Decoupling optical and potentiometric band gaps in pi-conjugated materials

Syntheses, optical spectroscopy, potentiometric studies, and electronic structural calculations are reported for two classes of conjugated (porphinato)metal oligomers that feature a meso-to-meso ethyne-bridged linkage topology. One set of these systems, bis[(5,5'-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethyne (DD), 5,15-bis[[5'-10',20'-bis[3,5-di(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethynyl]-10,20-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)phenyl]porphinato)zinc(II) (DDD), and 5,15-bis[[15' '-(5'-10',20'-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5' '-10' ',20' '-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)phenyl]porphinato)zinc(II)]ethyne]ethynyl]-10,20-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)phenyl]porphinato)zinc(II) (DDDDD), constitute highly soluble analogues of previously studied examples of this structural motif having simple 10,20-diaryl substituents, while a corresponding set of conjugated oligomers, [(5-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5'-15'-ethynyl-10',20'-bis[10,20-bis(heptafluoropropyl)porphinato)zinc(II)]ethyne (DA), 5,15-bis[[5'-10',20'-bis[3,5-di(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethynyl]-10,20-bis(heptafluoropropyl)porphinato]zinc(II) (DAD), and 5,15-bis[[15' '-(5'-10',20'-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5' '-(10' ',20' '-bis(heptafluoropropyl)porphinato)zinc(II)]ethyne]ethynyl]-10,20-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)phenyl]porphinato)zinc(II) (DADAD), features alternating electron-rich and electron-poor (porphinato)zinc(II) units. Electrooptic and computational data for these species demonstrate that it is possible to engineer conjugated oligomeric structures that possess highly delocalized singlet (S1) excited states yet manifest apparent one-electron oxidation and reduction potentials (E1/20/+ and E1/2-/0 values) that are essentially invariant with respect to those elucidated for their constituent monomeric precursors.     

Potentiometric, electronic structural, and ground- and excited-state optical properties of conjugated bis[(porphinato)zinc(II)] compounds featuring proquinoidal spacer units

We report the synthesis, optical, electrochemical, electronic structural, and transient optical properties of conjugated (porphinato)zinc(II)-spacer-(porphinato)zinc(II) (PZn-Sp-PZn) complexes that possess intervening conjugated Sp structures having varying degrees of proquinoidal character. These supermolecular PZn-Sp-PZn compounds feature Sp moieties {(4,7-diethynylbenzo[c][1,2,5]thiadiazole (E-BTD-E), 6,13-diethynylpentacene (E-PC-E), 4,9-diethynyl-6,7-dimethyl[1,2,5]thiadiazolo[3,4-g]quinoxaline (E-TDQ-E), and 4,8-diethynylbenzo[1,2-c:4,5-c']bis([1,2,5]thiadiazole) (E-BBTD-E)} that regulate frontier orbital energy levels and progressively increase the extent of the quinoidal resonance contribution to the ground and electronically excited states, augmenting the magnitude of electronic communication between terminal (5,-10,20-di(aryl)porphinato)zinc(II) units, relative to that evinced for a bis[(5,5',-10,20-di(aryl)porphinato)zinc(II)]butadiyne benchmark (PZnE-EPZn). Electronic absorption spectra show significant red-shifts of the respective PZn-Sp-PZn x-polarized Q state (S0 --> S1) transition manifold maxima (240-4810 cm(-1)) relative to that observed for PZnE-EPZn. Likewise, the potentiometrically determined PZn-Sp-PZn HOMO-LUMO gaps (E1/2(0/+) - E1/2(-/0)) display correspondingly diminished energy separations that range from 1.88 to 1.11 eV relative to that determined for PZnE-EPZn (2.01 eV). Electronic structure calculations provide insight into the origin of the observed PZn-Sp-PZn electronic and optical properties. Pump-probe transient spectral data for these PZn-Sp-PZn supermolecules demonstrate that the S1 --> S(n) transition manifolds of these species span an unusually broad spectral domain of the NIR. Notably, the absorption maxima of these S1 --> S(n) manifolds can be tuned over a 1000-1600 nm spectral region, giving rise to intense excited-state transitions approximately 4000 cm(-1) lower in energy than that observed for the analogous excited-state absorption maximum of the PZnE-EPZn benchmark; these data highlight the unusually large quinoidal resonance contribution to the low-lying electronically excited singlet states of these PZn-Sp-PZn species. The fact that the length scales of the PZn-Sp-PZn species (approximately 25 angstrom) are small with respect to those of classic conducting polymers, yet possess NIR S1 --> S(n) manifold absorptions lower in energy, underscore the unusual electrooptic properties of these conjugated structures.     

