Photon Upconversion Using Baird-Type (Anti)Aromatic Quinoidal

Oct 2, 2017 - A naphtho-p-quinodimethane (QDM) exhibiting Baird's 4n – π antiaromaticity was used as green photons-harvesting chromophore to sensit...
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Photon Upconversion Using Baird-Type (Anti)Aromatic Quinoidal Naphthalene Derivative as a Sensitizer Siamak Shokri, Gary P. Wiederrecht, David J Gosztola, and A. Jean-Luc Ayitou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08373 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017

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Photon Upconversion Using Baird-Type (Anti)Aromatic Quinoidal Naphthalene Derivative as a Sensitizer Siamak Shokri‡, Gary P. Wiederrecht◊, David J. Gosztola◊ and A. Jean-Luc Ayitou*,‡ ‡ ◊

Department of Chemistry, Illinois Institute of Technology, Chicago, IL 60616, USA Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439, USA

Supporting Information Placeholder ABSTRACT: A naphtho-p-quinodimethane (QDM) ex-

hibiting Baird’s 4n-p anti-aromaticity was used as green photons harvesting chromophore to sensitize perylene (Per) leading to upconverted blue photoluminescence. The solution phase QDM®Per triplet energy transfer (TET) could not be unraveled via the Stern-Volmer method; but, transient absorption measurements revealed that the kinetics of T1®Tn for QDM (t = 1.4 µs) was one order of magnitude reduced (t = 0.17 µs) as a result of 3(Per)* formation. Furthermore, we demonstrated that incident light with power densities in the microwatt regime is sufficient to perform photon upconversion using the present set of molecular systems.

Introduction Photon upconversion (UC) is a non-linear process that transforms low energy radiation into high-energy photons. Besides singlet fission from which two pairs of electron-holes can be generated per input photon, UC is considered as an alternative multi-excitonic process that could help circumvent the theoretical solar power conversion limit (also known as the Shockley-Queisser limit1) in photovoltaic (PV) devices.2,3 UC materials can be used to tune the spectral distribution of solar irradiation to match the wavelength range where a PV device works.4-6 Continuing research is also underway to tailor UC for sustainable chemical synthesis, biological imaging, night vision and multi-dimensional display applications.7-11 In organic or organometallic matrices, UC is a bimolecular process that requires a light harvesting donor molecule (D) and a luminescent acceptor (A) having thermodynamically matched photophysical energies. In this process, low energy photons are first stored into metastable excitons (Scheme 1i); then, energy transfer to acceptor molecular units facilitates population of much higher energy state(s) via acceptor exciton-exciton annihilation resulting in photoluminescence that generates higher energy photons (Scheme 1ii & 1iii).8,12 UC based

on triplet-triplet annihilation (TTA) falls under this mechanism, where the donor molecule is a triplet sensitizer, and the acceptor entity is a fluorophore with a moderate to high emission quantum yield (Scheme 1).1315

Scheme 1. Mechanism for triplet-triplet annihilation photon upconversion (low-energy to high-energy): D = donor (sensitizer) and A = acceptor (annihilator), ISC = intersystem crossing, TET = triplet energy transfer, TTA = triplet-triplet annihilation. +



3D* +

A

D

2 x 3 A*

TTA

1D* ISC

TET

1 A* +

3 A* +

A

3D*

(i)

D

(ii)

hν' + 2 x A

(iii)

Over of the course of recent years, organometallic triplet sensitizers (based on metal-to-ligand charge transfer processes 3(MLCT)*)4,6,16-24 that have been developed for UC research are not only synthetically challenging but also may become unstable under ambient conditions. These limitations have inspired the design of purely organic light harvesting sensitizers that have been used as energy donors for UC based on TTA (TTA-UC) research with various degrees of success.23,25,26 Moreover, it was also documented that in TTA-UC, the triplet energy transfer (TET) requires maximum overlap of the wave functions of the donors and acceptors species (for efficient Dexter type energy transfer), among other parameters such as solvent viscosity, temperature and lifetime of the excited donor molecules that contribute to the efficiency of the whole UC process.27-29 While fine tuning those parameters would improve the rate of TET and the resulting quantum yield of UC (FUC), it will be ideal to engineer sensitizers that are structurally (and

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energetically) similar/complementary to the acceptor entities to have optimum intermolecular interactions (maximum overlap of the wavefunctions) regardless of solvent viscosity and temperature of the matrix. Donor R N

Acceptor S

S2 S1

N R

S

3

(π,π*)

R N

* S

(n,π*)

ΦF = 0.002

(π,π*)

T2

(n,π*)

T1 N R

S

antiaromatic S0 aromatic triplet (4n)π (4n + 2)π R = n-C8H17 or HC(C 2H 5) 2 QDM

Perylene (Per)

Figure 1. Chemical structures of Baird-type (4n)p antiaromatic QDM and corresponding triplet excited state species exhibiting (4n + 2)p aromatic character. Structure of perylene (acceptor) is also shown. The schematic of the energy level illustrates the probable electronic transitions and configurations (solid arrows) and less probable transitions (dashed arrows).

