Tailoring Panchromatic Absorption and Excited-State Dynamics of

Article pubs.acs.org/JACS

Tailoring Panchromatic Absorption and Excited-State Dynamics of Tetrapyrrole−Chromophore (Bodipy, Rylene) ArraysInterplay of Orbital Mixing and Configuration Interaction Amit Kumar Mandal,† James R. Diers,§ Dariusz M. Niedzwiedzki,‡ Gongfang Hu,∥ Rui Liu,∥ Eric J. Alexy,∥ Jonathan S. Lindsey,*,∥ David F. Bocian,*,§ and Dewey Holten*,† †

Department of Chemistry and ‡Photosynthetic Antenna Research Center, Washington University in St. Louis, St. Louis, Missouri 63130-4889, United States § Department of Chemistry, University of CaliforniaRiverside, Riverside, California 92521-0403, United States ∥ Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States S Supporting Information *

ABSTRACT: Three sets of tetrapyrrole−chromophore arrays have been examined that exhibit panchromatic absorption across large portions of the near-ultraviolet (NUV) to nearinfrared (NIR) spectrum along with favorable excited-state properties for use in solar-energy conversion. The arrays vary the tetrapyrrole (porphyrin, chlorin, bacteriochlorin), chromophore (boron-dipyrrin, perylene, terrylene), and attachment sites (meso-position, β-pyrrole position). In all, seven dyads, one triad, and nine benchmarks in toluene and benzonitrile were studied using steady-state and time-resolved absorption and fluorescence spectroscopy. The results were analyzed with the aid of density functional theory (DFT) and time-dependent DFT calculations. Natural transition orbitals (NTOs) were constructed to assess the net change in electron density associated with each NUV−NIR absorption transition. The porphyrin− perylene dyad P-PMI displays the most even spectral coverage from 400 to 700 nm, with an average ε ∼ 43 000 M−1 cm−1. A significant contributor is a chromophore-induced reduction in the configuration interaction involving the four frontier molecular orbitals of benchmark porphyrins and associated constructive/destructive transition-dipole interference that results in intense (ε ∼ 400 000 M−1 cm−1) NUV and weak (1 ns) S1 state in both polar and nonpolar media. We have extended the approach from dyads to arrays that include up to four perylenes per porphyrin.22 Recently,

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MATERIALS AND METHODS

Synthesis. Syntheses of the compounds have been reported previously, as noted: meso-ethynylporphyrin dyads (P-PMI,22 CPMI,21 BC-PMI,21 P-BDPY,28 P-TMI28), β-linked dyads (P-PMI13, C-PMI13),28 β-linked triad (BC-PMI3,13),28 and benchmarks (P-Ph,21 C-Ph,21 BC-Ph,21 P-Ph13,28 C-Ph13,28 BC-Ph3,13,29 BDPY-Ph,28 PMIPh,28 TMI-Ph28).

Chart 1. Meso-Linked Porphyrin−Chromophore Dyads

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DOI: 10.1021/jacs.7b09548 J. Am. Chem. Soc. 2017, 139, 17547−17564

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Journal of the American Chemical Society Chart 2. Meso-Linked Tetrapyrrole−Perylene Dyads

Chart 3. β-Linked Tetrapyrrole−Perylene Dyads

Photophysical Measurements. Photophysical studies of the arrays and benchmarks in toluene and benzonitrile were carried out on dilute (micromolar) argon-purged solutions. Fluorescence quantum yields were measured using an integrating sphere (Quanta-φ, Horiba). S1 lifetimes were determined by transient absorption (TA) spectroscopy employing ∼100 fs excitation flashes from an ultrafast laser system (Spectra Physics). Acquisition of difference spectra (400−900 nm) from ∼100 fs to ∼7.5 ns utilized a spectrometer that employed ∼100 fs white-light probe pulses (Ultrafast Systems, Helios) or probing on the time scale from ∼100 ps to ∼0.5 ms using a white-light

pulsed laser (∼1 ns rise time) in 100 ps time bins (Ultrafast Systems, EOS). TA studies also afforded the yield of S1 → T1 intersystem crossing by comparing the magnitude of bleaching of the ground-state absorption bands due to T1 at the asymptote of the S1 decay versus the bleaching magnitude due to S1 immediately after excitation. TA data sets were analyzed at individual wavelengths and globally using Surface Xplorer (Ultrafast Systems), CarpetView (Light Conversions), and OriginPro (OriginLab). Time profiles were fit to the convolution of the instrument response with a series of exponentials plus a constant. 17549


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Journal of the American Chemical Society Chart 4. Benchmark Compounds for Photophysical Studies

Evolution-associated difference spectra (EADS) assumed a sequential model for the excited-state dynamics. Density Functional Theory Calculations. DFT calculations were performed with Gaussian 09 version D.01. Calculations were performed using the PCM model for the arrays in toluene except for a few studies using benzonitrile as noted.30 Molecular geometries were fully optimized using the hybrid B3LYP functional and the basis set 631++G**. These calculations used Gaussian defaults with the exception of the keywords SCF = (YQC, NoVarAcc, NoIncFock) to aid in SCF convergence. TDDFT calculations were performed using the long-range-corrected ωB97XD functional and the basis set 6-31+ +G**. These calculations used Gaussian defaults with the exception of the keywords TD (nStates = 16) and SCF = (maxcycle = 512, NoVarAcc, NoIncFock).

