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1.
Phys Chem Chem Phys ; 26(19): 14228-14243, 2024 May 15.
Article in English | MEDLINE | ID: mdl-38690612

ABSTRACT

The development of chromophores that absorb in the near-infrared (NIR) region beyond 1000 nm underpins numerous applications in medical and energy sciences, yet also presents substantial challenges to molecular design and chemical synthesis. Here, the core bacteriochlorin chromophore of nature's NIR absorbers, bacteriochlorophylls, has been adapted and tailored by annulation in an effort to achieve absorption in the NIR-II region. The resulting bacteriochlorin, Phen2,1-BC, contains two annulated naphthalene groups spanning meso,ß-positions of the bacteriochlorin and the 1,2-positions of the naphthalene. Phen2,1-BC was prepared via a new synthetic route. Phen2,1-BC is an isomer of previously examined Phen-BC, which differs only in attachment via the 1,8-positions of the naphthalene. Despite identical π-systems, the two bacteriochlorins have distinct spectroscopic and photophysical features. Phen-BC has long-wavelength absorption maximum (912 nm), oscillator strength (1.0), and S1 excited-state lifetime (150 ps) much different than Phen2,1-BC (1292 nm, 0.23, and 0.4 ps, respectively). These two molecules and an analogue with intermediate characteristics bearing annulated phenyl rings have unexpected properties relative to those of non-annulated counterparts. Understanding the distinctions requires extending concepts beyond the four-orbital-model description of tetrapyrrole spectroscopic features. In particular, a reduction in symmetry resulting from annulation results in electronic mixing of x- and y-polarized transitions/states, as well as vibronic coupling that together reduce oscillator strength of the long-wavelength absorption manifold and shorten the S1 excited-state lifetime. Collectively, the results suggest a heuristic for the molecular design of tetrapyrrole chromophores for deep penetration into the relatively unutilized NIR-II region.


Subject(s)
Porphyrins , Spectroscopy, Near-Infrared , Porphyrins/chemistry , Naphthalenes/chemistry , Molecular Structure , Bacteriochlorophylls/chemistry
2.
Phys Chem Chem Phys ; 25(3): 1781-1798, 2023 Jan 18.
Article in English | MEDLINE | ID: mdl-36597966

ABSTRACT

A new pentad array designed to exhibit panchromatic absorption and charge separation has been synthesized and characterized. The array is composed of a triad panchromatic absorber (a bis(perylene-monoimide)-porphyrin) to which are appended an electron acceptor (perylene-diimide) and an electron donor/hole acceptor (bacteriochlorin) in a crossbar arrangement. The motivation for incorporation of the bacteriochlorin versus a free-base or zinc chlorin utilized in prior constructs was to facilitate hole transfer to this terminal unit and thereby achieve a higher yield of charge separation across the array. The intense S0 → S1 (Qy) band of the bacteriochlorin also enhances absorption in the near-infrared spectral region. Due to synthetic constraints, a phenylethyne linker was used to join the bacteriochlorin to the core porphyrin of the panchromatic triad rather than the diphenylethyne linker employed for the prior chlorin-containing pentads. Static and time-resolved photophysical studies reveal enhanced excited-state quenching for the pentad in benzonitrile and dimethyl sulfoxide compared to the prior chlorin-containing analogues. Success was only partial, however, as a long-lived charge separated state was not observed despite the improved energetics for the final ground-state hole/electron-shift reaction. The apparent reason is more facile competing charge-recombination due to the shorter bacteriochlorin - porphyrin linker that increases electronic coupling for this process. The studies highlight design criteria for balancing panchromatic absorption and long-lived charge separation in molecular architectures for solar-energy conversion.


Subject(s)
Perylene , Porphyrins , Energy Transfer
3.
Phys Chem Chem Phys ; 25(3): 1827-1847, 2023 Jan 18.
Article in English | MEDLINE | ID: mdl-36601996

