A Computational and Spectroscopic Analysis of Î²-Indandione

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A: Spectroscopy, Photochemistry, and Excited States

A Computational and Spectroscopic Analysis of #-Indandione Modified Zinc Porphyrins Joseph I. Mapley, Pawel Wagner, David Leslie Officer, and Keith C. Gordon J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02746 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

A Computational and Spectroscopic Analysis of βIndandione Modified Zinc Porphyrins Joseph I. Mapley†, Pawel Wagner‡, David L. Officer‡, Keith C. Gordon*† †

‡

Department of Chemistry, University of Otago, PO Box 56, Dunedin, New Zealand

Intelligent Polymer Research Institute and ARC Centre of Excellence for Electromaterials Science, University of Wollongong, NSW 2522, Australia Author Contributions

Abstract

Porphyrins have characteristic optical properties which give them potential to be used in a range of applications. In this study a series of β-indandione modified zinc porphyrins, systematically changed in terms of linker length and substituent, resulted in dramatically different absorption spectra than is observed for the parent zinc porphyrin (ZnTXP - 5,10,15,20-tetrakis(3,5dimethylphenyl)porphyrinato zinc (II)). These changes include strong absorptions at 420, 541 and 681 nm (110.2, 57.5 and 29.2 mM−1 cm−1, respectively) for the most perturbed compound. Computational studies were conducted and showed the different optical effects are due to a reorganisation of molecular orbitals (MOs) away from Gouterman’s four orbital model. The substituent effects alter both unoccupied and occupied MOs. An increased length of linker group raised the energy of the HOMO-2 such that it plays a significant role in the observed transitions. The degenerate LUMO (eg) set are split by substitution and this splitting may be increased by use

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of a propylidenodinitrile group which shows the lowest energy transitions and the greatest spectral perturbation from the parent zinc porphyrin complex. These data are supported by resonance Raman spectroscopy studies which show distinct enhancement of phenyl modes for high energy transitions and indandione modes for lower energy transitions.

1. Introduction The optoelectronic properties of metalloporphyrins are of interest for a range of applications including photodynamic therapy,1 non–linear optics,2-3 organic light emitting diodes4 and dye sensitized solar cells5-7 (DSSCs). Perturbation of the characteristic UV/vis absorption profile of metalloporphyrins, to obtain greater spectral coverage, is of importance for their applications as sensitizers in DSSCs.6 The absorption profile of many metalloporphyrins arises from configuration interactions between energetically isolated frontier molecular orbitals (FMOs) which is explained by Gouterman’s four orbital model.8 The FMOs consist of two degenerate LUMOs, of eg symmetry, and the near degenerate HOMO and HOMO-1 of a2u and a1u symmetry respectively. An additive configuration interaction ((a2u → eg) + (a1u → eg)) results in a high energy, high intensity B band transition while a subtractive combination ((a2u → eg) - (a1u → eg)) produces the low energy, low intensity Q band transitions. Substitution of mettaloporphyrins at either β- or meso-pyrrolic positions lifts the degeneracy of the FMOs resulting in a broadening of the absorption spectra and a change in the B:Q intensity ratio.9-11 The use of an electron accepting substituent introduces the possibility for charge transfer (CT) type transitions which further perturbs the absorption spectra.11-15 In contrast to meso-substituted porphyrins, which are well studied,6,

16-20

β-substituted porphyrins are less

extensively investigated.


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Walsh et al. performed a computational study on a series of zinc complexes of tetraarylporphyrins (TPPs) substituted by cyanoacrylic acids with various conjugated linker lengths.11 One of the porphyrin LUMOs combined with the substituent LUMO resulting in two new mixed MOs with porphyrin and substituent character while the other porphyrin LUMO remained unperturbed by substitution. Mixing of transitions involving the now non degenerate LUMOs leads to a broadening of the absorption bands. Mixing of low lying porphyrin MOs with substituent MOs produced a new HOMO-2 which approaches the energy of the near degenerate HOMOs. The new LUMOs were calculated to decrease in energy with increased conjugation length while the new HOMO increased in energy resulting in a redshift of the absorption spectra. Lind et al. studied a similar series of β-substituted porphyrins containing conjugated thiophene linker groups but the same cyanoacrylic acid electron withdrawing group.14 The calculated MOs of this series is analogous to the series studied by Walsh et al.11 with mixing of a porphyrin LUMO with a substituent MO while the other porphyrin LUMO is unaltered. This is observed experimentally with a broadening and redshifting of the B and Q bands. Additional transitions were also observed between the B and Q bands and were assigned as CT in character. The effects of various electron withdrawing β-substituents have also been investigated.12-13, 15 Earles et al. examined a range of different electron withdrawing groups connected to porphyrins via an alkyne linker.13 The absorption spectra of porphyrins were shown to be resilient to perturbation with only cyanoacrylic acid producing notable variations in the absorption profile. Incorporation of the cyanoacrylic acid withdrawing groups resulted in a new transition to the red of the B band analogous to a CT band found by Lind et al.14 Stronger electron withdrawing groups, propylidenodinitrile and 1,3-diethyl-2-thiobarbituric acid have been explored in NiTPP systems which display broadened B and Q bands as well as an


