Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 3567−3572
pubs.acs.org/JPCL
Controlling the Emissive Activity in Heterocyclic Systems Bearing CP Bonds Sunandan Sarkar,†,§ John D. Protasiewicz,*,‡ and Barry D. Dunietz*,† †
Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242, United States Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, United States
‡
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S Supporting Information *
ABSTRACT: The photophysical properties of a series of heteroatom substituted indoles are explored to identify chemical means to control their emissive activity. In particular, we consider impacts of changes in the conjugated backbone, where the CN bonds of benzoxazoles are replaced by CP bonds (benzoxaphospholes). The effects of extending the π-conjugation, incorporating various secondary heteroatoms (X−CP), and enforcing planar rigidity are also examined. Our computational analysis explains the higher fluorescence efficiency observed with extended π-conjugation and highlights the importance of maintaining molecular planarity at both ground- and emissive-state geometries.
F
Furthermore, while 1,3-benzoxaphospholes (R-BOP, Chart 1) can show luminescent activity,19 the closely related benzazaphosphole (BAP) materials where the oxygen atom is replaced by an N-R group seem to not be luminescent. Recently an exception to this trend has been reported for materials involving a carbazole framework.26 In this work, we consider several related molecules including the 1,3-benzoxaphospholes.19 The understanding of their optoelectronic properties remains scarce despite being synthesized as early as the 1980s.27−29 The fluorescence (FL) quantum yield (QY) is determined by the ratio of the radiative decay (kfl) over the total deactivation [including also the competing nonradiative processes (knr)]:30
luorescent organic materials are widely used as sensors, whitening agents, wavelength converters, and probes of biological systems and in many other capacities.1−3 In particular π-conjugated organic molecules and materials are well-suited to function as organic light-emitting diodes (OLEDs) owing to their relatively low cost and the potential for their use in mass production of flexible devices as they can be readily processed from solutions. In this Letter we examine strategies to tune optical properties in conjugated molecular systems featuring pπ−pπ double bonds involving main group elements.4−18 In particular, we find that most molecules with CP double bonds are usually associated with diminished photoluminescence compared to analogous molecules with unfunctionalized CC double bonds. Chart 1 shows some molecules that were Chart 1. Examples of Luminescent Compounds Containing pπ−pπ PC bonds
Φfl =
(1)
Nonradiative decay from the lowest singlet excited state can be due to relaxation to the ground state, energy transfer through excited state, coupling to the molecular environment, and intersystem crossing (ISC) between singlet and triplet states. Below we assume the decay to be dominated by a unimolecular ISC. The FL rate constant (kfl) is determined by the oscillator strength (f) of the transition responsible for the emission process and the frequencies corresponding to absorption and emission energies (νab and νem).31,32 A simplified and widely used form for kfl is given by32
recently synthesized by us that present surprising exceptions.19−24 Indeed, to date reports of such molecular “photocopies” that exhibit significant photoluminescence activity are very limited.15−18,25 Several materials with conjugated CX double bonds are investigated computationally to identify conditions that significantly enhance the photoluminescence activity. The investigated organic molecules bear an aromatic ring with a pair of heteroatoms, such as [N, O], [P, O] or [P, N]. As the understanding of such materials remains underdeveloped, there is increasing urgency to address the noted large impacts subtle chemical changes can have on the photophysical activity. © XXXX American Chemical Society
k fl k fl + k nr
Received: April 4, 2018 Accepted: June 8, 2018
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DOI: 10.1021/acs.jpclett.8b01045 J. Phys. Chem. Lett. 2018, 9, 3567−3572
Letter
The Journal of Physical Chemistry Letters
Figure 1. Calculated excitation energies (eV) of molecular derivatives of BOZ, BOP, and BAP at their S1 minimum-energy structure. For each case, the left/right column (pink/blue lines) indicates energies of singlet/triplet excited states.
Table 1. Computed Absorption (Eab) and Emission (Eem) Energies (in eV) along with their Oscillator Strengths (fab and fem), and Fluorescence (kfl) and Intersystem Crossing (kisc) Rate Constants (in sec−1)a molecule
Eab
fab
Eem
fem
H-BOZ Ph-BOZ H-BOP Ph-BOP H-BAP(NH) Ph-BAP(NH) H-BAP(NCH3) Ph-BAP(NCH3)
6.01 4.76 5.24 4.05 4.92 4.29 4.80 4.46
0.222 1.077 0.180 0.870 0.203 0.551 0.227 0.381
5.11 3.99 4.59 3.39 4.39 3.37 4.31 3.15
0.099 1.283 0.231 0.872 0.209 0.623 0.232 0.454
kfl 2.14 1.51 3.76 7.39 3.17 4.90 3.41 2.80
× × × × × × × ×
kisc 108 109 108 108 108 108 108 108
3.55 4.77 2.51 1.90 4.85 6.74 4.68 2.70
× × × × × × × ×
105 106 1011 106 108 107 108 1010
a
The measured absorption and emission peaks of Ph-BOZ43 are 299 nm (4.15 eV) and 332 nm (3.73 eV), respectively, and those of Ph-BOP19 are 337 nm (3.68 eV) and 425 nm (2.92 eV).
