Excimer Formation in Carboxylic Acid-Functionalized Perylene

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C: Energy Conversion and Storage; Energy and Charge Transport

Excimer Formation in Carboxylic Acid Functionalised Perylene Diimides Attached to Silicon Dioxide Nanoparticles Jeffrey Gorman, Raj Pandya, Jesse R Allardice, Michael B Price, Timothy W. Schmidt, Richard H. Friend, Akshay Rao, and Nathaniel J.L.K. Davis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12061 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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The Journal of Physical Chemistry

Excimer Formation in Carboxylic Acid Functionalised Perylene Diimides Attached to Silicon Dioxide Nanoparticles Jeffrey Gormana, Raj Pandyaa, Jesse R. Allardicea, Michael B. Priceb, Timothy W. Schmidtc, Richard H. Frienda, Akshay Raoa and Nathaniel J. L. K. Davisb* Contacts Jeffrey Gorman, Raj Pandya, Jesse R. Allardice, Richard H. Friend and Akshay Rao a Cavendish

Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge, CB3 0HE, UK

Michael B. Price and Nathaniel J. L. K. Davis b School

of Chemical and Physical Sciences, Victoria University of Wellington, Wellington 6140, New

Zealand. * Email: [email protected] Timothy W. Schmidt c ARC

Centre of Excellence in Exciton Science, School of Chemistry, UNSW Sydney, Australia

Abstract The creation of artificial light harvesting complexes involves the ordered arrangement of chromophores in space. To guarantee efficient energy-transfer processes, organic dyes must be brought into close proximity, often leading to aggregation and the formation of excimer states. In recent years the attachment of ligand-based chromophores to nanoparticles has also generated interest in relation to improved solar harvesting and spin-dependent electronic interactions such as singlet fission and upconversion. We explore the covalent attachment of two novel perylene-diimide (PDI) carboxylic acid ligands to silicon dioxide nanoparticles. This allows us to study electronic interactions between the ligands when attached to nanoparticles, since these cannot couple to the wide-bandgap silicon dioxide. One of the synthesized PDI ligands has sterically hindering phenols in the bay position and undergoes minimal optical changes upon attachment, but the other forms an excimer state with a red-shifted and long-lived florescence. As such, molecular structure changes offer a method to tune weak and strong interactions between ligand layers on a nanocrystal surfaces. Introduction To fully explore the versatility and functionality of solar energy, we need to create and characterise artificial light harvesting systems that efficiently trap, transport and convert solar energy to useable forms1–3. These criteria are ensured in natural light harvesting antenna systems by well-defined spatial arrangements of chromophores4,5. The characterisation of self-assemblies of organic chromophore dyes in nature has lead research to explore ways to exploit biomimetic supramolecular assembles for tailored artificial photon processing, leading to functional molecular dye aggregates1,6–10. As such, there is much interest in the self-assembly of organic dyes into highly organized aggregates with varying morphologies11–18. Stabilisation of these self-assembled superstructures is of great practical

