
==== Front
J Phys Chem A
J Phys Chem A
jx
jpcafh
The Journal of Physical Chemistry. a
1089-5639
1520-5215
American Chemical Society

39226435
10.1021/acs.jpca.4c03821
Article
Influence of Substituents on the Vectorial Difference Static Dipole Upon Excitation in Synthetic Bacteriochlorins
https://orcid.org/0000-0003-2908-5573
Ketteridge Maia N. †
Watt Devan R. †
Duncan Katelyn M. †
https://orcid.org/0000-0002-9936-7720
Barcenas German †
Shaw Kaden †∥
https://orcid.org/0000-0003-3018-2207
Knowlton William B. †‡
https://orcid.org/0000-0003-3913-2855
Yurke Bernard †‡
https://orcid.org/0000-0002-1302-1770
Pensack Ryan D. †
https://orcid.org/0000-0002-2309-2644
Mass Olga A. *†
https://orcid.org/0000-0003-3870-8437
Li Lan *†§
† Micron School of Materials Science and Engineering, Boise State University, Boise, Idaho 83725, United States
‡ Department of Electrical and Computer Engineering, Boise State University, Boise, Idaho 83725, United States
§ Center for Advanced Energy Studies, Idaho Falls, Idaho 83401, United States
* Email: olgamass@boisestate.edu.
* Email: lanli@boisestate.edu.
03 09 2024
12 09 2024
128 36 75817592
08 06 2024
21 08 2024
16 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Organic dye aggregates have been shown to exhibit exciton delocalization in natural and synthetic systems. Such dye aggregates show promise in the emerging area of quantum information science (QIS). We believe that the difference in static dipole (Δd) is an essential dye parameter in the development of molecular QIS systems. However, a foundational understanding of the structural factors influencing Δd remains elusive. Bacteriochlorins play a vital role in photosynthesis due to their exceptional photophysical properties. Therefore, bacteriochlorins are particularly suitable dyes for the construction of aggregate systems for QIS. Synthetic bacteriochlorins further offer stability and tunability via chemical modifications. Here, the influence of substituents on the Δd of monomeric (nonaggregated) dyes was investigated via density functional theory (DFT) and time-dependent (TD)DFT in a set of 5-methoxybacteriochlorins progressively substituted with ethynyl, phenyl, and phenylethynyl substituents at the 3,13 and 3,13,15 positions of the macrocycle. Symmetrically substituted 5-methoxybacteriochlorins were shown to have the largest Δd. The increase in Δd in the series of dyes was largely due to changes in the orientation of the static dipole upon excitation rather than large changes in magnitude. In addition, the transition dipole (μ) and the angle between Δd and μ (ζ) were calculated. Three 5-methoxybacteriochlorins with large predicted Δd and μ values were synthesized and characterized spectroscopically. The trend in Δd values empirically determined using the solvatochromic Stokes shift method was comparable to the DFT calculations.

Office of Experimental Program to Stimulate Competitive Research 10.13039/100005714 DE-SC0020089 document-id-old-9jp4c03821
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pmcIntroduction

Natural and synthetic organic dyes are molecules capable of absorbing visible and near-infrared light due to their π-conjugated systems. Many such dyes have been observed to aggregate in a variety of systems, including natural photosynthetic antennas1−3 and synthetic systems such as DNA templated aggregates,4−8 covalently linked dye molecules,9,10 as well as concentrated solutions.11,12 In a molecular aggregate, the excitation energy can be delocalized across two or more neighboring dye molecules in a wave-like manner, forming a collective excitation known as a molecular exciton. Collective effects arising from excitonic delocalization, such as superradiance, may enable highly efficient energy transfer in natural photosynthetic antennas.13 Exciton delocalization can also be advantageous for a variety of applications such as light harvesting,13−17 medical imaging,18−20 photodynamic therapy,21−23 and organic optoelectronics.24−27 In addition, there is an emerging interest in applying exciton delocalization exhibited by dye aggregates in quantum computing and quantum information science (QIS).28−31 The behavior of delocalized excitons is described by the Frenkel Hamiltonian, which has a form similar to that of a multiparticle quantum walk. This similarity theoretically allows molecular excitons to be used to perform room-temperature quantum computing operations.28,31−35 This work focused on understanding dye monomer properties relevant to QIS dye aggregate systems, although these properties are also of general importance for most dye aggregate applications.

The photophysics of molecular excitons can be described by two key parameters from the Frenkel Hamiltonian: the exciton hopping parameter (Jm,n)36,37 and biexciton interaction energy (Km,n), both of which must be maximized for effective QIS dye aggregate systems. The parameter Jm,n is the energy associated with the hopping of an exciton between neighboring dyes in a dimer, and thus, it is responsible for exciton delocalization. The term Km,n is associated with the strength of the interaction between the two excitons. When the extended dipole approximation is applied, the Jm,n and Km,n parameters of a dye dimer are proportional to the following terms38:1

2

where lm and ln are the length of chromophore m and n, respectively; n is the refractive index of the solvent; and ε0 is the vacuum permittivity constant.

The term Jm,n is proportional to the product of the magnitudes of the transition dipoles (μ) of participating dyes (eq 1), while Km,n is proportional to the product of the difference static dipoles (Δd) of participating dyes (eq 2). The quantity Δd is defined as the vector difference between the excited-state electric dipole (de) and the ground-state electric dipole (dg) (Figure 1). The vector difference between de and dg is attributed to the redistribution of the electron density upon excitation. In addition, both Jm,n and Km,n depend on the dye intermolecular distance and orientation.

Figure 1 Schematic representation of the vectorial relationship between static dipole of the ground state (dg), static dipole of the excited state (de), the vector difference of de and dg (Δd), and the associated transition dipole (μ) of the first singlet excited state (Qy) transition. Vectors are overlaid on 5-methoxy bacteriochlorin (BC). Heteroatoms are omitted for clarity.

