Photophysical Properties and Electronic Structure of Porphyrins


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Photophysical Properties and Electronic Structure of Porphyrins Bearing Zero to Four meso-Phenyl Substituents: New Insights into Seemingly Well Understood Tetrapyrroles Amit Kumar Mandal,† Masahiko Taniguchi,‡ James R. Diers,§ Dariusz M. Niedzwiedzki,†,∥ Christine Kirmaier,† Jonathan S. Lindsey,*,‡ David F. Bocian,*,§ and Dewey Holten*,† †

Department of Department of § Department of ∥ Photosynthetic ‡

Chemistry, Washington University, St. Louis, Missouri 63130-4889, United States Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States Chemistry, University of California, Riverside, California 92521-0403, United States Antenna Research Center, Washington University, St. Louis, Missouri 63130-4889, United States

S Supporting Information *

ABSTRACT: Six free base porphyrins bearing 0−4 meso substituents have been examined by steady-state and time-resolved absorption and fluorescence spectroscopy in both toluene and N,N-dimethylformamide (DMF). The lifetime of the lowest singlet excited state (S1) decreases with an increase in the number of meso-phenyl groups; the values in toluene are H2P-0 (15.5 ns) > H2P-1 (14.9 ns) > H2P-2c (14.4 ns) > H2P-2t (13.8 ns) ∼ H2P-3 (13.8 ns) > H2P-4 (12.8 ns), where “H2P” refers to the core free base porphyrin, the numerical suffix indicates the number of meso-phenyl groups, and “c” and “t” refer to cis and trans, respectively. The opposite trend is found for the fluorescence quantum yield; the values in toluene are H2P-0 (0.049) < H2P-1 (0.063) ∼ H2P-2c (0.063) < H2P-2t (0.071) < H2P-3 (0.073) < H2P-4 (0.090). Similar trends occur in DMF. All radiative and nonradiative (internal conversion and intersystem crossing) rate constants for S1 decay increase with the increasing number of meso-phenyl groups. The increase in the rate constant for fluorescence parallels an increase in oscillator strength of the S0 → S1 absorption manifold. The trend is reproduced by timedependent density functional theory calculations. The calculations within the context of the four-orbital model reveal that the enhanced S0 ↔ S1 radiative probabilities derive from a preferential effect of the meso-phenyl groups to raise the energy of the highest occupied molecular orbital, which underpins a parallel bathochromic shift in the S0 → S1 absorption wavelength. Polarizations of the S1 and S2 excited states with respect to molecular structural features (e.g., the central proton axis) are analyzed in the context of historical conventions for porphyrins versus chlorins and bacteriochlorins, where some ambiguity exists, including for porphine, one of the simplest tetrapyrroles. Collectively, the study affords fundamental insights into the photophysical properties and electronic structure of meso-phenylporphyrins that should aid their continued widespread use as benchmarks for tetrapyrrole-based architectures in chemical, solar-energy, and life-sciences research.

1. INTRODUCTION

chlorophyll c1, which constitute the ligand for heme and the light-harvesting pigment in marine chromophyte algae, respectively, are illustrative (Chart 1). Although the photophysical properties of meso-tetraphenylporphyrin provide a benchmark against which to compare all other meso-tetraarylporphyrins, the core chromophore of all porphyrins is porphine. Despite the first synthesis over 80 years ago,1 porphine has traditionally not been widely utilized due to low yield syntheses and poor solubility. Recent advances have now overcome many of these problems and have provided robust access to porphine, which is a valuable building block.1−3 Advances in synthesisboth in strategies4−8 and in the

Studies in tetrapyrrole chemistry have long focused on synthetic analogues of the naturally occurring macrocycles. The most prevalent such analogue is meso-tetraphenylporphyrin (commonly named H2TPP, here referred to as H2P-4) given the ease of synthesis, ease of handling (crystallization, solubilization, robustness toward metalation, etc.), rectilinear arrangement of substituents and the versatility with which diverse analogues thereof can be prepared. Such analogues derive from variation in the nature and the number of mesosubstituents. Regardless, the arrangement of substituents in meso-substituted porphyrins stands in sharp contrast with that of naturally occurring porphyrins, which typically contain a full complement of β-pyrrolic substituents and few meso-substituents. The structures of porphyrins protoporphyrin IX and © XXXX American Chemical Society

Received: September 19, 2016

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distributions) obtained from density functional theory (DFT) calculations, and excited-state compositions and the energies and polarizations of associated absorption spectral transitions derived from time-dependent DFT (TDDFT) calculations. Simulations of the absorption spectra were also obtained using the Gouterman four-orbital model. In addition to insights into the photophysical properties of the set of free base porphyrins, a point with broader ramifications that will emerge concerns the historical conventions surrounding the polarizations of the S0 → S1 and S0 → S2 transitions and their orientations with respect to the central proton axis for the various free base porphyrins (and other tetrapyrroles). Another point is the historical context for designation of which transition is described as x or y polarized. The convention for reduced porphyrins (chlorins and bacteriochlorins) is clear because, in addition to the proton axis, the reduced pyrrole ring(s) provide an additional structural feature to define the molecular coordinates. This point is illustrated in Chart 3, which shows the structures of

Chart 1. Representative Naturally Occurring Porphyrins

availability of key pyrromethane intermediates9−12also have made available all intermediate members of the homologous set from porphine (H2P-0) to tetraphenylporphyrin (H2P-4), namely, the monophenylporphyrin (H2P-1), cis-diphenylporphyrin (H2P-2c), trans-diphenylporphyrin (H2P-2t), and triphenylporphyrin (H2P-3) (Chart 2).