k fl = 0.6671[cm 2 s−1]
νem 3 2 nf νab
protocol follows recent benchmarking for analyzing the optoelectronic properties of various organic systems.36,37 1,3-Benzoxazoles are a well-established related class of luminescent materials that offer additional benchmark systems. The observed low FL QY of benzoxazole (H-BOZ)38 can be understood from the relatively small OS for the emission process (Table 1). Furthermore, the competing ISC involves S2 (ππ*) and 3(nπ*)39,40 that are nearly degenerate at S1 geometry of H-BOZ (Figure 1). Nevertheless, as shown recently, the FL can be enhanced through structural modification.19 In particular, extending the conjugation system by a phenyl ring reduces the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gap to redshift both the absorption and emission energies and to substantially increase the OS. The increase in the OS reflects a larger transition dipole across a larger conjugation system that increases the FL rate. However, further red-shifting is required for practical applications. Toward this goal, we consider replacement of the nitrogen in BOZs by phosphorus (i.e., BOPs). The resulting BOPs present several important electronic structure changes that affect the optical properties. The extension of π-conjugation in H-BOP by phenyl group (Ph-BOP), which maintains overall planarity both in ground and emissive states, results in a smaller HOMO−LUMO gap. The absorption and emission energies are red-shifted by about 1.2 eV, and their OSs are also significantly increased (Table 1). More specifically, the lowest absorbing state (S1) of H-BOP dominated by HOMO−LUMO transition (H → L, ππ*) is
(2)
where n is the refractive index of the solvent. The ISC rate constant (kisc) between the lowest singlet excited state (S1) and a triplet excited state (Tm) is obtained by the Marcus rate expression for a thermally activated process:33−35 k isc =
soc |V(T |2 m,S1)
ℏ
⎡ (λ + ΔE)2 ⎤ π exp⎢ − ⎥ 4λkBT ⎦ kBTλ ⎣
(3)
Here Vsoc (Tm,S1) ≡ ⟨ Tm|HSO|S1⟩ is the spin−orbit coupling matrix element, λ the reorganization energy, and ΔE the free-energy difference. The ISC process is assumed to involve the lowest singlet excited state, S1, and a triplet state, Tm, and may or may not follow the selection rules depending on the energy differences. To discuss the photoluminescence efficiency, both the FL and ISC rate constants of the considered molecular series (shown in Figure 1) are calculated. We consider structural modifications that incorporate various heteroatoms, extend the π-conjugation, or enforce planar rigidity. The calculated absorption and emission energies and their oscillator strengths (OSs) are strongly affected by chemical functionalization (listed in Table 1). Further computational data on the spectral trends are provided in the Supporting Information. The methods used to calculate the electronic structure parameters including the electronic coupling36 are summarized below, where the 3568
DOI: 10.1021/acs.jpclett.8b01045 J. Phys. Chem. Lett. 2018, 9, 3567−3572
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The Journal of Physical Chemistry Letters ∼0.75 eV smaller than that of the S2 (also H → L, ππ*) absorbing state of H-BOZ (Table S2 and Figure S2). The emission energy of H-BOP is red-shifted by ∼0.5 eV with respect to that of H-BOZ (Table 1). Surprisingly, the OS of the emission process is larger in H-BOP. This trend is due to the H → L contribution of S1 that is changing from 0.86 to 0.95 between those calculated at the ground- and emissive-state geometries (Tables S2 and S3). Phenylation of H-BOP increases the OS to 0.87 from 0.23, while it remains less than that in Ph-BOZ (1.3). Therefore, the FL rate of Ph-BOP is predicted to be less than that for Ph-BOZ. Another chemical change of the heterocycle ring is considered by replacing the α oxygen atom of −O−CP− in BOPs with a nitrogen atom, resulting in the −N(R)−CP− skeleton of BAPs. The increased aromaticity in H-BAP(NH) results in further red-shifting the absorption (∼0.3 eV) and emission (∼0.2 eV) energies with respect to H-BOP (Table 1). Aromaticity in five-membered heterocycles increases with the decrease of electronegative differences between the heteroatom and the adjacent carbon atoms.41,42 Importantly, the OS of HBAP(NH) emitting state is larger than for H-BOZ while slightly smaller than for H-BOP. Therefore, the trend of the calculated FL rate constants is as follows H-BOP > HBAP(NH) > H-BOZ. In contrast to Ph-BOP, the ground-state structure of PhBAP(NH) is nonplanar because of the steric hindrance between hydrogen atoms at the bay region (N site of BAP and C site of phenyl group). The dihedral angle between the two rings is about 35° (Table S1). Interestingly, the planarity is regained at the emissive geometry (Figure 2). These structural
