Molecular Road Map to Tuning Ground State Absorption and Excited State Dynamics of Long-Wavelength Absorbers

The impact of spin-orbit coupling on fine-structure and spin polarisation in photoexcited porphyrin triplet states

The photoexcited triplet states of porphyrins show great promise for applications in the fields of opto-electronics, photonics, molecular wires, and spintronics. The magnetic properties of porphyrin triplet states are most conveniently studied by time-resolved continuous wave and pulse electron spin resonance (ESR). This family of techniques is singularly able to probe small yet essential details of triplet states: zero-field splittings, g-anisotropy, spin polarisation, and hyperfine interactions. These characteristics are linked to spin-orbit coupling (SOC) which is known to have a strong influence on photophysical properties such as intersystem crossing rates. The present study explores SOC effects induced by the presence of Pd2+ in various porphyrin architectures. In particular, the impact of this relativistic interaction on triplet state fine-structure and spin polarisation is investigated. These properties are probed using time-resolved ESR complemented by electron-nuclear double resonance. The findings of this study could influence the future design of molecular spintronic devices. The Pd2+ ion may be incorporated into porphyrin molecular wires as a way of controlling spin polarisation.

Investigation of Broadband Optical Nonlinear Absorption and Transient Dynamics in Orange IV Containing Azobenzene

Broadband reverse saturable absorption is systematically investigated via Z-scan, transient absorption spectrum (TAS). The excited state absorption and negative refraction of Orange IV are observed in the Z-scan experiment at 532 nm. Meanwhile, two-photon-induced excited state absorption and pure two-photon absorption are observed at 600 nm and 700 nm with the pulse width of 190 fs, respectively. An ultrafast broadband absorption in the visible wavelength region is observed via TAS. The different nonlinear absorption mechanisms at multiple wavelengths are discussed and interpreted from the results of TAS. In addition, the ultrafast dynamics of negative refraction in the excited state of Orange IV are investigated via a degenerate phase object pump-probe, from which the weak long-lived excited state is extracted. All studies indicate that Orange IV has the potential to be further optimized into a superior broadband reverse saturable absorption material and also has certain reference significance for the study of optical nonlinearity in organic molecules containing azobenzene groups.

Ultrafast optical limiting ability of trans-stilbene enhanced and broadened by a donor-π-acceptor structure

It has been shown that trans-stilbene (TSB) has great potential as an ultrafast optical limiting material through the process of three-photon absorption (3PA)-induced excited state absorption (ESA). The present paper shows that the main transitions in the absorption bands of TSB are mostly local excitation. In order to improve the optical limiting performance of TSB, a series of TSB derivatives with an electron donor-π-acceptor structure are designed. The analysis of π electron localized orbital locators (LOL-π) reveals that the distribution of π electrons in the derivatives of TSB is much more continuous compared to that in the original TSB. This results in the main transitions in the ground state absorption (GSA) and ESA of the TSB derivatives showing obvious charge transfer characteristics, and the GSA, ESA and 3PA bands are largely enhanced and broadened compared to those of the original TSB molecule. These observations are well supported by the enlarged transition dipole moments of the main transitions in GSA and ESA. With these results, it is clearly shown that the TSB derivatives are promising optical limiting materials. Our observations provide clues for the development of optical limiting materials based on TSB.

Modulating the Conduction Band Energies of Si Electrode Interfaces Functionalized with Monolayers of a Bay-Substituted Perylene Bisimide