Chart 3. Structures of the Parent Porphyrin (Porphine), Chlorin, and Bacteriochlorin Macrocycles and Native Photosynthetic Pigments Pheophytin a and Bacteriopheophytin aa

Chart 2. Porphyrins Studied Here

a

The macrocycle-position numbering scheme and molecular-axis convention typically used for chlorins and bacteriochlorins are also shown. There is ambiguity in the historical conventions for free base porphyrins (see text).

unsubstituted free base porphyrin (H2P-0), chlorin, and bacteriochlorin along with the native photosynthetic pigments pheophytin a (a chlorin) and bacteriopheophytin a (a bacteriochlorin), which are the free base analogues of chlorophyll a and bacteriochlorophyll a, respectively. The native pigments contain another relevant structural feature, the fused keto-bearing five-membered ring. The common convention shown for the simple free base chlorin at the top of Chart 3, which encompasses synthetic and native chlorins and bacteriochlorins, is as follows: (1) the x axis bisects the reduced pyrrole ring(s) (one in chlorins and two in bacteriochlorins), (2) the y axis is coincident with the central proton axis of the free base forms, and (3) the y axis bisects the nonreduced pyrrole ring to which the fused five-membered ring is attached in the native photosynthetic pigments. The present findings on the set of six free base porphyrins reveals ambiguity

Given the availability of the collection of porphyrins bearing 0−4 phenyl groups, we have examined how the photophysical properties and electronic structure change along the series from H2P-0 to H2P-4. The porphyrins shown in Chart 2 were characterized by static and time-resolved spectroscopy to elucidate the yields and rate constants for the three singlet excited-state decay pathways (fluorescence, intersystem crossing, and internal conversion). The spectral properties of all six porphyrins were further analyzed with the help of molecular orbital (MO) characteristics (energies and electron-density B

DOI: 10.1021/acs.jpca.6b09483 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A in the historical convention(s) for porphyrins regarding the issues noted above (polarizations of S1 versus S2, the proton axis, and x versus y) that dates back over 50 years. Collectively, the studies reveal the systematic effect of meso-phenyl substituents on the photophysical properties of porphyrins and give new insights into electronic structure and spectra that are of broad relevance for tetrapyrrole research.

2. MATERIALS AND METHODS 2.1. Porphyrins. Porphine (H2P-0),1,3,13 5-phenylporphyrin (H2P-1),14,15 5,10-diphenylporphyrin (H2P-2c),15,16 5,15diphenylporphyrin (H2P-2t),5,6 5,10,15-triphenylporphyrin (H2P-3),17 and 5,10,15,20-tetraphenylporphyrin (H2P-4)4,18 are all known compounds. New routes to H2P-1, H2P-2c, and H2P-3 are described in the Supporting Information.9,10,19−22 2.2. Photophysical Measurements. Spectral and photophysical studies of the six porphyrins in toluene and DMF were carried out on dilute (μM) argon-purged samples as described previously.23−25 In brief, fluorescence quantum yields were obtained by absolute measurement using an integrating sphere (Quanti-Phi, Horiba). Singlet excited-state lifetimes were determined by two methods. The first method is timecorrelated-single-photon-counting (TCSPC) using a SimpleTau 130 system (Becker and Hickl) with an instrument response function of <200 ps fwhm and ∼100 fs excitation pulses (at 8 MHz). The second method is transient absorption (TA) spectroscopy employing ∼100 fs excitation flashes from an ultrafast laser system (Spectra Physics) and acquisition of difference spectra (360−900 nm) using a white-light pulsed laser (∼1 ns rise time) in 100 ps time bins with variable pump− probe spacing up to 0.5 ms (Ultrafast Systems, EOS). Transient absorption measurements employed excitation both on the long-wavelength side of the Soret absorption (420−440 nm) and in the Q-band region (530−570 nm). Time profiles of ΔA at ∼1.5 nm spacing across the 380−880 nm region were analyzed at a number of individual wavelengths and also globally. The excited-state lifetimes obtained from the transient absorption and fluorescence measurements were the same to within the experimental uncertainty on the reported value in the tables. Transient absorption studies were also used to measure the yield of S1 → T1 intersystem crossing by comparing the extent of bleaching of the ground-state absorption bands due to T1 at the asymptote of the S1 decay with the extent due to S1 immediately after the excitation flash. 2.3. Density Functional Theory Calculations. DFT calculations were performed with Gaussian 09 version D.01. All calculations were in toluene using the polarizable continuum model (PCM). Molecular geometries were fully optimized using the hybrid B3LYP functional and basis set 6-31++G**.26 Single-point calculations of the ground-state MOs and TDDFT were performed using the long-range corrected wB97XD functional and basis set 6-31++G**. These calculations used Gaussian defaults with the exception of the keywords SCF=(YQC, NoVarAcc, NoIntFock), POP=(Full, NBO), and TD(nStates=16). MO images and transition dipole moment (TDM) vector images were generated from Gaussian outputs using the Chemcraft program (http://www.chemcraftprog. com).

Figure 1. Absorption spectrum of H2P-4 in toluene. The traditional assignments of the spectral features are given in blue and those used herein in red. The spectrum in the 460−700 nm region has been multiplied ×20 relative to that in the 380−460 nm region.

The traditional assignments of the bands (e.g., Qx, Qy, Bx, By) are given in blue font and the designations used herein (e.g., S1, S2, S3, S4) in red font. The x and y conventions are not used herein because the members of the H2P-# set with lower symmetry than H2P-4 (and H2P-0) exhibit TDM orientations that differ from the axes defined by the nitrogen atoms with and without the central protons (vide inf ra). Furthermore, as noted in the Introduction, even for the free base porphyrins with the highest symmetry (H2P-4 and H2P-0) there is ambiguity regarding the historical designations of x and y. Figure 2 shows the electronic absorption and fluorescence spectra of all six free base porphyrins in toluene. The absorption spectra (blue solid and dashed lines) show the standard features for free base porphyrins with the increasing number of phenyl groups. With the designations indicated in red in Figure 1, these are the strong near-ultraviolet (∼390− 425 nm) Soret band with overlapped S0 → S3 and S0 → S4 components, a pair of blue-green (∼480−550 nm) S2(1,0) and S2(0,0) bands, and a pair of orange-red (∼560−650) S1(1,0) and S1(0,0) bands. The near-UV (S4, S3) region is the focus in Figure 3A, and similarly for the visible (S2, S1) region in Figure 3B. The spectra in Figures 2 and 3 are normalized to the same total (300−900 nm) intensity obtained with integrations performed on spectra plotted versus wavenumbers, which is linear in energy. This presentation method complements others on the basis of peak normalizations and is convenient for showing trends in spectral characteristics of related tetrapyrroles and allows comparison with spectral simulations (vide inf ra). The peak wavelengths and intensity ratios are listed in Table 1. The absorption maxima for the six free base porphyrins in DMF are shifted hypsochromically versus toluene by 1−4 nm (Table 1). The following presentation focuses on spectra in toluene. All absorption bands shift bathochromically with increasing number of phenyl groups. For example, the Soret positions in toluene are H2P-0 (396 nm) < H2P-1 (402 nm) < H2P-2c (408 nm) = H2P-2t (408 nm) < H2P-3 (413 nm) < H2P-4 (419 nm). Figure 3A shows that this bathochromic shift is accompanied by a hyperchromic effect on peak intensity (molar absorptivity) paralleled by a decrease in overall bandwidth of the near-UV (B) feature. The decreasing spectral width derives in part from increasing overlap of the underlying S3 and S4 components. Similarly, the S2 and S1 (Q) absorption bands shift increasingly to red wavelengths with increasing number of phenyl groups. This point is visualized in Figure 3B and Figure