To realize multiparticle entangled states applicable for QIS, both Jm,n and Km,n should be maximized. According to eqs 1 and 2, both Jm,n and Km,n depend on the dye intermolecular distance and orientation in an aggregate. Hence, high Jm,n and Km,n values can be achieved by decreasing the distance and adjusting the angle between the constituent dye molecules. The distance and angle between the constituent dye molecules are governed by formation conditions and determine aggregate type. When dyes self-aggregate in a concentrated solution,11,12 the distance and angle between dyes depends on the concentration, dye chemical structure, solvent. In systems templated by DNA4−8,39,40 or photosynthetic proteins,1−3 the dye orientations also depend on the template design. Attaching dyes to a template via linkers (covalent attachment) provides control over the number of dyes per aggregate, which is not available in self-aggregation.

As stated above, Jm,n, and Km,n also depend on μ or Δd, which are intrinsic to the dye chemical structure. The influence of chemical structure on the magnitude of μ for organic dyes is well-studied,41,42 and many dyes with large μ have been synthesized. However, limited investigation has been devoted to the influence of the dye chemical structure on Δd. In principle, an increase in Δd could be achieved either by a substantial difference in static dipole magnitude or via changes in static dipole direction upon excitation. A number of dyes have been reported with a "push–pull" dye design, where strong electron-withdrawing and electron-donating groups were placed on the opposing side of the π-system. These dyes exhibited large Δd due to the difference in static dipole magnitude.18,43,44 A systematic computational screening to elucidate the influence of electron-withdrawing and donating groups on Δd has been performed for dyes such as cyanines and squaraines.45−47 These studies found that asymmetric substitution of indolenine rings with strong electron-withdrawing and -donating substituents led to higher Δd and μ. Substitution patterns with higher structural asymmetry in cyanines and squaraines also resulted in a further increase of Δd to up to 3.5 D in vacuum.45−47 However, the means to increase Δd by changing the orientation of the static dipole have not yet been demonstrated.

Moreover, we believe that another relevant parameter for QIS applications is the angle between Δd and μ, termed here ζ. Both nearly orthogonal (ζ ≈ 90°) and nearly parallel (ζ ≈ 0°) vectorial relationships between Δd and μ can be useful for the design of QIS systems.28,34,35 Therefore, we seek to gain insight into how dye substituents influence ζ.

Bacteriochlorin dyes are vital photosynthetic pigments. In the photosynthetic apparatus of purple bacteria, closely positioned molecules of bacteriochlorophyll a embedded in protein matrix exhibit exciton delocalization, absorb sunlight, and transfer the excitation energy to the reaction center at room temperature with unity energy transfer efficiency.13,48 The core structure of bacteriochlorin is a reduced porphyrin macrocycle consisting of two opposing pyrroles and two opposing pyrroline rings. The unique cyclic conjugated system of bacteriochlorins results in four characteristic absorption bands: Bx and By, Qx, and a long-wavelength Qy.49 Unlike porphyrins, the near-infrared Qy(0;0) absorption band (S0–S1 transition) of bacteriochlorins is very intense (εmax ∼ 120,000 cm–1 M–1) and narrow1,13,50 making bacteriochlorins highly advantageous for creating molecular excitons. Bacteriochlorins have been shown to exhibit excitonic coupling in natural photosynthetic antenna13 and, recently, in a fully synthetic system.4 In addition, de novo bacteriochlorins developed by Lindsey are stable due to a geminal dimethyl group attached to each of the pyrroline rings and can be subjected to a diverse range of synthetic modifications to tune their chemical and photophysical properties.51−54

This study focused on investigating how to enhance bacteriochlorin monomer properties Δd and ζ while maintaining high μ. To do so, we computationally evaluated the influence of substituents on μ, Δd, and ζ in a set of synthetically accessible de novo bacteriochlorins using density functional theory (DFT) and time-dependent (TD-)DFT. The relationship between the substituents of synthetic bacteriochlorins and properties of interest (μ, Δd, and ζ) was examined. Based on computational results, all substituents, with the exception of the 5-methoxy group, increased μ. Meanwhile, asymmetry introduced with the 5-methoxy group served as a basis for nonzero Δd. The major contributor to large Δd was a change in the static dipole direction upon excitation rather than a change in its magnitude. Extending the conjugated system with identical substituents along the molecular y-axis (3,13-substituted bacteriochlorins) yielded the largest Δd. A “push-pull” type structure, where electron donating and withdrawing groups were located on opposing phenylethynyl substituents of the macrocycle, resulted in lower Δd values than symmetrically substituted bacteriochlorins. The angle ζ was unaffected by the substituent type or pattern, remaining less than 12° for most computationally screened bacteriochlorins. A subset of three bacteriochlorins with high predicted Δd was then synthesized, and their Δd values were determined empirically via a solvatochromic method. Empirical Δd values confirmed the general trend in the substituent effect that was predicted computationally.

Methods

Computational Methods

The CAM-B3LYP hybrid functional55 and 6-31+g(d,p) basis set56 were chosen due to their high performance for porphyrin/hydroporphyrin dyes57,58 and sufficient accuracy for intramolecular charge transfer states.59 Ground-state geometry was optimized in vacuum, toluene, and water. Using the ground-state geometry, a vertical excitation was performed to probe Qy (S0–S1) excited state. μ and Δd were determined using vertical excited state results. The Δd parameter was determined by taking the vector difference between de (of the first excited state (Qy)) and dg, using the following equations60:3

4

where dgi and dei refer to the ith Cartesian component of the static dipole at the ground and excited states, respectively. dg, de, θg,e, and ζ were also reported.

To estimate the solvent effects in water and toluene, implicit solvation using the integral equation formalism polarizable continuum model (IEFPCM)61 was applied to ground-state geometry optimizations and excited-state single-point calculations. Nonequilibrium solvation conditions were applied for all solvated TD-DFT calculations to simulate excitation in the solvents of interest.