To understand photoluminescence efficiency, we compare the FL rates to those of the competing ISC that are assumed to dominate the nonradiative prcoesses. The resulting rates are included in Table 1, and the energetic parameters are included in Table S4. We use the energy difference, 1ΔST, between the two key electronic states, the lowest singlet (S1) and a triplet excited state (Tm) at the singlet excited-state geometry, to identify the most relevant ISC transitions. The energy levels are illustrated in Figure 1, and their energy differences with coupling values are listed in Table 2. As discussed below it Table 2. Energy Differences with Coupling Valuesa molecule H-BOZ Ph-BOZ H-BOP Ph-BOP H-BAP(NH) Ph-BAP(NH) H-BAP(NCH3) Ph-BAP(NCH3)
ΔSTp
Vsoc (Tp,S1)
1
0.11 (T4) −0.08 (T2) −0.15 (T3) 0.32 (T2) −0.02 (T3) 0.06 (T2) 0.02 (T3) 0.17 (T2)
0.03 0.12 1.02 0.25 1.06 0.40 1.04 11.34
0.63 1.04 0.24 1.14 0.69 1.27 0.80 1.38
1
ΔSTq (T6) (T9) (T4) (T6) (T5) (T7) (T6) (T7)
Vsoc (Tq,S1) 2.80 13.54 47.30 54.63 46.23 61.09 44.32 57.06
ΔST represents the calculated energy difference (eV) between S1 and excited triplet state at S1 geometry, where Tp indicates the triplet state with same state character as S1 state and Tq indicates a lowest energy triplet state of different character than the S1 state (represented soc corresponds to the spin-orbit coupling elements in Figure 1). V(T m,S1) (cm−1) between S1 and triplet state at S1 geometry. a1
appears that all the ISC processes, except for H-BOP, involve formally forbidden transitions between singlet and triplet states that are both π−π* transitions.44,45 We begin by relating to H-BOZ. Here the calculated 3(nπ*) triplet excited-state energy is significantly higher (0.63 eV) than the S1 state 1(ππ*), in good agreement with an earlier study finding of 0.78 eV energy difference.43 Reiser et al., on the other hand, suggested that the 3(nπ*) triplet-state energy is ∼0.22− 0.35 eV lower than the lowest singlet excited state 1(ππ*).38 For Ph-BOZ, the 3(nπ*) state energy is relatively even higher by ∼1.0 eV than the 1(ππ*) state. While the SOC between these states is quite large (∼13.5 cm−1). With such substantial energy differences these ISC transitions are expected to be negligible (Figure 1 and Table 2). The SOC values for the alternate excited ππ* triplet states of better energy alignment are 0.12 cm−1 for S1−T2 of Ph-BOZ and 0.03 cm−1 for S1−T4 of H-BOZ (Table 2), with the resulting rates confirming slow ISC processes. Conseqently, the ISC channel for deactivating the singlet excited state is ruled out, emphasizing the predicted high FL QY for the Ph-BOZ system. As discussed above, the fundamental orbital gap in the phosphorus functionalized ring molecule, BOP, is relatively small, affecting also the triplet excited states. The triplet 3(nπ*) state energy of H-BOP is lower by ∼1.0 eV than that of HBOZ. The H-BOP singlet−triplet splitting (1ΔST) between the S1 (1(ππ*)) and the lowest 3(nπ*) (T4) state at S1 optimized geometry is ∼0.2 eV (compare to ∼0.6 eV in H-BOZ). The T4 state minimum energy is 0.21 eV lower than that of the S1 state minimum energy. Furthermore, the SOC between S1 and T4 states of 47.3 cm−1 is significant because of their different character (El-Sayed’s allowed44,45). As a result, the ISC process in H-BOP is associated with a large rate constant (Table S4). The ISC rate of H-BOP is 3 orders of magnitude faster than the
Figure 2. Ground-state (S0) and excited-state (S1) structural views and frontier molecular orbitals of Ph-BAP(NH) and Ph-BAP(NCH3).