3. RESULTS AND DISCUSSION 3.1. Absorption and Fluorescence Spectra. Figure 1 shows the electronic absorption spectrum of H2P-4 in toluene. C

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4. The latter figure shows the spectra in the green-red region normalized to the S2(1,0) absorption band (with peak-intensity ratios listed in Table 1). Both figures reveal that the bathochromic shift of the S2(0,0) and S1(0,0) absorption positions is accompanied by a hyperchromic effect with the increasing number of phenyl groups. On the contrary, the bathochromic shift of the S2(1,0) and S1(1,0) vibronic satellites is not accompanied by a significant change in overall intensity. This point is best seen in Figure 3B, in which the spectra are normalized to the total (300−900 nm) spectral intensity. The intensity of the S2(1,0) and S1(1,0) absorption features is expected to remain roughly constant for related porphyrins because the absorption strength derives primarily from vibronic borrowing from the strong S3/S4 (Soret) transitions via Herzberg−Teller coupling, which supplements and far exceeds the intensity from normal Franck−Condon progressions.27 Figure 4 shows that the S1(1,0) absorption characteristic of H2P-4 becomes increasingly rich with fewer phenyl groups. Multiple features spanning the range 555−606 nm are partially resolved for parent H2P-0, marked by arrows in the figure. The shifts from the S1(0,0) position are roughly 1780, 1530, 1310, 980, 715, and 270 cm−1. Vibrations near these values are found in the normal-mode analysis of H2P-0, emission spectra of the molecule in rare gas matrices, and resonance Raman spectra of metalloporphines.28,29 The S1 → S0 fluorescence spectra for the porphyrins in toluene are shown in Figure 2 (red dotted lines). The shortest wavelength feature is the S1(0,0) band, which is shifted by 1−3 nm from the corresponding S1(0,0) absorption maximum (Table 1). Each emission spectrum also contains a prominent S1(0,1) feature centered ∼1500 cm−1 to lower energy than the origin band. The S1(0,0)/S1(1,0) peak-intensity ratio (Table 1 second to last column) increases in the order H2P-1 (0.24) < H2P-2c (0.37) < H2P-0 (0.47) < H2P-2t (0.70) < H2P-3 (0.91) < H2P-4 (1.4). The value for H2P-0 is out of order with respect to the number of phenyl groups, likely in part because the S1(0,0) band is sharper for H2P-0. The trend in S1(0,0) integrated intensity relative to the total fluorescence (Table 1 last column) is in the following order: H2P-1 (0.15) < H2P-0 (0.20) ∼ H2P-2c (0.20) < H2P-2t (0.33) < H2P-3 (0.40) < H2P-4 (0.50). Regardless of details, it is clear that the relative intensity of the origin transition is substantially larger for H2P-4 compared to H2P-0 with increases between. Such trends in emission parallel the trends in relative S1(0,0) absorption intensity (Figure 1 and Table 1). Similar results are obtained for the porphyrins in DMF (Table 1). The trends can be traced to substituent effects on the relative energies of the frontier MOs (vide inf ra). 3.2. Excited-State Properties. The measured excited-state properties of the porphyrins are the lifetime (τS) of the lowest singlet excited state (S1), the S1 → S0 fluorescence quantum yield (Φf), and the yield of S1 → T1 intersystem crossing (Φisc). The yield of S1 → S0 internal conversion is obtained by the difference Φic = 1 − Φf − Φisc. The fluorescence (kf), intersystem crossing (kisc), and internal conversion (kic) rate constants are obtained from these data via the calculation kx = Φx/τS, where x = f, isc, or ic. Figure 5 shows representative TA difference spectra and kinetic traces for H2P-4 in toluene, obtained using 100 fs excitation flashes at 560 or 570 nm. Data are shown for samples that were purged with argon to remove O2 (panels A and B) and with ambient O2 (panels C and D). As expected, the TA difference spectra are identical in the two cases. The spectrum

Figure 2. Absorption (blue) and fluorescence (red dashed) spectra of porphyrins in toluene. The absorption spectra are normalized to the total (300−900 nm) intensity obtained with integrations performed on spectra plotted versus wavenumbers. Emission spectra were acquired using excitation at the Soret maxima and normalized to each other (at an amplitude for convenient display) at the S1(0,1) maximum. In this way, the progressive decrease (top to bottom) in the relative S1(0,0) emission intensity can be visualized, along with the parallel decrease in relative S1(0,0) absorption strength. Shown are the wavelengths of Soret absorption (blue) and S1(0,0) fluorescence (red), the latter typically lying ∼1 nm to longer wavelength than the S1(0,0) absorption peak.

Figure 3. Absorption spectra of porphyrins in toluene emphasizing the near-UV B-band region (A) and visible Q-band region (B). The spectra are normalized to the total intensity as in Figure 1, with a 20× expanded vertical scale in panel B versus panel A. The arrows in panel B depict the coupled hyperchromic and bathochromic shifts of the two (0,0) bands with increasing number of phenyl groups whereas bathochromic shifts of the two (1,0) vibronic satellites are accompanied by relatively constant intensities.