The confinement of π-conjugated chromophores on silicon (Si) electrode surfaces is a powerful approach to engineer electroresponsive monolayers relevant to microelectronics, electrocatalysis, and information storage and processing. While common strategies to functionalize Si interfaces exploit molecularly dissolved building blocks, only a handful number of studies have leveraged the structure-function relationships of π-aggregates to tune the electronic structures of hybrid monolayers at Si interfaces. Herein, we show that the semiconducting properties of n-type monolayers constructed on Si electrodes are intimately correlated to the initial aggregation state of π-conjugated chromophore precursors derived from bay-substituted perylene bisimide (PBI) units. Specifically, our study unravels that for n-type monolayers engineered using PBI π-aggregates, the cathodic reduction potentials required to inject negative charge carriers into the conduction bands can be stabilized by 295 mV through reversible switching of the maximum anodic potential (MAP) that is applied during the oxidative cycles (+0.5 or +1.5 V vs Ag/AgCl). This redox-assisted stabilization effect is not observed with n-type monolayers derived from molecularly dissolved PBI cores and monolayers featuring a low surface density of the redox-active probes. These findings unequivocally point to the crucial role played by PBI π-aggregates in modulating the conduction band energies of n-type monolayers where a high MAP of +1.5 V enables the formation of electronic trap states that facilitate electron injection when sweeping back to cathodic potentials. Because the structure-function relationships of PBI π-aggregates are shown to modulate the semiconducting properties of hybrid n-type monolayers constructed at Si interfaces, our results hold promising opportunities to develop redox-switchable monolayers for engineering nonvolatile electronic memory devices.

Three-Photon-Induced Singlet Excited-State Absorption for the Tunable Ultrafast Optical-Limiting in Distyrylbenzene: A First-Principles Study

The ground and first singlet excited state absorption in distyrylbenzene(DSB) are simulated based on the linear-response time dependent density functional theory(LR-TDDFT). It is found that distyrylbenzene shows strong reverse saturable...

Excited-State Dynamics and Nonlinear Optical Properties of Hyperpolarizable Chromophores Based on Conjugated Bis(terpyridyl)Ru(II) and Palladium and Platinum Porphyrinic Components: Impact of Heavy Metals upon Supermolecular Electro-Optic Properties

A new series of strongly coupled oscillators based upon (porphinato)Pd, (porphinato)Pt, and bis(terpyridyl)ruthenium(II) building blocks is described. These RuPPd, RuPPt, RuPPdRu, and RuPPtRu chromophores feature bis(terpyridyl)Ru(II) moieties connected to the (porphinato)metal unit via an ethyne linker that bridges the 4'-terpyridyl and porphyrin macrocycle meso-carbon positions. Pump-probe transient optical data demonstrate sub-picosecond excited singlet-to-triplet-state relaxation. The relaxed lowest-energy triplet (T₁) excited states of these chromophores feature absorption manifolds that span the 800-1200 nm spectral region, microsecond triplet-state lifetimes, and large absorptive extinction coefficients [ε(T₁ → T_n) > 4 × 10⁴ M^-1 cm^-1]. Dynamic hyperpolarizability (β_λ) values were determined from hyper-Rayleigh light scattering (HRS) measurements carried out at several incident irradiation wavelengths over the 800-1500 nm spectral region. Relative to benchmark RuPZn and RuPZnRu chromophores which showed large β_HRS values over the 1200-1600 nm range, RuPPd, RuPPt, RuPPdRu, and RuPPtRu displayed large β_HRS values over the 850-1200 nm region. Generalized Thomas-Kuhn sum (TKS) rules and experimental hyperpolarizability values were utilized to determine excited state-to-excited state transition dipole terms from experimental electronic absorption data and thus assessed frequency-dependent β_λ values, including two- and three-level contributions for both β_zzz and β_xzx tensor components to the RuPPd, RuPPt, RuPPdRu, and RuPPtRu hyperpolarizability spectra. These analyses qualitatively rationalize how the β_zzz and β_xzx tensor elements influence the observed irradiation wavelength-dependent hyperpolarizability magnitudes. The TKS analysis suggests that supermolecules related to RuPPd, RuPPt, RuPPdRu, and RuPPtRu will likely feature intricate dependences of experimentally determined β_HRS values as a function of irradiation wavelength that derive from substantial singlet-triplet mixing, and complex interactions among multiple different β tensor components that modulate the long wavelength regime of the nonlinear optical response.