features explain the absorption blue-shifting (by ∼0.25 eV) of Ph-BAP(NH), whereas the emission energy is insignificantly affected in comparison to Ph-BOP values. The red-shift of absorption energy due to π-extension by a phenyl group is smaller in Ph-BAP(NH) (0.63 eV red-shifted) compared to PhBOZ (1.24 eV red-shifted) and Ph-BOP (1.19 eV red-shifted). Though the emissive geometry of Ph-BAP(NH) is planar, the emission state OS remains lower than the Ph-BOP value. Consequently, the FL rate is not enhanced as much upon phenylation of H-BAP(NH). In addition, because the emissive geometry is planar in contrast to that of the ground state, there is only a small vibrational overlap. The calculated FL rate constant of Ph-BAP(NH) remains smaller than the Ph-BOP constant. 3569
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nonparallel. Consequently, the SOC between two ππ* states becomes significant (see the S1 and T2 states in Figure S3h). The SOC is increased from 0.4 (in Ph-BAP(NH)) to 11.34 cm−1 (in Ph-BAP(NCH3)). The resulting ISC of Ph-BAP(NCH3) that involves two closely spaced singlet (S1) and triplet (T2) excited states becomes significant (three orders higher than that of Ph-BAP(NH) and two orders larger than that of H-BAP(NCH3) [Table S4]). Hence, the Ph-BAP(NCH3) molecule is nonfluorescent, where it reveals molecular means to emphasize ISC. In summary, we study computationally the photophysical properties of π-conjugated classes of molecules with heavier pπ−pπ PC bonds. The structure−function relationship analysis in a related series of organic molecules finds that significant fluorescence efficiency requires not only large πconjugated planar structures but also an unfavorable competing nonradiative transition. In particular, Ph-BOZ and Ph-BOP are found to be of a potential to exhibit strong fluorescence because both conditions are satisfied. By contrast, replacing the phenyl group by hydrogen (H-BOP) encourages fast ISC rates due to strong spin−orbit coupling between 1(ππ*) and 3(nπ*) excited states, which greatly limits fluorescent activity. The FL QYs of unsubstituted BAPs are also inhibited because excited-state depopulation can occur through ISC transitions. It is also predicted that Ph-BAP(NH) can emit radiation, but with efficiency that is lower than in Ph-BOZ or Ph-BOP because of slower FL rates. It is revealed that coplanarity maintained in the emissive geometry enhances the FL QYs. Strong steric hindrance in Ph-BAP(NCH3), which leads to distortions out of planarity, are predicted to yield small OS and strong SOC and therefore lead to low FL and high ISC rates. Consequently, the Ph-BAP(NCH3) is unlikely to be a fluorescent material. We find that high fluorescence yield in extended π-conjugated systems requires molecular palanarity in both the ground and excited states. In this way we report a rational basis for designing fluorescent functional π-conjugated materials that include a heavier main group pπ−pπ bond.
FL rate (Table 1), and therefore the FL efficiency of H-BOP is expected to be low. In Ph-BOP, the 3(nπ*) triplet excited state is stabilized only slightly, whereas the S1 (1(ππ*)) state is significantly stabilized by 1.2 eV (Figure 1). Here, the energy of the lowest 3(nπ*) state, the T6, is ∼1.1 eV higher than the S1 state compared to only ∼0.2 eV difference in H-BOP case. Thus, in spite of the strong SOC between S1 and T6 states (54.6 cm−1), the ISC process remains unfavorable. For completeness, as in the other case we consider the ISC between S1 (1(ππ*)) and T2 (3(ππ*)) states of the much smaller SOC (0.25 cm−1; Table S4) confirming the slow ISC. For H-BAP(NH) we find a negligible activation barrier of 0.3 meV for the S1 → T3 transition with a slightly bigger SOC that enables the ISC process. This nonradiative channel becomes comparable in rate constant to the radiative process and therefore appears to limit the FL QY (Table 1). The ISC process due to S1 → T2 is a noncompeting channel in PhBAP(NH) because of the lower SOC. The 3(nπ*) state energy is