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solvent

H2P-0

toluene DMF toluene DMF toluene DMF toluene DMF toluene DMF toluene DMF

H2P-1 H2P-2c H2P-2t H2P-3 H2P-4

S3,4(0,0)

S2(1,0)

S2(0,0)

S1(1,0)

S1(0,0)

S3,4(0,0)

S2(0,0)

S1(1,0)

S1(0,0)

S1(0,0)

S1(0,1)

S1(0,0)

λabs (nm)

λabs (nm)

λabs (nm)

λabs (nm)

λabs (nm)

Iabs

Iabs

Iabs

Iabs

λem (nm)

λem (nm)

Iem

Σem

396 392 402 399 408 405 408 405 413 411 419 417

489 487 495 494 502 500 502 501 508 507 514 514

519 518 527 525 533 531 536 534 541 541 548 548

563 560 569 567 576 574 576 574 583 582 591 589

616 612 623 620 633 629 631 629 640 637 647 645

16.1 16.7 17.8 19.5 19.0 20.5 21.1 23.0 21.3 22.8 24.4 25.0

0.16 0.20 0.13 0.13 0.13 0.12 0.29 0.32 0.29 0.30 0.40 0.44

0.32 0.35 0.30 0.30 0.28 0.28 0.30 0.31 0.29 0.29 0.27 0.29

0.055 0.056 0.025 0.022 0.038 0.037 0.08 0.11 0.11 0.15 0.19 0.23

616 612 625 621 634 631 633 631 641 640 648 649

682 677 690 687 699 695 698 697 707 706 715 715

0.47 0.41 0.24 0.31 0.37 0.49 0.70 0.93 0.91 1.1 1.4 1.7

0.20 0.20 0.15 0.19 0.20 0.24 0.33 0.39 0.40 0.45 0.50 0.56

S1(0,0)

a All values were measured using Ar-purged samples at room temperature. The absorption intensities (Iabs) are the peak value relative to that of the S2(1,0) band (∼485−515 nm). The emission intensity (Iem) is the peak value relative to that of the S1(0,1) band (∼675−715 nm). The emission intensity (Σem) is the integrated S1(0,0) intensity relative to the total.

subsection, the measured τs values and Φf values for the six porphyrins are compared with each other, and for H2P-4, with the range of literature values given the widespread use of H2P-4 as a benchmark for studies of other tetrapyrroles. In the subsequent three subsections, the trends in the yields and rate constants for the three S1 decay routes among the six porphyrins are described in turn. 3.2.1. Singlet Lifetimes and Fluorescence Yields. The τs and Φf values of the six porphyrins in toluene and DMF are listed in Table 2. The τS for the compounds in toluene decreases with the addition of meso-phenyl groups as follows (Figure 6): H2P0 (15.5 ns) > H2P-1 (14.9 ns) > H2P-2c (14.4 ns) > H2P-2t (13.8 ns) ∼ H2P-3 (13.8 ns) > H2P-4 (12.8 ns). The Φf for the compounds in toluene increases with increasing number of meso-phenyl substituents along the series: H2P-0 (0.049) < H2P-1 (0.063) ∼ H2P-2c (0.063) < H2P-2t (0.071) < H2P-3 (0.073) < H2P-4 (0.090). The compounds in DMF exhibit the same trends in τs and Φf (Table 2). Given that H2P-4 is widely used as a benchmark for studies of the photophysics of porphyrins and other tetrapyrroles, a brief comparison of our results with the range of values in the literature is useful. The τS values for H2P-4 in a variety of solvents and conditions reported over the years range from 8 to 16 ns with an average of ∼11 ns. Our values for H2P-4 in toluene of 12.8 ns with O2 removed (by purging with argon) and 9.9 ns with atmospheric O2 present are thus consistent with prior work. The Φf values for H2P-4 in diverse solvents and extent of sample deoxygenation reported over the years range from 0.04 to 0.13. The typical literature Φf values of H2P-4 used as a reference to determine Φf for other tetrapyrroles are 0.13, 0.11, and 0.090, with latter being the least common. For example, in early experiments we utilized the 0.11 literature value as a standard. Over years of repeated comparisons of Φf (and τS and derived kf) for H2P-4, zinc chelate ZnP-4, chlorophyll a, and numerous synthetic chlorins and bacteriochlorins (both referenced against each other and determined independently by absolute integrating-sphere measurements), we now believe that the correct Φf for H2P-4 in toluene is the value of 0.090, which was also one of the first values derived (in propanol).31 The value is reduced to 0.070 for H2P-4 in toluene in the presence of atmospheric O2. The latter solution affords a

Figure 4. Absorption spectra of porphyrins in toluene normalized to the S2(1,0) maximum (∼490−515 nm, not shown). The arrows mark positions of S1 vibronic features for H2P-0, the most intense of which is normally denoted the S1(1,0) band; approximate energy shifts from the S0(0,0) position are given in the text.

at 0.5 ns is due to the S1 excited state and that at 95 ns is due to the T1 excited state. The kinetic traces show that S1 decays with a lifetime of ∼13 ns with O2 removed and ∼10 ns with ambient O2. The T1 excited state does not decay over the time scale of the measurements (100 ns) with O2 removed (the lifetime is ≥1 ms30) and with a time constant of ∼300 ns with ambient O2. Panels A and B of Figure 5 show that the yield of the T1 excited state is ∼0.8 based on the extent of bleaching of the ground-state absorption bands (referenced to the relatively featureless excited-state absorption) for the T1 state (at long time) versus the S1 state (at early time). Such results for all the porphyrins in Ar-purged toluene and DMF are summarized in Table 2 along with the yields and rate constants for the three S1 excited-state decay pathways (fluorescence, intersystem crossing, and internal conversion). Figure 6 plots these data as a function of the number of phenyl groups. In the following E

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Figure 5. TA data for H2P-4 in Ar-purged toluene (A, B) or ambient O2 (C, D) acquired using 100 fs excitation flashes at 560 or 570 nm. The time constants derived from global analysis are indicated.

Table 2. Photophysical Parameters as a Function of the Number of Phenyl Substituentsa cmpd

solvent

S1 energy (eV)b

τs (ns)

Φf

Φisc

Φic

kf−1 (ns)

kisc−1 (ns)

kic−1 (ns)

H2P-0

toluene DMF toluene DMF toluene DMF toluene DMF toluene DMF toluene DMF

2.014 2.021 1.985 2.000 1.945 1.968 1.963 1.973 1.939 1.942 1.915 1.916

15.5 15.4 14.9 14.8 14.4 14.0 13.8 13.4 13.8 13.0 12.8 12.4

0.049 0.051 0.063 0.059 0.063 0.057 0.071 0.084 0.073 0.085 0.090 0.090

0.92 0.90 0.90 0.89 0.90 0.90 0.87 0.86 0.86 0.85 0.80 0.80

0.031 0.049 0.037 0.051 0.037 0.043 0.059 0.056 0.067 0.065 0.11 0.11

320 300 240 250 230 250 190 160 190 150 140 140

17 17 17 17 16 16 16 16 16 15 16 15

500 310 400 290 390 330 230 240 210 200 120 110

H2P-1 H2P-2c H2P-2t H2P-3 H2P-4

All values were measured using Ar-purged samples at room temperature. The typical errors (percent of value) are τS (±5%), Φf (±5%), Φisc (±5%), Φic (±10%), kf (±10%), kisc (±15%), and kic (±20%). bAverage energy of the S1(0,0) absorption and emission maxima in Table 1.