All DFT and TD-DFT calculations were performed in the Gaussian 16 software package.62 Input structures were prepared using the GaussView GUI.63 Vectoral representations of static dipoles (Figure 3) were generated using the ChimeraX molecular visualization software.64,65

Experimental Methods

General Synthesis Section

1H NMR (600 MHz) and 13C NMR (150 MHz) spectra were collected at room temperature in CDCl3. Chemical shifts (δ) were calibrated using a solvent residual proton signal at 7.26 ppm (1H NMR) and a solvent residual carbon signal at 77.23 ppm (13C NMR). All solvents and commercially available reagents were used as received. Palladium coupling reactions were performed by using commercially available anhydrous N,N-dimethylformamide. Known compounds 1(51) and 2(4) were synthesized following reported procedures. Initial absorption was undertaken as follows. UV–vis spectra were recorded at room temperature on a single-beam Cary 5000 UV–vis–NIR spectrophotometer (Agilent Technologies) in a quartz cuvette with a path length of 0.4 cm (Starna). Absorption spectra were collected over a 230–900 nm wavelength range with a 1.0 nm data interval. The fluorescence measurements are described further below.

5-Methoxy-8,8,18,18-tetramethyl-3,13-di[(4-nitrophenyl)ethynyl]bacteriochlorin (N3,13-BC)

Following a reported procedure with modifications,54 a mixture of 1(51) (15.0 mg, 0.027 mmol), 1-ethynyl-4-nitrobenzene (14.6 mg, 0.099 mmol), N,N-dimethylformamide (5.4 mL), and triethylamine (2.7 mL) in a 25 mL Schlenk flask was degassed via three freeze–pump–thaw cycles and allowed to warm to room temperature under argon. Bis(triphenylphosphine)palladium dichloride (1.84 mg, 0.0026 mmol) was added, and the reaction mixture was stirred at 80 °C under argon. After 5 h, the mixture was cooled to room temperature and diluted with ethyl acetate (35 mL). The organic solution was washed with water (40 mL) and brine (40 mL) and dried (anhydrous Na2SO4). After the solvent was removed using a rotatory evaporator, the crude product was dried under high vacuum and purified by flash column chromatography (silica, 3:1 dichloromethane/n-hexanes, then dichloromethane). The resulting solid was treated with n-hexanes (1 mL) and sonicated. The supernatant was decanted to afford N3,13-BC as a red solid (6 mg, 32%). The monosubstituted bacteriochlorin 2(4) was also isolated [8 mg, 44%]. 1H NMR (CDCl3, 600 MHz) δ, ppm: −1.57 (bs, 1H), −1.37 (bs, 1H), 1.958 (s, 6H), 1.963 (s, 6H), 4.43 (s, 4H), 4.48 (s, 3H), 7.97 (d, J = 8.7 Hz, 2H), 8.01 (d, J = 8.7 Hz, 2H), 8.36 (d, J = 8.7 Hz, 2H), 8.38 (d, J = 8.7 Hz, 2H) 8.54 (s, 1H), 8.56 (s, 1H), 8.82 (d, J = 2.1 Hz, 1H), 8.84 (d, J = 1.7 Hz, 1H), 8.87 (s, 1H); 13C NMR (CDCl3, 150 MHz) δ, ppm: 31.1, 31.2, 45.8, 45.9, 47.8, 52.1, 64.7, 90.3, 92.2, 93.5, 95.1, 96.7, 97.7, 98.2, 111.5, 115.8, 124.1, 125.2, 126.1, 130.7, 131.6, 132.0, 132.4, 132.6, 134.9, 135.9, 136.2, 138.6, 147.1, 147.4, 155.8, 162.0, 170.7, 171.2; λabs (toluene) 362, 537, 767 nm; λem (toluene, λex 535 nm) 775 nm; HRMS (ESI/Q-TOF) m/z: [M+] calcd for C41H34N6O5 690.2591; found: 690.2565.

5-Methoxy-8,8,18,18-tetramethyl-3-[(4-(N,N-dimethylamino)phenyl)ethynyl]-13-[(4-nitrophenyl)ethynyl]bacteriochlorin (D3N13-BC)

Following a reported procedure66 with modifications, a mixture of 2(4) (11.0 mg, 0.018 mmol), 1-ethynyl-4-(N,N-dimethyl)aniline (5.1 mg, 0.035 mmol), N,N-dimethylformamide (3.5 mL), and triethylamine (1.8 mL) in a 25 mL Schlenk flask was degassed via three “freeze–pump–thaw” cycles and allowed to warm to room temperature under argon. Bis(triphenylphosphine)palladium dichloride (1.2 mg, 0.0017 mmol) was added, and the reaction mixture was stirred at 80 °C under argon. After 22 h, the reaction mixture was cooled down to room temperature and diluted with ethyl acetate (50 mL). The organic solution was washed with water (50 mL) and brine (45 mL), dried (anhydrous Na2SO4), and concentrated using a rotary evaporator. The resulting crude product was dried under high vacuum and purified by flash column chromatography (silica, 1:1 dichloromethane/n-hexanes, then dichloromethane) to afford D3N13-BC as a red solid (4 mg, 33%). 1H NMR (CDCl3, 600 MHz) δ, ppm: −1.85 (bs, 1H), −1.49 (bs, 1H), 1.96 (s, 12H), 3.08 (s, 6H), 4.43 (s, 2H), 4.46 (s, 2H), 4.51 (s, 3H), 6.83 (d, J = 8.0 Hz, 2H), 7.75 (d, J = 8.8 Hz, 2H), 7.99 (d, J = 8.8 Hz, 2H), 8.36 (d, J = 8.8 Hz, 2H), 8.52 (s, 1H), 8.58 (s, 1H), 8.79 (d, J = 2.2 Hz, 1H), 8.80 (d, J = 2.2 Hz, 1H), 8.89 (s, 1H); 13C NMR (CDCl3, 150 MHz) δ, ppm: 31.1, 31.2, 40.6, 45.3, 46.0, 46.3, 48.4, 51.5, 64.9, 85.3, 91.3, 94.5, 96.3, 96.4, 96.6, 98.4, 112.3, 113.4, 116.0, 124.0, 124.1, 125.7, 131.1, 132.4, 132.5, 133.2, 134.4, 135.8, 136.4, 137.6, 147.2, 157.3, 160.2, 169.2, 171.8; λabs (toluene) 373, 538, 768 nm; λem (toluene, λex 538 nm) 776 nm; HRMS (ESI/Q-TOF) m/z: [M+] calcd for C43H40N6O3 688.3162; found 688.3137.