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interest as it potentially improves aggregate stability, which in turn leads to improved light harvester functionality and longevity. The performance of electronic and photonic processes in aromatic π-π stack materials depends strongly on the optical properties of the assembled chromophores19–22. The exciton transport mechanisms in these materials23,24 can be understood according to a Frenkel exciton model where the electronic excitation is spread over many monomer units1,25, which is similar to the quantum coherence found in natural photosynthetic systems1,7. Organic dye chromophores have been used extensively in the fluorescence labelling of biomolecules. The standard approach of using a two component system for wavelength shifting26 or fluorescence quenching27,28 via Förster resonance energy transfer (FRET) 29 leads to complexities due to the multiple species involved. An alternative approach is to harness excimer emission from a single type of chromophore, which offers the advantages of a large stokes shift and long fluorescent lifetimes and potentially reduces background fluorescence at the desired detection wavelength, which is useful for time-resolved biosensing and bioimaging30,31. As such there is much interest in developing a clear understanding of the intramolecular processes that underlie excimer formation with the goal of eventually controlling them. Perylene-diimides (PDIs) are one of the most heavily studied types of organic chromophores due to their range of hues from red to violet, their high molar absorptivity, high photoluminescence quantum efficiency and their superior chemical, thermal and photochemical stability32–34. The PDI’s tendency to form self-assembled aggregates mean they are versatile chromophores for the construction of novel functional super molecular assembles34–36. By changing the functionality of both the imide and bay positions of the PDIs’ various derivatives has been explored for applications in supramolecular assembly, liquid crystals, artificial light harvesting systems and organic photovoltaics37,38. Aggregation can be decreased by functionalising the diimide end-groups with bulky substituents that impede co-facial π–π interactions between the PDIs34,39–41. Due to their extensive π-systems, at high concentrations PDIs regularly form H or J aggregates42, as well as excimer like states20,42–44. H-aggregates are characterised by face-on stacking, blue-shifted absorption and quenched luminescence, while J aggregates form from edge on stacking and have red-shifted luminescence23,42. The role of the excimer state in PDIs has been extensively studied 20,45– 49. Excimer state emission generally comes from PDIs that exhibit H-aggregate formation facilitated by the addition of bulky groups attached to the imide positions of the PDI44,48,50. Understanding the formation of these low energy excimer states in molecular aggregates is of great interest11,51,52 as it could allow for increased functionality, stability and device performance. The excimer state can act as an exciton trap site, decreasing quantum yield or device performance if the trapping occurs at a faster rate than exciton diffusion53,54, charge transfer and transport20,55. Recently there has been an increase in research into the field of chromophore ligands attached to nanocrystal quantum dots56–59 or semiconductor scaffolds60, in particular, the replacement of ligands that offer colloidal stability with ones that add additional optoelectronic properties. These ligands have potential applications for increased solar absorption, efficient transport of excitations, upconversion and singlet fission56–59. While studies of the interaction between the chromophores and the nanocrystal quantum dots directly leads to more potential applications, the growth of the field necessitates the study of the interactions between chromophores on the surface of nanoparticles in the absence of the quantum dot interactions. We present a new method for the attachment of chromophores to small optically inert SiO2 nanoparticles and study the interactions between two different PDI-based ligands. One of these ligands undergoes minimal optical changes upon attachment compared to the monomer form while the other undergoes extensive aggregation and exhibits excimer state emission. We find that steric-hindrance is crucial to self-interaction in nanocrystal surface layers. This study presents a useful general methodology for characterising inter-ligand interactions for many other systems of interest. 2 ACS Paragon Plus Environment

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Methods All chemicals were purchased from Sigma Aldrich and used as received. Synthesis: Full synthetic details can be found in the SI. Attachment of perylene-diimides to silicon dioxide nanoparticles: The surface of the silicon dioxide nanoparticles

(nanopowder,

10-20

nm

particle

size)

were

initially

treated

with

(3-

Aminopropyl)triethoxysilane (APTES) 5% with 20 mg ml-1 nanoparticles in toluene. The solution was left over night and the nanoparticles were washed via flocculation with acetone (1:10 toluene:acetone) and centrifugation. The attachment of the PDI to the aminated silicon dioxide nanoparticles was performed in chloroform with 1.1 × molar equivalents of (2-(1H-benzotriazol-1-yl)-1,1,3,3tetramethyluronium hexaﬂuorophosphate) (HBUT) and 1

