Fused polycyclic acenes are a foundational class of organic compounds in physical organic chemistry that present a unique combination of optoelectronic properties and reactivity. 1,2 The thermodynamics of acene aromaticity influences rates and regiochemistry of their reactions, with reactivity increasing with acene length. [3][4][5] In addition to aromatic substitution reactions, acenes can be excellent diene reaction partners in [4+2] and [4+4] cycloaddition reactions. The cycloadditions of anthracenes, especially those with highly reactive dieneophiles and in photochemical [4+4] dimerizations have proven especially useful in various materials applications. [6][7][8] However, for these longer acenes with more than three fused arenes, the most important cycloadditions are with 1 O2 as the dieneophile to produce endoperoxides transistors and materials for singlet fission prioritize acene stability and persistence for performance in optoelectronic devices and during processing. Most acenes photosensitize 1 O2 efficiently and cycloadd rapidly with 1 O2, resulting in rapid photochemical oxidation of acenes that destroys their favorable optoelectronic properties. 9 On the other hand, our research group has leveraged these rapid endoperoxidations in responsive polymers and nanomaterials, where acene oxidation widens their frontier molecular orbital gap, preventing them from accepting energy from excited donor chromophores and yielding ratiometric fluorescence changes in response to 1 O2. [10][11][12][13][14] Given the importance of this class of reactions, it is critical to understand how the chemical structures of acenes influences acene-1 O2 reactivity. Various approaches have emerged to mitigate these photooxidations, 15 such as steric hinderance of cycloaddition, [16][17][18] or designing acenes that are poor sensitizers of 1 O2. 19,20 However, electronic substituent effects remain highly popular, especially ethynyl substituents on acenes, 21 which can enhance acene persistence under photo-oxidative conditions. The sp carbon atoms are electron withdrawing and therefore make the acene less reactive as a diene in cycloadditions, 22 and additional electron withdrawing silicon atoms as in TIPS-pentacene or TIPS-anthradithiophene, and electronic substituent effects on 9,10-diethynylanthracenes also influence the oxidation rate. 23 Beyond these incremental electronic effects, ethynyl groups also stabilize anthracenes and pentacenes specifically through different mechanisms: endoperoxides of 9,10-diethynylanthracenes cyclorevert rapidly at room temperature, 24 while 6,13-diethynylpentacenes physically quench 1 O2, possibly through an energy transfer process from 1 O2 to acene-based low energy triplet states. 23 Between these structural bookends of anthracene and pentacene, however, a range of important classes of 4-and 5ring (hetero)acenes exist. 22,25 Symmetrically substituted 5,12diethynyltetracenes are the simplest structures to fall into this category, and are important in 1 O2-responsive polymeric materials, 26 emitters in chemiluminescence applications, 27,28 materials for singlet fission, 20,[29][30][31][32][33][34][35] LEDs, 36 photocatalysts, 37 and supramolecular assemblies. 38 Therefore, our goal is to understand the extent to which electronic effects on the alkyne groups of 5,12-diethynyltetracenes influence both their electronic structures, especially their optical properties for the purposes of 1 O2-responsive luminescent materials, and reactivity with 1 O2, which is important in all applications of long acenes. Herein, we report that electronic effects of various aryl groups on the ethynyl substituents yield both broad spectral coverage in the visible part of the spectrum as well as a 10-fold difference in rate of cycloaddition that correlates with HOMO energies.
We targeted eight symmetrically substituted 5,12diarylethynyltetracene derivatives, the difference between which are the aryl (Ar) groups conjugated to the ethynyl substituents. As summarized in Chart 1, we chose unsubstituted Ar = phenyl (1) as a reference compound, as well as three aryl groups typically considered electron donating (Ar = thiophene, triarylamine, and dimethylaniline) as compounds 2-4, and three aryl groups typically considered electron withdrawing (Ar = pyridine, pyrazine, and benzothiadiazole) as compounds 5-7. Finally, we installed Ar = tetraphenylethylene (8), which is known to induce aggregation induced emission (AIE), 39 to understand any impact this moiety may have on luminescence and reactivity, given our prior conclusion that rapid nonradiative decay can slow direct photo-oxidation of acenes. Compounds 4-8 are unreported structures, and while compounds 2 and 3 are unreported to our knowledge, they differ from previously reported compounds by only modest non-conjugated units. 36 All new compounds except for 3 were prepared by Sonogashira coupling of the corresponding aryl iodide with 5,12-diethynyltetracene, which was prepared as reported previously, while 3 was prepared by direct addition of the lithium salt of methylated 4-ethynylaniline and 5,12tetracenequinone.
Chart 1. Structures of the eight diarylethynyltetracenes studied in this work.
