Soft chromophore featured liquid porphyrins and their utilization toward liquid electret applications

Optoelectronically active viscous liquids are ideal for fabricating foldable/stretchable electronics owing to their excellent deformability and predictable π-unit-based optoelectronic functions, which are independent of the device shape and geometry. Here we show, unprecedented 'liquid electret' devices that exhibit mechanoelectrical and electroacoustic functions, as well as stretchability, have been prepared using solvent-free liquid porphyrins. The fluidic nature of the free-base alkylated-tetraphenylporphyrins was controlled by attaching flexible and bulky branched alkyl chains at different positions. Furthermore, a subtle porphyrin ring distortion that originated from the bulkiness of alkyl chains was observed. Its consequences on the electronic perturbation of the porphyrin-unit were precisely elucidated by spectroscopic techniques and theoretical modelling. This molecular design allows shielding of the porphyrin unit by insulating alkyl chains, which facilitates its corona-charged state for a long period under ambient conditions

Conformational Relaxation Dynamics of Poly(3-hexylthiophene) Photoexcited in Solution as Studied by Femtosecond Time-Resolved Stimulated Raman Spectroscopy in 1190–1550 nm Region

When a conjugated polymer is photoexcited in solution, its effective conjugation length in the singlet exciton state often increases through the conformational relaxation of the polymer main chain and/or hopping of the excitation. We measured femtosecond time-resolved near-IR stimulated Raman spectra of poly(3-hexylthiophene) (P3HT) photoexcited in four organic solvents for understanding the dynamics of the exciton elongation through the conformational relaxation separately from that through the exciton hopping. In the ring CC stretch frequency region, a band appears at around 1415 cm–1 and decays, while a new band rises at around 1370 cm–1. The average time constant of the change is estimated to be 8.7–19 ps and correlated almost linearly with the viscosity of the solvents. These results suggest that the main chain of P3HT in the singlet exciton state relaxes from a twisted form to a planar form in the 0–100 ps range when it surmounts an activation barrier of 5.8–7.8 kJ mol–1, generated possibly by the steric effect of the hexyl side group. When the rise of the 1370 cm–1 band is analyzed in detail, it is reproduced with two exponential rise functions with time constants of 0–3.3 and 16–22 ps. The two rise components suggest that a portion of P3HT forms a cluster in solution, while the other portion of P3HT is isolated

Relaxation Mechanism of β‑Carotene from S₂ (1B_u⁺) State to S₁ (2A_g^–) State: Femtosecond Time-Resolved Near-IR Absorption and Stimulated Resonance Raman Studies in 900–1550 nm Region

Carotenoids have two major low-lying excited states, the second lowest (S₂ (1B_u⁺)) and the lowest (S₁ (2A_g^–)) excited singlet states, both of which are suggested to be involved in the energy transfer processes in light-harvesting complexes. Studying vibrational dynamics of S₂ carotenoids requires ultrafast time-resolved near-IR Raman spectroscopy, although it has much less sensitivity than visible Raman spectroscopy. In this study, the relaxation mechanism of β-carotene from the S₂ state to the S₁ state is investigated by femtosecond time-resolved multiplex near-IR absorption and stimulated Raman spectroscopy. The energy gap between the S₂ and S₁ states is estimated to be 6780 cm^–1 from near-IR transient absorption spectra. The near-IR stimulated Raman spectrum of S₂ β-carotene show three bands at 1580, 1240, and 1050 cm^–1. When excess energy of 4000 cm^–1 is added, the S₁ CC stretch band shows a large upshift with a time constant of 0.2 ps. The fast upshift is explained by a model that excess energy generated by internal conversion from the S₂ state to the S₁ state is selectively accepted by one of the vibronic levels of the S₁ state and is redistributed among all the vibrational modes

Direct Observation of Structure and Dynamics of Photogenerated Charge Carriers in Poly(3-hexylthiophene) Films by Femtosecond Time-Resolved Near-IR Inverse Raman Spectroscopy

The initial charge separation process of conjugated polymers is one of the key factors for understanding their conductivity. The structure of photogenerated transients in conjugated polymers can be observed by resonance Raman spectroscopy in the near-IR region because they exhibit characteristic low-energy transitions. Here, we investigate the structure and dynamics of photogenerated transients in a regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend film, as well as in a pristine P3HT film, using femtosecond time-resolved resonance inverse Raman spectroscopy in the near-IR region. The transient inverse Raman spectrum of the pristine P3HT film at 50 ps suggests coexistence of neutral and charged excitations, whereas that of the P3HT:PCBM blend film at 50 ps suggests formation of positive polarons with a different structure from those in an FeCl3-doped P3HT film. Time-resolved near-IR inverse Raman spectra of the blend film clearly show the absence of charge separation between P3HT and PCBM within the instrument response time of our spectrometer, while they indicate two independent pathways of the polaron formation with time constants of 0.3 and 10 ps

Quest for a Rational Molecular Design of Alkyl–Distyrylbenzene Liquid by Substitution Pattern Modulation

Alkyl-π functional molecular liquids (FMLs) are of interest for fabricating soft electronic devices due to their fluidic nature and innate optoelectronic functions from the π-conjugated moiety. However, predictable development of alkyl-π FMLs with the desired liquid and optoelectronic properties is challenging. A series of alkyl–distyrylbenzene (DSB) liquids was studied in terms of the substituent position effect by attaching 2-octyldodecyl chains at (2,4-), (2,5-), (2,6-), and (3,5-). The effect of the alkyl chain length was investigated by attaching 2-hexyldecyl, 2-decyltetradecyl (C10C14), and 2-dodecylhexadecyl at the (2,5-) substituent position. The 2,5-C10C14 substituent pattern constructed a superior alkyl–DSB liquid with a lower viscosity, intrinsic optical properties, and high thermal- and photo-stabilities. The discovered 2,5-C10C14 was applied to dicyanostyrylbenzene and comparable liquid physical and optical superiorities were confirmed. This molecular design is useful for creating alkyl-π FMLs with the aforementioned advantages, which are applicable for deformable and flowable optoelectronic devices

Charge resonance character in the charge transfer state of bianthryls: effect of symmetry breaking on time-resolved near-IR absorption spectra

We study the effects of symmetry breaking on the photogenerated intramolecular charge transfer (CT) state of 9,9'-bianthryl (BA) with femtosecond time-resolved near-IR spectroscopy. The time-resolved near-IR spectra are measured in acetonitrile for a symmetric substituted derivative of 10,10'-dicyano-9,9'-bianthryl (DCBA) and asymmetric substituted derivatives of 10-cyano-9,9'-bianthryl (CBA) and 9-(N-carbazolyl)anthracene (C9A), as well as nonsubstituted BA. The transient near-IR absorption spectrum of each compound at 0 ps has a locally excited (LE) absorption band, which agrees with the transient absorption band of the corresponding monomer unit. At 3 ps after the photoexcitation, the symmetric compounds show a broad charge transfer (CT) absorption band, whereas no absorption peak appears in the spectra of the asymmetric compounds. The broad CT absorption at 1250 nm only observed for the symmetric compounds can be attributed to the charge resonance transition associated with two equivalent charge separated states