a

convenient reference for relative Φf determinations without the need for rigorous O2 removal. The magnitude of the reduction of the Φf of H2P-4 in the presence versus the absence of atmospheric oxygen (0.070 versus 0.090), and the reduction in τS (9.9 versus 12.8 ns; Table 2) are consistent with prior observations.31,32 The T1 lifetime is reduced from milliseconds30,32 to ∼300 ns (Figure 5). Triplet lifetimes of ∼100−300 ns are common for tetrapyrroles in organic media exposed to ambient O2 at room temperature.33 Such reductions in S1 and T1 lifetimes occur because the excited-state decay-rate expression in the absence of O2 is supplemented in the presence of O2 by a term kq[O2],34 where atmospheric [O2] in toluene is centered in the 1−3 mM range34−37 for many organic solvents. The quenching rate constant kq for singlets derived from fluorescence quenching experiments on various fluorophores38,39 is typically (1−3) × 1010 M−1 s−1 and that for triplets is usually 1/9 that magnitude due to spin statistics.32,34,39,40 Such values explain the reduced

S1 and T1 lifetimes for H2P-4 in toluene with versus without atmospheric O2. 3.2.2. Radiative Probabilities. The combined Φf and τs values afford the rate constant kf for spontaneous emission. The kf in turn is proportional, via the Einstein coefficients,34 to the rate constant for absorption, which governs the intensity of the S0 → S1 transition. Because vibronic (Herzberg−Teller) coupling is responsible for a large fraction of the intensity in the S1(1,0) and S2(1,0) bands,27,30 those features may not change substantially among porphyrins. Thus, trends in S0 → S1 and S1 → S0 radiative probabilities are reflected in the S1(0,0)/ S1(1,0) absorption-intensity ratio and S1(0,0)/S1(0,1) fluorescence-intensity ratio. These ratios usually follow the order of H2P-1 < H2P-0 ∼ H2P-2c < H2P-2t < H2P-3 < H2P-4 (Table 1). The same trend in kf is obtained from the Φf/τs calculation, which for the porphyrins in toluene is as follows: H2P-0 [(320 ns)−1] < H2P-1 [(240 ns)−1] < H2P-2c [(230 ns)−1] < H2P-2t [(190 ns)−1] ∼ H2P-3 [(190 ns)−1] < H2P-4 [(140 ns)−1] F

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and the associated internal-conversion time constant decreases (the rate constant increases) from 500 ns for H2P-0 to 120 ns for H2P-4 in toluene (Figure 6G and Table 2). The change in the time constant for the two bookend compounds is consistent with their relative S1 energies (1.92 and 2.01 eV), as one would expect from the energy-gap law for nonradiative decay.34 The Φic and kic values for the six porphyrins in DMF are also listed in Table 2. 3.3. Molecular-Orbital, Excited-State, and Spectral Properties. The electronic-structure characteristics that give rise to the trends in spectral properties of the six porphyrins bearing different numbers of meso-phenyl groups (Figures 7−9, Table 3) were evaluated by DFT and TDDFT calculations. The four subsections below have the following content: (1) characteristics of the frontier MOs obtained from the DFT calculations; (2) results of TDDFT calculations on the excitedstate properties derived from combinations of one-electron promotions between frontier MOs; (3) absorption properties predicted by the TDDFT calculations and simulations using the four-orbital model compared with measured spectra; (4) a synopsis of key results of the analysis in historical context. 3.3.1. Molecular-Orbital Characteristics. The energies and electron-density distributions of the four frontier MOs obtained from the DFT calculations are shown in Figure 7. Note that the porphyrin with two phenyl groups in a cis configuration (Chart 1) is unique among the set in that the two tautomeric forms for positions of the central protons are not identical (denoted H2P2c and H2P-2c′). Both forms are shown in Figure 7 because the DFT calculations indicate that the two isomers have the same total energy to within ∼20 cm−1 and thus contribute similarly to molecular properties (vide inf ra). Table S1 of the Supporting Information lists the MO energies along with various energy gaps. The changes in MO energies and energy gaps with the number of phenyl groups are visualized in Figure 8. As phenyl groups are successively added to H2P-0 building toward H2P-4, the orbital primarily affected is the highest occupied MO (HOMO). The overall changes in the energies of the lowest unoccupied MO (LUMO), the LUMO+1, and the HOMO−1 are 7−9-fold lower than for the HOMO. The HOMO is the orbital primarily affected by addition of meso-phenyl groups because of the electron-density distributions shown in Figure 7. The DFT calculations show that the HOMO for H2P-0 and the other five porphyrins is basically the a2u(π) orbital of a porphyrin with D4h symmetry (e.g., zinc chelate), and is designated b1u(π) in the D2h point group of H2P-0 and H2P-4. (The b1u(π) designation assumes that the axis containing the central protons is defined as y.) This orbital places substantial electron density at the meso-carbons to which the phenyl groups are attached and therefore is progressively destabilized by successive addition of these substituents. In contrast, HOMO−1 has far less electron density at the mesopositions and is far less affected. The HOMO−1 is basically the a1u(π) orbital of a D4h porphyrin, denoted au(π) in D2h symmetry. Figure 7 reveals variations in the nature and ordering of the two lowest unoccupied MOs among the set of porphyrins. For all the molecules, the LUMO and LUMO+1 are extremely close in energy. The LUMO and LUMO+1 of H2P-0 and H2P4 are direct descendants of the eg(π*) degenerate pair for a D4h porphyrin and are b2g(π*) and b3g(π*) in the D2h point group (again, if y is the central proton axis). However, the LUMO of H2P-0 has a reflection plane of symmetry perpendicular to the central proton axis and is b2g(π*), whereas the LUMO of H2P-

Figure 6. Photophysical properties as a function of number of phenyl groups for all porphyrins in toluene. (A) Singlet excited-state lifetime, (B) fluorescence yield, (C) intersystem-crossing yield, (D) internalconversion yield, (E) radiative time constant, (F) intersystem-crossing time constant, and (G) internal-conversion time constant.

