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SYNOPSISNearly complete vibrational assignments have been obtained for a heme model, nickel etioporphyrin-I ( NiEPI) , using variable-wavelength resonance Raman (RR) , and FTRaman (FT-R), as well as infrared (IR) spectroscopy, on a series of isotopomers labeled at positions in the skeleton ( 15N, ,8-13C, meso-d4, 15N-meso-d4) and in the peripheral substituents ( methyl-d12, ethyl-d8, and ethyl-d12). The vibrational bands are assigned to the porphyrin skeletal and substituent modes on the basis of the mode description scheme developed for nickel octaethylporphyrin ( NiOEP) with the aid of a normal-mode analysis of NiEPI, explicitly including the peripheral substituents, i.e., the methyl and ethyl groups. The previously reported NiOEP force field was refined to account for the observed isotope shifts of NiEPI isotopomers. An important result is the requirement of relatively large, long-range force constants for methine bridge bonds on opposite sides of the porphyrin ring. These 1-8 and 1-9 interaction force constants are required to reproduce the frequencies and isotope shifts of six C,-C, stretching modes and especially to predict the relative order of the two highest-frequency E, modes, v( C,-C,) ( v 3 ' , -1570 cm-') and v( Co-Co) ( -1600 cm-') . Most of the substituent (methyl and ethyl) vibrations are located in the RR and IR spectra. Strong RR enhancement of some substituent modes can be attributed to hyperconjugative interaction of the aliphatic groups with the porphyrin a,, orbital, as well as vibrational mixing of substituent modes with the nearby skeletal modes. 0 1996 John Wiley & Sons, Inc. I NTRODUCTI 0 NT h e potential of resonance Raman (RR) spectroscopy as a probe of structure and function in heme proteins'-' motivates a continuing effort t o elucidate t h e vibrational modes of t h e heme chromop h~r e .~-'~ Excitation into t h e intense visible absorption bands in both the electronically allowed B-state and vibronically active Q-transitions of hemes a n d metalloporphyrins affords a rich array of spectral features. Some of these bands have been found t o convey information about t h e spin * To whom all correspondence should be addressed. Biospectroscopy, Vol. 1,395-412 (1995)

A framework is developed for modeling the resonance Raman (RR) intensities of metalloporphyrins, with a view toward rationalizing the enhancement patterns observed in the spectra of heme proteins. The geometry of the S 2 excited state of nickel(II) porphine is computed using INDO/1s methods, and the structural changes resulting from S 0 -S 2 photoexcitation are projected onto the ground-state normal modes to calculate the intensity of each Raman-active vibration. The RR intensities derive mainly from expansion of the C R C m and C C bonds in the excited state, with the relative intensities strongly influenced by the phasing between C R C m and C C stretching coordinates. Analysis of the ν 8 overtone shows the INDO predicted geometry changes to be about 25% too large. Results are compared at successive levels of approximation, demonstrating that inclusion of displacements along bending coordinates in the excited state are essential, as are frequency-dependent scaling factors which are determined from the absorption spectrum by the transform approach to RR scattering. Finally, the activation of non-totally symmetric modes by an A-term mechanism is modeled by distortion of the excited state along a b 1g coordinate. Enhancement of the experimentally observed non-totally symmetric modes is correctly predicted, although quantitative modeling of their intensity requires the inclusion of nonCondon coupling.

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Resonance Raman (RR) and FT-IR spectra are reported for nickel(II) 1,5-dihydroxy-1,5-dimethyloctaethylbacteriochlorin [Ni(HOEBC)] and its meso-d4 isotopomer. All the in-plane skeletal RR-active modes and most IR-active modes are assigned with the aid of a normal mode analysis by using a force field developed for nickel(II) octaethylporphyrin and by scaling the bond stretch force constants to bond lengths revealed in the crystal structure of nickel(H) octaethylbacteriochlorin. The calculated eigenvectors provide insight into the essential vibrational characteristics of metallobacteriochlorins. The RR spectra of Ni(HOEBC) were acquired with a variety of excitation wavelengths, near resonance with the Bx, Qx, and transitions. The enhancement pattern of the observed RR intensities reveals that the Bx-and near-Q^-resonant spectra are dominated by Franck-Condon-active modes while the Qx-resonant spectrum is dominated by vibronically active modes. The Bx-resonant spectrum also shows significant vibronic scattering, via coupling between the Bx-and B^-excited states. Frequencies correlate well among Ni(II) complexes of octaethylporphine (OEP) and hydroporphyrins for modes containing similar local mode contributions, when allowance is made for C^-C^b ond order reduction and the effects of symmetry lowering. Assignments are proposed for the existing RR data on bacteriochlorophyll a.