significantly higher than the S1 state (Figure 1). To fully understand the effects of extended π-conjugation on FL and ISC processes, we turn to consider the BAP system where we introduce further steric hindrance. In all systems discussed above, extending the conjugation plane by a phenyl ring enhances the FL while maintaining a low rate for ISC. Next, we add a methyl group to the nitrogen atom in the considered BAPs (H-BAP(NCH3) and Ph-BAP(NCH3)) (Figure 1). As a result, both the ground- and excited-state geometries of Ph-BAP(NCH3) are nonplanar, where the dihedral angle between the two ring planes in Ph-BAP(NCH3) is ∼55° at ground-state geometry and ∼20° at the lowest singlet excited-state geometry (Figure S1 and Table S1). The H-BAP(NCH3) does not present any significant structural changes compared to H-BAP(NH). The inductive effect of methyl group leads to a more conjugated fivemembered π-system. The emission energy of Ph-BAP(NCH3) is 1.16 eV red-shifted from the absorption energy compared to 0.34 eV in H-BAP(NCH3) (Table 1). The absorption energy of Ph-BAP(NCH3) is blue-shifted by 0.17 eV with respect to PhBAP(NH) energy. The LUMO energy of Ph-BAP(NCH3) is much higher compared to Ph-BAP(NH) LUMO because of the larger planar distortion in the ground-state geometry (a larger dihedral angle between the BAP and phenyl units) resulting in a blue-shifted absorption. The emission energy of PhBAP(NCH3), on the other hand, is red-shifted (0.22 eV). The LUMO energies of Ph-BAP(NCH3) and Ph-BAP(NH) at their S1 geometry are lower than that of ground state by 0.9 and 0.6 eV, respectively. For both systems, the HOMO energy at S1 geometry is higher almost by a similar amount (around 0.5 eV) than that at equilibrium geometry. At the S1 geometry of PhBAP(NCH3), while the overall structure tends to become planar (as opposed to the ground-state geometry), an overall out-of-planarity distortion remains (Figure 2). As a consequence, the π* orbitals across the two rings involve the P atom and a C atom from the phenyl group (the blue arrow in the figure of the LUMO). Therefore, we find red-shifting in emission energy of Ph-BAP(NCH3) with respect to PhBAP(NH) energy. The resulting FL rate constant of PhBAP(NCH3) is almost half that for Ph-BAP(NH), where the nonplanarity leads to low OS (Table 1). On the other hand, the SOC is quite enhanced because of distorted planarity. In the bent structure of the emissive geometry, the nodal planes of the π and π* orbitals are
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COMPUTATIONAL DETAILS
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ASSOCIATED CONTENT
Density functional theory (DFT) is used to calculate groundstate properties, and the Tamm−Dancoff approximation (TDA) of the time-dependent density functional theory (TDDFT) is used to obtain excited-state properties.46−49 The range-separated hybrid (RSH) ωB97X-D functional,50−52 which also accounts for long-range Coulomb and dispersion interactions, is employed. The 6-31G(d,p) basis set is used in all calculations. A dielectric constant of 8.93 and refractive index of 1.425 in polarizable continuum solvation model (PCM) is used to represent the dichloromethane solvent. Geometry optimizations of the higher triplet excited states follow a statetracking scheme.53 The S1−Tm spin−orbit coupling elements are calculated54 using the Tamm−Dancoff approximation of TDDFT. All the calculations were performed using the QChem 4.4 package.55
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b01045. Tables S1−S4 and Figures S1−S3 as described in the text (PDF) 3570
DOI: 10.1021/acs.jpclett.8b01045 J. Phys. Chem. Lett. 2018, 9, 3567−3572