ABSTRACT

Bacteriochlorophylls, nature's near-infrared absorbers, play an essential role in energy transfer in photosynthetic antennas and reaction centers. To probe energy-transfer processes akin to those in photosynthetic systems, nine synthetic bacteriochlorin-bacteriochlorin dyads have been prepared wherein the constituent pigments are joined at the meso-positions by a phenylethyne linker. The phenylethyne linker is an unsymmetric auxochrome, which differentially shifts the excited-state energies of the phenyl- or ethynyl-attached bacteriochlorin constituents in the dyad. Molecular designs utilized known effects of macrocycle substituents to engineer bacteriochlorins with S0 → S1 (Qy) transitions spanning 725-788 nm. The design-predicted donor-acceptor excited-state energy gaps in the dyads agree well with those obtained from time dependent density functional theory calculations and with the measured range of 197-1089 cm-1. Similar trends with donor-acceptor excited-state energy gaps are found for (1) the measured ultrafast energy-transfer rates of (0.3-1.7 ps)-1, (2) the spectral overlap integral (J) in Förster energy-transfer theory, and (3) donor-acceptor electronic mixing manifested in the natural transition orbitals for the S0 → S1 transition. Subtle outcomes include the near orthogonal orientation of the π-planes of the bacteriochlorin macrocycles, and the substituent-induced shift in transition-dipole moment from the typical coincidence with the NH-NH axis; the two features together afforded the Förster orientation term κ2 ranging from 0.55-1.53 across the nine dyads, a value supportive of efficient excited-state energy transfer. The molecular design and collective insights on the dyads are valuable for studies relevant to artificial photosynthesis and other processes requiring ultrafast energy transfer.


Subject(s)
Acetylene , Photosynthesis , Energy Transfer
4.
J Phys Chem A ; 126(50): 9353-9365, 2022 Dec 22.
Article in English | MEDLINE | ID: mdl-36508586

ABSTRACT

A panchromatic triad and a charge-separation unit are joined in a crossbar architecture to capture solar energy. The panchromatic-absorber triad (T) is comprised of a central free-base porphyrin that is strongly coupled via direct ethyne linkages to two perylene-monoimide (PMI) groups. The charge-separation unit incorporates a free-base or zinc chlorin (C or ZnC) as a hole acceptor (or electron donor) and a perylene-diimide (PDI) as an electron acceptor, both attached to the porphyrin via diphenylethyne linkers. The free-base porphyrin is common to both light-harvesting and charge-separation motifs. The chlorin and PDI also function as ancillary light absorbers, complementing direct excitation of the panchromatic triad to produce the discrete lowest excited state of the array (T*). Attainment of full charge separation across the pentad entails two steps: (1) an initial excited-state hole/electron-transfer process to oxidize the chlorin (and reduce the panchromatic triad) or reduce the PDI (and oxidize the panchromatic triad); and (2) subsequent ground-state electron/hole migration to produce oxidized chlorin and reduced PDI. Full charge separation for pentad ZnC-T-PDI to generate ZnC+-T-PDI- occurs with a quantum yield of ∼30% and mean lifetime ∼1 µs in dimethyl sulfoxide. For C-T-PDI, initial charge separation is followed by rapid charge recombination. The molecular designs and studies reported here reveal the challenges of balancing the demands for charge separation (linker length and composition, excited-state energies, redox potentials, and medium polarity) with the constraints for panchromatic absorption (strong electronic coupling of the porphyrin and two PMI units) for integrated function in solar-energy conversion.


Subject(s)
Perylene , Porphyrins , Electron Transport , Imides
5.
Molecules ; 27(19)2022 Oct 01.
Article in English | MEDLINE | ID: mdl-36235037

ABSTRACT

The syntheses of two triads are reported. Each triad is composed of two perylene-monoimides linked to a porphyrin via an ethyne unit, which bridges the perylene 9-position and a porphyrin 5- or 15-position. Each triad also contains a single tether composed of an alkynoic acid or an isophthalate unit. Each triad provides panchromatic absorption (350-700 nm) with fluorescence emission in the near-infrared region (733 or 743 nm; fluorescence quantum yield ~0.2). The syntheses rely on the preparation of trans-AB-porphyrins bearing one site for tether attachment (A), an aryl group (B), and two open meso-positions. The AB-porphyrins were prepared by the condensation of a 1,9-diformyldipyrromethane and a dipyrromethane. The installation of the two perylene-monoimide groups was achieved upon the 5,15-dibromination of the porphyrin and the subsequent copper-free Sonogashira coupling, which was accomplished before or after the attachment of the tether. The syntheses provide relatively straightforward access to a panchromatic absorber for use in bioconjugation or surface-attachment processes.