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intense CT band that redshifts with increasing linker length providing increased spectral coverage.12 However the effects of these strong electron withdrawing groups on the porphyrin MOs has not been determined. 1-3 Indandione (IND) is a strongly electron withdrawing unit and is often used in various dyes as terminal electron accepting end groups.21-22 IND is a stronger electron withdrawing group than cyanoacrylic acids which have been the subject of a number of studies in β-substituted porphyrins.11, 13-14, 23 This provides an opportunity to further examine how porphyrin MOs are perturbed by different substituents which will improve the rational design of porphyrin based systems. In this report the exploration of three β-substituted porphyrins (Figure 1) with computational and spectroscopic techniques, is described. ZnTPP-A and B explores the effect of altering the linker length while ZnTPP-C examines the addition of a propylidenodinitrile group to the substituent.

Figure 1: Structures of β-substituted porphyrins studied in this work. 2. Experimental 2.1 Synthesis. Syntheses of 2-formyl-5,10,15,20-tetrakis(3,5-dimethylphenyl)porphyrin and (E)-3-[5,10,15,20-terakis(3,5-dimethylphenyl)porphyrin-2-yl]2-propenal

were

described


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previously.24 The other chemicals were commercially available and used without further purification. NMR spectra were recorded on Bruker Avance 400 spectrometer. The following abbreviations were used: s = singlet, d = doublet, dd = doublets of doublets, br s –broader singlet, m = multiplet. All coupling constants J were measured in hertz (Hz). Chemical shifts are reported in parts-per-million (ppm). Tetramethylsilane was used as the internal reference. Mass spectra were recorded on Hewlett Packard 5973. The synthetic procedures were not optimised. All compounds were synthesized via Knoevenagel condensation of appropriate aldehyde and one of three CH-acids. The ‘free base’ porphyrins were converted to their zinc complexes in reaction with zinc acetate. The specific procedures are described below. 2-[5,10,15,20-tetrakis(3,5-dimethylphenyl)porphyrin-2-ylmethylidene]-1,3-indandione:

2-

formyl-5,10,15,20-tetrakis(3,5-dimethylphenyl)porphyrin (135 mg, 1.8 x 10-4 mol) and 1,3indandione (78 mg, 5.4 x 10-4 mol) were dissolved in THF (15 mL), DBU (3 drops) was added and the resulting mixture was stirred at room temperature overnight. Afterwards, the solvent was removed under vacuum at 50oC, the resulting solid was redissolved in minimal amount of dichloromethane,

filtered

through

pad

of

silica

then

recrystallized

from

dichloromethane/methanol mixture. Yield: 63%; 1H NMR (500 MHz, CDCl3, TMS): 9.81 d, 1H, J = 0.7 Hz, β3-pyrrolic), 8.99 (d, 1H, J = 5.0 Hz, β-pyrrolic), 8.93 (d, 1H, J = 5.0 Hz, β-pyrrolic), 8.88 (d, 1H, J = 5.0 Hz, βpyrrolic), 8.85 (d, 1H, J = 5.0 Hz, β-pyrrolic), 8.79 (d, 1H, J = 4.8 Hz, β-pyrrolic), 8.77 (d, 1H, J = 4.8 Hz, β-pyrrolic), 8.06 – 7.98 (m, 4H, Ind-H, Ar-H), 7.91 (d, 1H, J = 0.7 Hz, Methine-H), 7.85 – 7.76 (m, 8H, Ind-H, Ar-H), 7.46 – 7.34 (m, 4H, Ar-H), 2.70 (s, 6H, CH3), 2.61 (s, 6H, CH3), 2.60 (s, 6H, CH3), 2.45 (s, 6H, CH3), -2.48 (br s, 2H, NH); HRMS (ESI, (M+1)+): found: 883.4015, requires for C62H51N4O2: 883.4007.