BDPY), perylene (P-PMI), or terrylene (P-TMI) linked to a common porphyrin at the meso-position in toluene are shown in Figure 2. [Spectra in benzonitrile are given in Figures S1 and S2 of the Supporting Information (SI).] Each panel of the figure also shows the spectra of the benchmark chromophore (BDPY-Ph, PMI-Ph, TMI-Ph) and the benchmark porphyrin (P-Ph), for which the intense near-ultraviolet (NUV) Soret band has been halved to keep it on scale with the other features and spectra. The spectrum of each dyad contains features not present in the spectra of the benchmarks and is not just the sum of the spectra of constituents. Relative to the porphyrin benchmark, the spectrum of each dyad illustrates a dramatic shift of intensity from the porphyrin NUV (Bx, By) absorption into the longer-wavelength (Qx, Qy) regions. The bathochromic shift of the longest-wavelength feature of the dyad versus the common porphyrin benchmark follows the order P-PMI < P-

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RESULTS Absorption and Fluorescence Spectra. The absorption spectra for the three dyads consisting of a boron-dipyrrin (P17550


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Journal of the American Chemical Society

Figure 2. Absorption spectra in toluene of porphyrin−chromophore dyads (red), benchmark porphyrin (blue), and benchmark chromophores (green), which are boron-dipyrrin (A), perylene (B), and terrylene (C). The spectra are plotted to reflect approximate molar absorptivity (ε), with the vertical tick marks incremented by 50 000 M−1 cm−1.28 The NUV Soret band of porphyrin benchmarks (dashed blue) have been multiplied by 0.5 to keep on scale with the other features.

Figure 3. Absorption (solid) and fluorescence (dashed) spectra of tetrapyrrole−chromophore arrays in toluene. The amplitude of the vertical arrow at the top (corresponding to a molar absorptivity of 50 000 M−1 cm−1) applies to all absorption spectra,28 except for the NUV Soret band of P-BDPY, which has been reduced in intensity by a factor of 3 to stay on the same scale as the other features.

BDPY < P-TMI. This trend tracks the peak absorption wavelength of the parent perylene (536 nm), boron-dipyrrin (544 nm), and terrylene (655 nm) benchmark. The distribution of intensity across the spectral region encompassed is most even and the uniformity of the panchromatic absorption is greatest for P-PMI. The absorption spectra of the same three meso-linked porphyrin dyads (Chart 1) are reproduced in Figure 3 (blue). These spectra are compared with those for dyads in which the common perylene is attached at the meso-position of a chlorin (C-PMI) or bacteriochlorin (BC-PMI). Figure 3 also shows the spectra of arrays in which the perylene is attached instead at the β-pyrrole position of the porphyrin (P-PMI13), chlorin (CPMI13), or bacteriochlorin (BC-PMI3,13), with the latter being a triad. [Comparison of the spectra of all arrays and benchmarks in toluene is given in Figure S3 (SI).] Pairwise comparison of arrays in which the perylene is attached to the porphyrin at the β-pyrrole versus meso-positions shows the greatest difference in the case of the perylene, with a hypsochromic shift in the long-wavelength absorption feature that affords a collapse in the wavelength spread. In the case of the chlorin pair and the bacteriochlorin pair, β-pyrrole attachment results in a small bathochromic shift relative to meso attachment. The longest wavelength reached is greater for both (meso or β-pyrrole attached) bacteriochlorins compared to both chlorins, consistent with the relative Qy positions of the benchmarks. Fluorescence spectra of all eight arrays in toluene are shown in Figure 3 (dashed lines). Emission spectra for the benchmarks

and arrays in toluene are compared in Figure S3 (SI). The fluorescence of each array is shifted by a variable amount to longer wavelength than the S0 → S1 absorption band. The spectral properties are listed in Table S1 (SI). The fluorescence spectrum of most arrays is similar to that of the corresponding tetrapyrrole benchmark but is shifted bathochromically, consistent with the shift in the S0 → S1 absorption band of the array versus the benchmark. Compared to the other arrays, the fluorescence of P-TMI is broader, and that for P-BDPY is broader still. In general, as the fluorescence features become broader, there is a larger shift between the S1 → S0 fluorescence and S0 → S1 absorption peaks. This (Stokes) shift (in cm−1) for the arrays in toluene increases in the following order: C-PMI13 (44) < C-PMI (132) < P-PMI (146) < BC-PMI (167) < BCPMI3,13 (283) < P-PMI13 (344) < P-TMI (539) < P-BDPY (1088). The fluorescence spectra of the arrays and benchmarks in benzonitrile (Figure S2, SI) are generally similar to those in toluene. However, there is a substantial quenching of S1 for some arrays in the more polar solvent versus the same arrays in toluene and compared to the corresponding tetrapyrrole benchmarks in either solvent. For such arrays, some emission from more highly fluorescent trace species (residual monomeric tetrapyrrole or chromophore) is evident when the emission spectrum is plotted on a greatly expanded vertical scale required to see the (quenched) emission from S1 of the array. This effect distorts the overall emission shape and precludes reliable determination of the absorption−fluorescence shift for these arrays. 17551


17 10 19 20 19 40 24 11 40 25 27 35 80 13 20

toluene PhCN toluene PhCN toluene PhCN toluene PhCN toluene PhCN toluene PhCN toluene PhCN toluene PhCN