Figure 1 shows solution-state absorbance spectra of molecules 1-8, each of which shows a strong absorbance peak in the visible range and a sharp absorbance peak in the ultraviolet. Distinct vibrational features exist in the spectra for most molecules, except 3 and 4, each of which contain amine substitutions. Introducing potent electron withdrawing or electron donating groups and/or extending the size of the conjugated system redshifted the absorbance maxima of these tetracenes. Among this set, molecule 6, containing the benzothiadiazole and additional alkyne unit, shows the most red-shifted absorbance (68 nm shifted from 1) and the highest extinction coefficient. The other two molecules with extended conjugation systems, 4 and 8, also exhibit substantially higher extinction coefficients than the other molecules, all of which are similar. All eight tetracenes are fluorescent with spectra spanning the yellow and orange regions of the visible spectrum (Figure 2). Like their absorbance spectra, 3 and 4 show broadened fluorescence spectra and loss of vibronic resolution at room temperature, which we attribute to the substantial electronic delocalization of the conjugated anilines into the acene chromophore, as reflected in their HOMO energies as determined electrochemically, and the electronic distributions of HOMOs and LUMOs computed by DFT (vide infra). Notably, 3
Please do not adjust margins Please do not adjust margins has a larger red shift than the more conjugated 4, which we attribute to a stronger electron-donating ability of dimethylaniline as opposed to triphenylamine. Again, benzothiadiazole-containing 6 has the most red-shifted spectrum, at 653 nm, 88 nm shifted from reference compound 1, albeit with the lowest quantum yield of emission at 26%. All other molecules have quantum yields of luminescence above 50%, with triarylamine-containing 4 and pyrazine-containing 7 above 80%. We also note that, unlike its analog without triple bonds, 40 molecule 8 does not show any evidence of aggregation induced emission, as its luminescence is not substantially quenched by the tetraarylethylene unit part of the chromophore. We attribute this effect to the additional distance created by triple bonds and the lack of any distortion due to the crowded alkene not impacting the behavior of the more distant acene chromophore. Furthermore, the spectra of 3 displays positive solvatochromism in fluorescence of 23 nm (absorbance) and 56 nm (emission) between hexanes and CH2Cl2, indicating the strong donor-acceptor feature of this molecule due to the dimethylaniline substituents. 41, 42 In addition, we determined the first oxidation potentials of these molecules by cyclic voltammetry (CV). Compared with an external ferrocene reference, we calculate the energy of the highest occupied molecular orbital (HOMO) from either the peak currents during oxidation (for those molecules that show some reversibility of oxidation) or the onsets of the first oxidation peak (for those molecules that show fully irreversible oxidations), compared with the corresponding features for ferrocene. Along with theoretically computed energy levels, the results are listed in Table 2. In general, the molecules with electron-donating groups have elevated HOMO energies, and those with electron-withdrawing groups have lower energies. For instance, pyrazine-containing 7 shows the lowest HOMO energy at -5.3 V, while dimethylaniline-substituted 3 has the highest HOMO energy of -4.9 V, consistent with a larger electron donation character of the dimethylaniline group revealed in solvatochromism trends. a Radiative rate of relaxation, and b Non-radiative rate of relaxation. Each of these is determined using a combination of the quantum yield of fluorescence and excited state lifetime.
To better understand the trends in electronic structures of these molecules, we performed density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. 43 Their geometries are optimized with the B3LYP functional and the 6-31G+(d,p) basis set, using a polarizable dielectric continuum model for chloroform. The frontier molecular orbitals (FMOs) and vertical excitations are then calculated on these optimized structures with the 6-311+(d,p) basis set. All eight molecules show the aryl rings directly connected to the ethynyl groups highly coplanar with the tetracene unit. Deviations from coplanarity of formally conjugated occur only with crowded moieties: the triarylamines of 3 showing a familiar propeller arrangement of the three aryl rings, and the tetraarylethenes of 8 displaying all four rings skewed from each other, while each maintains coplanarity between the ethynylphenyl and the tetracene core. As summarized in Table 2, the trends in calculated HOMO energies agree with those determined using cyclic voltammetry in CH2Cl2, with those molecules bearing conjugated substituents that are considered electron donating (2-4) showing higher energy HOMO levels than those that have electron withdrawing conjugated units (5-7).
As summarized in the ESI, the lowest energy singlet excited states computed for all molecules involve primarily or exclusively electronic transition from HOMO to LUMO, and the trend of calculated excited state energies agree with those we measure by UV/vis spectrophotometry in solution. Visual inspection of the FMOs (two examples are in Figure 3) only show three clear outliers with respect to their electron distributions, compared to 1, which shows roughly homogeneous electron distribution across the whole structure for the comparing HOMO and LUMO. Molecules 3 and 4, which have conjugated amine substituents, show clear donoracceptor behavior, with substituent phenyl rings having substantially more electron density in their HOMOs than their LUMOs. Benzothiadiazole-containing 6 shows the opposite behavior, with the BTD unit bearing increased electron density in the LUMO. These calculations indicate that the tetracene cores can be substantially polarized by electronic characteristics of the substituent rings that are conjugated through ethynyl linkers. This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins Please do not adjust margins
We also sought to determine the impact of these functional substitutions on the reactivity of these molecules with singlet oxygen ( 1 O2). We followed the consumption of each tetracene derivative by irradiating an external singlet oxygen photosensitizer, Methylene Blue (MB), in the presence of each acene. Under constant irradiation and constant concentration of MB, we assume the concentration of 1 O2 in the reaction is constant while irradiating, enabling us to use a pseudo firstorder kinetic model-each kinetic analysis fits a first-order reaction model well. Thus, we determine the trend in tetracene reactivity quantitatively by comparing the relative rate constants, which we report here in relation to compound 1. Relative rate constants of oxidation determined in CHCl3, presented as mean values ± standard error of the mean of three replicates.