Polycyclic aromatic diimides and related parent hydrocarbons meet the above criterion; these scaffolds have been shown to form intrinsic pairs of intermolecu-

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lar/supramolecular donor-acceptor systems due to the planarity and rigidity of their molecular backbone as well as reduced steric crowding and electronic complementarity (p-acid•••p-base interaction).32-35 For example, naphthalene and perylene diimides (NDIs and PDIs respectively) are well-known fluorescent dyes (with short excited state lifetime) that have been used in various photonic applications.36 Although the efficient steady-state photoluminescence of these dyes may not be useful for UC research, this property can be altered or manipulated after subtle functionalization or modification of the molecular core (with molecular synthons) leading to efficient population of their corresponding lowest triplet state.37-41 Very recently, Zhao et. al. have demonstrated that heavy atom(s) containing NDIs (spinorbit coupling and 3(MLCT)*) can be utilized as triplet sensitizers for TTA-UC.25,42-44 These seminal contributions stimulated our interest to devise new light harvesting NDI derivatives for TTA-UC using polycyclic aromatic hydrocarbons as acceptors. Herein we present a new approach for maximizing the TET process based on reduced steric crowding around the interacting molecular scaffolds (Figures 1 & 2). For the first time, using planar anti-aromatic naphtho-p-quinodimethane (QDM) as a triplet sensitizer30 and perylene (Per) as annihilator, we successfully performed TTA-UC of green-to-blue photons with ultra-low-power incident radiation. Interestingly, no additional synthon (bulky or supramolecu-

Figure 2. A) Normalized absorption and emission spectra of QDM (R = n-C8H17) and Per in 1:1 EtOH:DCM. B) Phosphorescence decay kinetics of QDM (t = 395 µs). C) Qualitative Jablonski diagram illustrating the overall TTA30 31 UC process between QDM and Per. D) Molecular single crystal structures of QDM (R = CH(C2H5)2 and Per : H atoms are omitted for clarity.

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lar) was introduced on the scaffold of the new sensitizer to yield the corresponding triplet state as previously done.45-48 Fortunately, QDM also exhibits lower pacidity (compared to NDI), and change in its intrinsic aromaticity was inferred to induce the observed attractive photophysical behaviors according to Baird’s rule of anti-aromaticity.49

Results and Discussion In the present investigation, we specifically designed the triplet sensitizer QDM to exhibit ground state 4n pelectrons anti-aromaticity and we ascertained that QDM should form metastable aromatic triplet species upon photoexcitation according to Baird’s rule of excited state aromaticity (vide supra). The ground state UV-Vis absorption spectrum of QDM shown in Figure 2A suggests that the new chromophore can harvest green photons up to 570 nm (2.17 eV). The steady state emission of QDM shows a profile with vibronic features that resembles that of the parent NDI.30 However, this emission has a low quantum yield (FF = 0.002) suggesting a highly competitive intersystem crossing to produce phosphorescence. The emission spectrum of QDM at 77K in 1:1 ethanol:dichloromethane (EtOH:DCM) glassy matrix exhibits a phosphorescence profile centered at 725 nm ( lonset = 700 nm) and lifetime of t = 395 µs (Figure 2A and 2B) indicating that the lowest triplet state of QDM is at ca. 40.8 kcal•mol-1 above its ground state. As mentioned above, for an efficient TTA-UC, it is important to produce a long-lived triplet state of the light-harvesting sensitizer. A lifetime value of ca. 395 µs for our Baird’s (anti)aromatic QDM chromophore is compelling vis-a-vis NDI derivatives. For example, heavy atoms based NDIs used for TTA-UC have triplet radiative decay lifetimes of a few microseconds25 whereas complex structural modifications of transition-metal based sensitizers are necessary to attain values of 100s of µs.50,51 To achieve a thermodynamically favored energy transfer, we used Per (ET ≈ 35 kcal•mol-1) as the acceptor/annihilator since its fluorescence also exhibits high quantum yield (FF = 0.75) centered near 470 nm and it has a good thermal and photochemical stability (Figure 2A).52 It is worth noting that under our UC experimental conditions, the (delay) fluorescence of Per could be diminished (inner-filter-effect) due to the high concentration needed to perform efficient annihilation of the corresponding triplet excitons. Additionally, the partial overlap of Per emission band and QDM ground state absorption could possibly compromise the overall upconverted emission quantum yield of Per. We performed quenching experiments (Figure 3) to unravel the bimolecular interactions of 3(QDM)* and Per as well as to decipher the kinetics/dynamics of the