EPR of Photoexcited Triplet-State Acceptor Porphyrins

The photoexcited triplet states of porphyrin architectures are of significant interest in a wide range of fields including molecular wires, nonlinear optics, and molecular spintronics. Electron paramagnetic resonance (EPR) is a key spectroscopic tool in the characterization of these transient paramagnetic states singularly well suited to quantify spin delocalization. Previous work proposed a means of extracting the absolute signs of the zero-field splitting (ZFS) parameters, D and E, and triplet sublevel populations by transient continuous wave, hyperfine measurements, and magnetophotoselection. Here, we present challenges of this methodology for a series of meso-perfluoroalkyl-substituted zinc porphyrin monomers with orthorhombic symmetries, where interpretation of experimental data must proceed with caution and the validity of the assumptions used in the analysis must be scrutinized. The EPR data are discussed alongside quantum chemical calculations, employing both DFT and CASSCF methodologies. Despite some success of the latter in quantifying the magnitude of the ZFS interaction, the results clearly provide motivation to develop improved methods for ZFS calculations of highly delocalized organic triplet states.

Electronic structure and photophysics of a supermolecular iron complex having a long MLCT-state lifetime and panchromatic absorption

The main hurdle that prevents earth-abundant iron-based complexes from replacing environmentally unfriendly and expensive heavy metal [e.g., Ru(II), Os(II), Ir(III)] complexes in solar-energy conversion applications is the typical ultrashort (femtosecond timescale) charge-transfer state lifetime of Fe(II) chromophores. We provide a design roadmap to a generation of efficient iron-based photosensitizers and present an Fe(II) complex archetype, FeNHCPZn, which features a profoundly extended metal-to-ligand charge-transfer (³MLCT) lifetime and a large transition-dipole moment difference between its ground and metal-to-ligand charge-transfer states. This supermolecular design promotes superior visible photon harvesting over classic metal complexes while assuring a triplet excited-state oxidation potential appropriate for charge injection into the conduction bands of common semiconductor electrode materials, highlighting its photosensitizing utility in dye-sensitized solar-cell architectures.

Exploiting earth-abundant iron-based metal complexes as high-performance photosensitizers demands long-lived electronically excited metal-to-ligand charge-transfer (MLCT) states, but these species suffer typically from femtosecond timescale charge-transfer (CT)-state quenching by low-lying nonreactive metal-centered (MC) states. Here, we engineer supermolecular Fe(II) chromophores based on the bis(tridentate-ligand)metal(II)-ethyne-(porphinato)zinc(II) conjugated framework, previously shown to give rise to highly delocalized low-lying ³MLCT states for other Group VIII metal (Ru, Os) complexes. Electronic spectral, potentiometric, and ultrafast pump–probe transient dynamical data demonstrate that a combination of a strong σ-donating tridentate ligand and a (porphinato)zinc(II) moiety with low-lying π*-energy levels, sufficiently destabilize MC states and stabilize supermolecular MLCT states to realize Fe(II) complexes that express ³MLCT state photophysics reminiscent of their heavy-metal analogs. The resulting Fe(II) chromophore archetype, FeNHCPZn, features a highly polarized CT state having a profoundly extended ³MLCT lifetime (160 ps), ³MLCT phosphorescence, and ambient environment stability. Density functional and domain-based local pair natural orbital coupled cluster [DLPNO-CCSD(T)] theory reveal triplet-state wavefunction spatial distributions consistent with electronic spectroscopic and excited-state dynamical data, further underscoring the dramatic Fe metal-to-extended ligand CT character of electronically excited FeNHCPZn. This design further prompts intense panchromatic absorptivity via redistributing high-energy absorptive oscillator strength throughout the visible spectral domain, while maintaining a substantial excited-state oxidation potential for wide-ranging photochemistry––highlighted by the ability of FeNHCPZn to photoinject charges into a SnO₂/FTO electrode in a dye-sensitized solar cell (DSSC) architecture. Concepts enumerated herein afford opportunities for replacing traditional rare-metal–based emitters for solar-energy conversion and photoluminescence applications.