5-Methoxy-8,8,18,18-tetramethyl-3,13-di[(4-(N,N-dimethylamino)phenyl)ethynyl]bacteriochlorin (D3,13-BC)

Following a reported procedure66 with modifications, a mixture of 1(51) (20.0 mg, 0.036 mmol), 4-ethynyl-N,N-dimethylaniline (19.2 mg, 0.132 mmol), N,N-dimethylformamide (7.2 mL), and triethylamine (3.6 mL) in a 25 mL Schlenk flask was degassed via three “freeze–pump–thaw”cycles and allowed to warm to room temperature under argon. Bis(triphenylphosphine)palladium dichloride (2.5 mg, 0.0036 mmol) was added, and the reaction mixture was stirred at 80 °C under argon. After 16 h, the mixture was cooled down to room temperature and diluted with ethyl acetate (50 mL). The organic solution was washed with water (50 mL) and brine (50 mL) and dried (anhydrous Na2SO4). The solvent was removed by using a rotatory evaporator. The crude was dried under high vacuum and purified by column chromatography (silica, 2:1 dichloromethane/n-hexanes, then dichloromethane). The resulting solid was treated with methanol (1 mL), and the mixture was sonicated. The supernatant was decanted to afford a red solid (14 mg, 56%). 1H NMR (CDCl3, 600 MHz) δ, ppm: −1.74 (bs, 1H), −1.53 (bs, 1H), 1.94 (s, 6H), 1.95 (s, 6H), 3.07 (s, 6H), 3.08 (s, 6H), 4.41 (s, 2H), 4.43 (s, 2H), 4.50 (s, 3H), 6.81–6.84 (m, 4H), 7.74–7.77 (m, 4H), 8.50 (s, 1H), 8.51 (s, 1H), 8.71–8.73 (m, 2H), 8.91 (s, 1H); 13C NMR (CDCl3, 150 MHz) δ, ppm: 11.3, 14.2, 23.2, 24.2, 29.2, 30.8, 31.0, 31.2, 39.1, 40.48, 40.51, 45.7, 45.8, 47.7, 52.0, 64.6, 68.0, 82.8, 85.6, 95.1, 96.4, 96.9, 97.2, 98.1, 110.5, 111.5, 112.18, 112.24, 113.6, 118.0, 123.8, 124.3, 129.7, 131.4, 133.0, 133.2, 134.6, 135.7, 136.2, 138.5, 150.3, 150.5, 154.6, 161.1, 166.2, 169.6, 170.2; λabs (toluene) 367, 388, 527, 762 nm; λem (λex = 527 nm, toluene) 769 nm; HRMS (ESI/Q-TOF) m/z: [M+] calcd for C45H46N6O 686.3733, found 686.3727.

Steady-State Absorption and Fluorescence Spectroscopy

The procedure to measure the steady-state absorption and fluorescence spectra of N3,13-BC, D3,13-BC, and D3N13-BC in toluene, dichloromethane (CH2Cl2), acetone, diethyl ether (Et2O), 1,4-dioxane, tetrahydrofuran (THF), chloroform, dimethyl sulfoxide (DMSO), and dimethylformamide (DMF) to extract the Stokes shift was done as follows. Each sample was freshly prepared to obtain the desired absorbance of ∼0.1 in the vicinity of the Qy transition, which corresponds to a concentration of ∼1 μM. All absorption and fluorescence measurements were carried out in the same 1 cm path length quartz cuvette (Starna Cells, Atascadero, CA, USA), with the absorption spectrum measured first and the fluorescence spectrum subsequently measured. The absorption measurements were performed for all 3 dyes in each of the solvents listed above using the same instrument parameters for each dye in all solvents. For steady-state absorption measurements, a Cary 5000 UV–vis-NIR spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) was used. Each absorption spectrum was measured from 700 to 850 nm with a 0.2 nm data interval, 2 nm spectral bandwidth, and 0.2 s averaging time. The wavelength accuracy of the Cary 5000 spectrometer for UV–vis was ±0.08 nm (manufacturer specification). For steady-state fluorescence measurements, a Horiba Fluorolog-QM spectrophotometer (Horiba Scientific, Edison, NJ, USA) was used. The solutions containing N3,13-BC, D3,13-BC, and D3N13-BC were excited at 535, 527, and 538 nm, respectively. Each fluorescence spectrum was collected from 720 to 820 nm with 0.2 nm data interval and 2 nm bandpass resulting in 1.5 and 0.7 mm slit widths for excitation and emission, respectively. The wavelength accuracy of the Horiba Fluorolog-QM spectrophotometer was ±0.3 nm (manufacturer specification). Each fluorescence scan was integrated for 0.2 s with the exception of the D3,13-BC and D3N13-BC dyes in CH2Cl2, DMF, and DMSO, which was integrated for 0.8 s. The fluorescence spectra were corrected for the wavelength dependence of the detection system response using the correction curve provided by the manufacturer. The absorption and fluorescence peak maxima were obtained in OriginPro 2021 by using a PeakAnalyzer.

Results and Discussion

Computational Results

To examine the effects of substituent types and substitution patterns on μ, Δd, and ζ, a DFT and TD-DFT survey was performed on a set of bacteriochlorins progressively functionalized with phenyl, ethynyl, and phenylethynyl substituents (Figure 2). The phenylethynyl substituents were chosen for their abilities to form a strong conjugation with the macrocycle4,67 and incorporate electron-donating or electron-withdrawing groups, such as nitro and dimethylamino groups. These substituents were placed at positions 3 and 13 along the molecular y-axis aligned with the Qy transition. Two such bacteriochlorins (D3N13-BC and D3N13-HBC) were designed as “push-pull” dyes with opposing electron-donating dimethylamino and withdrawing nitro groups. An unsubstituted macrocycle (HBC) and a macrocycle with a methoxy group at the 5-position (BC) were utilized as benchmarks. To further examine the influence of a substituent position, two dyes with the 3,13,15-substitution pattern were investigated. All surveyed bacteriochlorins could be, in principle, accessed synthetically via established synthetic methodologies.51−54

Figure 2 Chemical structures of bacteriochlorins evaluated by DFT and TD-DFT for μ, Δd, and ζ. Bacteriochlorins are categorized by a substitution pattern. Electron-donating dimethylamino groups are colored orange, and electron-withdrawing nitro groups are depicted in blue. Carbons are numbered for HBC and apply to all structures.