TET process. While this study should help interpret the TET process (at various donor:acceptor ratios), we were surprised to find that the phosphorescence of QDM was readily quenched by Per at concentration 19.8 µM (1:1.1 ratio) at room temperature under anaerobic conditions (Figure 3A). Unfortunately, we were unable to vary the added amount of Per to deduce the kinetics of TET process; the phosphorescence decay traces at various concentrations of Per did not change the observed kinetics. Moreover, we performed the quenching study in a rigid sample of the donor–acceptor system. Figure 3B shows the phosphorescent decay traces of 3(QDM)* at 77 K in the presence of Per. Surprisingly, the phosphorescence decay in the glassy matrix also shows slow kinetics even at sub-stoichiometric concentration of Per (Figure 3B). We hypothesize that the condensed phase quenching of QDM by Per is most likely due to their structural complementarity/similarity which renders a perfect overlap of their transition dipole moment (or wavefunction). Since we could not ascertain the kinetics of the TET process from the quenching studies, and to substantiate our hypothesis for the bimolecular dynamics (vide supra), we investigated the quenching of the triplet transient of QDM by nanosecond time-resolved transient absorption spectroscopy (Figure 4). A

B 300

[Per] (µM) at r.t.

1000

[Per] (µM) at 77 K 0

0 19.8

4.2 12.6 42

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0 0.0

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0.2 0.3 Time (ms)

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0.2 Time (ms)

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C QDM

QDM

QDM + Per

Figure 3. Phosphorescence (monitored at 725 nm) decay traces of QDM (R = HC(C2H5)2, conc. = 1.71x10-5 M) in the presence of Per at A) room temperature in deaerated 1:1 EtOH:DCM (4 cycles of freeze-pump-thaw) and B) at 77 K in 1:1 EtOH:DCM glassy matrix. C) Photograph of photoluminescence of QDM (left) and Per upconverted photoluminescence (delayed fluorescence) in deaerated THF (right) using a 532 nm commercial laser (30-40 mW•cm-2).

As shown in Figure 4A, upon laser excitation (lExc = 520 nm) of a deaerated (by freeze-pump-thaw) dilute tetrahydrofuran (THF: this was chosen for practicability

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10 -3 0 x10 -10

600

700

800

QDM

900

5

-5

τ465 = 1.33 µs

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D

2.5 µs 5.3 µs

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600 700 Wavelength (nm)

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20 10 -3 0 x10 -10 -20

800 600 400 200

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QDM + Per (1:1)

0 Pump @ 520 nm

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600 700 Wavelength (nm)

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τ590 = 0.2 µs

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delayed fluorescence from 2 x 3(Per)*

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10 15 20 Time (µs)

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Figure 4. Nanosecond time-resolved transient absorption 2D intensity maps and spectra for QDM (R = HC(C2H5)2) in deaerated THF (4 cycles of freeze-pump-thaw). A) Spectra of QDM (optical density: O.D = 0.27, lExc = 520 nm, 1 µJ); B) Corresponding decay trace monitored at 465 and 590 nm; C) Spectra of QDM in the presence of Per (optical density: O.D = 0.33, lExc = 520 nm, 1 µJ); D) Decay trace monitored at 465, 590 and 650 nm; E) kinetics of Per delay fluorescence monitored at 465 nm.

Moreover, the absorption band at 400 – 515 nm with positive O.D. is indicative of Per delay fluorescence. Qualitative analyses of the kinetics for the transient absorption features from Figure 4C produced monoexponential fits of 0.17 µs and 0.2 µs for the lifetime of QDM (590 nm) and Per (650 nm) transients respectively (Figure 4D). Likewise, the kinetics of the delay photoluminescence (PL) from 3(Per)* at 400 – 515 nm was computed for t = 5.8 µs, a value that equals about thou-

B

200 x10

upconverted Per emission λExc = 532 nm

3300 3100 3050 2320

150

E

545 ns 1.72 µs

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4 6 Time (µs)

τ465 = 0.17 µs

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T1Wavelength Tn (nm)

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sand times the actual lifetime of prompt fluorescence from a dilute solution of Per. The above results demonstrate efficient TET 3(QDM)*®Per. Now that we have established the dynamics and kinetics of the TET process, it is important to determine the power regime (lowest to highest) of the incident light, where one should be able to realize the present TTA-UC using QDM and Per. Illustrated in Figure 5 is the PL of per (partial profile) obtained upon excitation at 532 nm (Supporting Information) of a mixture of QDM and Per in deaerated THF (1:3 molar ratio). At power density as low as 45 µW•cm-2 of the incident green light, we observed a moderate Per PL centered at 465 nm from the sample even though Per does not absorb green photons. This emission increases as we increase the power density up to 3300 µW•cm-2. The upconverted PL intensity was fitted with the best quadratic (x2) function vs. power density (Figure 5 B), where the corresponding double logarithmic plot produced the best linear fit with slope of 1.75 (Figure 5B Inset).