Driving high quantum yield NIR emission through proquinoidal linkage motifs in conjugated supermolecular arrays

High quantum yield NIR fluorophores are rare. Factors that drive low emission quantum yields at long wavelength include the facts that radiative rate constants increase proportional to the cube of the emission energy, while nonradiative rate constants increase in an approximately exponentially with decreasing S₀-S₁ energy gaps (in accordance with the energy gap law). This work demonstrates how the proquinoidal BTD building blocks can be utilized to minimize the extent of excited-state structural relaxation relative to the ground-state conformation in highly conjugated porphyrin oligomers, and shows that 4-ethynylbenzo[c][1,2,5]thiadiazole (E-BTD) units that terminate meso-to-meso ethyne-bridged (porphinato)zinc (PZnn) arrays, and 4,7-diethynylbenzo[c][1,2,5]thiadiazole (E-BTD-E) spacers that are integrated into the backbone of these compositions, elucidate new classes of impressive NIR fluorophores. We report the syntheses, electronic structural properties, and emissive characteristics of neoteric PZn-(BTD-PZn)n, PZn2-(BTD-PZn2)n, and BTD-PZnn-BTD fluorophores. Absolute fluorescence quantum yield (ϕ _f) measurements, acquired using a calibrated integrating-sphere-based measurement system, demonstrate that these supermolecules display extraordinary ϕ _f values that range from 10-25% in THF solvent, and between 28-36% in toluene solvent over the 700-900 nm window of the NIR. These studies underscore how the regulation of proquinoidal conjugation motifs can be exploited to drive excited-state dynamical properties important for high quantum yield long-wavelength fluorescence emission.

Excited-state absorption for zinc phthalocyanine from linear-response time-dependent density functional theory

The mechanism for zinc phthalocyanine (ZnPc) showing optical-limiting character is related to the first singlet excited-state absorption (ESA). Two distinct band peaks in this ESA spectrum (1.97 eV and 2.56 eV) were observed in experiments. However, the origin of the absorption is not well understood. In the present work, we perform accurate quantum mechanical calculations and analysis of the absorption of ZnPc in the first singlet excited state. It is found that the transitions of S1 → S3 and S1 → S24 are the origin of the first and second band peaks, respectively. Charge transfer character is observed between the edges and central parts of ZnPc for those two transitions, but occurs in opposite directions. It is gratifying to note that the absorption can be modified smoothly through the substitution of nitrogen atoms in ZnPc with methyne or benzene rings. The aggregation phenomenon is also investigated with ZnPc dimers. The present calculations show that the absorptions of two ZnPc molecules with cofacially stacked and slightly shifted cofacially stacked configurations both result in an obvious blueshift compared with the zinc phthalocyanine monomer. The present observations may be utilized in tuning the optical-limiting character of ZnPc.

An Optical Power Limiting and Ultrafast Photophysics Investigation of a Series of Multi-Branched Heavy Atom Substituted Fluorene Molecules

A common molecular design paradigm for optical power limiting (OPL) applications is to introduce heavy atoms that promote intersystem crossing and triplet excited states. In order to investigate this effect, three multi-branched fluorene molecules were prepared where the central moiety was either an organic benzene unit, para-dibromobenzene, or a platinum(II)–alkynyl unit. All three molecules showed good nanosecond OPL performance in solution. However, only the dibromobenzene and Pt–alkynyl compounds showed strong microsecond triplet excited state absorption (ESA). To investigate the photophysical cause of the OPL, especially for the fully organic molecule, photokinetic measurements including ultrafast pump–probe spectroscopy were performed. At nanosecond timescales, the ESA of the organic molecule was larger than the two with intersystem crossing (ISC) promoters, explaining its good OPL performance. This points to a design strategy where the singlet-state ESA is balanced with the ISC rate to increase OPL performance at the beginning of a nanosecond pulse.

