Sung Ik Yang

Ground and excited state electronic properties of halogenated tetraarylporphyrins. Tuning the building blocks for porphyrin-based photonic devices

The rational design of molecular photonic devices relies on the ability to select components with predictable electronic structure, excited state lifetimes and redox chemistry. Electronic communication in multiporphyrin arrays depends critically on the relative energies and electron density distributions of the frontier molecular orbitals, especially the energetically close highest occupied molecular orbitals (a2u and a1u). To explore how these ground and excited state properties can be modulated, we have synthesized and characterized 40 free base (Fb), magnesium and zinc tetraarylporphyrins. The porphyrins bear meso-substituents with the following substitution patterns: (1) four identical substituents (phenyl, o-chlorophenyl, p-chlorophenyl, o,o-difluorophenyl, pentafluorophenyl, mesityl); (2) one, two, three or four o,o-dichlorophenyl substituents; (3) one p-ethynylphenyl group and three mesityl or pentafluorophenyl groups; (4) one p-ethynyl-o,o″-dichlorophenyl or p-ethynyl-o,o″-dimethylphenyl and thr...

Synthesis and excited-state photodynamics of phenylethyne-linked porphyrin–phthalocyanine dyads

Free base, zinc and magnesium mono-ethynyl phthalocyanines have been prepared as building blocks for constructing phthalocyanine-containing molecular devices. The phthalocyanine building blocks were prepared by the statistical reaction of 4,5-diheptylphthalonitrile and 4-(3-hydroxy-3-methylbut-1-ynyl)phthalonitrile, followed by chromatographic separation and subsequent deprotection. Seven porphyrin–phthalocyanine dyads in various metalation states have been prepared (M1PM2Pc; M1, M2 = Zn2+, Mg2+, or 2H+). ZnPH2Pc and ZnPZnPc were synthesized by Pd-coupling reactions of an ethynylphthalocyanine and an iodoporphyrin. Five other dyads (H2PH2Pc, MgPH2Pc, MgPMgPc, H2PMgPc, ZnPMgPc) were prepared by selective metalation and demetalation reactions starting from ZnPH2Pc, based on the stability differences of metalloporphyrins and metallophthalocyanines. Transient absorption and static emission experiments indicate the following: (1) Excited singlet-state intramolecular energy transfer from the porphyrin to the phthalocyanine moiety is very fast (≤10xa0ps). (2) The efficiency of the energy-transfer process is very high (typically ≥90%), and is greatest in dyads in which competing charge transfer is inhibited on energetic grounds (e.g. >98% for H2PH2Pc). (3) Charge transfer involving the excited phthalocyanine and the porphyrin occurs to a limited degree (typically <10%) depending on the redox characteristics of the chromophores. (4) The desirable strong emission properties of monomeric phthalocyanines are retained in most of the dyads (Φf = 0.37–0.75). This paper establishes the foundation for utilizing phthalocyanines as strong-red absorbers, energy-transfer acceptors, and bright emitters in conjunction with porphyrin-based molecular photonic devices.

Synthesis and properties of weakly coupled dendrimeric multiporphyrin light-harvesting arrays and hole-storage reservoirs

A convergent synthesis employing porphyrin building blocks has afforded dendrimeric multiporphyrin arrays containing n Zn-porphyrins (nxa0=xa04, 8, or 20) and one free base- (Fb-) porphyrin joined via diarylethyne linkers. Size exclusion chromatography was used extensively for purification. The arrays have sufficient solubility in toluene or other solvents for routine handling. With increasing size, the intense near-UV Soret (S0xa0→xa0S2) absorption band broadens, splits, and red shifts due to inter-porphyrin exciton coupling. In contrast, the weaker visible bands (S0xa0→xa0S1) remain essentially unchanged in position or width in proceeding from the monomer all the way to the 21-mer; however, the molecular extinction coefficients of the visible bands scale with the number of porphyrins. Similarly, the one-electron oxidation potentials of the porphyrins are virtually unchanged as the arrays get larger. These results are indicative of relatively weak (but significant) electronic coupling between ground states and between the photophysically relevant lowest-excited-singlet states of the diarylethyne-linked porphyrins; thus, the characteristic properties of the individual units are retained as the architectures increase in complexity. Efficient excited-singlet-state energy transfer occurs among the Zn-porphyrins and ultimately to the sole Fb-porphyrin in each of the arrays, with the overall arrival time of energy at the trapping site increasing modestly with the number of Zn-porphyrinsxa0=xa01 (45xa0ps), 2 (90xa0ps), 8 (105xa0ps), and 20 (220xa0ps). The overall energy-transfer efficiencies are 98%, 96%, 96%, and 92% in the same series. The ground-state hole-storage properties of the 21-mer (Zn20Fb) were examined. Bulk electrolysis indicates that 21 (or more) electrons can be removed from this array (e.g., one hole resides on each porphyrin) to yield a stable “super-charged” nπ-cation radical. Taken together, these results indicate that the convergent building-block synthesis approach affords dendrimeric multiporphyrin arrays with favorable properties for light-harvesting and hole storage.