×

molar equivalents of N, N-

Diisopropylethylamine (DIPEA) compared to the PDI. The solution was left over night and the nanoparticles were washed via flocculation with acetone and centrifugation. Transmission electron microscopy: TEM samples were prepared by drop-casting an ca. 1 mg mL-1 silicon dioxide nanoparticle solution in octane on a TEM Grid (200 mesh Cu, Agar Scientific). TEM measurements were conducted on a FEI Philips Tecnai 20 microscope, operating at 200 kV. Images were capture with a GATAN CCD camera. Steady-state spectral measurements: Absorption spectra were measured using a HP 8453 spectrophotometer. Dyes were dispersed in chloroform at a concentration of ca. 1 mg and a 1 cm path length was used. Photoluminescence measurements (1 mg in chloroform in a 1 mm cuvette) and powders including two-dimensional scans were measured on an Edinburgh Instruments FLS90 fluorimeter. PLQE measurements: Antenna complexes were placed in an integrating sphere and were photoexcited using a 405 nm or 532 nm continuous-wave laser. The laser and the emission signals were measured and quantified using a calibrated Andor iDus DU490A InGaAs detector for the determination of PL quantum efficiency. PLQE was calculated as per de Mello, et al. 61. Transient Photoluminescence: To record the timeresolved emission scan or photoluminescence decay of the samples, time-correlated single photon counting (TCSPC) was performed. Samples were excited with a pulsed laser (PicoQuant LDH400 40 MHz) at 375 nm, 407 nm and 470 nm, with the resulting photoluminescence decay collected collected on a 500 mm focal length spectrograph (Princeton Instruments, SpectraPro2500i) with a cooled CCD camera. The instrument response was determined by scattering excitation light into the detector using a piece of frosted glass; a value of 265 ps was obtained. The excitation fluence for all samples was similar to that used in transient absorption measurements and kept sufficiently low (~ 6-12 μJ/cm2) to avoid nonlinear effects e.g. from exciton-exciton annihilation. 3 ACS Paragon Plus Environment


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Pico-second transient absorption: The ps-TA experiments were performed using an Yb-based amplified system (PHAROS, Light Conversion) providing 14.5 W at 1030 nm and 38 kHz repetition rate. The probe beam is generated by focusing a portion of the fundamental in a 4 mm YAG substrate and spans from 520 to 900 nm. The pump beam is generated by seeding a portion of the fundamental to a narrow band optical parametric oscillator (ORPHEUS-LYRA, Light Conversion). The pump pulse was set to 500 nm. Sample solutions were placed in 1 mm path length cuvettes (Helma). The pump and probe beams were focussed to a size of 300 μm x 330 μm and 60 μm x 70 μm and respectively. The pump fluence was typically 45 μj/cm2. The white light is delayed using a computer-controlled piezoelectric translation stage (Newport), and a sequence of probe pulses with and without pump is generated using a chopper wheel (Thor labs) on the pump beam. The probe pulse transmitted through the sample was detected by silicon photodiode array camera (Stresing Entwicklunsbüro; visible monochromator 550 nm blazed grating). Results and discussion In order to create our PDI ligands, we synthesise two different asymmetric PDIs with carboxylic acid functional groups. One imide substituent acts to increase the solubility of the perylene core, while the other offers the functionality. Having a long alkyl chain between the perylene core and the carboxylic acid ensures isolation of the spectral properties of the perylene core upon attachment to SiO2 nanoparticles. PDI carboxylic acid ligands 4 a red PDI derivative, and 8 a orange PDI derivative were synthesised as shown in Scheme 1. Orange PDI derivatives contain branched 7-tridecyl N-imide substituents to increase solubility and limit stacking, whilst red PDI derivatives contain diisopropylphenyl N-imide chains to also give good solubility and limit stacking; tetraphenoxy substitution of the bay position induce torsion strain and further extends π-conjugation of the perylene core resulting in the red shifted absorption and emission34. The synthesis of the Red PDI 4 began from previously reported62 alcohol-functionalised tetraphenoxy bay-substituted PDI 1. Single-step Jones oxidation of the alcohol to an acid was prohibited owing to additional hydrolysis of the imide positions. Instead 1 was substituted with an azide to give 2 in two steps. Copper-Catalysed Azide-Alkyne Cycloaddition (CuAAC) with ethyl propiolate yielded ester 3 in near unity followed by base-mediated deprotection to give 4 in near-quantitative yield. Orange PDI 662 was converted in a similar manner to yield the acid 8.