Spin Delocalization in the Radical Cations of Porphyrin Molecular Wires: A New Perspective on EPR Approaches

The spin delocalization in the radical cations of a series of ethyne-linked oligoporphyrins was investigated using EPR spectroscopy. The room-temperature spectral envelope for these oligomers deviates significantly from the benchmark N^-0.5 trend in line width expected for a completely delocalized spin density, in contrast to the butadiyne-linked analogues measured previously. Here, we show, using DFT calculations and complementary low-temperature ENDOR measurements, that this deviation is primarily driven by a more pronounced inequivalence of the ¹⁴N spins within individual subunits for the ethyne-linked oligoporphyrins. Once this ¹⁴N inequivalence is taken into consideration, the room-temperature and ENDOR spectra for both butadiyne-linked and ethyne-linked oligomers, up to N = 5, can be simulated by similar static delocalization patterns. This work highlights the importance of EPR in exploring such spin delocalization phenomena while also demonstrating that the N^-0.5 trend should not be interpreted in isolation but only in combination with careful simulation and theoretical modeling.

Proton-Coupled Electron Transfer Induced by Near-Infrared Light

A proton-coupled electron transfer reaction induced by near-infrared light (>710 nm) has been achieved using a dye that shows intense NIR absorption property and electron/proton-accepting abilities. The developed system generated long-lived radical species and showed high reversibility and robustness. Mechanistic investigations suggested that the rate-determining step of the reaction involves the proton transfer process.

Heuristics from Modeling of Spectral Overlap in Förster Resonance Energy Transfer (FRET)

Among the photophysical parameters that underpin Förster resonance energy transfer (FRET), perhaps the least explored is the spectral overlap term ( J). While by definition J increases linearly with acceptor molar absorption coefficient (ε_(A) in M^-1 cm^-1), is proportional to wavelength (λ⁴), and depends on the degree of overlap of the donor fluorescence and acceptor absorption spectra, the question arose as to the value of J for the case of perfect spectral overlap versus that for representative fluorophores with incomplete spectral overlap. Here, Gaussian distributions of absorption and fluorescent spectra have been modeled that encompass varying degrees of overlap, full-width-at-half-maximum (fwhm), and Stokes shift. For ε_(A) = 10⁵ M^-1 cm^-1 and perfect overlap, the J value (in M^-1 cm^-1 nm⁴) ranges from 1.15 × 10¹⁴ (200 nm) to 7.07 × 10¹⁶ (1000 nm), is almost linear with λ⁴ (average of λ_abs and λ_flu), and is nearly independent of fwhm. For visible-region fluorophores with perfectly overlapped Gaussian spectra, the resulting value of J ( J_G-0) is ∼0.71 ε_(A)λ⁴ (M^-1 cm^-1 nm⁴). The experimental J values for homotransfer, as occurs in light-harvesting antennas, were calculated with spectra from a static database of 60 representative compounds (12 groups, 5 compounds each) and found to range from 4.2 × 10¹⁰ ( o-xylene) to 5.3 × 10¹⁶ M^-1 cm^-1 nm⁴ (a naphthalocyanine). The degree of overlap, defined by the ratio of the experimental J to the model J_G-0 for perfectly overlapped spectra, ranges from ∼0.5% (coumarin 151) to 77% (bacteriochlorophyll a). The results provide insights into how a variety of factors affect the resulting J values. The high degree of spectral overlap for (bacterio)chlorophylls prompts brief conjecture concerning the relevance of energy transfer to the question "why chlorophyll".