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(14) Smith, R. C.; Protasiewicz, J. D. A Fluorescent (E)-Poly(pphenylenephosphaalkene) Prepared by a Phospha-Wittig Reaction. Inorg. Chem. 2003, 42, 5468−5470. (15) Wright, V. A.; Gates, D. P. Poly(p-Phenylenephosphaalkene): A π-Conjugated Macromolecule Containing PC Bonds in the Main Chain. Angew. Chem., Int. Ed. 2002, 41, 2389−2392. (16) Qiu, Y.; Worch, J. C.; Chirdon, D. N.; Kaur, A.; Maurer, A. B.; Amsterdam, S.; Collins, C. R.; Pintauer, T.; Yaron, D.; Bernhard, S.; Noonan, K. J. T. Tuning Thiophene with Phosphorus: Synthesis and Electronic Properties of Benzobisthiaphospholes. Chem. - Eur. J. 2014, 20, 7746−7751. (17) Worch, J. C.; Chirdon, D. N.; Maurer, A. B.; Qiu, Y.; Geib, S. J.; Bernhard, S.; Noonan, K. J. T. Synthetic Tuning of Electronic and Photophysical Properties of 2-Aryl-1,3-Benzothiaphospholes. J. Org. Chem. 2013, 78, 7462−7469. (18) Sklorz, J. A. W.; Hoof, S.; Rades, N.; Rycke, N. D.; Konczol, L.; Szieberth, D.; Weber, M.; Wiecko, J.; Nyulaszi, L.; Hissler, M.; Muller, C. PyridylFunctionalised 3H1,2,3,4Triazaphospholes: Synthesis, Coordination Chemistry and Photophysical Properties of LowCoordinate Phosphorus Compounds. Chem. - Eur. J. 2015, 21, 11096−11109. (19) Washington, M. P.; Gudimetla, V. B.; Laughlin, F. L.; Deligonul, N.; He, S.; Payton, J. L.; Simpson, M. C.; Protasiewicz, J. D. Phosphorus Can Also Be a ”Photocopy. J. Am. Chem. Soc. 2010, 132, 4566−4567. (20) Wu, S.; Rheingold, A. L.; Protasiewicz, J. D. Luminescent materials containing multiple benzoxaphosphole units. Chem. Commun. 2014, 50, 11036−11038. (21) Wu, S.; Deligonal, N.; Protasiewicz, J. D. An unusually unstable ortho-phosphinophenol and its use to prepare benzoxaphospholes having enhanced air-stability. Dalton Trans. 2013, 42, 14866−14874. (22) Laughlin, F. L.; Deligonul, N.; Rheingold, A. L.; Golen, J. A.; Laughlin, B. J.; Smith, R. C.; Protasiewicz, J. D. Fluorescent heteroacenes with multiply-bonded phosphorus. Organometallics 2013, 32, 7116−7121. (23) Laughlin, F. L.; Rheingold, A. L.; Deligonul, N.; Laughlin, B. J.; Smith, R. C.; Higham, L. J.; Protasiewicz, J. D. Naphthoxaphospholes as examples of fluorescent phospha-acenes. Dalton Trans. 2012, 41, 12016−12022. (24) Washington, M. P.; Payton, J. L.; Simpson, M. C.; Protasiewicz, J. D. Redox Behavior of 2-Substituted 1,3-Benzoxaphospholes and 2,6Substituted Benzo[1,2-d:4,5-d]bisoxaphospholes. Organometallics 2011, 30, 1975−1983. (25) Phosphorus-Carbon Heterocyclic Chemistry: The Rise of a New Domain; Mathey, F., Ed.; Pergamon: Amsterdam, 2001. (26) Wu, S.; Rheingold, A. L.; Golen, J. A.; Grimm, A. B.; Protasiewicz, J. D. Synthesis of a Luminescent Azaphosphole. Eur. J. Inorg. Chem. 2016, 2016 (5), 768−773. (27) Heinicke, J.; Tzschach, A. 1,3-Benzoxaphosphole-Heterocyclen Mit Phosphor der Koordinationszahl 2. Phosphorus Sulfur Relat. Elem. 1985, 25, 345−356. (28) Heinicke, J.; Tzschach, A. Zur Oxydation von (y-PC)Derivaten; Untersuchungen an 2-tert-Butyl-1,3-benzoxaphosphol. Z. Chem. 1983, 23, 439−440. (29) Heinicke, J.; Tzschach, A. Synthese von 2-tert-Butyl-1,3benzoxaphosphol. Z. Chem. 1980, 20, 342−343. (30) Lakowicz, J. R. Radiative Decay Engineering: Biophysical and Biomedical Applications. Anal. Biochem. 2001, 298, 1−24. (31) Strickler, S. J.; Berg, R. A. Relationship between Absorption Intensity and Fluorescence Lifetime of Molecules. J. Chem. Phys. 1962, 37, 814. (32) Kwon, M. S.; Jordahl, J. H.; Phillips, A. W.; Chung, K.; Lee, S.; Gierschner, J.; Lahann, J.; Kim, J. Multi-luminescent switching of metal-free organic phosphors for luminometric detection of organic solvents. Chem. Sci. 2016, 7, 2359−2363. (33) Marcus, R. A. Electron transfer reactions in chemistry. Theory and experiment. Rev. Mod. Phys. 1993, 65, 599−610. (34) Beljonne, D.; Shuai, Z.; Pourtois, G.; Bredas, J. L. Spin-Orbit Coupling and Intersystem Crossing in Conjugated Polymers: A
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Barry D. Dunietz: 0000-0002-6982-8995 Present Address §
S.S.: Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, India.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS B.D.D. acknowledges support from NSF grant CHE-1362504. J.D.P. acknowledges support from NSF grant CHE-1464855. We are also grateful to generous resource allocations on the Ohio Supercomputer Center and the Kent State University, College of Arts and Sciences Computing Cluster.