P-BDPY

0.27 0.04 0.31 0.15 4.7 1.3 1.3 0.025 3.2 0.14 4.4 0.035 2.8 0.03 1.8 0.05b

τ2 (ns) 0.020 0.005 0.053 0.015 0.38 0.21 0.27 BC-PMI (2.8) > BCPMI3,13 (1.8) > P-PMI13 (1.3). For comparison, the lifetimes (in nanoseconds) of the benchmarks are as follows: P-Ph (11), P-Ph13 (11), C-Ph (13), C-Ph13 (8.2), BC-Ph (3.9), and BCPh3,13 (3.5). The intersystem-crossing yields for the tetrapyrrole−perylene arrays in toluene are in the range 0.19−0.54 compared to 0.36−0.80 for the benchmarks, generally following the (usual) trend: porphyrin > chlorin > bacteriochlorin. Similar results are found for terrylene analogue P-TMI, except that the S1 lifetime is shortened to 310 ps. The S1 lifetime is shortened further and the TA spectra take on additional characteristics when the chromophore is switched to a boron-dipyrrin in P-BDPY (Figure 4D−F). The TA difference spectrum at 0.5 ps in this case (Figure 4D) contains bleaching of all ground-state absorption bands (Figure 4F, solid) and stimulated emission in the vicinity of the spontaneous fluorescence (Figure 4F, dashed), for a variety of excitation wavelengths. The TA difference spectrum evolves with time constants of 17 and 270 ps and has decayed to zero by ∼1 ns. The 17 ps EADS from global analysis (Figure 4E) shows prominent stimulated emission expected for S1. The 270 ps component could be a product of S1 decay, such as a chargeseparated state, or is the relaxed form of S1 in which the stimulated emission is weaker, broader, and further shifted from the S0 → S1 bleaching, consistent with the spontaneous fluorescence (Figure 4F), thereby making a less prominent contribution to the difference spectrum. Regardless, P-BDPY and P-TMI in toluene show significant shortening of the S1 lifetime relative to the six tetrapyrrole−perylene arrays in this solvent, with no improvement in, and in some respects less, panchromaticity of the absorption spectrum (Figure 3). The short S1 lifetime for P-BDPY and P-TMI in toluene gives no detectable T1 and thus Φisc < 0.03 (Table 1). Turning to the excited-state properties of the arrays in benzonitrile, the overarching finding is a shortening of the S1 lifetime compared to the same arrays in toluene. The lifetime reduction is the least substantial (a factor of 3.6) for P-PMI, which has τS = 1.3 ns in benzonitrile compared to 4.7 ns in toluene. The shortening is the most dramatic (a factor of 125) for C-PMI13, which displays τS = 35 ps in benzonitrile versus 4.4 ns in toluene. The S1 lifetime varies among the tetrapyrrole−chromophore arrays in benzonitrile as follows: P-PMI (1.3 ns) > P-TMI (150 ps) > C-PMI (140 ps) > BCPMI3,13 (50 ps) > P-BDPY (40 ps) > C-PMI13 (35 ps) > BCPMI (30 ps) > P-PMI13 (25 ps). The fluorescence yield of each array is generally reduced in benzonitrile relative to that in toluene in proportion to the reduction in τS (Table 1); this finding is consistent with the solvent effect being primarily on the rate constant for S1 → S0 internal conversion with the radiative rate constant being relatively unchanged (vide infra). The triplet yield in all cases is Φisc ≤ 0.05 (Table 1). The arrays in benzonitrile also have a (relaxation) component with a shorter lifetime (10−40 ps) than in toluene (Table 1). In addition to the shortened S1 lifetime, most of the arrays in benzonitrile (Figures S4−S14, SI) show more substantial spectral changes as a function of time than observed for the arrays in toluene. For example, the raw TA data at 0.5 ps (Figure 4G) and 25 ps EADS (Figure 4H) for C-PMI in benzonitrile show bleaching of the S0 → S1 absorption bands and S1 → S0 stimulated emission. The raw spectra at longer times and the 140 ps EADS show a broad excited-state absorption centered at ∼700 nm that masks part of the 17554


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Figure 5. MO correlation diagram for dyad P-PMI, porphyrin benchmark P-Ph, and perylene benchmark PMI-Ph. The MOs for the dyad that are indicated with the blue font are essentially porphyrin MOs (nodes at the linker attachment site); those indicated in the red font are mixtures of the perylene and porphyrin orbitals.

comparisons by summarizing the energies of the frontier MOs for all eight arrays (and benchmarks) absent the orbitals. Figures S30−S33 (SI) give the permanent dipole moments and directions for the S0 and S1 states of the arrays from TDDFT. The orbitals shown for the tetrapyrrole benchmarks are the highest occupied MO (HOMO), the lowest unoccupied MO (LUMO), the HOMO−1, and the LUMO+1. Within the Gouterman four-orbital model,32−34 the NUV−NIR absorption spectra involve mixtures of excited-state configurations that result from one-electron promotions among these four orbitals. For the meso-linked dyads, the HOMO or HOMO−1 of the tetrapyrrole has little or no electron density at the attachment site. Thus, this orbital has little or no linker-mediated interaction with the chromophore HOMO, and the associated orbital of the dyad has little or no electron density on the chromophore. This is seen to be the case for the HOMO−1 of P-Ph (−7.11 eV), which gives rise to the HOMO−2 of dyad PPMI (−7.26 eV), indicated by the blue font in Figure 5. The same is true of the LUMO+1 of P-Ph (−1.35 eV), which affords the LUMO+2 of P-PMI (−1.33 eV). On the other hand, the HOMO of P-Ph (−6.67 eV) mixes with the perylene HOMO (−7.06 eV) to give the P-PMI HOMO (−6.57 eV) and HOMO−1 (−7.10 eV), split by 0.53 eV, with electron density on both units, albeit perhaps not equal. Similarly, interaction of the P-Ph LUMO (−1.60 eV) and PMI-Ph LUMO (−1.63 eV) affords the P-PMI LUMO (−1.80 eV) and LUMO+1 (−1.36 eV), split by 0.44 eV, with electron density on both units. The energies of the orbitals of P-PMI that result from mixing of the porphyrin and perylene orbitals are indicated in the red font in Figure 5. The same is