Differential quenching of the angular momentum of the B and Q bands of a porphyrin as a result of extended ring π-conjugation

A novel porphyrin, whose [Formula: see text]-system has been extended via the presence of two additional carbon–carbon triple bonds on opposite meso-positions and by fusion of a single naphthalene unit simultaneously bridging the third meso-position and the [Formula: see text]-carbon of one of the pyrroles, has been synthesized in good yield. Absorption, magnetic circular dichroism, emission, and theoretical spectra are reported for the fused and unfused trans-naphthalene free base and zinc porphyrins. The fusing of one of the naphthalene moieties results in significant changes to the absorption spectrum and, very unusually, the bridged meso-[Formula: see text]-pyrrole fusion results in quenching of the MCD Faraday pseudo-A term in the porphyrin’s B band (S2). This unique effect was interpreted as resulting from the origin of the electronic structure of the second excited state (the B state). The [Formula: see text] and [Formula: see text] polarizations are completely mixed by the electronic effects of the non-symmetric extended conjugation of the [Formula: see text] ring. Analysis of the origin of the MCD signal indicates that the presence of this novel mixed polarization leads to negligible angular momentum in the important B state. To our knowledge, this is the first report in which the magnetic moment in a porphyrin’s intensely absorbing B band has been quenched while the angular momentum in the Q band, the first excited state, remains as normal. This implies that the photophysical properties of the B state are likely very different than those of the Q state, which has novel and significant implications for applications, especially in non-linear spectroscopy.

Tailored homo- and hetero- lanthanide porphyrin dimers: a synthetic strategy for integrating multiple spintronic functionalities into a single molecule

We present molecular magnetic systems that contain all elements necessary for spin-valve control in molecular spintronic devices in a single molecule.

We present the design, synthesis and magnetic properties of molecular magnetic systems that contain all elements necessary for spin-valve control in molecular spintronic devices in a single molecule. We investigate the static and dynamic magnetic properties and quantum spin properties of butadiyne-linked homo- and hetero-nuclear lanthanide-porphyrin dimers. A heterometallated porphyrin dimer containing both Tb^III and Dy^III centres is created rationally by the stepwise oxidative homocoupling of distinct lanthanide-porphyrin monomers. Tb^III and Dy^III mononuclear porphyrin complexes, homodimers and heterodimers all exhibit slow magnetic relaxation below 10 kelvin under a static magnetic field. The coherence times for Gd^III porphyrin monomers and dimers are found to be in excess of 3.0 μs at 2 K, allowing distinct magnetic manipulations in low temperature transport experiments.

Ultrastrong Absorption Meets Ultraweak Absorption: Unraveling the Energy-Dissipative Routes for Dye-Sensitized Upconversion Luminescence

Dye sensitization is becoming a new dimension to highly improve the upconversion luminescence (UCL) of lanthanide-doped upconversion nanoparticles (UCNPs). However, there is still a lack of general understanding of the dye-UCNPs interactions, especially the confused large mismatch between the inputs and outputs. By taking dye-sensitized NaYF₄:Yb/Er@NaYF₄:Nd UCNPs as a model system, we not only revealed the in-depth energy-dissipative process for dye-sensitized UCL but also confirmed the first ever experimental observation of the energy back transfer (EBT) in the dye-sensitized UCL. Furthermore, this energy-dissipative EBT restricted the optimal ratio of dyes to UCNP. By unearthing all of the energy loss behind the EBT, energy transfer, and energy migration processes, this paper sheds light on the further design of effective dye-sensitized nanosystems for UCL or even downconversion luminescence.

Enhanced phosphorescence properties of a Pt-porphyrin derivative fixed on the surface of nano-porous glass

Room-temperature phosphorescence of a Pt-porphyrin derivative was enhanced in quantum yield and lifetime by fixing it on the surface of nanometer-scale pores of porous glass.

Molecular-based upconversion in homo/heterogeneous liquids and in micro/nanostructured solid materials

A step-by-step investigation to move TTA-UC from homogeneous solutions to nanostructured solid materials for new technological perspectives.