To estimate solvent effects, all calculations were performed in vacuum and toluene. Supplementary calculations in water were also performed (Tables S1–S3). Solvation was modeled implicitly using the integral equation formalism polarizable continuum model (IEFPCM).61 Ground-state geometry was optimized in vacuum, toluene, and water, although no significant differences in atomic positions were observed. The vertical transition wavelength, λvert, was determined to verify the calculations. In toluene, λvert of HBC was 645 nm, which was less than the reported experimental value of 713 nm (Table 1).66 Such underestimation of the vertical transition energy is a well-known limitation of TD-DFT.68

Table 1 DFT-Calculated Values of Vertical Excitation Energy and μ for the First Singlet Excited State (Corresponding to the Qy Transition) in the Computationally Screened Bacteriochlorinsa

substitution pattern	dye	λvert-vac (nm)	λvert-toluene (nm)	μvac (D)	μtoluene (D)	
unsubstituted (controls)	HBC	645	669	5.77	7.32	
BC	610	620	5.61	7.02	
3,13	E3,13-BC	634	646	7.15	8.72	
Ph3E13-BC	654	650	7.24	8.61	
P3,13-BC	651	663	9.40	10.9	
D3N13-BC	647	661	10.5	12.3	
D3N13-HBC	629	643	10.6	12.2	
E3N13-BC	686	699	8.95	10.6	
N3,13-BC	663	677	10.4	12.0	
N3,13-HBC	712	725	10.7	12.3	
D3,13-BC	654	668	10.4	11.9	
D3,13-HBC	705	723	10.6	12.2	
3,13,15	E3,13D15-BC	647	713	8.12	10.0	
E3,13N15-BC	682	669	8.11	9.96	
a All calculations were performed in vacuum and implicit toluene using the IEFPCM61 method.

First, we assessed the impact of the bacteriochlorin substituents on μ. A benchmark HBC was investigated first. The calculated magnitude of μ associated with the Qy transition for HBC was 5.77 D in vacuum (Table 1). In toluene, however, the μ of HBC was higher, with a magnitude of 7.32 D. Next, we evaluated the influence of substituents on μ. All substituents, with the exception of the 5-methoxy group, increased μ compared to that of HBC. Uniquely, the 5-methoxy was the only group that decreased μ (by 0.2–0.3 D), which was evident in the following comparisons. Varying only in the presence of the 5-methoxy group, dyes HBC and BC exhibited μ values (in vacuum) of 5.77 and 5.61 D, respectively. Another dye pair, N3,13-HBC and N3,13-BC, which also differ only in the presence of the 5-methoxy group, exhibited μ magnitudes in vacuum of 10.7 and 10.4 D, respectively. The same pattern in μ magnitudes was found when comparing the pairs D3,13-BC/D3,13-HBC and D3N13-BC/ D3N13-HBC, which suggests that the 5-methoxy group slightly reduces μ. The largest μ was calculated for N3,13-HBC, with values of 10.7 D under vacuum and 12.3 D in toluene.

Next, we proceeded to characterize Δd of benchmark HBC. In vacuum, Δd was near zero (0.018 D), as expected due to structural symmetry. However, in implicit toluene, the Δd value of HBC increased to 2.07 D (Table 2). Since the solvent-optimized geometries did not significantly differ from those in vacuum, increased ground- and excited-state static dipole moments were likely caused by the polarization of the electron density rather than differences in atomic positions. Notably, the magnitude of the static electric dipole in the excited state (de) was slightly smaller than the magnitude of the static electric dipole in the ground state (dg). Electrostatic potential surfaces of HBC showed a marginally more negative charge on the pyrrole ring in the excited state compared to the ground state (Figures S1 and S2).

Table 2 DFT-Calculated dg, de, and Δd in the Computationally Screened Bacteriochlorinsa

substitution pattern	dye	dg-vac (D)b	de-vac (D)c	θg,e °d	Δdvac (D)e	Δdtoluene (D)f	
unsubstituted (controls)	HBC	0.018	0.028	16	0.012	2.07	
BC	2.05	1.32	84	2.32	4.16	
3,13	E3,13-BC	2.38	1.53	106	3.16	5.56	
Ph3E13-BC	2.56	1.06	52	2.09	5.14	
P3,13-BC	3.03	1.85	124	4.35	6.96	
D3N13-BC	14.7	10.9	7	4.07	5.78	
D3N13-HBC	15.4	11.0	9	4.83	6.41	
E3N13-BC	7.31	6.80	3	0.597	5.87	
N3,13-BC	3.53	2.27	113	4.90	7.73	
N3,13-HBC	0.167	0.187	7	0.029	5.55	
D3,13-BC	3.41	1.90	131	4.87	7.47	
D3,13-HBC	0.064	0.061	19	0.020	4.92	
3,13,15	E3,13D15-BC	5.43	2.99	38	3.58	4.65	
E3,13N15-BC	7.10	9.02	10	2.34	5.71	
a All calculations were performed in vacuum and implicitly solvated in toluene using the IEFPCM method. All reported values correspond to the Qy transition.

b Ground-state static electric dipole magnitude in vacuum.

c First excited state static electric dipole magnitude in vacuum.

d Angle between ground and excited state static electric dipoles in vacuum.

e Difference static dipole in vacuum.

f Difference static dipole in toluene.