bleaching (and perhaps stimulated emission) in this region; such a feature would be consistent with a state that has some perylene anion character. Spectral changes in the red region are also seen when the chromophore is changed from perylene to terrylene or boron-dipyrrin, as indicated by the TA data for PBDPY in benzonitrile in Figure 4J−L. In this case, fitting the time evolution of the TA difference spectrum requires two components with time constants of 10 and 40 ps; the former has pronounced S1 → S0 stimulated emission, whereas the latter has a more prominent red-region transient absorption feature. Regardless of assignments, the combined results for the arrays in benzonitrile show that only P-PMI retains a long (>1 ns) S1 lifetime in the polar solvent. Molecular-Orbital Characteristics. DFT and TDDFT calculations were performed to aid analysis of the ground-state absorption spectra and excited-state properties of the tetrapyrrole−chromophore arrays. Key molecular orbital (MO) characteristics from DFT are highlighted first. Then the absorption spectra obtained from TDDFT are compared with the measured spectra, and the electronic configurations contributing to each excited state (transition) are given. This is followed by a description of the natural transition orbitals (NTOs), which give insight into changes in electron density that accompany the transition.31 Figure 5 is an MO correlation diagram that shows the energies and electron-density distributions of the frontier MOs of dyad P-PMI and benchmarks P-Ph and PMI-Ph obtained from DFT; Figures 6 and 7 give analogous diagrams for PBDPY and P-TMI, respectively, and their benchmarks. The same types of plots for the other five dyads (and their benchmarks) are given in Figures S15−S19 (SI). Figure 8 aids 17555


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Figure 6. MO correlation diagram for dyad P-BDPY, porphyrin benchmark P-Ph, and boron-dipyrrin benchmark BDPY-Ph. The MOs for the dyad that are indicated with the blue font are essentially porphyrin MOs (nodes at the linker attachment site); those indicated in the red font are mixtures of the boron-dipyrrin and porphyrin orbitals.

overlap with the measured one, and the two spectra were normalized at the highest peak. Figure 9D shows a similar comparison for the porphyrin benchmark P-Ph. Figure 9B,C and 9E,F gives such results for dyads P-BDPY and P-TMI and benchmarks BDPY-Ph and TMI-Ph. Figures S20 and S21 (SI) show the comparisons for the other five arrays (and benchmarks). An important question is the extent to which charge is shifted from one constituent to the other as a consequence of any of the S0 → Sn transitions. This question arises because various electronic configurations contribute to each excited state (and absorption band) for each array, and many of the MOs involved have electron density on both the tetrapyrrole and chromophore to varying degrees. This issue is particularly important for formation and decay of S1, which dominates the photophysical properties summarized in Table 1, including the solvent-polarity dependence. For example, even given the list of excited-state configurations from TDDFT in Table 2 and the MOs in Figure 5 for P-PMI in toluene, it is difficult to deduce if the S0 → S1 transition is accompanied by a significant shift of electron density from porphyrin to perylene (or vice versa). This type of information is better represented in the characteristics of the NTOs.31 The construction of these orbitals takes into account the contributing configurations to give the net electron-density distributions of the occupied orbital and the virtual (empty) orbital as if a single one-electron promotion were involved. The NTOs for formation of the first five excited states (absorption transitions) for P-PMI in toluene are shown in Figure 10, those for P-BDPY are shown in Figure 11, and those

true for P-BDPY (Figure 6), P-TMI (Figure 7), and all the other arrays (Figures S15−S19, SI). The arrays that utilize the β-pyrrole position of the tetrapyrrole (P-PMI13, C-PMI13, BC-PMI3,13) have some amount of electron density at the linker site for all four frontier MOs, affording some extent of interaction with the perylene HOMO or LUMO. Additionally, for triad BCPMI3,13, the presence of two perylenes means that this array will have four filled orbitals and four empty orbitals; one filled and one empty triad orbital has mainly perylene character, as is indicated by the green font in Figures 8 and S19 (SI). Table 2 shows the calculated wavelengths and oscillator strengths ( f) of absorption from the ground state (S0) to the first five excited states (S1−S5) of P-PMI in toluene obtained from TDDFT calculations. Tables S3−S18 (SI) give this information for the seven other arrays and benchmarks. These tables also identify the electronic configurations that contribute to the excited state along with the percent contributions. A key point that emerges from the TDDFT calculations (e.g., Table 2 for P-PMI) is that each of the first five excited states is comprised of multiple electronic configurations. For example, S1 (and the lowest energy absorption band) for P-PMI is derived from four configurations with contributions of 12−38% and four more that contribute 3−9%. Figure 9A compares the measured spectrum for P-PMI with that calculated by TDFFT, using the data in Table 2. The calculated transitions (black sticks) for states S1−S16 were each given a 10 nm Gaussian skirt and summed to produce the spectrum shown (blue line). The calculated spectrum was shifted by an arbitrary amount (1200 cm−1) to provide good 17556


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Figure 7. MO correlation diagram for dyad P-TMI, porphyrin benchmark P-Ph, and terrylene benchmark TMI-Ph. The MOs for the dyad that are indicated with the blue font are essentially porphyrin MOs (nodes at the linker attachment site); those indicated in the red font are mixtures of the terrylene and porphyrin orbitals.