PL. Intensity

300

B -3

800 600 400 200

Time

A

ΔA (OD x 10 )

purposes) solution of QDM, a transient absorption developed between 400 and 800 nm. The absorption at 400 – 500 nm with negative optical density was assigned to the ground state depletion, whereas the positive feature beyond 550 nm indicates the T1®Tn absorption for 3 (QDM)*, which was readily quenched by molecular oxygen. A mono-exponential fit of the corresponding decay trace monitored at 590 nm gave a lifetime value of 1.4 µs for the transient of QDM (Figure 4B). Expectedly, this triplet transient was also quenched by Per (Figure 4C). The transient absorption profile for the mixture of 1:1 molar ratio of QDM + Per displays minor spectral changes compared to the profile of QDM alone. The new absorption feature appearing near 650 nm was attributed to the absorption of Per transient53 that forms during the TET process.

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50 0 0

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Power density (µW•cm )

Figure 5. A) Intensity-dependent upconverted emission of Per in the presence of QDM (R = HC(C2H5)2 excited at 532 nm ([QDM] = 7.04x10-5 M; [Per] = 21.12x10-5 M) in deaerated THF (4 cycles of freeze-pump-thaw); a 500 nm shortpass filter was used to discard the overlapping emission from QDM as well as cutting off the incident radiation. B) Best quadratic fit (x2) of the PL intensity at 465 nm. Inset: Double logarithmic plot PL intensity vs. power density of the incident light producing a linear fit with slope = 1.75.

Knowing that incident photons with power densities in the microwatt regime are sufficient to produce Per upconverted emission in this investigation, we used noncoherent radiation (500 nm from CW Xe-lamp, 105–

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1400 µW•cm-2) to determine the relative TTA-UC quantum yield using Ru(bpy)3Cl2•6H2O (FF = 0.028 in water)54 as photon counter standard (Figure S5). We adopted the widely-accepted formula (Equation 1) to determine the relative FUC of emission at various concentrations of Per and QDM. 𝑄"# = 2×𝑄'() ×

*+,*./

×

0./ 0+,-

×

1./ 2 1+,-

toluminescence data related to quantum yield evaluation; methods/techniques and information of instrumentation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors [email protected]

Equation 1

Where, 𝑄"# , 𝐴"# , 𝐼"# , and 𝜂"# represent the quantum yield, absorbance (O.D.) at lExc = 500 nm, integrated photoluminescence and refractive index of the solvent (THF) respectively. Similarly, the corresponding terms for “std” are for the standard/reference. First, the concentration of the acceptor was varied (12.4 – 72 µM) to obtain a maximum quantum yield of 0.16% at [QDM] = 7.91 µM (Figure S6-A). It is worth noting that this value did not increase or change even at higher power densities of the incident light (noncoherent CW Xe-lamp). On the other hand, keeping the concentration of Per constant (52.7 µM), the amount of QDM was gradually increased giving much lower values for the FUC (Figure S6-B). One possible reason that accounts for the lowering of FUC is the partial overlap of the emission band of Per with the absorption of QDM (Figure 2) signifying a likely reabsorption of the upconverted blue photons via FRET mechanism. We are currently exploring other derivatives of sensitizer QDM, which have better spectral properties to avoid such photophysical dichotomies.

Conclusion In summary, we successfully performed photon upconversion using Baird-type anti-aromatic sensitizer that showcases photo-physically matched energetics and structural similarity/complementarity to the emitter perylene. We demonstrated efficient 3(QDM)*®Per TET and the resulting TTA annihilation of the resulting 3 (Per)* excitons to produce upconverted blue photons from green radiation. The kinetics of the energy transfer process were unraveled via nanosecond transient absorption spectroscopy techniques. By modulating the intensity of the incident green light, we demonstrated that it is possible to realize TTA-UC at ultra-low powers using non-coherent photons. The present investigation highlights structure–efficiency relationship for TTA-UC with regard to the triplet sensitizer which exhibits attractive photophysical behaviors as well. We hope that the structural characteristics of the sensitizer–acceptor systems used in this study will inspire the creation of novel organic chromophoric systems for TTA-UC research. ASSOCIATED CONTENT Supporting Information. Synthetic procedures, additional UV– Vis absorption spectra, emission spectra and supplementary pho-

Notes

The authors declare no financial interest.

ACKNOWLEDGMENT AJA thanks IIT for funding this research. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract N° DE-AC0206CH11357. The authors thank to Dr. Benjamin Diroll and Dr. Richard Shaller at the CNM-ANL for assistance with the intensity dependent fluorescence measurements.

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