(Table 2 and Figure 6E). A similar trend of kf occurs for the six free base porphyrins in DMF (Table 2). 3.2.3. Intersystem Crossing. For all six free base porphyrins, Φisc (the triplet yield) is 0.80−0.92, indicating that S1 → T1 intersystem crossing dominates decay of the S1 excited state (Table 2). This finding is consistent with prior work on free base and light-metal porphyrins (e.g., zinc chelates).33,37,41−49 The value obtained here for H2P-4 in toluene (0.80) is near the upper end of the range of literature values in various solvents (0.63−0.84).31,37,45,48,50,51 The time constant for intersystem crossing decreases from (17 ns)−1 for H2P-0 to (16 ns)−1 for H2P-4 in toluene (Table 2 and Figure 6F). 3.2.4. Internal Conversion. Early work on porphyrins, fueled by sparse Φisc measurements, led to uncertainty concerning whether the internal-conversion quantum yield Φic is insignificant in free base and light-metal porphyrins (e.g., zinc or magnesium chelates), and thus if Φf + Φisc ∼ 1.30,31 It has become clear that Φic is not zero for such porphyrins, although the value is typically ≤0.1. Thus, internal conversion contributes measurably, but not greatly, to the nonradiative decay process. The series of porphyrins studied here display a general trend for internal conversion. The Φic value increases modestly from 0.031 for H2P-0 to 0.11 for H2P-4 (Figure 6D and Table 2), G

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Figure 7. MO correlation diagram for the porphyrins.

energy. Because the four excited-state configurations have comparable transition-dipole strengths, a constructive linear combination gives rise to a strong transition in the near-UV (a B band) and a destructive combination of the same pair gives a weak transition in the visible (a Q-band) for porphyrins. Table 3 shows that the TDDFT calculations are consistent with the model for H2P-0, H2P-2c (and H2P-2c′) and H2P-4. Each excited state of each of these porphyrins is composed primarily of either a combination of HOMO → LUMO and HOMO−1 → LUMO+1 or a combination of HOMO−1 → LUMO and HOMO → LUMO+1. For each porphyrin, S1 (weak Q-band) and S3 (strong B band) arise from one pair, and S2 (weak Q-band) and S4 (strong B band) arise from the other pair. Furthermore, for H2P-0, H2P-2c (and H2P-2c′), and H2P4, the TDMs for S1 and S3 lie along the proton axis, and the TDMs for S2 and S4 lie along the perpendicular axis (Figure 9). This is not the case for H2P-1, H2P-2t, and H2P-3, where all four configurations contribute substantially to each excited state, and the TDMs have variable direction with regard to the proton axis. Additionally, S1 no longer lies on the same axis as S3, S2 no longer lies on the same axis as S4, and the orthogonality within the pairs is often lost. The differences between the two trios of porphyrins (D2h symmetry versus lower symmetry) are connected to the abovenoted differences in MO characteristics, particularly of the LUMO and LUMO+1 (Figure 7). More explicitly, for symmetry groups in which the Cartesian axes that lie in the plane of the macrocycle belong to different one-dimensional irreducible representations (or the same two-dimensional representation), the four one-electron promotions among the

4 has a reflection plane containing the central proton axis and is b3g(π*). The switch in ordering of the LUMO and LUMO+1 generally occurs upon addition of two phenyl substituents. Both H2P-2c and H2P-2c′ have C2v symmetry, and the general appearance of the LUMO and LUMO+1 is similar to that of H2P-0 and H2P-4. [Note that the frame of reference for H2P2c′ is rotated 90° with respect to H2P-2c and the other molecules in the set, owing to the switch of the central proton axis.] In contrast, H2P-1, H2P-2t, and H2P-3 have no in-plane symmetry, and the LUMO and LUMO+1 of this trio no longer have the characteristics of the original D4h eg(π*) pair but rather appear to contain mixtures of them. In other words, if the in-plane directions are x and y (using any molecular reference), the LUMO and LUMO+1 of H2P-1, H2P-2t, and H2P-3 have mixed x−y character. The consequences on the excited-state compositions and TDM directions are given in the following subsection. 3.3.2. Excited-State Properties. The electronic compositions of the S1, S2, S3, and S4 excited states and predicted spectral characteristics for the six porphyrins obtained from the TDDFT calculations are given in Table 3. The table shows the transition wavelength/energy, oscillator strength ( f), and TDM directions with respect to the central proton axis for the associated absorption bands. The TDM directions are graphed in Figure 9. The four excited states within the Gouterman fourorbital model30,41,52 arise from pairwise linear combinations of one-electron promotions (with the same symmetry) between the filled and empty frontier MOs. The extent of mixing depends on the relative energies of the two excited-state configurations and the magnitude of configuration-interaction H

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The Journal of Physical Chemistry A Table 3. Excited-State Compositions and Absorption Characteristics from TDDFT Calculationsa cmpd H2P-0

H2P-1

H2P-2c b

H2P-2c′ c

H2P-2t

H2P-3

H2P-4

state S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0 S0

→ → → → → → → → → → → → → → → → → → → → → → → → → → → →

S1 S2 S3 S4 S1 S2 S3 S4 S1 S2 S3 S4 S1 S2 S3 S4 S1 S2 S3 S4 S1 S2 S3 S4 S1 S2 S3 S4

H→L

H−1 → L+1

0.50

0.50

0.49 0.25 0.31 0.22 0.20 0.57

0.50 0.18 0.26 0.32 0.23 0.43

0.37

0.56

0.54

0.46

0.44 0.27 0.32 0.24 0.15 0.32 0.28 0.23 0.16 0.60

0.54 0.18 0.23 0.40 0.19 0.21 0.19 0.37 0.21 0.38

0.35

0.60

H−1 → L

H → L+1

λ (nm)

E (eV)

f

TDM (deg)

0.46

0.54

0.53

0.40

0.26 0.21 0.23 0.29

0.31 0.21 0.18 0.28

0.47

0.53

0.54 0.43

0.45 0.57

0.56

0.38

0.25 0.22 0.18 0.35 0.20 0.24 0.21 0.35

0.30 0.23 0.14 0.32 0.27 0.29 0.15 0.28

0.42

0.58

0.58

0.41

587 533 374 368 594 539 379 373 601 544 384 378 600 545 384 378 602 547 384 379 610 553 389 384 616 557 393 387

2.11 2.33 3.31 3.37 2.09 2.30 3.27 3.32 2.06 2.28 3.23 3.28 2.07 2.28 3.23 3.28 2.06 2.27 3.23 3.27 2.03 2.24 3.19 3.23 2.01 2.22 3.16 3.20

0.0006 0.0014 1.35 1.55 0.0010 0.0028 1.48 1.65 0.0021 0.0024 1.54 1.74 0.0016 0.0038 1.52 1.77 0.0079 0.018 1.68 1.72 0.010 0.019 1.67 1.85 0.013 0.024 1.71 1.94

0 90 0 90 20 65 80 5 0 90 0 90 90 0 90 0 36 35 10 80 18 65 8 90 0 90 0 90

a

The calculations were performed for the structures in Chart 2 in toluene. The MOs are abbreviated as HOMO−1 (H−1), HOMO (H), LUMO (L), and LUMO+1 (L+1). A TDM angle of 0° is vertically up on the structures as shown in Figure 9. bThe proton axis does not lie between the two phenyl groups (Figure 9). cThe proton axis lies between the two phenyl groups (Figure 9).