productive use requires favorable S1 properties to drive subsequent processes, such as energy/electron transfer, that initiate chemistry or photocurrent. A significant unknown has been how to simultaneously afford (1) panchromatic absorption across a broad span of the NUV−NIR spectrum, (2) a discrete S1 state that is produced quantitatively no matter the excitation wavelength (analogous to internal conversion in a simple chromophore), and (3) a long S1 lifetime. Our recent work in this regard has explored novel molecular lightharvesting architectures comprised of a tetrapyrrole and one or more chromophores that communicate strongly with one another via direct ethyne linkages, including tetrapyrrole− chromophore dyads,21,22,28 several triads,22,27 and a few larger arrays.22 The present work has quantified and greatly extended these initial studies by establishing the connections among (1) absorption spectra, (2) excited-state properties, including the yields and rate constants of the S1 decay routes, and (3) the MO properties and electronic structure of the arrays. These relationships are analyzed in the following sections. We first discuss the effect of configuration interaction (CI) on the optical properties of the tetrapyrrole−chromophore arrays relative to tetrapyrrole benchmarks. We then turn to the effects of orbital mixing on the excited-state manifolds and spectral properties. Finally, we relate these factors to the excited-state properties, including the S1 lifetime. Effects of Configuration Interaction. The NUV−NIR absorption spectra of monomeric tetrapyrroles are dominated by transitions from the ground state (S0) to four excited states (S1−S4). For the free base phenylethyne-substituted tetrapyrrole benchmarks studied here, which include P-Ph, C-Ph, and

for P-TMI are shown in Figure 12. The NTOs for the other arrays and key benchmarks are given in Figures S22−S29 (SI). The NTOs show that in most cases the transitions do not involve a significant shift in electron density from one part of an array to another. For example, NTOs for P-PMI indicate that transitions from S0 to S1 (704 nm), S2 (641 nm), S4 (429 nm), and S5 (423 nm) are predominantly localized on the porphyrin, whereas a transition to S3 (532 nm) is essentially localized on the perylene (Figure 10). The same is true for P-BDPY (Figure 11). Indeed, the HOMO → LUMO promotion using the standard MOs (Figure 6) suggests that S0 → S1 might have significant porphyrin-to-boron-dipyrrin charge-transfer character. However, this promotion makes only a 23% contribution to the overall transition (Table S5, SI) and the NTOs indicate that it is largely localized on the porphyrin, with a small amount of boron-dipyrrin-to-porphyrin charge-transfer character (opposite the charge-transfer character that would be inferred by only considering the HOMO−LUMO promotion). In contrast to PPMI and P-BDPY, the occupied and virtual NTOs of S1 (711 nm) and S2 (651 nm) for P-TMI have electron density spread across the porphyrin and terrylene. The S0 → S3 (604 nm), S0 → S4 (438 nm), and S0 → S5 (423 nm) transitions primarily involve the porphyrin. Thus, for P-TMI, there is no absorption transition in the visible region like S0 → S3 for P-PMI, PBDPY, and all the other PMI-based arrays that is predominantly chromophore based.

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DISCUSSION Panchromatic absorption is important for a light-harvesting system to make maximum use of the solar spectrum, yet 17557


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Figure 8. MO correlation diagram for the tetrapyrrole−chromophore arrays and benchmarks. The MOs for the arrays that are indicated with the blue font are essentially porphyrin MOs; those indicated in the red font are mixtures of orbitals of the tetrapyrrole and chromophore that are connected by the dashed lines. The MOs for triad BC-Ph3,13 that are indicated with the green font are essentially perylene MOs. The LUMO of BCPh and BC-Ph3,13 and the LUMO+2 of BC-PMI and BC-PMI3,13 are off-scale at higher energy, as shown.

constructive and destructive interference gives rise to strong B-bands and weak Q-bands.35 In proceeding to chlorin C-Ph and bacteriochlorin BC-Ph, the successive reduction of one and then two pyrrole rings preferentially raises the HOMO and LUMO+1 in energy (Figure 8), decreasing the constructive/destructive transitiondipole interference and increasing the intensity and bathochromic shift of Qy with a commensurate decrease and hypsochromic shift of By (Figures S2 and S3, SI). Similarly, peripheral substituents on a tetrapyrrole can alter MO energies and thereby the magnitude of CI and shift intensity from By to Qy (and Bx to Qx) as the B-band generally moves to higher energy and the Q-band to lower energy.32−34,36 These macrocycle and substituent effects primarily alter the electronic mixing (Θ) by adjusting the MO energy spacings and thus the energy-denominator (ΔE) in the mixing formula (eq 1)

BC-Ph (Chart 4), the four excited states from lower to higher energy (absorption bands from longer to shorter wavelength) using the common nomenclature are Qy (S1), Qx (S2), Bx (S3), and By (S4). These excited states and associated absorption transitions arise from linear combinations of the one-electron promotions involving the four frontier MOs, i.e., CI. These MOs are shown for P-Ph on the left side of Figure 5 (and again in Figures 6 and 7). Using D4h symmetry notation, the MOs are the a1u(π)-like HOMO−1, the a2u(π)-like HOMO, and the LUMO and LUMO+1, which are mixtures of egx(π*)- and egy(π*)-like orbitals. Under the four-orbital model for a highly symmetric porphyrin, the strong NUV By-band and the much weaker red-region Qy-band derive from additive and subtractive combinations, respectively, of the [HOMO → LUMO] and [HOMO−1 → LUMO+1] configurations, in which the associated transition dipoles constructively or destructively interfere. Similarly, Bx and Qx are derived from [HOMO → LUMO+1] and [HOMO−1 → LUMO]. Although these pairwise combinations give way to mixtures of all four configurations when the symmetry is reduced, the same