Subject(s)
Perylene , Porphyrins
6.
J Phys Chem Lett ; 13(34): 7906-7910, 2022 Sep 01.
Article in English | MEDLINE | ID: mdl-35980198

ABSTRACT

The impact of vibrational-electronic resonances on the rate of excited-state energy transfer is examined in a set of bacteriochlorin dyads that employ the same phenylethyne linker. The donor/acceptor excited-state energy gap is tuned from ∼200 to ∼1100 cm-1 using peripheral substituents on the donor and acceptor bacteriochlorin macrocycles. Ultrafast energy transfer is observed with rate constants of (0.3 ps)-1 to (1.7 ps)-1, which agree with those predicted by Förster theory to within a factor of 2. Furthermore, the measured rates follow a trend-line with only small deviations that do not correlate with the density of vibrations at the donor/acceptor excited-state energy gap. Thus, if vibrational-electronic resonances occur in any of these dyads, which seems likely, the impact on the rate of energy transfer is small.


Subject(s)
Porphyrins , Vibration , Electronics , Energy Transfer
7.
J Phys Chem A ; 126(31): 5107-5125, 2022 Aug 11.
Article in English | MEDLINE | ID: mdl-35901315

ABSTRACT

Electronic interactions between tetrapyrroles are utilized in natural photosynthetic systems to tune the light-harvesting and energy-/charge-transfer processes in these assemblies. Such interactions also can be employed to tailor the electronic properties of tetrapyrrolic dyads and larger arrays for use in materials science and biomedical research. Here, we have utilized static and time-resolved optical spectroscopy to characterize the optical absorption and emission properties of a set of chlorin and bacteriochlorin dyads with varying degrees of through-bond (TB) and through-space (TS) interactions between the constituent macrocycles. The dyads consist of two chlorins or two bacteriochlorins joined by a linker that utilizes a triple-double-triple-bond (enediyne) motif in which the double-bond portion is an ester-substituted ethylene or o-phenylene unit. The photophysical studies are coupled with density functional theory (DFT) calculations to probe the ground-state molecular orbital (MO) characteristics of the dyads and time-dependent DFT calculations (TDDFT) to elucidate excited-state properties. The latter include electronic characteristics of the singlet excited-state manifold and the absorption transitions to these states from the electronic ground state. A comparison of the MO and calculated spectral properties of each dyad with the linker present versus disrupted (by eliminating the double-bond portion) gives insight into the relative contributions of TB versus TS interactions to the electronic properties of the dyads. The results show that the TB and TS contributions are additive (constructively interfere), which is not always the case for molecular dyads. Most of the dyads have shorter lifetimes of the lowest singlet excited state compared to the parent monomer, which derives from increased S1 → S0 internal conversion. The enhancement is greater for the dyads in benzonitrile than in toluene. The studies provide insights into the nature of the electronic interactions between the constituents in the tetrapyrrole arrays and how these interactions dictate the spectral properties and excited-state decay characteristics.


Subject(s)
Photosynthesis , Tetrapyrroles , Electronics , Energy Transfer , Spectrum Analysis , Tetrapyrroles/chemistry
8.
Sci Adv ; 8(1): eabk0953, 2022 Jan 07.
Article in English | MEDLINE | ID: mdl-34985947

ABSTRACT

We report two-dimensional electronic spectroscopy (2DES) experiments on the bacterial reaction center (BRC) from purple bacteria, revealing hidden vibronic and excitonic structure. Through analysis of the coherent dynamics of the BRC, we identify multiple quasi-resonances between pigment vibrations and excitonic energy gaps, and vibronic coherence transfer processes that are typically neglected in standard models of photosynthetic energy transfer and charge separation. We support our assignment with control experiments on bacteriochlorophyll and simulations of the coherent dynamics using a reduced excitonic model of the BRC. We find that specific vibronic coherence processes can readily reveal weak exciton transitions. While the functional relevance of such processes is unclear, they provide a spectroscopic tool that uses vibrations as a window for observing excited state structure and dynamics elsewhere in the BRC via vibronic coupling. Vibronic coherence transfer reveals the upper exciton of the "special pair" that was weakly visible in previous 2DES experiments.