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(E)-2-{3-[5,10,15,20-terakis(3,5-dimethylphenyl)porphyrin-2-yl]2-propenylidene}-1,3indandione: (E)-3-[5,10,15,20-terakis(3,5-dimethylphenyl)porphyrin-2-yl]2-propenal (78 mg, 1.0 x 10-4 mol) and 1,3-indandione (44 mg, 3.0 x 10-4 mol) were dissolved in THF (10 mL) and methanol (0.5 mL), DBU (3 drops) was added and the resulting mixture was stirred at room temperature overnight. Afterwards, the solvent was removed under vacuum at 50oC, the resulting solid was purified on silica using dichloromethane as an eluent then recrystallized from dichloromethane/methanol mixture. Yield: 37%; 1H NMR (500 MHz, CDCl3, TMS): 9.21 (d, 1H, J = 0.6 Hz, β3-pyrrolic), 8.89 (d, 1H, J = 4.8 Hz, β-pyrrolic), 8.56 – 8.82 (m, 3H, β-pyrrolic), 8.80 (d, 1H, J = 5.0 Hz, β-pyrrolic), 8.78 (d, 1H, J = 5 Hz, β-pyrrolic), 8.50 (dd, 1H, J = 11.9 and 14.9 Hz, Propen2-H), 8.04 – 7.96 (m, 2H, Ind-H), 7.86 – 7.74 (m, 12 H, Ind-H, Ar-H), 7.57 – 7.38 (m, 4H, Ar-H), 7.29 (dd, 1H, J = 0.6 and 11.9 Hz, Propen3-H), 6.95 (d, 1H, J = 14.9 Hz, Propen1-H), 2.66 (s, 3H, CH3), 2.60 (br s, 6H, CH3), 2.57 (s, 3H, CH3), -2.51 (br s, 2H, NH); HRMS (ESI, (M+1)+): found: 909.4171, requires for C64H53N4O2: 909.4163. (E)-2-{3-[5,10,15,20-terakis(3,5-dimethylphenyl)porphyrin-2-yl]2-propenylidene}-(3-oxo-2,3dihydro-1H-inden-1-ylidene)malononitrile:

(E)-3-[5,10,15,20-terakis(3,5-

dimethylphenyl)porphyrin-2-yl]2-propenal (78 mg, 1.0 x 10-4 mol) and 2-(3-oxo-2,3-dihydro1H-inden-1-ylidene)malononitrile (58 mg, 3.0 x 10-4 mol) were dissolved in THF (6.0 mL) and methanol (6.0 mL) and the resulting mixture was stirred at room temperature overnight. The product was precipitated directly from the reaction mixture by methanol, filtered off and dried. Yield: 83%; 1H NMR (500 MHz, CDCl3, TMS): 9.26 (d, 1H, J = 0.6 Hz, β3-pyrrolic), 8.97 – 8.88 (m, 2H, β-pyrrolic, Propen2-H), 8.87 – 8.81 (m, 3H, β-pyrrolic), 8.78 (d, 1H, J = 5.1 Hz, βpyrrolic), 8.76 (d, 1H, J = 5.1 Hz, β-pyrrolic), 8.75 – 8.71 (m, 1H, Ind-H), 8.18 (dd, 1H, J = 0.6