Synthesis, excited-state dynamics, and reactivity of a directly-linked pyromellitimide-(porphinato)zinc(II) complex

N-[5-(10,20-Diphenylporphinato)zinc(II)]-N'-(octyl)pyromellitic diimide (PZn-PI), a meso-pyromellitimide-substituted (porphinato)zinc(II) compound, has been fabricated from the reaction of (5-amino-10,20-diphenylporphinato)zinc(II) with pyromellitic dianhydride in the presence of octylamine. Interrogation of the photoinduced charge separation (CS) and thermal charge recombination (CR) electron-transfer (ET) dynamics for PZn-PI in CH(2)Cl(2) via pump-probe transient absorption spectroscopic methods shows that tau(CS) and tau(CR) are 770 and 5200 fs, respectively. These ET dynamics differ from those elucidated previously for closely related 5-quinonyl-substituted (porphinato)metal compounds, and derive from the fact that the low-lying excited states for PZn-PI are porphyrin-localized, possessing little charge-transfer character. The synthesis of N-(5-[15-(2-(triisopropylsilyl)ethynyl)-10,20-diphenylporphinato]zinc(II))-N'-(octyl)pyromellitic diimide demonstrates that PZn-PI can be halogenated at its 15-meso-position and used subsequently as a substrate in metal-catalyzed cross-coupling reactions; the reactivity of PZn-PI is unusual with respect to many directly linked donor-acceptor compounds in that it is stable to these oxidizing and reducing reaction conditions.     

Extreme electronic modulation of the cofacial porphyrin structural motif

The synthesis, electrochemistry, and optical spectroscopy of an extensive series of cofacial bis[(porphinato)zinc(II)] compounds are reported. These species were synthesized using sequential palladium-catalyzed cross-coupling and cobalt-mediated [2+2+2] cycloaddition reactions. This modular methodology enables facile control of the nature of macrocycle-to-macrocycle connectivity and allows unprecedented modulation of the redox properties of face-to-face porphyrin species. We report the synthesis of 5,6-bis[(5',5'-10',20'-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]indane (1), 5,6-bis[(2'-5',10',15',20'-tetraphenylporphinato)zinc(II)]indane (2), 5-([2'-5',10',15',20'-tetraphenylporphinato]zinc(II))-6-[(5"-10',20'-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]indane (3), 5-([2'-5',10',15',20'-tetrakis(trifluoromethyl)porphinato]zinc(II))-6-[(5' '-10' ',20' '-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]indane (4), 5-(2'-5',10',15',20'-[tetrakis(trifluoromethyl)porphinato]zinc(II))-6-[(2'-5',10',15',20'-tetraphenylporphinato)zinc(II)]indane (5), 5,6-bis([2'-5',15'-diphenyl-10',20'-(trifluoromethyl)porphinato]zinc(II))indane (6), and 5,6-bis([2'-5',10',15',20'-tetrakis(trifluoromethyl)porphinato]zinc(II))indane (7); 4-7 define the first examples of cofacial bis[(porphinato)metal] compounds in which sigma-electron-withdrawing perfluoroalkyl groups serve as macrocycle substituents, while 2, 6, and 7 constitute the first such structures that possess a beta-to-beta linkage topology. Cyclic voltammetric studies show that the electrochemically determined HOMO and LUMO energy levels of these cofacial bis(porphinato) complexes can be lowered by 780 and 945 mV, respectively, relative to the archetypal members of this class of compounds; importantly, these orbital energy levels can be modulated over well-defined increments throughout these wide potentiometric domains. Analyses of these cofacial bis[(porphinato)metal] potentiometric data, in terms of the absolute and relative frontier orbital energies of their constituent [porphinato]zinc(II) building blocks, as well as the nature of macrocycle-to-macrocycle connectivity, provide predictive electronic structural models that rationalize the redox behavior of these species.     