9.
Phys Chem Chem Phys ; 23(35): 19130-19140, 2021 Sep 15.
Article in English | MEDLINE | ID: mdl-34490865

ABSTRACT

Tetrapyrrole macrocycles serve a multitude of roles in biological systems, including oxygen transport by heme and light harvesting and charge separation by chlorophylls and bacteriochlorophylls. Synthetic tetrapyrroles are utilized in diverse applications ranging from solar-energy conversion to photomedicine. Nevertheless, students beginning tetrapyrrole research, as well as established practitioners, are often puzzled when comparing properties of related tetrapyrroles. Questions arise as to why optical spectra of two tetrapyrroles often shift in wavelength/energy in a direction opposite to that predicted by common chemical intuition based on the size of a π-electron system. Gouterman's four-orbital model provides a framework for understanding these optical properties. Similarly, it can be puzzling as to why the oxidation potentials differ significantly when comparing two related tetrapyrroles, yet the reduction potentials change very little or shift in the opposite direction. In order to understand these redox properties, it must be recognized that structural/electronic alterations affect the four frontier molecular orbitals (HOMO, LUMO, HOMO-1 and LUMO+1) unequally and in many cases the LUMO+1, and not the LUMO, may track the HOMO in energy. This perspective presents a fundamental framework concerning tetrapyrrole electronic properties that should provide a foundation for rational molecular design in tetrapyrrole science.


Subject(s)
Tetrapyrroles/chemistry , Bacteriochlorophylls/chemistry , Chlorophyll/chemistry , Density Functional Theory , Electrons , Oxidation-Reduction , Porphyrins/chemistry , Quantum Theory
10.
J Phys Chem A ; 125(36): 7900-7919, 2021 Sep 16.
Article in English | MEDLINE | ID: mdl-34472866

ABSTRACT

Panchromatic absorbers have potential applications in molecular-based energy-conversion schemes. A prior porphyrin-perylene dyad (P-PMI, where "MI" denotes monoimide) coupled via an ethyne linker exhibits panchromatic absorption (350-700 nm) and a tetrapyrrole-like lowest singlet excited state with a relatively long singlet excited-state lifetime (τS) and increased fluorescence quantum yield (Φf) versus the parent porphyrin. To explore the extension of panchromaticity to longer wavelengths, three arrays have been synthesized: a chlorin-terrylene dyad (C-TMI), a bacteriochlorin-terrylene dyad (B-TMI), and a perylene-porphyrin-terrylene triad (PMI-P-TMI), where the terrylene, a π-extended homologue of perylene, is attached via an ethyne linker. Characterization of the spectra (absorption and fluorescence), excited-state properties (lifetime, yields, and rate constants of decay pathways), and molecular-orbital characteristics reveals unexpected subtleties. The wavelength of the red-region absorption band increases in the order C-TMI (705 nm) < PMI-P-TMI (749 nm) < B-TMI (774 nm), yet each array exhibits diminished Φf and shortened τS values. The PMI-P-TMI triad in toluene exhibits Φf = 0.038 and τS = 139 ps versus the all-perylene triad (PMI-P-PMI) for which Φf = 0.26 and τS = 2000 ps. The results highlight design constraints for auxiliary pigments with tetrapyrroles to achieve panchromatic absorption with retention of viable excited-state properties.

11.
J Phys Chem A ; 124(38): 7776-7794, 2020 Sep 24.
Article in English | MEDLINE | ID: mdl-32926787

ABSTRACT

Six zinc(II) porphyrins bearing 0-4 meso-phenyl substituents have been examined spectroscopically and theoretically. Comparisons with previously examined free base analogues afford a deep understanding of the electronic and photophysical effects of systematic addition of phenyl groups in porphyrins containing a central zinc(II) ion versus two hydrogen atoms. Trends in the wavelengths and relative intensities of the absorption bands are generally consistent with predictions from time-dependent density functional theory calculations and simulations from Gouterman's four-orbital model. These trends derive from a preferential effect of the meso-phenyl groups to raise the energy of the highest occupied molecular orbital. The calculations reveal additional insights, such as a progressive increase in oscillator strength in the violet-red (B-Q) absorption manifold with increasing number of phenyls. Progressive addition of 0-4 phenyl substituents to the zinc porphyrins in O2-free toluene engenders a reduction in the measured lifetime of the lowest singlet excited state (2.5-2.1 ns), an increase in the S1 → S0 fluorescence yield (0.022-0.030), a decrease in the yield of S1 → T1 intersystem crossing (0.93-0.88), and an increase in the yield of S1 → S0 internal conversion (0.048-0.090). The derived rate constants for S1 decay reveal significant differences in the photophysical properties of the zinc chelates versus free base forms. The unexpected finding of a larger rate constant for internal conversion for zinc chelates versus free bases is particularly exemplary. Collectively, the findings afford fundamental insights into the photophysical properties and electronic structure of meso-phenylporphyrins, which are widely used as benchmarks for tetrapyrrole-based architectures in solar energy and life sciences research.

