 nm. To ascertain the observed inter-dyad TEnT to free DPA in solution, we recorded the photoluminescence emission of samples of the dyads + free DPA by varying the power density of the 532 nm incident light. The upconverted emission spectrum of DPA is shown in Fig. 9A & 9B with max = 430; this emission profile matches the intrinsic emission of DPA alone (max = 400 nm). The residual photoluminescence from the dyads, most likely from the (CT)* state, were also recovered around 540-750 nm. The non-linear/quadratic behavior of the TTA-UC process can be revealed by plotting the corresponding logarithmic values of the emission intensity and the power density (Fig. 9C and 9D). The comprehensive photophysical pathways for the two dyads are described in Fig. 8. 4 2 0 Intensity (x10 5 ) 800700600 Wavelength (nm) 500400 4 2 0 Intensity (x10 5 ) 800700600 Wavelength (nm) 500400 6 4 2 Log (UC Intensity), counts 4.03.53.02.52.01.5 Log (Power density), mW/cm 2 6 4 2 Log (UC Intensity), counts 4.03.53.02.52.01.5 Log (Power density), mW/cm 2 Slope = 2.01 Slope = 1.39 Ith = 1520 mW/cm2 5 + DPA in THF 5 + DPA in THF Slope = 2.00 Slope = 1.45 Ith = 3061 mW/cm2 4 + DPA in THF 4 + DPA in THF A) B) C) D) Upconverted PL from DPA Residual PL of dyad 4 Upconverted PL from DPA Residual PL of dyad 5 Power (mW/cm2) 0 • • • • • • 6012 2147 Power (mW/cm2) 0 • • • • • • 5807 1747 Fig. 9. Upconverted emission spectra of DPA in the presence of (A) 4 and (B) 5 excited at 532 nm (samples O.D. = 0.1 at 532 nm) in deaerated THF (3 cycles of freeze-pump-thaw). Double logarithmic plot of the upconverted emission (at max = 430 nm) for (C) 4 + DPA and (D) 5 + DPA as a function of the logarithmic of the power density of the 532 nm Nd:YAG laser. Fig. 10. Photophysical pathway of 4 and 5 (D–A) with TTA-UC process in the existence of free acceptor DPA (A). As one can see, the double logarithmic plots produced slope values of 2.0 and 1.4 for both dyads. The power density threshold (Ith), where maximum TTA-UC quantum yield values (UC) will be observed, can be determined by intersecting the low and high power density regimes. The calculated Ith for 4 and 5 were 3061 and 1520 mW/cm2, respectively. The higher value of Ith for dyad 4 suggests that the inter-dyad interaction/TEnT with free DPA molecules is competing with the formation of the CT state/species. On the other hand, the lower Ith value, for co-facial type dyad 5, correlates with previously reported data (for our group),36 where the TTA-UC process using free QDN and DPA chromophores gave UC = 2–4 % in PEG200 with the Ith value of 1790 mW/cm2. ■ Conclusions Using UV-vis emission and absorption and time-resolved transient spectroscopy tools, we demonstrated that molecular arrangement and interactions in triplet donor-acceptor dyads could influence the overall photophysics viz. intra-dyad TEnT and triplet sensitization. Photophysical characterization of dyads 4 and 5 indicated that while impacts resulting from subtle differences in optoelectronic energies between donor QDN and the acceptor (A1 or A2) chromophores cannot be ignored, the T1S1 ISC was greatly influenced by the donor/acceptor geometrical features. Slip-stacked dyad 4 was found to produce persistent CT species than the co-facial type dyad 5. On the other hand, the later dyad was found to perform better in terms of intermolecular/inter-dyad TEnT likely due to its larger surface area. Furthermore, dyad 5 can sensitize free DPA acceptor at a lower power density than dyad 4 (1520 vs. 3061 mW/cm2). Consequently, it was possible to achieve TTA-UC of DPA using these dyads as light-harvesting triplet sensitizer(s). The present study provides key/exciting results and a closer step in our ongoing investigation on triplet donoracceptor dyads, which could be used as single-component chromophores for TTA-UC in the solid state. SUPPORTING INFORMATION: Details of the synthetic procedures for all precursors of dyads 4 and 5: 1H and 13C NMR spectra, additional UV-vis absorption spectra, emission spectra, additional computational data. This material is available free of charge via the Internet at http://pubs.rsc.org AUTHOR INFORMATION: * A. Jean-Luc Ayitou (Corresponding Author) ORCID: 0000-0001-6355-2564 E-mail: [email protected] Gonzalo Jiménez-Osés ORCID: 0000-0003-0105-4337 Benjamin T. Diroll ORCID: David J. Gosztola ORCID: 0000-0003-2674-1379 Gary P. Widerrecht ORCID: 0000-0001-8821-932X Francesca Peccati ORCID: 0000-0002-7813-8216 Young Ju Yun ORCID: 0000-0002-8612-4244 FUNDING SOURCES National Science Foundation under a CAREER grant no. 1753012 Awarded to AJA. Illinois Tech Graduate Kilpatrick and Starr Fieldhouse Fellowships to YJY. Grant RTI2018-099592-B-C22 from the Agencia Estatal Investigacion of Spain (AEI) to G.J.O. NOTES The authors declare no competing financial interests. ACKNOWLEDGMENT: This material is based upon work supported by the National Science Foundation under a CAREER grant no. 1753012 awarded to AJA. YJY is thankful for the support from the Kilpatrick Graduate Fellowship and the Starr Fieldhouse Research Fellowship Programs at Illinois Tech. 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