After establishing the properties of the unsubstituted symmetric macrocycle HBC, we examined the asymmetric effect of the 5-methoxy group on Δd. The motivation to examine the effect of the 5-MeO group arose from its utility in synthetic modifications of de novo bacteriochlorins. The 5-MeO group can be introduced during the self-condensation of dihydrodipyrrin–methoxyacetal into a bacteriochlorin catalyzed by the Lewis acid trimethylsilyl triflate. Further, the 5-MeO group was found to be useful for the synthetic regioselective modifications of the bacteriochlorins as Sonogashira reactions54 and bromination.69 When 5-methoxy was added to the unsubstituted macrocycle (BC), introducing macrocycle asymmetry, Δd increased by ∼2 D both in vacuum and toluene (Table 2). Although the difference between the static dipole magnitudes in the ground and excited states was less than 1 D, the angle between de and dg (θg,e) was 84°, resulting in an appreciable Δd of 2.32 D (Figure 3). Relative directions and magnitudes of de and dg are visualized in Figure 3. As we discuss further, a similar large θg,e occurs in other 5-MeO substituted bacteriochlorins.

Figure 3 Vector representations of ground-state static dipoles (dg, purple) and first excited-state static dipoles (de, orange) for the investigated bacteriochlorins. Bacteriochlorins HBC, N3,13-HBC, and D3,13-HBC had near-zero dg and de magnitudes, and therefore, dg and de are represented by a sphere dye center.

Next, we examined the influence of substituents on Δd for the bacteriochlorins substituted at the 3,13 positions. Namely, the following series of bacteriochlorins were computationally characterized: E3,13-BC, P3,13-BC, N3,13-BC, D3,13-BC, and D3N13-BC. This series progressively builds on the previous dye, probing the effects of the constituents of 3,13 phenylethynyl-type substituents. First, the effects of the 3,13-ethynyl groups (in E3,13-BC) were evaluated. The dye E3,13-BC had a Δd of 3.16 D in vacuum and 5.56 D in toluene, which is an increase of 0.84 D in vacuum and 1.40 D in toluene compared to the benchmark BC without these substituents. Symmetric disubstitution at the 3 and 13 positions with phenylethynyl substituents resulted in P3,13-BC, where Δd was further increased to 4.35 D in vacuum and 6.96 D in toluene. Next, two electron-withdrawing nitro- or two electron-donating dimethylamino groups were added to the phenylethynyl moieties, resulting in D3,13-BC and N3,13-BC, respectively. In addition, the asymmetric substitution of P3,13-BC with nitro and dimethylamino groups afforded the “push-pull” bacteriochlorin D3N13-BC. Symmetrically substituted D3,13-BC and N3,13-BC exhibited the highest Δd values (7.47 and 7.73 D, respectively, in toluene) among all of the investigated bacteriochlorins. The appreciable Δd in 5-methoxy bacteriochlorins with the symmetric 3,13-substituents (D3,13-BC, N3,13-BC, E3,13-BC, and P3,13-BC) was due to a large difference in direction between the ground- and excited-state dipoles (θg,e > 100°), as illustrated in Figure 3. As expected, symmetrically substituted bacteriochlorins D3,13-HBC and N3,13-HBC without the 5-MeO substituent lacked static dipoles in both ground and excited states due to their full structural symmetry. Interestingly, the asymmetric “push-pull” bacteriochlorins D3N13-BC and D3N13-HBC afforded a smaller Δd than the symmetrically substituted 5-methoxy bacteriochlorins D3,13-BC and N3,13-BC. This is because neither static dipole magnitudes nor static dipole directions in D3N13-BC and D3N13-HBC changed significantly upon excitation. In contrast to the symmetric substitution of 5-methoxy bacteriochlorins, the asymmetry of the “push-pull” structure defined the same direction of both ground- and excited-state dipoles as quantified with θg,e angles of 7° and 9° in vacuum for D3N13-BC and D3N13-HBC, respectively. Thus, the asymmetric “push-pull” structures of D3N13-BC and D3N13-HBC, while providing larger static dipole magnitudes, did not provide an advantageous increase in Δd.

Two additional 3,13-substituted 5-methoxybacteriochlorins, Ph3E13-BC and E3N13-BC, were computationally examined to further investigate the effects of the asymmetric 3,13-substitution. The magnitudes of Δd for Ph3E13-BC (2.09 D in vacuum and 5.14 D in toluene) were smaller than those for symmetrically substituted E3,13-BC. Notably, the Δd magnitude of Ph3E13-BC in vacuum was less than that of the benchmark 5-methoxybacterioclorin (BC). Similarly, E3N13-BC had a Δd of 0.597 D under vacuum and 5.87 D in toluene. The comparison between Ph3E13-BC and E3N13-BC suggests that dye asymmetry is not a strong determining factor for a high Δd.

The effect of substitution at the 15 position was probed with E3,13D15-BC and E3,13N15-BC. These 3,13,15-substituted bacteriochlorins can be compared to E3,13-BC, which contains the same 3,13 substituents but is unsubstituted at the 15 position. The dyes E3,13D15-BC and E3,13N15-BC have Δd magnitudes of 3.58 and 2.34 D, respectively, in vacuum. The dyes E3,13D15-BC and E3,13N15-BC have similar Δd values to those of E3,13-BC, despite larger ground- and exited-state dipoles. Unlike E3,13-BC (θg,e = 106°), E3,13D15-BC and E3,13D15-BC have small θg,e values (38° and 10°, respectively). This decrease in θg,e is likely caused by increased molecular asymmetry.

We predict that the angle between Δd and μ (i.e., ζ) will be a significant factor in the design of exciton-based quantum information systems.28,34,35 Both parallel (ζ ≈ 0°) and perpendicular (ζ ≈ 90°) relative orientations of Δd and μ are advantageous for various QIS applications. With the exception of N3,13-HBC and HBC, Δd and μ remained largely parallel (0.6° < ζ < 12°) in the surveyed bacteriochlorins (Table S3). In the case of N3,13-HBC and HBC,ζ exceeded 12° in vacuum, although Δd was very small (<0.2 D), and thus, the direction was unreliable. Thereof, substitution at the 3, 13- or 15-positions does not appear to affect ζ. Future identification of bacteriochlorins with nonparallel Δd and μ vectors should entail a larger range of substitution positions.