Electronic modulation of hyperpolarizable (porphinato)zinc(II) chromophores featuring ethynylphenyl-, ethynylthiophenyl-, ethynylthiazolyl-, and ethynylbenzothiazolyl-based electron-donating and -accepting moieties

A series of conjugated (porphinato)zinc(II)-based chromophores structurally related to [5-(4-dimethylaminophenylethynyl)-15-(5-nitrothienyl-2-ethynyl)-10,20-bis(3,5-bis(3,3-dimethyl-1-butyloxy)phenyl)]zinc(II) were synthesized using metal-catalyzed cross-coupling reactions involving [5-bromo-15-triisopropylsilylethynyl-10,20-diarylporphinato]zinc(II), [5-bromo-15-(4-dimethylaminophenylethynyl)-10,20-diarylporphinato]zinc(II), [5-(4-dimethylaminophenylethynyl)-15-ethynyl-10,20-diarylporphinato]zinc(II), and [5-(4-nitrophenylethynyl)-15-ethynyl-10,20-diarylporphinato]zinc(II), along with appropriately functionalized aryl, thienyl (or thiophenyl), thiazolyl, benzothiazolyl, and carbazolyl precursors. The linear and nonlinear optical properties of these asymmetrically 5,15-substitued-(10,20-diarylporphinato)zinc(II) chromophores that bear either 2-(9H-carbazol-9-yl)-thiophen-5-yl-ethynyl, 4-dimethylaminophenylethynyl, or 2-(N,N-diphenylamino)thiophen-5-yl-ethynyl electron-releasing groups and an electron-withdrawing group selected from 2-formyl-thiophen-5-yl-ethynyl, 2-(2,2-dicyanovinyl)-thiophen-5-yl-ethynyl, 4-nitrophenylethynyl, 6-nitrobenzothiazol-2-yl-ethynyl, or 5-nitrothiazol-2-yl-ethynyl are reported. The dynamic hyperpolarizabilities of these compounds were determined from hyper-Rayleigh light scattering measurements carried out at a fundamental incident irradiation wavelength (lambda(inc)) of 1300 nm; these measured beta1300 values ranged from 690 --> 1400 x 10(-30) esu. These data (i) show that these neutral dipolar molecules express substantial beta1300 values, (ii) highlight that reductions in the magnitude of the aromatic stabilization energy of (porphinato)metal-pendant arylethynyl groups have a significant impact upon the magnitude of the molecular hyperpolarizability, and (iii) provide insights into advantageous design modifications for closely related structures having potential utility in long-wavelength electrooptic applications.     

Conjugated chromophore arrays with unusually large hole polaron delocalization lengths

We report variable temperature X-band EPR spectroscopic data for the cation radical states of meso-to-meso ethyne-bridged (porphinato)zinc(II) (PZnn) oligomers. These [PZn2-PZn7]+ species span an average 18-75 A length scale and display peak-to-peak EPR line widths (DeltaBp-p) that diminish with conjugation length. Analysis of these EPR data show that PZnn+ structures possess the largest hole polaron delocalization lengths yet measured; experiments carried out over a 4-298 K temperature domain demonstrate remarkably that the charge delocalization length remains invariant with temperature. These cation radical EPR data are well described by a stochastic, near barrierless, one-dimensional charge hopping model developed by Norris for N equivalent sites on a polymer chain, where the theoretical EPR line width is given by DeltaBp-p(N-mer) = (1/N1/2)DeltaBp-p(monomer); PZnn+ oligomers are the first such systems to verify a Norris-type hole delocalization mechanism over a substantial ( approximately 75 A) length scale. Given the time scale of the EPR measurement, these data show that either (i) Franck-Condon effects are incapable of driving charge localization in [PZn2-PZn7]+, resulting in cation radical wave functions which are globally delocalized over a spatial domain that is large with respect to established benchmarks for hole-doped conjugated materials, or (ii) polaron hopping rates in these oligomers exceed 107 s-1, even at 4 K. Finally, this study demonstrates that polymeric building blocks having low magnitude inner sphere reorganization energies enable the development of electronic materials having long polaron delocalization lengths.     