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REFERENCES
(1) Introduction to Fluorescence; Jameson, D. M., Ed.; CRC Press: Boca Raton, FL, 2014; p 313. (2) Sinkeldam, R. W.; Greco, N. J.; Tor, Y. Fluorescent Analogs of Biomolecular Building Blocks: Design, Properties, and Applications. Chem. Rev. 2010, 110, 2579−2619. (3) Principles of Fluorescence Spectroscopy; Lakowicz, J. R., Ed.; Plenum Press: New York, 1983; p 496. (4) Obeid, N. M.; Maus, L. K. D.; Zimmer, M.; Jeck, J.; Bejan, I.; White, A. J. P.; Huch, V.; Jung, G.; Scheschkewitz, D. (Oligo)aromatic species with one or two conjugated SiSi bonds: near-IR emission of anthracenyl-bridged tetrasiladiene. Dalton Trans. 2017, 46, 8839− 8848. (5) Hayakawa, N.; Nishimura, S.; Kazusa, N.; Shintani, N.; Nakahodo, T.; Fujihara, H.; Hoshino, M.; Hashizume, D.; Matsuo, T. π-Conjugation between a SiSi Double Bond and Thiophene Rings: Synthesis, Structural Characteristics, and Photophysical Properties of 1,2-Bis(thiophen-2-yl)disilene and 1,2-Bis(2,2-bithiophen-5yl)disilene. Organometallics 2017, 36, 3226−3233. (6) Yang, Y.; Mosquera, M. A.; Skinner, K.; Becerra, A. E.; Shamamian, V.; Schatz, G. C.; Ratner, M. A.; Marks, T. J. Electronic Structure and Potential Reactivity of Silaaromatic Molecules. J. Phys. Chem. A 2016, 120, 9476−9488. (7) Wang, Z.; Zhang, J.; Li, J.; Cui, C. NHC-Stabilized SiliconCarbon Mixed Cumulene. J. Am. Chem. Soc. 2016, 138, 10421−10424. (8) Kobayashi, M.; Hayakawa, N.; Matsuo, T.; Li, B.; Fukunaga, T.; Hashizume, D.; Fueno, H.; Tanaka, K.; Tamao, K. (Z)-1,2-Di(1pyrenyl)disilene: Synthesis, Structure, and Intramolecular ChargeTransfer Emission. J. Am. Chem. Soc. 2016, 138, 758−761. (9) Li, L.; Matsuo, T.; Hashizume, D.; Fueno, H.; Tanaka, K.; Tamao, K. Coplanar Oligo(p-phenylenedisilenylene)s as SiSi Analogues of Oligo(p-phenylenevinylene)s: Evidence for Extended Π-Conjugation through the Carbon and Silicon π-Frameworks. J. Am. Chem. Soc. 2015, 137, 15026−15035. (10) Wright, V. A.; Patrick, B. O.; Schneider, C.; Gates, D. P. Phosphorus Copies of PPV: π-Conjugated Polymers and Molecules Composed of Alternating Phenylene and Phosphaalkene Moieties. J. Am. Chem. Soc. 2006, 128, 8836−8844. (11) Gates, D. P. Expanding the analogy between PC and CC bonds to polymer science. Top. Curr. Chem. 2005, 250 ((New Aspects in Phosphorus Chemistry V)), 107−126. (12) Smith, R. C.; Protasiewicz, J. D. Systematic Investigation of PPV Analogue Oligomers Incorporating Low-Coordinate Phosphorus Centres. Eur. J. Inorg. Chem. 2004, 2004 (5), 998−1006. (13) Smith, R. C.; Protasiewicz, J. D. Conjugated Polymers Featuring Heavier Main Group Element Multiple Bonds: A Diphosphene-PPV. J. Am. Chem. Soc. 2004, 126, 2268−2269. 3571
DOI: 10.1021/acs.jpclett.8b01045 J. Phys. Chem. Lett. 2018, 9, 3567−3572
Letter
The Journal of Physical Chemistry Letters Configuration Interaction Description. J. Phys. Chem. A 2001, 105, 3899−3907. (35) Kohler, A.; Bassler, H. Triplet States in Organic Semiconductors. Mater. Sci. Eng., R 2009, 66, 71−109. (36) Sarkar, S.; Hendrickson, H. P.; Lee, D.; DeVine, F.; Jung, J.; Geva, E.; Kim, J.; Dunietz, B. D. Phosphorescence in Bromobenzaldehyde Can Be Enhanced through Intramolecular Heavy Atom Effect. J. Phys. Chem. C 2017, 121, 3771−3777. (37) Phillips, H.; Zheng, Z.; Geva, E.; Dunietz, B. D. Orbital gap predictions for rational design of organic photovoltaic materials. Org. Electron. 