Θi = ηi /ΔEi

(i = x , y )

(1)

where η is the CI energy. Thus, the greater the energy gap (e.g., between the [HOMO → LUMO] and [HOMO−1 → LUMO +1] configurations), derived from macrocycle or substituent 17558


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Journal of the American Chemical Society Table 2. TDDFT Results for P-PMI in Toluenea state

λ (nm)

shifted λ (nm)b

fc

filled MO

empty MO

%

S1

649

704

0.43

S2

595

641

0.40

S3

500

532

1.5

S4

409

430

1.4

S5

403

423

1.3

H−2 H−2 H−1 H−1 H−1 H H H H−2 H−2 H−1 H−1 H−1 H H H−2 H−2 H−1 H H H−2 H−2 H−1 H−1 H−1 H H H−2 H−2 H−1 H−1 H−1 H H H

L+1 L+2 L L+1 L+2 L L+1 L+2 L+1 L+2 L L+1 L+2 L L+2 L L+1 L+2 L L+1 L L+2 L L+1 L+2 L+1 L+2 L L+1 L L+1 L+2 L L+1 L+2

4 3 9 5 14 38 12 14 3 5 19 12 5 22 32 35 10 15 13 22 3 2 29 13 14 3 32 6 5 8 5 45 8 9 9

and [HOMO−1 → LUMO+1]. The By-, Bx-, Qx-, and Qy-bands would have approximately equal intensity (because the four transition dipoles have approximately equal intensity)32 and would be separated in energy/wavelength according to the relative energies of the four frontier MOs. For the tetrapyrrole− chromophore arrays, as η → 0, the states would have mixed parentage due to the lower symmetry caused by the attachment of the chromophore (and other substituents), and there would be an additional main absorption feature (generally between the B- and Q-bands) associated with the chromophore. Although this formulation is simplistic, the idea that the strongly coupled chromophore acts to reduce tetrapyrrole CI and associated constructive/destructive transition-dipole interference involving the four-orbital setprovides a zeroth-order lens to view the optical spectra of the tetrapyrrole− chromophore arrays. For example, the framework described above explains the dramatic shift in intensity from the strong NUV Soret (Bx, By) region into the much weaker visible bands (Qx, Qy) of porphyrin benchmark P-Ph to give the spectrum of P-PMI in which all main features across the absorption spectrum have comparable intensity. The spectrum of this dyad could be viewed as containing four main tetrapyrrole-like features (including two overlapped features in the NUV) with a perylene-derived feature in between (Figures 1 and 2B). As for any tetrapyrrole, each origin transition would be accompanied by one or more vibronic satellite features. Additionally, rotation of the porphyrin and perylene about the ethyne linker will alter their interaction, which is maximal for the lowest-energy, nominally coplanar arrangement indicated by the DFT calculations. Given that barriers to rotation are quite low (typically C-PMI13 (4.4) > C-PMI (3.2) > BCPMI (2.8) > BC-PMI3,13 (1.8) > P-PMI13 (1.3) > P-TMI (0.31) > P-BDPY (0.27). Table 1 shows that τS is shorter than 50 ps for all arrays in benzonitrile except for C-PMI (140 ps), P-TMI (150 ps), and P-PMI (1.3 ns). The S1 lifetime depends, of course, on rate constants for the three decay pathways via the expression τS = (kf + kisc + kic)−1. Note that these rates were determined for all the arrays using the longer of the two time constants to the excited-state time evolution (τ1 and τ2 in Table 1). This is reasonable given that the shorter lifetime in most cases clearly represents some relaxation in S1. Examination of Table 1 suggests that for the arrays in toluene, the trend in τS is governed largely by the interplay of the radiative rate (kf) and the internal conversion rate (kic). The value of kf affects both the lifetime and the absorption and emission spectra because of the relationships of the Einstein coefficients.38 Some variation in radiative rate (for absorption and emission) is expected for the arrays because, relative to the tetrapyrrole benchmarks, intensity has been shifted to varying degrees into the S0 ↔ S1 transitions, with the shift decreasing in the order porphyrins > chlorins > bacteriochlorins for the reasons noted above. Variations in kic may be expected among the arrays for various reasons. For example, compared to mesolinked arrays, the β-pyrrole-linked analogs have less steric hindrance and greater torsional motion about the linker that may facilitate nonradiative deactivation. With regards to PBDPY, excited boron-dipyrrins are known to undergo deformation from planarity that facilitates nonradiative decay.39 Thus, to the extent that excitation visits the borondipyrrin in the S1 state of P-BDPY, such nonplanar deformations may contribute to the short S1 lifetime in both toluene and benzonitrile (Table 1). The reason for the relatively short S1 lifetime of P-TMI is unclear. Perhaps the electron delocalization associated with the orbitals involved in S1 (and S2) for this array plays a role, making the electronic properties more sensitive to relative motions of the tetrapyrrole and terrylene than other arrays. In view of all of the above analysis, the least clear aspect is the origin of the shortened S1 lifetimes when the solvent polarity is increased. The usual explanation would be stabilization of some charge-transfer/separated state to make it accessible from S1 or affording increased contribution of a charge-transfer configuration to the S1 state itself. However, the ground-state absorption spectra do not show significant differences in toluene and benzonitrile [Figures 3 and S1−S3 (SI)] that would suggest a solvent effect on the relative contributions of one or more charge-transfer configurations to the various states/bands. Additionally, the NTOs of the various arrays do not generally indicate any clear shift in electron density between the tetrapyrrole and the chromophore accompanying