Design, synthesis, linear, and nonlinear optical properties of conjugated (porphinato)zinc(II)-based donor-acceptor chromophores featuring nitrothiophenyl and nitrooligothiophenyl electron-accepting moieties

An extensive series of conjugated (porphinato)zinc(II)-based chromophores featuring nitrothiophenyl and nitrooligothiophenyl electron-accepting moieties has been synthesized using metal-catalyzed cross-coupling reactions involving [5-bromo-15-triisopropylsilylethynyl-10,20-diarylporphinato]zinc(II) and an unusual electron-rich Suzuki-porphyrin synthon, [5-(4-dimethylaminophenylethynyl)-15-(4',4',5',5'-tetramethyl[1',3',2']dioxaborolan-2'-yl)-10,20-diarylporphinato]zinc(II), with appropriately functionalized aryl and thienyl precursors. These donor-acceptor chromophores feature thiophenyl, [2,2']bithiophenyl, and [2,2';5',2' ']terthiophenyl units terminated with a 5-nitro group; one series of structures features these acceptor moieties appended directly to the porphyrin macrocycle meso-carbon position, while a second set utilizes an intervening meso-ethynyl moiety to modify porphyrin-to-thiophene conjugation. The dynamic hyperpolarizability of these compounds was determined from hyper-Rayleigh light scattering (HRS) measurements carried out at fundamental incident irradiation wavelengths (lambda(inc)) of 800 and 1300 nm; interestingly, measured beta(1300) values ranged from 650 --> 4350 x 10(-30) esu. The combined linear and nonlinear optical properties of these compounds challenge the classical concept of the nonlinearity/transparency tradeoff in charge-transfer chromophores: the magnitude of the molecular hyperpolarizability is observed to vary substantially despite approximately uniform ground-state absorptive signatures for a given porphyrin-to-thiophene linkage topology. These data show that these neutral dipolar molecules can express substantial beta(1300) values; such conjugated, electronically asymmetric porphyrin-thiophene chromophores may thus find utility for electrooptic applications at telecom-relevant wavelengths.     

Molecular engineering of intensely near-infrared absorbing excited states in highly conjugated oligo(porphinato)zinc-(polypyridyl)metal(II) supermolecules

A new series of chromophores, MPZn(n), which combine ethyne-bridged bis(terpyridyl)metal(II)-(porphinato)zinc(II) (MPZ(n)) and oligomeric, ethyne-bridged (porphinato)zinc(II) (PZn(n)) architectures, have been synthesized and characterized, along with a series of derivatives bearing pyrrolidinyl electron-releasing groups on the ancillary terpyridine units (Pyr(m)MPZn(n)). Cyclic voltammetric studies, as well as NMR, electronic absorption, fluorescence, and femtosecond pump-probe transient absorption spectroscopies, have been employed to study the ground- and excited-state properties of these unusual chromophores. All of these species possess intensely absorbing excited states having large spectral bandwidth that penetrate deep in the near-infrared (NIR) energy regime. Electronic structural variation of the molecular framework shows that the excited-state absorption maximum can be extensively modulated [lambdamax(T(1) --> T(n))] (880 nm < lambdamax < 1126 nm), while concomitantly maintaining impressively large T(1) --> T(n) absorption manifold spectral bandwidth (full width at half-maximum, fwhm, approximately 2000-2500 cm(-1)). Furthermore, these studies enable correlation of supermolecular electronic structure with the magnitude of the excited-state lifetime (tau(es)) and demonstrate that this parameter can be modulated over 4 orders of magnitude ( approximately 1 ns < tau(es) < 45 micros). Terpyridyl pyrrolidinyl substituents can be utilized to destabilize terpyridyl ligand pi(*) energy levels and diminish the E1/2 (M3+/2+) value of the bis(terpyridyl)metal(II) center: such perturbations determine the relative energies of the PZn(n)-derived 1pi-pi(*) and bis(terpyridyl)metal(II) charge-transfer states and establish whether the T(1)-state wave functions of MPZn(n) and PyrmMPZn(n) species manifest the extensive electronic delocalization and charge-separated (CS) features characteristic of long-lived triplet states that absorb strongly in the NIR.     