2014, 15, 1509−1520. (38) Reiser, A.; Leyshon, L. J.; Saunders, D.; Mijovic, M. V.; Bright, A.; Bogie, J. Fluorescence of Aromatic Benzoxazole Derivatives. J. Am. Chem. Soc. 1972, 94, 2414−2421. (39) O’Dwyer, M. F.; El-Bayoumi, M. A.; Strickler, S. J. Observation of Anomalous Phosphorescence-Fluorescence Intensity Ratio in Excitation of Upper Electronic States of Certain Aromatic Hydrocarbons. J. Chem. Phys. 1962, 36, 1395. (40) Li, Y. H.; Lim, E. C. Intersystem Crossing from Higher Excited Singlet States of Phthalazine. J. Chem. Phys. 1972, 56, 1004. (41) Horner, K. E.; Karadakov, P. B. Shielding in and around Oxazole, Imidazole, and Thiazole: How Does the Second Heteroatom Affect Aromaticity and Bonding? J. Org. Chem. 2015, 80, 7150−7157. (42) Nyulaszi, L.; Varnai, P.; Veszpremi, T. About the aromaticity of five-membered heterocycles. J. Mol. Struct.: THEOCHEM 1995, 358, 55−61. (43) Catalan, J.; Mena, E.; Fabero, F. The role of the torsion of the phenyl moiety in the mechanism of stimulated ultraviolet light generation in 2-phenylbenzazoles. J. Chem. Phys. 1992, 96, 2005− 2016. (44) Lower, S. K.; El-Sayed, M. A. The triplet state and molecular electronic processes in organic molecules. Chem. Rev. 1966, 66, 199− 241. (45) El-Sayed, M. A. The triplet state. Its radiative and nonradiative properties. Acc. Chem. Res. 1968, 1, 8−16. (46) Casida, M. E.; Gutierrez, F.; Guan, J.; Gadea, F.-X.; Salahub, D.; Daudey, J.-P. Charge-transfer correction for improved time-dependent local density approximation excited-state potential energy curves: Analysis within the two-level model with illustration for H2 and LiH. J. Chem. Phys. 2000, 113, 7062. (47) Peach, M. J. G.; Williamson, M. J.; Tozer, D. J. Influence of triplet instabilities in TDDFT. J. Chem. Theory Comput. 2011, 7, 3578−3585. (48) Sears, J. S.; Koerzdoerfer, T.; Zhang, C.-R.; Brédas, J.-L. Communication: orbital instabilities and triplet states from timedependent density functional theory and long-range corrected functionals. J. Chem. Phys. 2011, 135, 151103. (49) Peach, M. J. G.; Tozer, D. J. Overcoming low orbital overlap and triplet instability problems in TDDFT. J. Phys. Chem. A 2012, 116, 9783−9. (50) Chai, J.-D.; Head-Gordon, M. Systematic optimization of longrange corrected hybrid density functionals. J. Chem. Phys. 2008, 128, 084106. (51) Chai, J.-D.; Head-Gordon, M. Long-range corrected hybrid density functionals with damped atom-atom dispersion corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615−20. (52) Chai, J.-D.; Head-Gordon, M. Long-range corrected hybrid density functionals with damped atom-atom dispersion corrections. Chem. Phys. Lett. 2008, 467, 176−178. (53) Closser, C. D.; Gessner, O.; Head-Gordon, M. Simulations of the dissociation of small helium clusters with ab initio molecular dynamics in electronically excited states. J. Chem. Phys. 2014, 140, 134306. (54) Ou, Q.; Subotnik, J. E. Electronic relaxation in benzaldehyde evaluated via TD-DFT and localized diabatization: intersystem crossings, conical intersections, and phosphorescence. J. Phys. Chem. C 2013, 117, 19839−19849.
(55) Shao, Y.; et al. Advances in molecular quantum chemistry contained in the Q-Chem 4 program package. Mol. Phys. 2015, 113, 184−215.
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DOI: 10.1021/acs.jpclett.8b01045 J. Phys. Chem. Lett. 2018, 9, 3567−3572