3.3.3. Comparison of Measured and Predicted Spectral Properties. Figure 10 compares the measured spectra (panel A), the spectra predicted by the TDDFT calculations (panel B) and spectra simulated using the four-orbital model (panel C) for the two bookend porphyrins (H2P-0 and H2P-4). In progressing from H2P-0 to H2P-4, the TDDFT calculations correctly predict bathochromic shifts in the near-UV (overlapped) S3 and S4 (B) bands and bathochromic shifts in the visible S1 and S2 (Q) bands. The hyperchromic effects on the S1 and S2 transitions are predicted. However, the TDDFT calculations predict that the visible bands are far weaker compared to the near-UV bands than is observed. Note that the spectra are normalized as described for Figure 2; the different peak intensity of the blue-violet feature on each spectrum stems primarily from variation in overlap of the underlying S3 and S4 contributions (and the Gaussian widths given to the features in the calculated and simulated spectra). Simulations using DFT MO energies input into the fourorbital model also reproduce the bathochromic shifts in all absorption bands with increasing number of phenyl groups (Figure 10C). The simulations also give the hyperchromic effect on the S1 and S2 transitions with relative intensities that more closely resemble the measured spectra. That such simulations would reproduce the spectral trends can be seen from the outset from the trends in MO energies in Figure 8, which are the only variables among the porphyrins in the simulations. Again, the preponderant effect in progressing from zero to four phenyl groups is the increasing energy of the

Figure 8. Energies of frontier MOs versus number of phenyl groups. The MO energies are from DFT calculations on structures in toluene. The values are the slopes of the trend lines.

four-orbital set factor into two symmetry adapted pairs. The TDMs for the four transitions lie along the Cartesian axes, which necessary bisect either the pyrrole/pyrrolenine (i.e., nonprotonated pyrrolic) nitrogen atoms or the methine carbons. In the absence of in-plane symmetry, the four-one electron promotions can mix in any proportion, and the TDMs can assume any orientation in the plane of the macrocycle. Such mixing of states of different polarizations, and the additional effects of vibronic coupling have been noted for a number of chlorophyllides.53 I

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Figure 10. Comparison of absorption spectra that are measured (A), calculated using TDDFT (B), and simulated using the four-orbital model (C), all normalized as described for the measured spectra in Figure 2. Peak positions are given for the S1(0,0) and S2(0,0) bands. Spectra obtained from TDDFT calculations employed data given in Table 3. Spectra simulated using the four-orbital model employed a shift of 0.4 eV to align the energy centers of gravity of the MO and spectral domains and configuration-interaction energies of 0.45 and 0.35 eV for configurations polarized along or perpendicular to the proton axis, respectively, to split the Q bands. Gaussian skirts of 400 cm−1 for S1 and S2 bands and 600 cm−1 for S3 and S4 bands were used for TDDFT and simulated spectra.

and oscillator strengths with phenyl substitution are proportionately far greater for S1 and S2 than those for S3 and S4. The S1 and S2 transitions are much weaker than S3 and S4 and have near minimum intensity for H2P-0. Thus, as the S1 and S2 transitions (Q bands) gain intensity at the expense of S3 and S4 transitions (B bands) with incorporation of phenyl groups, the fractional change in intensity of the former pair is much more pronounced than the latter pair. There may be additional factors at play that remain to be elucidated. 3.3.4. Synopsis of the Analysis in Historical Context. The MO results depicted in Figure 7 for H2P-4 indicate that the HOMO is the a2u(π)-like orbital and the LUMO is an eg(π*)like orbital with a reflection plane along the proton axis. Thus, the HOMO → LUMO electron promotion gives an excitedstate configuration with the TDM polarized along the proton axis, as does the HOMO−1 → LUMO+1 promotion. TDDFT calculations indicate that the S1 excited state is composed of these two configurations (Table 3) and confirm that the TDM is aligned with the proton axis (Figure 9). The end result is the same for H2P-0 but is derived from parallel swaps in empty orbitals and excited-state configurations. In particular, although for H2P-0 the S1 state is now composed of HOMO−1 → LUMO and HOMO → LUMO+1 (Table 3), because LUMO and LUMO+1 are swapped (Figure 7) the S1 TDM remains proton-axis polarized (Figure 9). At first glance, it might seem unexpected that the lowest excited state of H2P-0 is not borne from the HOMO →

Figure 9. TDM directions. The central proton axis is vertical in all cases except H2P-2c′. For each porphyrin, the TDM magnitudes have been multiplied times 5 for S1 and S2 and times 1 for S3 and S4. A TDM angle of 0° is vertically up on the structures as shown.

HOMO with relatively little change in the LUMO+1, LUMO, and HOMO−1 (Figure 8). Preferential destabilization of the HOMO has two main spectral effects. First, the increasingly lower average energy gap between filled and empty orbitals affords the bathochromic shift in the center of gravity of the absorption spectrum. Second, the increasing energy difference between excited-state configurations HOMO → LUMO/LUMO+1 versus HOMO−1 → LUMO/LUMO+1 reduces configurational mixing, reduces destructive interference between transition dipoles, and increases the S1 and S2 transition intensities at the expense of the S3 and S4 transitions. This framework gives insight into the finding from the TDDFT calculations that variations in TDMs J