Electron transfer reactions of rigid,cofacially compressed, π-stacked porphyrin–bridge–quinone systems

A 1,8-naphthyl pillaring motif can be utilized to enforce sub van der Waals interplanar separations between juxtaposed porphyryl, aromatic bridge, and quinonyl components of donor–spacer–acceptor (D–Sp–A) compounds. Such structures, synthesized via metal-mediated cross-coupling of appropriately functionalized (porphinato)zinc(II), arene, and quinone precursors, manifest unusual conformational rigidity in the condensed phase, and significant electronic communication between the cofacially aligned D, Sp, and A components. NMR experiments provide rigorous determination of the ambient temperature structures of these species in solution, while computational methods offer insight into the fragment molecular orbital interactions that give rise to the strong D–A coupling evident in these assemblies. The distance-, temperature-, and solvent-dependences of photoinduced charge separation (CS) and thermal charge recombination (CR) rate constants in these systems have been evaluated using femtosecond visible pump/vis–NIR probe and visible pump/mid-IR probe transient dynamical methods. These experiments: (i) demonstrate that simple aromatic building blocks like benzene, which are characterized by highly stabilized filled molecular orbitals and large HOMO–LUMO gaps, provide substantial D–A electronic coupling when organized within a π-stacked structural motif that features a modest degree of arene–arene interplanar compression; (ii) assess directly the degree of ground and excited state charge transfer (CT) in these donor–spacer–acceptor (D–Sp–A) structures, (iii) reveal unusual CS dynamics from vibrationally relaxed and unrelaxed S₁ states, and (iv) show a photoinduced CS mechanistic transition from the nonadiabatic to the solvent-controlled adiabatic regime, to one where CS becomes decoupled from solvent dynamics and is determined by the extent to which the vibrationally unrelaxed S₁ state is populated.     

Electrode grafting by oxidation of an amine catalyzed by a ferrocenyl “antenna” through intramolecular electron transfer

Two aminoferrocene complexes were studied by electrochemical techniques. Molecules retain the redox properties of both ferrocene and amine groups, but fundamentally different behaviours were observed depending on whether the linker between the two redox end groups was saturated (ethyl bridge) or not (ethynyl bridge). The possibility of an intramolecular electron transfer from the amine to the ferricenium moiety through the π-conjugated linker was demonstrated and the ethynyl bridge is expected to have a dual effect by facilitating both the oxidation of the amine into the cation radical and the production of aminyl radical, due to its strong electron withdrawing effect. Because of this synergy of properties, grafting of the conjugated aminoferrocene complex can occur just by oxidizing the ferrocene group without the presence of a base in solution.     

Highly conjugated (polypyridyl)metal-(porphinato)zinc(II) compounds: long-lived, high oscillator strength, excited-state absorbers having exceptional spectral coverage of the near-infrared

Transient dynamical studies of ruthenium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2';6',2' '-terpyridine)2+ bis-hexafluorophosphate (Ru-PZn), osmium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2';6',2' '-terpyridine)2+ bis-hexafluorophosphate (Os-PZn), ruthenium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-15-(4'-nitrophenyl)ethynyl-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2';6',2' '-terpyridine)2+ bis-hexafluorophosphate (Ru-PZn-A), osmium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-15-(4'-nitrophenyl)ethynyl-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2';6',2' '-terpyridine)2+ bis-hexafluorophosphate (Os-PZn-A), and ruthenium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-ruthenium(II)-15-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-bis(2,2';6',2' '-terpyridine)4+ tetrakis-hexafluorophosphate (Ru-PZn-Ru), and ruthenium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-osmium(II)-15-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-bis(2,2';6',2' '-terpyridine) tetrakis-hexafluorophosphate (Ru-PZn-Os) show that these highly conjugated supermolecular chromophores feature electronically excited states that absorb over broad NIR spectral windows with considerable oscillator strength and manifest lifetimes (1-50 mus) that are extraordinarily long relative to those of classic low band-gap organic materials. The excited-state absorptive domains of these strongly coupled multipigment ensembles can be extensively modulated. For sequential one-photon absorptive processes, these compounds evince large sigmae, sigmae/sigmag, and sigmae - sigmag values. As the combination of all these properties within single chromophoric entities have heretofore lacked precedent within the NIR, these and closely related structures may find particular utility in a variety of technologically important optical-limiting applications.     