12.
J Phys Chem Lett ; 9(18): 5219-5225, 2018 Sep 20.
Article in English | MEDLINE | ID: mdl-30136848

ABSTRACT

The bacterial reaction center (BRC) serves as an important model system for understanding the charge separation processes in photosynthesis. Knowledge of the electronic structure of the BRC is critical for understanding its charge separation mechanism. While it is well-accepted that the "special pair" pigments are strongly coupled, the degree of coupling among other BRC pigments has been thought to be relatively weak. Here we study the W(M250)V mutant BRC by two-color two-dimensional electronic spectroscopy to correlate changes in the Q x region with excitation of the Q y transitions. The resulting Q y-Q x cross-peaks provide a sensitive measure of the electronic interactions throughout the BRC pigment network and complement one-color 2D studies in which such interactions are often obscured by energy transfer and excited-state absorption signals. Our observations should motivate the refinement of electronic structure models of the BRC to facilitate improved understanding of the charge separation mechanism.


Subject(s)
Bacteria/metabolism , Bacterial Proteins/chemistry , Photosynthetic Reaction Center Complex Proteins/chemistry , Bacterial Proteins/genetics , Bacterial Proteins/metabolism , Electrons , Kinetics , Models, Molecular , Mutagenesis, Site-Directed , Photosynthesis , Photosynthetic Reaction Center Complex Proteins/genetics , Photosynthetic Reaction Center Complex Proteins/metabolism , Quantum Theory , Rhodobacter sphaeroides/metabolism , Spectrophotometry, Infrared
13.
J Phys Chem A ; 122(36): 7181-7201, 2018 Sep 13.
Article in English | MEDLINE | ID: mdl-30152691

ABSTRACT

Panchromatic absorbers that have robust photophysical properties enable new designs for molecular-based light-harvesting systems. Herein, we report experimental and theoretical studies of the spectral, redox, and excited-state properties of a series of perylene-monoimide-ethyne-porphyrin arrays wherein the number of perylene-monoimide units is stepped from one to four. In the arrays, a profound shift of absorption intensity from the strong violet-blue (B y and B x) bands of typical porphyrins into the green, red, and near-infrared (Q x and Q y) regions stems from mixing of chromophore and tetrapyrrole molecular orbitals (MOs), which gives multiplets of MOs having electron density spread over the entire array. This reduces the extensive mixing between porphyrin excited-state configurations and the transition-dipole addition and subtraction that normally leads to intense B and weak Q bands. Reduced configurational mixing derives from moderate effects of the ethyne and perylene on the MO energies and a more substantial effect of electron-density delocalization to reduce the configuration-interaction energy. Quantitative oscillator-strength analysis shows that porphyrin intensity is also shifted into the perylene-like green-region absorption and that the ethyne linkers lend absorption intensity. The reduced porphyrin configurational mixing also endows the S1 state with bacteriochlorin-like properties, including a 1-5 ns lifetime.

14.
Opt Express ; 26(17): 22327-22341, 2018 Aug 20.
Article in English | MEDLINE | ID: mdl-30130927

ABSTRACT

Fluorescence-detected two-dimensional electronic spectroscopy (F-2DES) projects the third-order non-linear polarization in a system as an excited electronic state population which is incoherently detected as fluorescence. Multiple variants of F-2DES have been developed. Here, we report phase-modulated F-2DES measurements on a strongly coupled symmetric bacteriochlorin dyad, a relevant 'toy' model for photosynthetic energy and charge transfer. Coherence map analysis shows that the strongest frequency observed in the dyad is well-separated from the excited state electronic energy gap, and is consistent with a vibrational frequency readily observed in bacteriochlorin monomers. Kinetic rate maps show a picosecond relaxation timescale between the excited states of the dyad. To our knowledge this is the first demonstration of coherence and kinetic analysis using the phase-modulation approach to F-2DES.