Experimental Results

Three bacteriochlorins, N3,13-BC, D3,13-BC, and D3N13-BC, with the predicted highest μ and Δd, were synthesized and characterized. The synthesis of N3,13-BC, D3,13-BC, and D3N13-BC relied on the 3,13-dibromo-5-methoxy bacteriochlorin building block 1(51) via the Pd-mediated Sonogashira coupling under previously developed conditions.54,70 First, the treatment of bacteriochlorin 1 with 1-ethynyl-4-nitrobenzene catalyzed by (PPh3)2PdCl2 in triethylamine/dimethylformamide at 80 °C afforded N3,13-BC in 32% yield. Similarly, 1 was treated with 4-ethynyl-N,N-dimethylaniline to afford D3,13-BC in a 56% yield. For the synthesis of D3N13-BC, 1 was converted into 2 in 56% yield as previously described in ref (4). Then, 2 was treated with 1-ethynyl-4-(N,N-dimethyl)aniline in the presence of (PPh3)2PdCl2 in triethylamine/dimethylformamide at 80 °C to afford asymmetrically substituted D3N13-BC in 33% yield (Figure 4). Newly synthesized compounds N3,13-BC, D3,13-BC, and D3N13-BC were characterized by 1H NMR, 13C NMR, HRMS, steady-state absorption, and fluorescence spectroscopies (Figures S3–S10 and Tables S4–S6).

Figure 4 Synthetic route to bacteriochlorins N3,13-BC, D3,13-BC, and D3N13-BC.

The spectral properties of the synthesized N3,13-BC, D3,13-BC, and D3N13-BC were first characterized by steady-state absorption and fluorescence emission spectroscopy in toluene (Figure 5). The bacteriochlorins exhibited four characteristic absorption bands49: By (Soret), Bx (Soret), Qx, and Qy. The dyes D3N13-BC and N3,13-BC exhibited similar absorption profiles. Namely, in D3N13-BC and N3,13-BC, the Qy(0;0) absorption peak was located at 769 and 768 nm, respectively, while the Qx peak was located at 538 and 537 nm. In contrast, the Qy and Qx absorption peaks of D3,13-BC were blue-shifted, peaking at 762 and 527 nm, respectively. The magnitudes of the Qy(0;0) transition dipoles were qualitatively compared using an established method of calculating the ratio of the B to the Qy(0;0) band areas52,66,71 (Table 3). This method is based on the assumption that the magnitude of the B transitions does not change upon macrocycle substitution; hence, the ratio of the Qy(0;0) area to the B(By,x) area (i.e., ∑Qy/∑B) is directly proportional to the Qy(0;0) transition dipole. The ratio of areas (∑Qy/∑B) was the largest in D3N13-BC, with a value of 1.59. The ratios of areas for D3E13-BC and N3,13-BC were close in value, which is consistent with the results obtained from TD-DFT calculations in toluene. Fluorescence emission was measured at wavelengths longer than the Qy(0;0) absorption band and peaked at 775 nm in both D3N13-BC and N3,13-BC. The observed fluorescence emission is attributed to the Qy transition; the observed values in absorption and fluorescence result in Stokes shifts of 101 cm–1 (6.0 nm) and 135 cm–1 (7.0 nm) for D3N13-BC and N3,13-BC, respectively. The D3,13-BC exhibited fluorescence at 769 nm with the Stokes shift of 119 cm–1 (7.0 nm). Observed small Stokes shifts are indicative of minimal energy loss, that is, structural relaxation and solvent reorganization, following photoexcitation.

Figure 5 Acquired steady-state absorption (a) and fluorescence (b) in toluene with 1.0 nm data interval at room temperature of D3,13-BC, N3,13-BC, and D3N13-BC. Absorbance and fluorescence were normalized at the Qy(0;0) peak maximum. The inset in (a) shows a magnification of the Qy region.

Table 3 Experimental Optical Properties of Bacteriochlorins in Toluenea

dye	λabs By,Bx/nm	λabs Qx/nm	λabs Qy/nm	λem Qy/nm	∑Qy/∑Bb	υ̅a – υ̅f nm (cm–1)	Δdexpc (D)	
D3N13-BC	373	538	768	776	1.30	6.0 (101)	2.01	
N3,13-BC	362	537	767	775	1.35	7.0 (135)	2.82	
D3,13-BC	367, 388	527	762	769	1.59	7.0 (119)	2.68	
a At room temperature.

b Ratio of the intensities of the By(0,0) and Qy(0,0) bands. Ratio of the integrated intensities of the B (By and Bx; 320–420 nm) and Qy [Qy(0,0) and vibrational progression including Qy(1,0); 660–850 nm] bands.

c Value of difference static dipole (Δdexp), determined using a solvatochromic method.

To estimate Δd experimentally in D3,13-BC, N3,13-BC, and D3N13-BC, we chose to utilize a solvatochromic method that relates the changes of Stokes shifts to solvent polarity. The theory of the solvatochromic method is built on Onsager’s reaction field theory, where a dye molecule is represented by a nonpolarizable point dipole in a spherical cavity surrounded by a continuous dielectric medium.72,73 With the further assumption that no specific dye–solvent interactions (e.g., hydrogen bonding) take place, a linear relation between the Stokes shifts and the solvent polarity is derived, from which Δd can be extracted.72 A number of solvatochromic models, such as models of Lippert–Mataga72 and Bakhshiev,73 directly correlate Stokes shift with solvent polarity functions in terms of dielectric constant and refractive index. We, however, chose the Reichardt and Ravi approach74,75 for the following advantages. First, the Reichardt and Ravi approach utilizes an empirical solvent polarity scale, ET,30 based on the negative solvatochromism of the standard dye 2,6-diphenyl-4-(2,4,6- triphenylpyridinium-1-yl)phenolate (also known as Betaine 30). Thus, the ET30 scale is thought to reflect the solvent polarity specific to the electrostatic interactions between the solvent and solute static dipole moments.75 Second, in the Reichardt and Ravi approach, Δd of the target molecule is normalized to the changes in Δd of the referenced dye Betaine 30 in different solvents. Thus, this approach diminishes the error in the estimation of the Onsager radius, which is challenging to accurately estimate for ellipsoidal molecules like N3,13-BC, D3,13-BC, and D3N13-BC. According to the Reichardt and Ravi approach, Stokes shifts in various solvents fit the following equation76:5

where ET30 is the empirical solvent polarity scale based on the negative solvatochromism of the standard dye 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (also known as Betaine 30),74,75 ΔdB (9.0 D) and aB (6.2 Å) are the difference static dipole and Onsager cavity radius of reference Betaine 30,77 Δd is the change in dipole moment upon excitation, and a is the molecular radius of the investigated molecule. Hence, the Δd of the investigated molecule is determined as6

where m is the slope obtained from the linear plot of Stokes shift versus ET(30) and a is the Onsager cavity radius, in angstroms.