Synthesis and excited-state photodynamics of perylene–porphyrin dyads Part 3. Effects of perylene, linker, and connectivity on ultrafast energy transfer

New perylene–porphyrin dyads have been designed that exhibit superior light-harvesting and energy-utilization activity compared with earlier generations of structurally related dyads. The new dyads consist of a perylene mono(imide) dye (PMI) connected to a porphyrin (Por) nvia an ethynylphenyl (ep) linker. The PMI–ep–Por arrays were prepared with the porphyrin as either a zinc or magnesium complex (Porxa0=xa0Zn or Mg) or a free-base form (Porxa0=xa0Fb). The absorption properties of the perylene complement those of the porphyrin. Following excitation of the perylene (forming PMI*) in toluene, each array exhibits ultrafast (kENTxa0≥xa0(0.5xa0ps)−1) and essentially quantitative energy transfer from PMI* to the ground-state porphyrin (forming Por*). In each nof the arrays, the properties of the excited porphyrin (lifetime, fluorescence yield, etc.) are basically unperturbed from those of the isolated pigment. Thus, following energy transfer, the excited porphyrin is not quenched by deleterious reactions involving the perylene accessory unit that would otherwise limit the ability of Por* to emit light or transfer energy to another stage in a molecular photonic device. Collectively, the PMI–ep–Por dyads represent the successful result of a molecular design strategy to produce arrays with excellent properties for use as light-input and energy-transduction elements for applications in molecular optoelectronics.

Synthesis and excited-state photodynamics of perylene-porphyrin dyads. 4. Ultrafast charge separation and charge recombination between tightly coupled units in polar media

New perylene-porphyrin dyads that have excellent light-harvesting and energy-utilization capabilities in nonpolar media are found to exhibit efficient, ultrafast and tunable charge-transfer activity in polar media. The dyads consist of a perylene-monoimide dye (PMI) connected to a porphyrin (Por) via an ethynylphenyl (ep) linker. The porphyrin constituent of the PMI-ep-Por arrays is either a zinc or magnesium complex (Por = Zn or Mg) or a free-base form (Por = Fb). Following excitation of the perylene in each array in acetonitrile, PMI* decays in ≤0.4 ps by a combination of energy transfer to the ground-state porphyrin (forming Por*) and hole transfer (forming PMI-Por+). The excited porphyrin formed by energy transfer (or via direct excitation) then undergoes effectively quantitative electron transfer back to the perylene (τ = 1, 1, 700 ps for Por = Mg, Zn, Fb). Subsequently, charge recombination within PMI- Por+ returns each dyad quantitatively to the ground state (τ = 2, 4, 8 ps for Por = Mg, Zn, Fb). The dynamics of the PMI Por* → PMI-Por+ and PMI- Por+ → PMI Por charge-transfer processes can be modulated by altering the type of polar solvent (acetonitrile, benzonitrile, tetrahydrofuran and 2,6-lutidine). The charge-separation times for PMI-ep-Zn are 1, 6, 9 and 22 ps in these solvents, while the charge-recombination times are 4, 24, 38 and 34 ps. The efficient, rapid and tunable nature of the charge-transfer processes in polar media makes the PMI-ep-Por dyads useful units for performing molecular-switching functions. These properties when combined with the excellent light-harvesting and energy-transfer capabilities of the same arrays in nonpolar media afford a robust perylene-porphyrin motif that can be tailored for a variety of functions in molecular optoelectronics.