6

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and 11.6 Hz, Propen3-H), 7.98 – 7.94 (m, 1H, Ind-H), 7.88 – 7.73 (m, 10H, Ind-H, Ar-H), 7.58 – 7.38 (m, 4H, Ar-H), 6.98 (d, 1H, J = 14.9 Hz, Propen1-H), 2.66 (s, 6H, CH3), 2.60 (s, 12H, CH3), 2.57 (s, 6H, CH3), -2.43 (br s, 2H, NH); HRMS (ESI, (M+1)+): found: 957.4289, requires for C67H53N6O: 957.4273. General procedure for syntheses of zinc complexes: Porphyrin (1 x 10-4 mol) was dissolved in dichloromethane (15 mL) and solution of zinc acetate dihydrate (31 mg, 1.5 x 10-4 mol) in methanol (1 mL) was added. The resulting mixture was stirred at room temperature for 1 hour then filtered through pad of silica using dichloromethane as an eluent. The solvents were removed under vacuum at 50oC and the resulting solid was recrystallized from dichloromethane – methanol mixture. 2-[5,10,15,20-tetrakis(3,5-dimethylphenyl)porphyrinato-2-ylmethylidene]-1,3-indandione zinc (II) (ZnTPP-A) Yield 99%, 1H NMR (500 MHz, CDCl3, TMS): 9.75 (s, 1H, β3-pyrrolic), 8.95 (d, 1H, J = 4.7 Hz, β-pyrrolic), 8.94 – 8.90 (m, 4H, β-pyrrolic), 8.88 (d, 1H, J = 4.7 Hz, β-pyrrolic), 7.99 – 7.94 (m, 1H, Ind-H), 7.93 – 7.82 (m, 7H, Methine-H, Ar-H), 7.79 – 7.70 (m, 3H, Ind-H), 7.66 – 7.60 (m, 2H, Ar-H), 7.44 – 7.38 (m, 2H, Ar-H), 7.17 – 7.11 (br s, 1H, Ar-H), 2.67 (s, 6H, CH3), 2.61 (s, 6H, CH3), 2.60 (s, 6H, CH3), 2.26 (br s, 6H, CH3); HRMS (ESI, (M+1)+): found: 945.3153, requires for C62H49N4O2Zn: 945.3141. (E)-2-{3-[5,10,15,20-terakis(3,5-dimethylphenyl)porphyrinato-2-yl]2-propenylidene}-1,3indandione zinc (II) (ZnTPP-B). Yield 99%, 1H NMR (500 MHz, CDCl3, TMS): 9.34 (d, 1H, J = 0.7 Hz, β3-pyrrolic), 8.95 (d, 1H, J = 4.7 Hz, β-pyrrolic), 8.93 – 8.90 (m, 4H, β-pyrrolic), 8.89 (d, 1H, J = 4.7 Hz, β-pyrrolic), 8.50 (dd, 1H, J = 12.1 and 15.1 Hz, Propen2-H), 7.98 – 7.95 (m, 1H, Ind-H), 7.88 – 7.71 (m,


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12H, Ind-H, Ar-H), 7.54 – 7.37 (m, 4H, Ar-H), 7.22 (dd, 1H, J = 0.7 and 12.1 Hz, Propen3-H), 7.00 (d, 1H, J = 15.1 Hz, Propen1-H), 2.65 (s, 6H, CH3), 2.60 (s, 6H, CH3), 2.59 (s, 6H, CH3), 2.54 (s, 6H, CH3); HRMS (ESI, (M+1)+): found: 971.3310, requires for C64H51N4O2Zn: 971.3298. (E)-2-{3-[5,10,15,20-terakis(3,5-dimethylphenyl)porphyrinato-2-yl]2-propenylidene}-(3-oxo2,3-dihydro-1H-inden-1-ylidene)malononitrile zinc (II) (ZnTPP-C). Yield 99%, 1H NMR (500 MHz, CDCl3, TMS): 9.40 (s, 1H, β3-pyrrolic), 9.00 – 8.88 (m, 7H, β-pyrrolic, Propen2-H), 8.70 – 8.63 (m, 1H, Ind-H), 8.19 (d, 1H, J = 11.4 Hz, Propen3-H), 7.96 – 7.91 (m, 1H, Ind-H), 7.88 – 7.71 (m, 11H, Ind-H, Ar-H), 7.58 – 7.38 (m, 4H, Ar-H), 7.05 (d, 1H, J = 15.2 Hz, Propen1-H), 2.66 (s, 6H, CH3), 2.60 (s, 12H, CH3), 2.56 (s, 6H, CH3); HRMS (ESI, (M+1)+): found: 1019.3434, requires for C67H51N6OZn: 1019.3410. 2.2 Spectroscopic Measurements. Spectroscopic grade dichloromethane was used for all spectroscopic measurements. Spectral data was analysed and manipulated using GRAMS v9.2 (Thermo Fisher Scientific) and OriginPro v9.0 (OriginLab Corporation). Uv-vis spectra were recorded on a Lambda 950 UV-vis spectrophotometer (PerkinElmer, Waltham, MA, USA) at room temperature in a 1 cm quartz cuvette. A scan rate of 100 nm min−1 was employed between 340 and 800 nm. Extinction coefficients were determined in dichloromethane by measuring a series of samples between 1 × 10−5 and 1 × 10−6 mol L−1. FT-Raman spectra were measured in solution, using a Bruker Optics MultiRAM spectrometer (Bruker, Billerica, MA, USA) and a liquid-nitrogen-cooled Model D418T germanium detector. The system was controlled by Bruker Opus v7.5 software. A 1064 nm Nd:YAG laser was used with a power of 150 mW. Spectra were measured with 5000 scans and a spectral resolution of 4 cm−1. Sample concentrations were 5 × 10−3 mol L−1 (CH2Cl2).