MO mixing is likely one of the origins of so many configurations contributing to the S1 state of P-PMI (double the number for P-Ph) and similarly for S2−S5 of this dyad. The excited states of the other tetrapyrrole−chromophore arrays also have a much larger number of contributing configurations than the tetrapyrrole benchmark. One way to view this situation is that the CI among the four orbital configurations in the parent tetrapyrrole has been distributed or diluted over more configurations in the tetrapyrrole−chromophore arrays, involving orbitals that according to DFT have significant electron density on both constituents. As a consequence, the significant constructive and destructive transition-dipole interference that gives rise to the strong Bx- and By-bands and weak Qx- and Qy-bands for P-Ph has been altered (reduced, redistributed, diluted) such that the spectrum for P-PMI has features spanning the violet to red regions of more comparable intensity. This intensity-distribution change from benchmark to dyad in the observed spectrum is captured by the TDDFT calculations (Figure 9A,D). The same is generally true for the other tetrapyrrole−chromophore dyads versus the benchmarks [Figures 9 and S20−S29 (SI)]. The existence of such a multitude of contributing configurations (e.g., Table 2 for P-PMI) and the characteristics of the orbitals involved (e.g., Figure 5 for P-PMI) together make it difficult to picture the extent to which electron density moves from one part of the molecule to the other during the transition, namely, the extent of charge transfer. The concept of NTOs was developed by Martin to overcome this problem and to avoid misleading inferences that might be drawn from individual orbitals involved in electronic transition due to the view that “... in principle all orbitals in DFT but the HOMO are devoid of physical significance”.31 For example, examination of the MOs for P-PMI (Figure 5) would lead one to believe that at least some of the optical absorption features for this dyad would involve transitions derived from both the porphyrin and perylene. However, the NTOs (Figure 10) show that transitions from S0 to S1, S2, S4, and S5 spanning the red to violet regions are primarily tetrapyrrole transitions. The central feature of the spectrum (S0 → S3) is essentially a perylene transition, which occurs close to the wavelength of the benchmark chromophore. From an empirical standpoint, it is as if main elements of the spectrum of P-PMI are derived from four tetrapyrrole-like features borne from those in the benchmark P-Ph but with redistributed intensity, altered wavelengths, broadened spectra, and modified vibronic activity, plus perylene absorption in the central, green region. One also concludes that S0 → S1, and by inference decay of S1 to S0, involves an insignif icant shift of electron density between the porphyrin and the perylene. Additionally, although some of the MOs of P-PMI clearly arise from interaction of orbitals on the two constituents (those denoted by the red font in Figure 5), the NTOs lead one to reassess the view that these orbitals have electron density “delocalized” across the perylene and porphyrin, at least to the extent that such delocalization plays a significant role in the optical transitions. The NTOs for P-BDPY (Figure 11) give more indication that transitions between S0 and S1 may involve the borondipyrrin to at least some extent and that some charge-transfer character could be involved; the latter is relevant to the excitedstate dynamics (vide infra). The NTOs for S1 and S2 for P-TMI (Figure 12) give the strongest indication of any of the arrays for the idea of electron delocalization spanning the tetrapyrrole and 17562


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Journal of the American Chemical Society the S0 ↔ S1 transitions, although there could be some small such contribution for P-BDPY (Figure 11). One might argue that the TDDFT calculations may fail to capture a large chargetransfer contribution to S 1 by placing charge-transfer configurations at a higher energy than they really are. However, usually the reverse is true and charge-transfer states are placed by TDDFT at a lower energy than they really are. This has been our experience and has been noted previously for tetrapyrrole-based complexes.40 Insights into the solvent polarity dependence of τS (and Φf) and also into the variation in the S1 lifetime among the arrays in a given solvent (Table 1) are provided by examination of the (Stokes) shift between the S0 → S1 absorption peak and the S1 → S0 fluorescence maximum. Figure 13A shows that the S1

motions or vibronic interactions, and the effect of medium. For tetrapyrrole monomers, there is usually little solvent reorientation and little shape change, giving small Stokes shifts. Perhaps for these arrays, to the extent to which there is electron delocalization across the tetrapyrrole and chromophore, there is a more substantial solvent reorientation effect on S1 vs S0 that could enhance internal conversion and contribute to the Stokes shift. Torsional motions of the tetrapyrrole with respect to the linker also may be different in S1 than S0. There could be interplay between these two factors. Such effects could contribute to the trend observed for the S1 lifetimes vs Stokes shift for the rylene-containing arrays in Figure 13. The effect would also provide a means to understand the shorter S1 lifetimes in benzonitrile. The difference in the dipole moments of the tetrapyrrole−chromophore arrays in S1 vs S0 from TDFFT calculations (Figures S30−S33, SI) does not show a trend with Stokes shift (or excited-state lifetime); however, these calculations assume the ground-state geometry for both states and would not capture a configuration change of the array or solvent (or both) following excitation that likely underlies the Stokes shift. Regardless of the exact mechanisms that underlie the short S1 lifetimes for many of the arrays in benzonitrile, the lifetime of P-PMI remains 1.3 ns in this solvent and is 4.7 ns in toluene. Most of the other tetrapyrrole arrays do not show significant improvements in panchromaticity over P-PMI in terms of the evenness of the spectral coverage from 400 to 700 nm, with an average extinction coefficient of ∼43 000 M−1 cm−1 (Figures 2B and 3). For more extended coverage (350−750 nm), the bacteriochlorin analogue BC-PMI would be the array of choice (τS = 2.8 ns in toluene). P-TMI has intermediate optical characteristics with an S1 lifetime of 310 ps in toluene and 150 ps in benzonitrile. Such tetrapyrrole−chromophore constructs afford panchromatic-like absorption across the visible and into the NIR region, as well as excited-state properties that are attractive for applications in solar-energy conversion.