Ultrafast singlet excited-state polarization in electronically asymmetric ethyne-bridged bis[(porphinato)zinc(II)] complexes

The excited-state dynamics of two conjugated bis[(porphinato)zinc(II)] (bis[PZn]) species, bis[(5,5'-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethyne (DD) and [(5,-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5',-15'-ethynyl-10',20'-bis(heptafluoropropyl)porphinato)zinc(II)]ethyne (DA), were studied by pump-probe transient absorption spectroscopy and hole burning techniques. Both of these meso-to-meso ethyne-bridged bis[PZn] compounds display intense near-infrared (NIR) transient S(1)-->S(n) absorptions and fast relaxation of their initially prepared, electronically excited Q states. Solvational and conformational relaxation play key roles in both DD and DA ground- and excited-state dynamics; in addition to these processes that drive spectral diffusion, electronically excited DA manifests a 3-fold diminution of S(1)-->S(0) oscillator strength on a 2-20 ps time scale. Both DD and DA display ground-state and time-dependent excited-state conformational heterogeneity; hole burning experiments show that this conformational heterogeneity is reflected largely by the extent of porphyrin-porphyrin conjugation, which varies as a function of the pigment-pigment dihedral angle distribution. While spectral diffusion can be seen for both compounds, rotational dynamics driving configurational averaging (tau approximately 30 ps), along with a small solvational contribution, account for essentially all of the spectral changes observed for electronically excited DD. For DA, supplementary relaxation processes play key roles in the excited-state dynamics. Two fast solvational components (0.27 and 1.7 ps) increase the DA excited-state dipole moment and reduce concomitantly the corresponding S(1)-->S(0) transition oscillator strength; these data show that these effects derive from a time-dependent change of the degree of DA S(1)-state polarization, which is stimulated by solvation and enhanced excited-state inner-sphere structural relaxation.     

Bridge-dependent electron transfer in porphyrin-based donor-bridge-acceptor systems.

Photoinduced electron transfer in donor-bridge-acceptor systems with zinc porphyrin (or its pyridine complex) as the donor and gold(III) porphyrin as the acceptor has been studied. The porphyrin moieties were covalently linked with geometrically similar bridging chromophores which vary only in electronic structure. Three of the bridges are fully conjugated pi-systems and in a fourth, the conjugation is broken. For systems with this bridge, the quenching rate of the singlet excited state of the donor was independent of solvent and corresponded to the rate of singlet energy transfer expected for a F?rster mechanism. In contrast, systems with a pi-conjugated bridging chromophore show a solvent-dependent quenching rate that suggests electron transfer in the Marcus normal region. This is supported by picosecond transient absorption measurements, which showed formation of the zinc porphyrin radical cation only in systems with pi-conjugated bridging chromophores. On the basis of the Marcus and Rehm-Weller equations, an electronic coupling of 5-20 cm(-)(1) between the donor and acceptor is estimated for these systems. The largest coupling is found for the systems with the smallest energy gap between the donor and bridge singlet excited states. This is in good agreement with the coupling calculated with quantum mechanical methods, as is the prediction of an almost zero coupling in the systems with a nonconjugated bridging chromophore.     

Tetrapyrrole derivatives substituted with ferrocenylethynyl moieties. synthesis and electrochemical studies

Up to eight redox-active ferrocenyl units have been incorporated, through the unsaturated ethynyl linkers, on the periphery of a series of cyclic tetrapyrrole derivatives including zinc(II) phthalocyanine and 2,3-naphthalocyanine, and nickel(II) meso-diphenylporphyrin. The synthesis of the former two macrocycles 4 and 7 involves the Sonogashira coupling reaction of ferrocenylethyne with 4,5-dichlorophthalonitrile (1) or 6,7-dibromonaphthalonitrile (5), respectively, followed by a base-promoted cyclization. The meso-bis(ferrocenylethynyl)porphyrin 11 has been prepared from the dibromo analogue 10 also by a palladium-catalyzed coupling reaction. These novel macrocyclic compounds have been spectroscopically and electrochemically characterized. As revealed by cyclic voltammetry, the ferrocenyl moieties appear to be electrochemically independent in these complexes and there is no significant electronic coupling among the iron(II) centers.     