15.
Proc Natl Acad Sci U S A ; 115(14): 3563-3568, 2018 04 03.
Article in English | MEDLINE | ID: mdl-29555738

ABSTRACT

In the initial steps of photosynthesis, reaction centers convert solar energy to stable charge-separated states with near-unity quantum efficiency. The reaction center from purple bacteria remains an important model system for probing the structure-function relationship and understanding mechanisms of photosynthetic charge separation. Here we perform 2D electronic spectroscopy (2DES) on bacterial reaction centers (BRCs) from two mutants of the purple bacterium Rhodobacter capsulatus, spanning the Q y absorption bands of the BRC. We analyze the 2DES data using a multiexcitation global-fitting approach that employs a common set of basis spectra for all excitation frequencies, incorporating inputs from the linear absorption spectrum and the BRC structure. We extract the exciton energies, resolving the previously hidden upper exciton state of the special pair. We show that the time-dependent 2DES data are well-represented by a two-step sequential reaction scheme in which charge separation proceeds from the excited state of the special pair (P*) to P+HA- via the intermediate P+BA- When inhomogeneous broadening and Stark shifts of the B* band are taken into account we can adequately describe the 2DES data without the need to introduce a second charge-separation pathway originating from the excited state of the monomeric bacteriochlorophyll BA*.


Subject(s)
Electrons , Models, Biological , Photosynthetic Reaction Center Complex Proteins/chemistry , Photosynthetic Reaction Center Complex Proteins/metabolism , Rhodobacter sphaeroides/metabolism , Spectrum Analysis/methods , Kinetics , Photosynthesis
16.
RSC Adv ; 8(42): 23854-23874, 2018 Jun 27.
Article in English | MEDLINE | ID: mdl-35540249

ABSTRACT

Achieving solar light harvesting followed by efficient charge separation and transport is an essential objective of molecular-based artificial photosynthesis. Architectures that afford strong absorption across the near-UV to near-infrared region, namely panchromatic absorptivity, are critically important given the broad spectral distribution of sunlight. A tetrapyrrole-perylene pentad array was synthesized and investigated as a means to integrate panchromatic light harvesting and intramolecular charge separation. The pentad consists of three moieties: (1) a panchromatically absorbing triad, in which a porphyrin is strongly coupled to two perylene-monoimides via ethyne linkages; (2) a perylene-diimide electron acceptor; and (3) a chlorin hole-trapping unit. Integrating the three components with diphenylethyne linkers generates moderate electronic coupling for intramolecular energy and hole/electron transfer. The construction of the array relies on a stepwise strategy for incorporating modular pigment building blocks. The key building blocks include a trans-A2BC porphyrin, a chlorin, a perylene-monoimide, and a perylene-diimide, each bearing appropriate (halo, ethynyl) synthetic handles for Pd-catalyzed Sonogashira coupling reactions. One target pentad, three tetrads, four triads, and four monomeric benchmark compounds were synthesized from six building blocks (three new, three reported) and 10 new synthetic intermediates. Four of the tetrapyrrole-containing arrays are zinc chelated, and four others are in the free base form. Absorption and fluorescence spectra and fluorescence quantum yields were also measured. Collectively, investigations of the arrays reveal insights into principles for the design of novel reaction centers integrated with a panchromatic antenna for artificial photosynthetic studies.

17.
J Am Chem Soc ; 139(48): 17547-17564, 2017 12 06.
Article in English | MEDLINE | ID: mdl-29160700

ABSTRACT

Three sets of tetrapyrrole-chromophore arrays have been examined that exhibit panchromatic absorption across large portions of the near-ultraviolet (NUV) to near-infrared (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, ß-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 (<20 000 M-1 cm-1) visible features. P-PMI has an S1 lifetime (τS) of 4.7 ns in toluene and 1.3 ns in benzonitrile. Bacteriochlorin analogue BC-PMI has more extended spectral coverage (350-750 nm) and τS = 2.8 ns in toluene and 30 ps in benzonitrile. Terrylene analogue P-TMI has intermediate optical characteristics with τS = 310 ps in toluene and 150 ps in benzonitrile. The NTOs for most arrays show that S0 → S1 primarily involves the tetrapyrrole, but for P-TMI the NTOs have electron density delocalized over the two units as a result of extensive orbital mixing. Collectively, the insights obtained should aid the design of tetrapyrrole-based architectures for panchromatic light-harvesting systems for solar-energy conversion.