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Scheme 1: Synthetic reaction scheme. Scheme 1: (a) TsCl, Et3N, DCM, 0 °C, 3h; then NaN3, DMF, 50 °C, 16h, 60% over two steps; (b) CuSO4.5H2O, THPTA, HC≡CC(O)OEt, Sodium L-ascorbate, THF:H2O, 50 °C, 1 h, 97%; (c) KOH, THF:H2O, 50 °C, 24 hr, 88%; (d) TsCl, Et3N, DCM, 0 °C, 3h; then NaN3, DMF, 50 °C, 16h, 76% over two steps; (e) CuSO4.5H2O, THPTA, HC≡CC(O)OEt, Sodium L-ascorbate, THF:H2O, 50 °C, 1 h, 97%; (f) KOH, THF:H2O, 50 °C, 24 hr, 78%.

The optical properties of the synthesised PDI carboxylic acids (4 and 8) in solution exhibit absorption and emission spectra similar to model symmetrical PDIs with similar core structures33,41,63(Figure 1. (a)). Photo-luminescent quantum yields of the synthesised Red 4 and Orange 8 PDIs in chloroform were measured to be 95% and 90% respectively with luminescence lifetimes of 5.5 ns and 4.3 ns respectively.

Figure 1: Attachment of PDIs to SiO2 NPS. (a) Extinction coefficients and emission (under 500 nm excitation) of the synthesised PDI carboxylic acids. (b) Reaction diagram of the created PDI coated SiO2 nanoparticles: CHCl3, DIPEA, HBTU, RT, 12 hr.

In order to attach the ligands to the surface of a nanoparticle, 15 nm SiO2 nanoparticles (NPs) were amine functionalised via the addition of 3-Aminopropyl)triethoxysilane (APTES) in DCM64. The PDI ligands 4 and 8 were amide coupled to amine-functionalised NPs using 2-(1H-benzotriazol-1-yl)1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) and N, N-Diisopropylethylamine (DIPEA) (Figure 1 (b)). The NPs were then washed extensively via flocculation with acetone/centrifugation and re-dispersal to remove any unbound PDI. These PDI attached nanoparticles exhibited strong absorption and luminescence compared to the unfunctionalised SiO2 beads (Figure S1). TEM images of the 5 ACS Paragon Plus Environment


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resulting particles (Figure S2.) showed no structural difference between the PDI attached and the clean SiO2 beads. However, the functionalised SiO2 NPs had higher colloidal stability in organic solvents (toluene and chloroform) as the PDI coated particles would stay dispersed in solution for > 8 hours compared to the unfunctionalised SiO2 NPs that showed settling after 1 hr. We assign this to increased colloidal stability offered via the bulky attached ligands. Note the SiO2 particles aggregate together upon drying, as seen in Figure S2, as such all measurements were performed in solution. We see only minimal subtle shift in the absorption spectra upon attachment (Figure S3.). In particular the vibronic structure of the Orange PDI’s absorption band changes, with a decrease in the strength of the 0,0 sub-band relative to the others. This difference is completely consistent with the effects of moderate aggregation and will be discussed later39. By using the measured extinction coefficients of the PDI carboxylic acids and a known weight of SiO2 NPs, the attachment density of the PDIs to SiO2 surface was calculated. The packing density of Red PDI on the SiO2 NPs was found to be 0.32 per nm2, while the Orange PDI was found to be 1.0 per nm2 (Assuming spherical Si02 nanoparticle with a Gaussian distribution between 10 and 20 nm). This gives attachments to the nanoparticles of around 715 Orange PDIs or 225 Red PDIs. This difference is most likely the result of steric hindrance by the phenol substituents in the bay positions of the Red PDI, preventing the higher loadings seen in the Oranges PDI NPs. The difference in loading has a direct effect on the photoluminescence from the PDI NPs (Figure 2.) The Red PDI NPs shows a very similar emission spectrum to the free monomer though we note the appearance of a second peak in its emission at 650 nm that appears to be vibronic in nature. However, the Orange PDI nanoparticle shows a large red shift and broadened emission. The PLQEs of the Red and Orange PDI NPs were 18.5% and 6.5% respectively. 2D excitation/emission scans (Figure 2 (c) and (d)) do show that we only have a single species emitting in our NP systems, as at all excitation wavelengths (300 – 600 nm) exhibit similar emission spectra.