Distance dependence of electron transfer in rigid,cofacially compressed,pi-stacked porphyrin-bridge-quinone systems

The electron-transfer (ET) dynamics of a series of unusually rigid pi-stacked porphyrin-quinone (P-Q) systems, in which sub-van der Waals interplanar distances separate juxtaposed porphyryl, aromatic bridge, and quinonyl components of these assemblies, are reported. The photoinduced charge separation (CS) and thermal charge recombination (CR) ET reactions of [5-[8'-(2',5'-benzoquinonyl)-1'-naphthyl]-10,20-diphenylporphinato]zinc(II) (1a-Zn), [5-[8'-(4'-[8'"-(2'"',5'"'-benzoquinonyl)-1'"-naphthyl]-1'-phenyl)-1'-naphthyl]-10,20-diphenylporphinato]zinc(II) (2a-Zn), and [5-(8'-[4'-(8'"-[4'"'-(8'"-[2'"',5'"'-benzoquinonyl]-1'"-naphthyl)-1'"'-phenyl]-1'"-naphthyl)-1'-phenyl]-1'-naphthyl)-10,20-diphenylporphinato]zinc(II) (3a-Zn) in CH(2)Cl(2) were investigated by pump-probe transient absorption spectroscopy. Analyses of these data show that the phenomenological ET distance dependence (beta) for both the CS and CR reactions in these systems is soft (beta(CS) = 0.43 A(-1); beta(CR) = 0.35 +/- 0.16 A(-1)). This work demonstrates that simple aromatic building blocks such as benzene, which are characterized by highly stabilized filled molecular orbitals and large HOMO-LUMO gaps, can provide substantial D-A electronic coupling when organized within a pi-stacked structural motif that features a modest degree of arene-arene interplanar compression.     

Unusual Stabilization of Dication Diradical Intermediate of Dizinc(II) Porphyrin Dimer

We have demonstrated here how the nature of a metal ion controls the reactivity of a metalloporphyrin π‐cation radical. One‐electron oxidations of diethylpyrrole‐bridged dicopper(II) and dipalladium(II) porphyrin dimers using iodine as an oxidant result in the formation of strongly interacting cofacial mixed‐valent π‐cation radical dimers. The mixed‐valent cation radical so generated being highly reactive drives a spontaneous and rapid transformation to form an indolizinium‐fused chlorin‐porphyrin heterodimer. In sharp contrast to this, similar addition of iodine leads to 1e‐oxidation of dizinc(II) porphyrin dimer, which is followed by a second oxidation to produce a dication diradical complex. The axial coordination of iodine upon 1e‐oxidation of dizinc(II) porphyrin dimer lowers the overall oxidation potential of the system, and thereby, making the second oxidation easily accessible. This has resulted in the stabilization of a dication diradical complex, in which two porphyrin π‐cation radicals undergo electronic communication through the bridging pyrrole group. Interestingly, despite being well‐separated from each other, the two radical spins undergo strong antiferromagnetic coupling to form a diamagnetic compound. The conjugation also leads to a change in identity of the bridge, which further highlights the critical role played by the bridge in the electronic communication between the two rings. DFT calculations clearly support the experimental observations.     

Optical spectra of protected diamine 10-bond-bridged intervalence radical cations related to N,N,N'N'-tetraalkylbenzidine

The optical absorption spectra of the delocalized intervalence radical cations of seven o,o'-linked benzidine derivatives that have the nitrogens protected as 9-(9-aza-bicyclo[3.3.1]nonan-3-one) derivatives are discussed and compared with that of the p-phenylene radical cation. The linking units are CH2, CH2CH2, NMe, S, SO2, and C=O, and we also studied H,H (the unlinked benzidine). The lowest-energy absorption band is assigned as the transition from the antibonding combination of symmetrical N and aromatic orbitals to the antibonding combination of the antisymmetric N and aromatic orbitals using TD-DFT calculations, and a good correlation between the observed transition energies and those calculated using the simple Koopmans theorem-based "neutral in-cation geometry" calculations on the UB3LYP/6-31G* structures is found. The use of the two-state model that equates the electronic interaction through the bridge between the amino groups with half of the lowest transition energy is seriously incorrect for these and other delocalized intervalence compounds. The problem of extracting the electronic interactions that actually are involved from calculated transition energies is discussed.     