18.
Photochem Photobiol ; 93(5): 1204-1215, 2017 10.
Article in English | MEDLINE | ID: mdl-28439932

ABSTRACT

Tolyporphins are tetrapyrrole macrocycles produced by a cyanobacterium-containing culture known as HT-58-2. Tolyporphins A-J are free base dioxobacteriochlorins, whereas tolyporphin K is an oxochlorin. Here, the photophysical characterization is reported of tolyporphin A and two synthetic analogues, an oxobacteriochlorin and a dioxobacteriochlorin. The characterization (in toluene, diethyl ether, ethyl acetate, dichloromethane, 1-pentanol, 2-butanone, ethanol, methanol, N,N-dimethylformamide and dimethylsulfoxide) includes static absorption and fluorescence spectra, fluorescence quantum yields and time-resolved data. The data afford the lifetime of the lowest singlet excited state and the yields of the nonradiative decay pathways (intersystem crossing and internal conversion). The three macrocycles exhibit only modest variation in spectroscopic and excited-state photophysical parameters across the solvents. The long-wavelength (Qy ) absorption band of tolyporphin A appears at ~680 nm and is remarkably narrow (full-width-at-half-maximum ~7 nm). The position of the long-wavelength (Qy ) absorption band of tolyporphin A (~680 nm) more closely resembles that of chlorophyll a (662 nm) than bacteriochlorophyll a (772 nm). The absorption spectra of tolyporphins B-I, K (which were available in minute quantities) are also reported in methanol; the spectra of B-I closely resemble that of tolyporphin A. Taken together, tolyporphin A generally exhibits spectral and photophysical features resembling those of chlorophyll a.


Subject(s)
Cyanobacteria/chemistry , Photochemistry , Porphyrins/chemistry , Spectrometry, Fluorescence , Spectrophotometry, Ultraviolet
19.
Photosynth Res ; 131(3): 291-304, 2017 Mar.
Article in English | MEDLINE | ID: mdl-27854005

ABSTRACT

Light-harvesting complex 2 (LH2) from the semi-aerobically grown purple phototrophic bacterium Rhodobacter sphaeroides was studied using optical (static and time-resolved) and resonance Raman spectroscopies. This antenna complex comprises bacteriochlorophyll (BChl) a and the carotenoid spheroidenone, a ketolated derivative of spheroidene. The results indicate that the spheroidenone-LH2 complex contains two spectral forms of the carotenoid: (1) a minor, "blue" form with an S2 (11B u+ ) spectral origin band at 522 nm, shifted from the position in organic media simply by the high polarizability of the binding site, and (2) the major, "red" form with the origin band at 562 nm that is associated with a pool of pigments that more strongly interact with protein residues, most likely via hydrogen bonding. Application of targeted modeling of excited-state decay pathways after carotenoid excitation suggests that the high (92%) carotenoid-to-BChl energy transfer efficiency in this LH2 system, relative to LH2 complexes binding carotenoids with comparable double-bond conjugation lengths, derives mainly from resonance energy transfer from spheroidenone S2 (11B u+ ) state to BChl a via the Qx state of the latter, accounting for 60% of the total transfer. The elevated S2 (11B u+ ) â†’ Qx transfer efficiency is apparently associated with substantially decreased energy gap (increased spectral overlap) between the virtual S2 (11B u+ ) â†’ S0 (11A g- ) carotenoid emission and Qx absorption of BChl a. This reduced energetic gap is the ultimate consequence of strong carotenoid-protein interactions, including the inferred hydrogen bonding.


Subject(s)
Carotenoids/metabolism , Light-Harvesting Protein Complexes/metabolism , Rhodobacter sphaeroides/metabolism , Energy Transfer , Photochemistry , Spectrometry, Fluorescence , Spectrum Analysis, Raman
20.
J Phys Chem A ; 120(49): 9719-9731, 2016 Dec 15.
Article in English | MEDLINE | ID: mdl-27973797

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 time-dependent 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.


Subject(s)
Electrons , Porphyrins/chemistry , Tetrapyrroles/chemistry , Fluorescence , Molecular Structure , Photochemical Processes , Quantum Theory
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