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Resonance Raman spectra were collected using a setup described previously.25-26 In summary, the laser beam was focused on a spinning NMR tube in a 135° backscattering geometry with a 50 µm entrance slit. Excitation wavelengths of 351, 406, and 413 nm were obtained using a Kr-ion laser (Innova I-302, Coherent, Inc. Santa Clara, CA, USA). Excitation at 458 and 515 nm was provided by solid state diodes (Cobolt, Solna, Sweden) and 448 and 532 nm (CrystaLaser, Reno, NV, USA). Notch filters (Kaiser Optical Inc., Ann Arbor, MI, USA) or long-pass filters (Semrock, Inc., Rochester, NY, USA) matched to these wavelengths was used to remove the laser excitation line. The beam was dispersed using 1200 mm−1 grating onto a PyLoN 400BR CCD (Princeton Instruments, Trenton, NJ, USA), cooled with liquid nitrogen to -120°C; Winspec/32 software v2.5.23 was used to control the CCD equipment. Sample concentrations were typically 1 × 10−5 mol L−1 (CH2Cl2) and spectra were obtained at room temperature. Spectra were calibrated at each excitation wavelength, using reference peaks of a 1:1 mixture of toluene and acetonitrile to within a pixel. 2.3 Computational Methods. Geometry optimisations and vibrational calculations were performed using density functional theory (DFT) calculations with B3LYP27-28 or CAMB3LYP29 functionals. Both functionals employed the basis set 6-31G(d). These were implemented with Gaussian 09 D0.1 (Gaussian Inc, Wallingford, CT, USA).30 Scaling factors of 0.975 and 0.95 for B3LYP and CAM-B3LYP, respectively, were applied to calculated vibrational frequencies as recommended previously.31 The vibrational modes were visualised using Molden.32 Time-dependent DFT (TD-DFT) methods were implemented on the optimised structures, using both B3LYP and CAM-B3LYP functionals to approximate electronic transition energies


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and oscillator strengths for each compound investigated. Calculations were conducted with and without a solvent field implemented using a polarised continuum model.33 3. Results and Discussion 3.1 Ground State Geometry. Density functional theory (DFT) was used to find optimized geometry of all compounds implementing both B3LYP and CAM-B3LYP functionals. B3LYP, a hybrid functional (20% Hartree Fock), is typically better for modelling short range electron interactions27-28 and has previously been shown to provide an accurate model for porphyrin systems.11,

13-14

CAM-B3LYP, a range corrected hybrid functional (20-60% Hartree Fock),

performs well for long range interactions29 such as charge transfer processes that may be occurring in these systems and may provide a more accurate model. The geometry of the porphyrin core was relatively unaltered by substitution for all compounds with the largest change in bond length occurring at the substituted pyrrole of up to 2.6 pm compared to unsubstituted ZnTPP. The bond lengths for the rest of the porphyrin ring were altered by less than 0.75 pm. The substituents were all found to be planar and connected to the porphyrin at a slight angle of 21°, 23° and 26° for ZnTPP-A, B and C respectively. The angle that the β-substituent is attached may affect how well the π systems of the porphyrin and substituent can interact; however, β-substituted porphyrins with similar dihedral angles have shown extended conjugation across the porphyrin and substituent.17 The optimized geometries were validated by comparing calculated and measured Raman spectra. Overall structures calculated using the B3LYP functional provided better correlation to experimental spectra than CAM-B3LYP especially for vibrations between 1200 and 1300 cm-1. The mean absolute deviation (MAD) between experimental and theoretical peak frequencies of greater than 20% relative intensity was calculated for each compound. For B3LYP a MAD of 5,


10

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5 and 4 cm−1 were found for ZnTPP-A, B and C respectively which indicates a good correlation between experimental data and DFT calculations.13 Larger MADs of 9, 7 and 11 cm-1 were found for spectra calculated with CAM-B3LYP. 3.2 Electronic Absorption Spectroscopy. The UV-vis spectra of all compounds (Figure 2) are markedly different from that of unsubstituted porphyrin. Additional transitions are observed along with the Q and B bands, which is typical of porphyrins with strong electron withdrawing β-substituents.11, 13-15 A large decrease in B band intensity is observed as well as broadening of the band which is consistent with the predictions of Gouterman’s four orbital model due to splitting in energy of the a2u → eg and a1u → eg configurations.8-9 There is no shift in the energy of the B band for ZnTPP-A, however both ZnTPP-B and C are redshifted by 400 cm−1. This is consistent with predicted changes due the increased length of conjugation due to the longer linker of ZnTPP-B and C.11, 14 The lowest energy transition for all compounds also undergoes redshifting, but to a much greater extent than for the B band. ZnTPP-A and B are redshifted by 1400 cm−1. Decreasing the energy of the lowest energy transition must be due to a decrease in the HOMO-LUMO band gap and as the HOMO has been shown to remain relatively unperturbed by substitution11, 13-14 the LUMO must be heavily influenced by the substituent. In ZnTPP-C the transition is even further redshifted, alongside an increase in intensity, with a difference of 2600 cm−1 compared with the unsubstituted porphyrin, showing that the LUMO is further stabilised by the presence of the propylidenodinitrile group.