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Figure 2: Absorption and emission data. (a) Absorption and emission of Orange PDI NP. (b) Absorption and emission of Red PDI NP. (c) Two-dimensional emission/excitation of Orange PDI NP. (d) Two-dimensional emission/excitation of Orange PDI NP.

To understand the nature of the intramolecular interactions within our PDI NPs we perform timeresolved photoluminescence measurements using time-correlated single photon counting (TCSPC). Upon attachment to the SiO2 NP the Red PDI shows no significant change in lifetime (5.5 ns unattached vs 5.6 ns with SiO2) and a very minimal redshift (Figure 3. (b) and (d) and Figure S4.). It’s worth noting that the decrease in PLQE associated with the Red PDI upon attachment may be attributed to localized dark states which decrease the PLQE but do not change the measured emission lifetime. As such, we assign these observations to minimal aggregation of the Red PDI monomer attached to the surface of the SiO2 NP. Compared to the unattached Orange PDI 8 the attached Orange PDI NP broadened and red shifted emission is longer-lived, 8.4 ns compared to 4.3 ns (Figure 3. (a) and (c) and Figure S4.). In the first few 100 ps, there also appears to be a red shift in the spectra, indicating an energy transfer process to lower-lying species. This long-lived red shifted emission is characteristic of excimer emission in PDIs17,30,31,45,46,54. Based on the measured PLQEs and lifetimes, we calculate a 30-fold decrease in the intrinsic radiative rate of the excimeric species compared to the unattached Orange PDI 8, along with a five-fold increase in the non-radiative rate. A similar but less drastic trend is seen with the Red PDI nanoparticles (Table S1.). This is indicative of H-aggregate behavior36 - where the radiative transition moments of the aggregated molecules partially cancel each other, thus increasing the lifetime of spontaneous emission.



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Figure 3: Transient photoluminescence of PDI monomers (4 and 8) compared to PDI NPs. (a) Transient photoluminescence of Orange PDI monomer 8 in chloroform. (b) Transient photoluminescence of Red PDI monomer 4 in chloroform. (c) Transient photoluminescence Orange PDI NPS in chloroform. (d) Transient photoluminescence of Red PDI NPs in chloroform.

The Orange PDI exhibits a significant quenching of photoluminesence quantum yield and red-shifting of emission on attachment to SiO2 nanoparticles. Similar behaviour has been observed in other PDI or acene systems upon aggregation, with the PL quenching attributed to excimer formation, charge transfer or singlet fission65–69. To examine the origin of this in PDI-Orange-SiO2 nanoparticle composites we carried out picosecond transient absorption spectroscopy. The transient absorption spectrum in Figure 4 shows three main features: a positive band between 550 – 650 nm, a broad negative signal, 600 – 900 nm, and a sharp negative peak at 700 nm - that rapidly decays. We overlay the steady state absorption and emission spectra with the transient spectral dynamics (the region around 600 nm is cut due to an experimental artefact) in Figure 4 (b). The positive band consists of a combination of ground state bleach and stimulated emission (near 550 nm). The broad negative signal (600 – 900 nm) corresponds to a photoinduced absorption (PIA) which is additionally overlaid with a sharp PIA peak at 700 nm. Within ~2 ps, this sharp 700 nm PIA peak decays, as does the ground state bleach/stimulated emission, and the broad PIA signal out to 900 nm rises.


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Figure 4: Picosecond transient absorption spectra of PDI-Orange-glass composite. (a) Transient absorption spectrum of PDIOrange-glass nanoparticle composite, following photoexcitation at 500 nm. The ΔT/T signal is plotted as functions of probe wavelength and time delay between pump and probe, where ΔT is the change in the transmission of the sample with/without the pump pulse, and T is the transmission without the pump pulse. The positive signal between 500 – 600 nm corresponds to an overlap of the ground state bleach and stimulated emission and the broad negative signal is due a photoinduced absorption. (b) We overlap the TA spectrum at various time slices with the steady state absorption (dark red) and emission (pink) spectra of the PDI-nanoparticle composites. The spectrum around 600 nm has been removed due to an experimental artefact. (c) Kinetic associated with GSB (550 nm), singlet PIA (705 nm) and broad excimer PIA (sampled at 840 – 850 nm) of the PDIglass nanoparticle composites. The singlet PIA sits on top of the broad excimer PIA and disappears within 2000 fs, corresponding to a reduction in the GSB.