To determine experimental Δd using the Reichardt and Ravi approach, absorption and fluorescence of bacteriochlorins D3N13-BC, N3,13-BC, and D3,13-BC were recorded in 9 different solvents (toluene, dichloromethane, acetone, diethyl ether, 1,4-dioxane, tetrahydrofuran, chloroform, dimethyl sulfoxide [DMSO], and dimethylformamide [DMF]). The bacteriochlorins exhibited fluorescence in most solvents, with Stokes shifts in the 101–135 cm–1 range (Tables S5–S7). However, the fluorescence intensity was strongly diminished for bacteriochlorins with the nitrophenylethynyl substituents (D3N13-BC and N3,13-BC) in certain solvents. In particular, the fluorescence intensity significantly decreased for N3,13-BC in dichloromethane, DMF, and DMSO and that for D3N13-BC in dichloromethane, DMF, DMSO, chloroform, and acetone. Next, bacteriochlorin Stokes shifts were plotted against the empirical solvent parameter ET.30 For symmetrically substituted bacteriochlorins N3,13-BC and D3,13-BC, the Stokes shift dependence on the solvent polarity afforded strong (R2 = 0.840 and 0.769, respectively) linear correlations with slope values (m) of 142.3 and 128.6, respectively (Figure 6). In contrast, for asymmetric D3N13-BC, the Stokes shift dependence on the solvent polarity was not observed, indicating a potentially strong role of specific interactions between the solvent and the dye. After the exclusion of abnormally small Stokes shifts in CH2Cl2 and DMSO to improve the fit to R2 = 0.521, the m value for D3N13-BC was 72.44. The Δd values were determined according to eq 6. The a values were calculated as half a distance between opposing nitrogen atoms of nitro or dimethylamino groups in the DFT-optimized vacuum ground-state structures. The resulting a value of 12.30 Å was the same for D3N13-BC, N3,13-BC, and D3,13-BC. Consequently, the Δd was determined to be 2.01, 2.82, and 2.68 D in D3N13-BC, N3,13-BC, and D3,13-BC, respectively. The relative increase in the experimental Δd in the order of D3N13-BC < D3,13-BC < N3,13-BC agreed with the computational results.

Figure 6 Bacteriochlorin Stokes shift versus empirical solvent polarity in ET30 scale for (a) N3,13-BC, (b) D3,13-BC, and (c) D3N13-BC. In plot (c), the stokes shifts of D3N3-BC in CH2Cl2 and DMSO were excluded from the linear fit.

Conclusions

We studied the influence of bacteriochlorin substituents on Δd, μ, and ζ requisite parameters for creating quantum information dye aggregate systems. A total of 14 bacteriochlorins, substituted at the 3,13 and 3,13,15 position(s), were investigated via DFT and TD-DFT, and several trends of the substituent effects were observed. The vector μ remained nearly parallel to Δd (i.e., small ζ) in all of the bacteriochlorins. The 5-methoxy group had an opposing effect on Δd and μ. Even though the presence of the 5-methoxy group slightly decreased μ, it increased Δd compared to that of the bacteriochlorins lacking this group. As the conjugated system of 5-methoxy bacteriochlorins was progressively extended with the symmetric 3,13-substituents, Δd increased. On the other hand, asymmetry in the substitution of 5-methoxy bacteriochlorins caused Δd to decrease. Notably, symmetrically substituted 5-methoxy bacteriochlorins (N3,13-BC and D3,13-BC) exhibited higher Δd than the comparable “push-pull” bacteriochlorin D3N13-BC, even though the latter had a higher magnitude of both excited and ground-state static dipoles. In the 5-methoxybacteriochlorins symmetrically substituted at the 3,13 positions, the angle between the ground and excited static dipoles was often >100°, resulting in a near inversion of the static dipole direction upon excitation. Despite smaller magnitudes of both dg and de, this large change in the orientation of dg and de was responsible for the generally larger Δd in symmetrically substituted 5-methoxybacteriochlorins compared with the asymmetric “push-pull” structure. Three bacteriochlorins (D3N13-BC, N3,13-BC, and D3,13-BC) with high predicted Δd were synthesized and characterized. The Δd of synthesized bacteriochlorins was experimentally estimated using a solvatochromic method, and the results exhibited a trend comparable to that of the computational results. These results provide a basic framework for the design of bacteriochlorins for QIS systems. Further investigation toward identifying bacteriochlorins with high ζ values will be beneficial.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.4c03821.Additional DFT and TD-DFT results, electrostatic potential maps, steady-state absorption and fluorescence spectroscopy of N3,13-BC, D3,13-BC, and D3N13-BC, and 1H and 13C NMR data (PDF)

Supplementary Material

jp4c03821_si_001.pdf

Author Present Address

∥ Brigham Young University, Rexburg, Idaho 83460, United States

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Acknowledgments

This research was supported wholly by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Materials Sciences and Engineering Division, and DOE’s Established Program to Stimulate Competitive Research (EPSCoR) (Award DE-SC0020089), except for the following. This research also made use of the resources of the High-Performance Computing Center at Idaho National Laboratory, which is supported by the Office of Nuclear Energy of the DOE and Nuclear Science User Facilities under Contract No. DE-AC07-05ID14517. Molecular graphics and analyses performed with UCSF ChimeraX, developed by the Resource for Biocomputing, Visualization, and Informatics at the University of California, San Francisco, with support from National Institutes of Health R01-GM129325 and the Office of Cyber Infrastructure and Computational Biology, National Institute of Allergy and Infectious Diseases.
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