Figure 13. Lifetime of the lowest singlet excited state (A) and natural logarithm of the excited-state decay rate constant (B) versus the (Stokes) shift between the long-wavelength absorption and emission maxima for the tetrapyrrole−chromophore arrays in toluene. The dashed lines are linear fits of the data for all the arrays except P-BDPY in part A and are guides to the eye. The labels on the points are as follows: P-BDPY (1), P-TMI (2), P-PMI13 (3), BC-PMI3,13 (4), BCPMI (5), P-PMI (6), C-PMI (7), C-PMI13 (8).

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ASSOCIATED CONTENT

S Supporting Information *

lifetimes of seven of the eight arrays in toluene correlate with the Stokes shift. P-BDPY does not fit on the line, perhaps because of the above-noted propensity of the boron-dipyrrin to distort in S1 versus S0, which may affect the electronic structure, lifetime, and fluorescence shape. Figure 13B shows the same data plotted as the natural logarithm of the excited-state decay rate constant (inverse of the lifetime) versus Stokes shift. The point for P-BDPY falls on the line for this representation, but the fact remains that the Stokes shift is much larger for this dyad than the other seven arrays. The general trends shown in Figure 13 suggest that there is some net configuration change involving the tetrapyrrole−chromophore array, the medium, or both that contribute to the S1 lifetime in toluene. The S1 lifetimes are generally short for the arrays in benzonitrile (Table 1) and there is no clear correlation with the Stokes shift in this solvent. Examination of the S1 decay rate constants in toluene (Table 1) shows that the variation in S1 lifetime among the tetrapyrrole−chromophore arrays and the connection with the Stokes shift largely stems from the nominal S1 → S0 nonradiative decay rate constant. A plot of ln(kic) vs the Stokes shift is very similar to that shown in Figure 13B. For these dyads, this nonradiative decay includes internal conversion, contributions of charge transfer or charge separation, torsional

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b09548. Absorption and fluorescence spectra of arrays and benchmarks in toluene and benzonitrile, TA data for arrays, MO correlation diagrams for arrays and benchmarks, results of TDDFT calculations, and measured vs calculated absorption spectra (Tables S1−S18 and Figures S1−S33) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected]. ORCID

Dariusz M. Niedzwiedzki: 0000-0002-1976-9296 Jonathan S. Lindsey: 0000-0002-4872-2040 David F. Bocian: 0000-0002-1411-7846 Dewey Holten: 0000-0003-3639-6345 Notes

The authors declare no competing financial interest. 17563


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(27) Amanpour, J.; Hu, G.; Alexy, E. J.; Mandal, A. K.; Kang, H. S.; Yuen, J. M.; Diers, J. R.; Bocian, D. F.; Lindsey, J. S.; Holten, D. J. Phys. Chem. A 2016, 120, 7434−7450. (28) Hu, G.; Liu, R.; Alexy, E. J.; Mandal, A. K.; Bocian, D. F.; Holten, D.; Lindsey, J. S. New J. Chem. 2016, 40, 8032−8052. (29) Kang, H. S.; Esemoto, N. N.; Diers, J. R.; Niedzwiedzki, D. M.; Greco, J. A.; Akhigbe, J.; Yu, Z.; Pancholi, C.; Bhagavathy, G. V.; Nguyen, J. K.; Kirmaier, C.; Birge, R. R.; Ptaszek, M.; Holten, D.; Bocian, D. F. J. Phys. Chem. A 2016, 120, 379−395. (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (31) Martin, R. L. J. Chem. Phys. 2003, 118, 4775−4777. (32) Gouterman, M. J. Chem. Phys. 1959, 30, 1139−1161. (33) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138−163. (34) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 3, pp 1−165. (35) Mandal, A. K.; Taniguchi, M.; Diers, J. R.; Niedzwiedzki, D. M.; Kirmaier, C.; Lindsey, J. S.; Bocian, D. F.; Holten, D. J. Phys. Chem. A 2016, 120, 9719−9731. (36) Vairaprakash, P.; Yang, E.; Sahin, T.; Taniguchi, M.; Krayer, M.; Diers, J. R.; Wang, A.; Niedzwiedzki, D. M.; Kirmaier, C.; Lindsey, J. S.; Bocian, D. F.; Holten, D. J. Phys. Chem. B 2015, 119, 4382−4395. (37) High, J. S.; Virgil, K. A.; Jakubikova, E. J. Phys. Chem. A 2015, 119, 9879−9888. (38) Birks, J. B. Photophysics of Aromatic Molecules; WileyInterscience: London, 1970; pp 142−192. (39) Li, F.; Yang, S. I.; Ciringh, Y.; Seth, J.; Martin, C. H., III; Singh, D. L.; Kim, D.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 1998, 120, 10001−10017. (40) Dreuw, A.; Head-Gordon, M. J. Am. Chem. Soc. 2004, 126, 4007−4016.

ACKNOWLEDGMENTS 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-FG0205ER15661). TA studies were performed 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.

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Tailoring Panchromatic Absorption and Excited-State Dynamics of

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