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Figure 2: UV-Vis spectra for ZnTPP, ZnTPP-A, B and C as labelled in CH2Cl2. ZnTPP shown at 0.3 times intensity in a) to aid comparison of spectra. Q band region showed in greater detail in b). An additional transition with considerable intensity is present at 482, 500 and 540 nm for ZnTPP-A, B and C, respectively. A significant redshift of this transition occurs with increasing conjugation length with an even greater redshift occurring with addition of the propylidenodinitrile group. This indicates that the substituent is heavily involved in this transition. Between this transition and the B band, another transition with lower intensity is present which shows a similar response across the series.


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3.3 Excited State Calculations. TD-DFT calculations were performed to model the excited states of all compounds. Porphyrin systems have been shown to be well modelled by the B3LYP functional.11, 13-14, 34 However this functional often perform poorly when estimating energy of CT transitions due to self-interaction errors.35 The CAM-B3LYP functional has been shown to perform well when modelling charge-transfer systems.36 TD-DFT calculations with both functionals were performed to develop the best model for each compound. Additionally, calculations were performed with and without a CH2Cl2 solvent field. CAM-B3LYP performed poorly for modelling these compounds with all transitions being overestimated (Figure S1 in the Supporting Information). B3LYP however performed very well with calculated transitions close to experimentally observed peaks indicating that the transitions have greater porphyrin character than CT. Application of the solvent field further improved correlation between calculated and experimental spectra. The higher energy transitions calculated for the B band are slightly overestimated and should be accounted for when interpreting the calculated spectra. As B3LYP provides a much better fit than CAM-B3LYP, the transitions of the compounds would be expected to have more π to π* character, similar to what is observed in unsubstituted porphyrins, rather than charge-transfer type transitions. Calculated electronic transitions for ZnTPP-B are shown in Table 1. ZnTPP-A and C show similar transitions to ZnTPP-B and are shown in the supplementary information (Table S1, S2). The lowest energy transitions can be described almost entirely by a single component; HOMO→LUMO and H-1→LUMO for the two lowest energy transitions. These transitions show CT character with a significant shift of electron density shifting from the porphyrin core to the linker and substituent.


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Table 1: Experimental absorption data and calculated electronic transitions for ZnTPP-B with B3LYP/6-31G(d) including a solvent field for CH2Cl2. Experimental

Calculated

λmax nm 632

ε

mM-1 cm-1 20.0

λ nm 639

0.15

568

11.2

593

0.35

499

63.1

497

0.66

495

0.49

ƒ

Mulliken charge density change (%) Configuration interactions H→L (92%)

459

41.1

448

0.66

420

76.2

401

0.76

395

0.20

393

0.17

388

0.16

384

0.22

H-1→L(81%), H→L+1 (18%) H-1→L+1 (25%), H→L+1 (23%), H→L+2 (33%) H-1→L+1 (15%), H→L+1 (33%), H→L+2 (24%) H-2→L (79%), H-1→L+2 (17%) H-1→L+1 (31%), H→L+2 (22%) H-8→L (19%), H-6→L (28%) H-8→L (16%), H-4→L (34%) H-11→L(25%), H-7→L (19%), H-2→L+1 (58%)

370

0.31

H-8→L (33%)

Core

Substituent

Linker

85→37 (-48)

1→23 (22)

3→38 (35)

Phenyl groups 11→3 (-8)

85→43 (-42)

3→20 (17)

8→34 (26)

5→3 (-2)

83→72 (-11)

3→10 (7)

6→14 (8)

8→4 (-4)

83→71 (-12)

3→10 (7)

6→14 (8)

8→4 (-4)

32→37 (5)

23→23 (0)

39→37 (-2)

6→3 (-3)

71→63 (-8)

4→19 (15)

9→14 (5)

16→4 (-12)

38→40 (2)

2→22 (20)

4→35 (31)

56→3 (-53)

30→41 (11)

3→22 (19)

5→34 (29)

62→3 (-59)

27→40 (13)

12→22 (10)

4→35 (31)

57→3 (-54)

21→74 (53)

19→8 (-11)

32→14 (-18)

28→4 (-24)

28→42 (14)

5→22 (17)

9→34 (25)