These observations, combined with the transient photoluminescence measurements, are consistent with a rapid evolution of photoexcited singlets excitons to excimers. Upon photoexcitation, an initial excited singlet exciton population (characterised by the sharp 700 nm PIA peak70 and enhanced stimulated emission) evolves in ~ 2ps to a different state with a broad PIA that persists and is the dominant species out to ~1 ns, which is the timescale at which the broad emissive species is observed in the transient PL. [We note that triplet signals would be observed as sharp PIA peaks70 (similar to the singlet signals), but we do not see any evidence for this, only transfer from the initial sharp singlet PIA to the broad excimeric PIA.] Thus, in the Orange PDI SiO2 nanoparticle system, we are confident in assigning the red-shifted, longlived, low PLQE emission with no absorption shift, to that of an excimer23,24. Due to the large overlap between the PDI absorption and emission spectrum, rapid energy migration through the ligands is expected. The self-Förster radius29, the distance at which 50% of all excitations lead to energy transfer from the one molecule to a like molecule was calculated, based on the absorbance and emission spectrum and the PLQE of our PDIs monomers (4 and 8) in chloroform, to be 1.52 nm and 1.66 nm for the Orange and Red PDIs respectively (the orientation factor was assumed to be 2/3). As the distance between PDIs on the SiO2 nanoparticles is 1 nm for the orange PDIs and 1.8 nm for the red PDIs, it is likely that FRET strongly out-competes monomer emission for the Orange PDI systems and competes with monomer emission in the red system as the distance is close to that of the Förster radius. It is also worth noting that the hexyl alkyl chain linker is relatively flexible, which could allow some molecules to sit closer together. Over the time scale of monomer luminescence decay, the excitation should be able to sample a large number of sites, finding an aggregated or excimer forming site. The difference in luminescence properties of the PDI nanoparticles is likely not solely due to increased FRET efficiency but also to the fact the red PDI has a more isolated luminophore core due to the out-of-plane 9 ACS Paragon Plus Environment


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phenols, which makes it less sensitive to microenvironments, aggregation and the formation of excimer states62,63. Migration of this excited state ends when the excitation reaches a site capable of excimer formation, which is more likely in the orange PDI nanoparticles. Conclusion We present the synthesis of two novel PDI carboxylic acid derivative for the use as chromophore ligands. These PDIs were covalently attached to the surface of SiO2 nanoparticles. Where the perylene Red derivative with its sterically hindering bay substituent phenol groups underwent only minor aggregation, the Orange PDI was found to form excimer states with long lived redshifted emission. This work represents an initial study into the photophsyical and aggregation effects of chromophore ligands when brought into close proximity as they attach to the surface of nanoparticles. As such, this work represents an important study in the context of the rise of chromophore ligand nanocrystal systems. Supporting Information The supporting information contains the following: Digital images of the PDI nanoparticles. TEM images of the PDI nanoparticles. Absorption and emission spectrum of the attached and unattached PDIs. Transient photoluminescent decays at peak emission of the attached and unattached PDIs. Calculated radiative and non-radiative luminescence rates. Synthetic details of molecules 2,3,4,6,7, and 8. Acknowledgments J.G., R.P., J.R.A., A.R and N.J.L.K.D. acknowledge funding from the EPSRC and the Winton Program for the Physics of sustainability. N.J.L.K.D. thanks the Ernest Oppenheimer Trust for a research fellowship. The research leading to these results has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement

No

670405).

The

data

underlying

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publication

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at

https://www.repository.cam.ac.uk/handle/1810/288077. References

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Excimer formation in carboxylic acid functionalised perylene diimides attached to silicon dioxide nanoparticles 136x108mm (150 x 150 DPI)

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Excimer Formation in Carboxylic Acid-Functionalized Perylene

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