Electronically coupled MM quadruply-bonded complexes (M = Mo or W) employing functionalized terephthalate bridges: toward molecular rheostats and switches

Toluene solutions of M2(O2C(t)Bu)4 (M = Mo, W; 2 equiv) react with a range of functionalized terephthalic acids, HO2CArCO2H (Ar = C6H4, C6F4, C6Cl4, C6H2-2,5-Cl2, C6H2-2,5-(OH)2, C6H3-2-F), to give [(tBuCO2)3M2]2[mu-O2CArCO2]. These compounds show intense ML(bridge)CT absorptions in the visible region of the electronic spectrum, and the terephthalate bridge serves to electronically couple the two M2 units via interactions between the M2 delta and bridge pi orbitals. Electronic structure calculations reveal how the degree of electronic coupling is controlled by the dihedral angles between the terephthalate C6 ring and the two CO2 units and the degree of interaction between the M4 delta MOs and the LUMO of the bridge. Both of these factors are controlled by the aryl substituents, and collectively these determine the thermochromism displayed by these complexes in solution together with the physical properties of the oxidized radical cations as determined by electrochemical studies (CV, DPV), UV-vis-NIR and EPR spectroscopic methods.     

Conformer-dependent electronic coupling for long-range triplet energy transfer in donor-bridge-acceptor porphyrin dimers

The electronic coupling for triplet energy transfer is calculated by time-dependent density functional theory (TD-DFT) for a set of tri-chromophoric systems based on a zinc(II) porphyrin donor and the corresponding free base acceptor covalently connected by different hydrocarbon bridging chromophores. The calculated electronic coupling, for systems with identical donor acceptor distances, is sensitive to the bridge electronic structure and shows a significant dependence for the bridge and donor-bridge conformations. The computational results compare quantitatively to measurements of triplet energy transfer rates in the corresponding donor-bridge-acceptor systems.     

Highly coupled dyads based on phthalocyanine-ruthenium(II) tris(bipyridine) complexes. Synthesis and photoinduced processes

A new series of multicomponent ZnPc-Ru(bpy)(3) systems, 1a-c, consisting of a zinc-phthalocyanine linked through conjugated and/or nonconjugated connections to a ruthenium(II) tris(bipyridine) complex, has been synthesized. The ruthenium complexes 1a-c were prepared from phthalocyanines 2a-c, bearing a 4-substituted-2,2'-bipyridine ligand by treatment with [Ru(bpy)2Cl2].2H2O. Different synthetic strategies have been devised to prepare the corresponding dyad precursors (2a-c). Compound 2a, for example, with an ethenyl bridge, was synthesized by statistical condensation of 4-tert-butylphthalonitrile and 5-[(E)-2-(3,4-dicyanophenyl)ethenyl]-2,2'-bipyridine (3) in the presence of zinc chloride. Compounds 2b and 2c, having, respectively, an amide or an ethynyl bridge, were prepared following a different synthetic approach. The method involves the coupling of an appropriate 5-substituted-2,2'-bipyridine to an unsymmetrical phthalocyanine suitably functionalized with an amino (4) or an ethynyl group (5). The photophysical properties of the dyads that are ZnPc-Ru(bpy)3 1a-c and related model compounds have been determined by a variety of steady-state (i.e., fluorescence) and time-resolved methods (i.e., fluorescence and transient absorption). Clearly, intramolecular electronic interactions between the two subunits dominate the photophysical events following the initial excitation of either chromophore. These intramolecular interactions lead, in the case of photoexcited ZnPc, to faster intersystem crossing kinetics compared to a ZnPc reference, while photoexcited [Ru(bpy)3]2+) undergoes a rapid and efficient transduction of triplet excited-state energy to the Pc.