58→3 (-55)

The transition at 499 nm is a result of a combination of electronic transitions between the FMOs similar to what is seen for the Q band in unsubstituted porphyrins8-9 with only a small shift of electron density occurring between the porphyrin and substituent. In a previous study by Lind et al. of β-substituted porphyrins the CT band was assigned as a higher energy transition than the Q band;14 however, the calculations predict that this is not the case for these complexes. Analysis of the electron density changes reveals that the transition at 459 nm involves very little net charge transfer (Table 1, bold). As this transition does not occur for unsubstituted porphyrin or for the substituent and that a CT process is not occurring this transition is assigned as π→π* arising from the extended length of conjugation from the porphyrin across the linker to


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the substituent. In ZnTPP-A this transition is raised in energy which is consistent with the shorter linker reducing the length of conjugation. A large number of transitions are calculated to occur within the B band. Many of these transitions occur from low lying MOs that can access the LUMO which must be considerably lowered in energy compared with unmodified ZnTPP. These low lying MOs have a large amount of electron density located on the phenyls which is shifted to the porphyrin and substituents for the calculated transitions. This involvement of phenyls has not previously been seen in β-substituted ZnTPPs and introduces the possibility for new donor-porphyrin-acceptor systems. To further understand the effects of substitution, the individual MOs can be examined. The 4 lowest lying unoccupied orbitals are energetically isolated from the other unoccupied orbitals and are shown in Figure 3. The energy of the FMOs is shown in Figure 4 and tabulated in the supplementary information alongside the Mulliken charge density distribution (Table S3). The calculated MOs for ZnTPP-A and B are consistent with the predictions for substitution to Gouterman’s four orbital model8-9 and are similar to previously reported β-substituted porphyrins.11, 14 The egy orbital is relatively unaffected by substitution, due to negligible electron density at the substituted β-carbon and retains the same electron distribution for all compounds. The egx orbital was able to mix with a substituent based MO resulting in two new MOs; Mix1 and Mix2 which have electron density spread across the porphyrin, linker and substituent. These MOs are split in energy with Mix2 being raised in energy while Mix1 is lowered relative to egx. The decreased LUMO energy is reflected by redshifting of the lowest energy transitions. Additionally Mix1 is energetically accessible to lower lying MOs allowing their involvement in B band transitions.


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Figure 3: MO diagrams for the 4 lowest lying unoccupied orbitals for ZnTPP-A, B and C.

Figure 4: Calculated MO energies from the B3LYP/6-31G(d) TD-DFT calculations. MO symmetry labels refer to the MOs of ZnTPP (D4h point group). An additional unoccupied orbital is present for ZnTPP-A and B which is located almost entirely on the indandione. This MO, despite being similar in energy to egy, is not calculated to be involved in any electronic transitions due to negligible wave function overlap with the HOMOs.


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The first two HOMOs, a2u and a1u, are relatively unperturbed by substitution with the majority of electron density located on the porphyrin core (supplementary information Figure S2). The energy of these MOs are similar to unsubstituted porphyrin (Figure 4) with only a small difference between the two MOs. The electron density for the LUMO is well suited for a CT transition with overlap of electron density occurring over the porphyrin core while there is also significant charge separation from electron density shifting to the substituent. While the first two HOMOs were unaffected by substitution HOMO-2 was heavily influenced. Mixing of a substituent MO with low lying porphyrin MOs (eg and b2u) produced HOMO-2 which approaches the energy of the first two HOMOs. The eg/b2u derived MO has electron density spread mostly along the linker. A transition from HOMO-2 to the LUMO was calculated to be a major component (79% contribution) for the π→π* absorption. Examination of this transition (Figure 5) shows electron density shifting to alternating bonds in the excited state compared with the ground state as is expected for a π→π* transition. This transition is redshifted (0.073 eV) for ZnTPP-B compared to ZnTPP-A which is consistent with the increased linker length increasing the length of conjugation.