Figure 4 shows exemplar pseudo first-order kinetic analysis and relative rate constant determination for tetracenes 3 and 7, while Table 2 contains the relative rate constants for all tetracenes reported here. In general, molecules with higher energy HOMO levels react faster than those with lower energy HOMO levels, as shown in Figure 4b. Therefore, arenes typically considered as electron-donating accelerate the reactions of diethynyltetracenes with 1 O2, while electron-withdrawing acenes decelerate these reactions. We note a ten-fold range of rate constants, with the pyrazine-substituted 7 slowest, and the dimethylaniline-substituted 3 reacting ten times faster, indicating that the wide panoply of available aromatic structures substantially impact cycloaddition reactivities of acenes.
Beyond measuring the intrinsic rates of reactivity of these acenes with 1 O2, we also examine here the impact of alkynespaced electron-donating/withdrawing groups on the bleaching of tetracenes upon direct irradiation. We isolate the 546 nm Hg emission line with a bandpass filter and start each irradiation experiment with the same absorbance value (0.4) at 546 nm. Figure 5 shows the degradation of each molecule upon irradiation. Surprisingly, all other molecules show a slower degradation rate than 1. In general, however, there is a reasonable general correlation with observed rate, as 6 and 7 react slowest, and besides 1, 3 reacts fastest. Here, we suspect that these results are due to a combination of photosensitizing efficiency and intrinsic rate of reaction. For example, 6 having the lowest quantum yield of fluorescence and slowest rate of degradation in this experiment suggests that, at least to some degree, non-radiative decay of the excited state of 6 competes with singlet oxygen photosensitization. Please do not adjust margins Please do not adjust margins
In addition to the changes in absorbance we use to monitor kinetics, all the molecules show a significant blue-shift in fluorescence upon reacting with singlet oxygen. We attribute this spectroscopic change to the destruction of the conjugated tetracene core. Such changes give these molecules potential for use in of singlet oxygen sensing applications. Among the molecules described here, the more highly conjugated tetracenes 4 and 6 show favorable ratiometric fluorescent signals, with emission peaks highly spectrally separated (140 nm for 4, and110 nm for 6) before and after the reaction, while maintaining strong fluorescent intensity (Figure 6).
Given the favorable photophysical and chemical properties of the tetracene derivatives here, we explored the possibility of applying them to a singlet oxygen responsive materials platform based on conjugated nanoparticles (CPNs) dispersed in water. We selected molecule 6 to examine because it has the most red-shifted emission spectrum. Following a nanoprecipitation method reported by McNeill group [44][45][46] and used previously in our group, 10 we doped 6 into a matrix of poly(9,9dioctylfluorene-alt-benzothiadiazole) (F8BT) with various weight percentages. Figure 7 shows the emission of the CPNs with different dopant levels. With 3% (w/w) of dopant, the original emission of F8BT is almost fully quenched by energy transfer (ET), 26 with emission emanating primarily from the dopant at 660 nm. Directly irradiating the CPNs generated singlet oxygen, which oxidized 6 and we monitored by fluorescence spectroscopy. Upon irradiation, a 120 nm hypochromic shift occurred, highlighting the application potential of 6 in ratiometric fluorescent singlet oxygen sensing.
This work therefore expands on the existing understanding of the often-beneficial impact of ethynyl groups on acenes longer than three rings showing that electronic characteristics of groups conjugated to those alkynes can also impact long acenes in substantial ways. Specifically, substituted phenyl groups and especially heteroaromatic arenes offer a wide panoply of design parameter space that can tune optoelectronic properties and photo-oxidation reactivity across a wide range within the diethynyltetracene structural class, yielding tetracenes that show favorable fluorescence behavior in responsive material platforms. Given that tetracenes offer two reactive rings, we anticipate these electronic effects, which here span 90 nm in fluorescence and 10-fold in intrinsic reactivity with 1 O2, could be larger in molecules that present one ring that is substantially more reactive than others, such as anthradithiophenes. This work highlights the potential of fully exploring and understanding the chemical space of conjugated materials beyond the portions that are typically thought of as most important.
6 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins Please do not adjust margins
This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5