Resonance Raman (RR) spectra are reported for the SI excited states of zinc(D) complexes of octaethylporphyrin (OEP) and protoporphyrin-IX-dimethyl ester (PPDME). Skeletal stretching modes exhibit large downshifts in the SI state of ZnOEP relative to the ground state, indicating an overall expansion of the macrocycle upon photoexcitation. Vibrational modes are assigned using I5N and meso-d4 isotopomers of OEP: the isotopic sensitivity is observed to be similar in the SO and SI states, implying that the ground-state normal mode structure is preserved in the singlet excited state. Excited-state bond orders are calculated using INDOUS methods, and the large observed downshifts of skeletal stretching modes v2, v3, and v4 in the S I state are found to be consistent with the description of this state as an admixture of (al, -eg) and (a2" -eg) excitations. Substituent vibrational frequencies in the SI state exhibit minor ('5 cm-') downshifts from their groundstate values, indicating that substituent bonds are minimally affected by (n,n*) excitation and that the strong SI-resonant enhancement of these modes in the ground state does not take place by an A-term mechanism. The frequency shift patterns for skeletal vibrational modes of ZnPPDME in the SI excited state closely resemble those of ZnOEP, implying a similar electronic configuration; the vinyl C=C stretching mode appears to be only slightly perturbed in the excited states, although the exact position cannot be accurately determined.The results of these studies support the description of metalloporphyrin singlet excited states provided by Gouterman's four-orbital model.

Nonvolatile resistive switching based memristor and memtransistor devices have emerged as a leading platform in neuromorphic computing. In this work, we have fabricated a multifunctional synaptic transistor (ST) using a conjugated polymer P3HT channel and a superionic rubidium silver iodide (RbAg4I5) thin film coated over a polyethylene oxide (PEO) layer as the gate dielectric. Large hysteresis in the transfer curve represents the memristive behavior with at least 105 current On/Off ratio. Enormously large specific capacitance induced by the electrical double layers at the interfaces of PEO/RbAg4I5 dielectric induces polaron (P3HT+) generation in the channel through bound states formation by the electrons with Ag+ ions and consequent movement of iodine (I−) counter ions toward the P3HT channel under a negative gate bias stress. This is strongly supported by the blue shift of the Raman peak from 1444.2 to 1447.9 cm−1 and the appearance of a new peak at 1464.6 cm−1. Interestingly, the proposed ST device exhibits various synaptic actions, which include an excitatory postsynaptic current, paired-pulse facilitation, and short-term potentiation to long-term potentiation after repeated rehearsal on top of standard nonvolatile data storage capability. Our ST also depicts an enhanced retention to 103 s and more than 103 discrete On- and Off-states during potentiation and depression function modulation, respectively, just by consuming a very low energy of about 2.0 pJ per synaptic event. These results are very significant to make this organic synaptic transistor as a potential candidate in terms of the desired metrics for neuromorphic computation at low cost and improved accuracy in the future.

The design of solution-processed transparent transistors with ultralow-voltage operations and a planar architecture can be a paradigm shift toward the realization of ultralow-power electronic circuits due to conformity with the existing complementary metal oxide semiconductor (CMOS) platform. We report a robust and solution-based device fabrication protocol to demonstrate near-steep-slope transparent oxide field-effect transistors (TO-FETs) with operating voltages at or below 0.5 V using a nanometer-thick amorphous indium–gallium–zinc oxide (a-IGZO) channel and an ultrathin anodized aluminum dielectric. The transmittance spectra confirm the excellent transparency (>98%) of the a-IGZO thin film for the entire visible range. Hysteresis-free transfer characteristics exhibit the film’s operation as an n-channel TO-FET with a low threshold voltage (∼96 mV), a near-thermionic subthreshold swing (SS) down to 85 mV/dec, and a high ON/OFF current ratio (>105). The consistency of these TO-FET results of ultralow-power operation with a near-steep-slope nature was demonstrated by enormously large specific capacitance values of ultrathin anodized aluminum gate dielectrics as the forcing factor. Moreover, half-volt operation of the TO-FET is also flawlessly demonstrated at room temperature with hysteresis-free characteristics. Hence, these planar TO-FETs could be a potential technological breakthrough for the future of cost-effective and high-performance transparent ultralow-power applications, including quantum and neuromorphic computation fields of research.