Figure 5: Major component of the calculated 448nm transition for ZnTPP-B. The addition of a propylidenodinitrile group to the substituent has a significant effect on the LUMOs of ZnTPP-C. While egy remained unperturbed by substitution the egx derived orbitals underwent further mixing with the indandione based MO resulting in 3 new mixed MOs. These new mixed MOs have increased substituent character compared with ZnTPP-B (supplementary


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information Table S3) and therefore ZnTPP-C would have greater CT character. This mixing caused the LUMO, Mix1, to be further lowered in energy which is reflected in the redshifting of the low energy CT bands. The electron density on the lowest energy of the new MOs, Mix1, includes the propylidenodinitrile group but not the carbonyl. In the highest energy mixed orbital, Mix2, electron density is located over the carbonyl but not the propylidenodinitrile group. The other mixed orbital, Mix3, has electron density over both of these groups. This pattern indicates that the relative energy of these MOs could be tuned by changing the electron withdrawing groups incorporated into the indandione. 3.4 Resonance Raman Spectroscopy. Resonance Raman spectra for ZnTPP-C are shown in Figure 6. Peak enhancement patterns can be seen to change as the excitation wavelength is tuned from 351 to 532 nm indicating multiple absorption processes are probed. Excitation to the red of 532 nm was precluded due to emission. Vibrational modes used in the interpretation of the resonance Raman spectra are shown in Figure 7. No resonance enhancement is observed for excitation at 351 nm due to ZnTPP-C having poor absorbance at this wavelength. Excitation at 406 and 413 nm probes the B band and shows enhancement of many bands. The calculated transitions for the B band predict electron density shifting across the entire molecule which is consistent with enhancement of a range of bands. The involvement of the phenyls is unique to transitions within the B band and is reflected by the 1078 cm-1 band, which is associated with phenyl vibrations, only being enhanced with excitation at 406 and 413 nm. Additionally the 1357 cm-1 band shows a large enhancement for the B band. The 1357 cm-1 vibration is mostly located on the porphyrin core however it does feature across the phenyl to


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porphyrin bonds. The large change in electron density of the phenyls, calculated for the B band transitions, would have a large effect on this bond consistent with the enhancement observed. Excitation at 448 and 458 nm display only a small enhancement of some bands which is consistent with the low extinction coefficients in this region. The enhancements that are observed are similar to those seen for the B band indicating that the enhancement is due to overlap of the B band and not from a new transition which is consistent with calculations.

Figure 6: Resonance Raman spectra of ZnTPP-C (0.01 mM, CH2Cl2) at a range of excitation wavelengths. Solvent peaks labelled with *. Excitation at 1064 nm was used for FT-Raman (1mM, CH2Cl2). Selected bands are labelled with associated vibrational modes (Figure 7), C=porphyrin core, Ph=phenyl, L=linker and I=indandione.


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Figure 7: Selected ZnTPP-C vibrations displaying resonance Raman enhancement. Labels indicate the location of vibrations within the molecule. C=porphyrin core, Ph=phenyl, L=linker and I=indandione. The π→π* transition is probed with excitation at 491 nm which shows enhancement of the 1518 and 1547 cm-1 bands. However, these bands are further enhanced with excitation at 515 and 532 nm indicating that this enhancement is due to overlap with the lower energy transition. The 1575 cm-1 band is also enhanced at 491 nm but does not show further enhancement at 515 and


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532 nm. The 1575 cm-1 vibration is located across the linker which is consistent with the assignment of the transition as π→π*. Enhancement of 1357 and 1547 cm-1 core bands and 1238 and 1518 cm-1 indandione bands is observed with excitation at 515 and 532 nm. The calculated transition predicts configuration interactions between the FMOs similar to the Q band of unsubstituted porphyrin. The large enhancement of core bands is consistent with the egy MO being accessed while enhancement of indandione bands show involvement of the mixed egx MOs. There is some overlap between the 1178 and 1193 cm-1 bands, however; it is clear that as the excitation wavelength is tuned from 406 to 532 nm that the enhancement of the 1178 cm-1 band decreases while the 1193 cm-1 band increases. The 1178 cm-1 vibration is located on the phenyls while the 1193 cm-1 vibration has greater linker involvement. This is consistent with the higher energy transitions having phenyl involvement while the lower energy transitions have greater substituent involvement. While bands at ~1490 cm-1 showed enhancement they could not be assigned to a specific vibrational mode as multiple vibrational modes were calculated within the 1490 cm-1 band. Additionally the peak location shifts as the excitation wavelength is changed indicating that different bands are being resonantly enhanced. The enhancement patterns for ZnTPP-A and B (supplementary information Figure S3, S4) are similar to those described for ZnTPP-C with the B band showing enhancement of phenyl and porphyrin bands, the π→π* transition showing enhancement of the linker and the Q band derived transition showing porphyrin and indandione band enhancement. Overall the calculated transitions for all compounds are in good agreement with the enhancements observed with


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resonance Raman spectroscopy although the lower energy transitions were unable to be probed due to emission. 4. Conclusion DFT calculations were shown to be effective for modelling the geometry of all β-substituted porphyrins which is demonstrated quantitatively by the small (

A Computational and Spectroscopic Analysis of Î²-Indandione

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