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axis along which the TDM of S1 is again directed as well as an axis that bisects the reduced ring(s). Resonance structures and extensive experiments indicate that the nitrogens of the reduced pyrrole ring(s) are not protonated in the lowest energy tautomer.55 The central proton axis has been designated y for chlorins and bacteriochlorins (Chart 3). [The inconsistency of the IUPAC nomenclature to protonate the nitrogen of the reduced pyrrole ring of free base chlorins and the opposite for bacteriochlorins has also been noted.56] The free base porphyrins studied herein present a selfconsistent set in which the HOMO/HOMO−1 energy ordering remains constant (Figure 7), the HOMO orbital smoothly increases in energy with the increasing number of phenyl substituents (Figure 8), and the observed progressive bathochromic shifts and hyperchromic effects on the S1 and S2 transitions (Q bands) can be qualitatively reproduced by TDDFT calculations and simulations using the four-orbital model (Figure 10). To avoid confusion, and because the free base porphyrins with one phenyl, two trans-phenyl, or three phenyl substituents would not allow it, we have avoided the conventional x and y designations of the B and Q bands (Figure 1, blue font). Regardless, our findings show that free base porphyrins (with D2h symmetry) have the S1 transition polarized along the proton axis as is also the case for synthetic chlorins and bacteriochlorins as well as natural photosynthetic pigments (Chart 3), revealing consistency across all three tetrapyrrole families. From an electronic-structure point of view, there is no reason why the conventional x and y labels on the Q and B bands for free base porphyrins (Figure 1) could not be reversed to display such consistency. One outcome of this presentation may be to clarify any misconceptions: (1) It may be incorrectly thought that the proton axis has been historically denoted y for free base forms of all three tetrapyrrole families (porphyrins, chlorins, bacteriochlorins) because of familiarity with the latter two classes, particularly the photosynthetic pigments. (2) It may be incorrectly thought that the S1 for porphyrins is not polarized along the proton axis because the long wavelength absorption band is traditionally designated Qx for these tetrapyrroles. In addition to a comprehensive analysis of factors that underlie spectral trends, the present work has demonstrated that the yields and rate constants for the fluorescence, intersystem-crossing and internal-conversion decay pathways of the S1 excited state vary relatively smoothly with successive incorporation of phenyl groups starting with parent porphine and culminating in tetraphenylporphyrin. Collectively, the study has afforded fundamental insights into the photophysical properties and electronic structure of the widely used mesophenylporphyrins.

LUMO promotion. The situation is a consequence of substantial configuration interaction along with very small energy gaps between the HOMO and HOMO−1 and between the LUMO and LUMO+1 that underlie small energy differences between the excited-state configurations. A crude calculation indicates that the MO energy gaps (Figures 7 and 8 and Table S1) weighted by the configurational contributions (Table 3) produce the expected lower energy for S1 than S2, even though S2 rather than S1 is derived in part from the HOMO → LUMO promotion. The two empty orbitals are so close in energy, however, that the order may change depending on the details of the DFT calculation (e.g., functional, in vacuum or solvent) for a given porphyrin (e.g., H2P-0 or H2P4). MINDO calculations on H2P-0 in the early 1980s also had S1 composed of HOMO−1 → LUMO mixed with HOMO → LUMO+1 promotions, and S2 arising from HOMO → LUMO mixed with HOMO−1 → LUMO+1 promotions.54 However, the ordering within the HOMO/HOMO−1 and LUMO/ LUMO+1 pairs were both reversed from the orderings found here by DFT calculations (Figure 7). Such a difference is not without precedent, as early calculations using a variety of methods differ in the orbital orderings, including whether the a2u(π)- or a1u(π)-like filled orbital is the HOMO.30 Regardless, the MINDO calculations, like our TDDFT calculations, have the S1 transition polarized along the proton axis. Gouterman presented an analysis in his seminal 1959 paper on the four-orbital model as to why the TDM for the S0 → S1 transition of H2P-0 would be polarized along the central proton axis.52 That view has been an apparent common outcome over the intervening decades from a variety of calculations, including those presented herein, despite differences in the HOMO/ HOMO−1 and LUMO/LUMO+1 energy orderings and the Cartesian coordinate chosen for the proton axis. The classic paper had the proton axis as x and the other in-plane axis as y,52 although other conventions have been used.54 Arguments were given that the HOMO of H2P-0 is the a1u(π)-like orbital and that HOMO−1 is the a2u(π)-like orbital,41,52 predating most quantum-chemistry calculations. The argument involved the relative intensities of the Q bands in spectra of H2P-0 versus analogues bearing eight β-pyrrole alkyl groups. It was suggested that the observations would be more easily explained if the a1u(π)-like orbital was the HOMO and did not cross the a2u(π)like orbital in energy when destabilized by the alkyl substituents compared to H2P-0. Although logical, the end result can be obtained for the other HOMO/HOMO−1 ordering depending on the relative magnitudes of the substituent effect and the original spacing of these orbitals along with the relative dispositions of the empty orbitals. A related quandary under the four-orbital model is that if the ordering of the HOMO and HOMO−1 is reversed but their spacing remains the same, the spectral positions may change but the relative intensities will not. The historical designation of the S1 state and the longwavelength absorption band as Qx rather than Qy (Figure 1 , blue font) was predicated on two views: (1) that the a1u(π) orbital is the HOMO for H2P-0, and (2) the x direction is coincident with the central proton axis. The (arbitrary) choice of the x axis as the proton axis for H2P-0 and the (correct) assignment that the TDM of the S1 state lies along the proton axis was soon recognized as posing an inconsistency for applications of the model to chlorins and bacteriochlorins.52 These reduced porphyrins (hydroporphyrins) have a proton



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b09483. Complete citation for reference 26, synthesis of H2P-1, H2P-2c, and H2P-3 along with complete characterization data, and a table of MO energies along with various energy gaps (PDF)



AUTHOR INFORMATION

Corresponding Authors

*J. S. Lindsey. E-mail: [email protected] Tel: 919-515-6406. K

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The Journal of Physical Chemistry A *D. F. Bocian. E-mail: [email protected] Tel: 951-8273660. *D. Holten. E-mail: [email protected] Tel: 314-935-6502.

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ORCID

Dariusz M. Niedzwiedzki: 0000-0002-1976-9296 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS



REFERENCES

This work was supported by a grant from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, of the U.S. Department of Energy (DE-FG0205ER15651). Excited-state lifetimes were measured in the Ultrafast Laser Facility of the Photosynthetic Antenna Research Center (PARC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0001035. Mass spectra were obtained at the Mass Spectrometry Laboratory for Biotechnology at North Carolina State University. Partial funding for the facility was obtained from the North Carolina Biotechnology Center and the National Science Foundation.

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