Coherent Nuclear Wave Packets in Q States by Ultrafast Internal Conversions in Free Base Tetraphenylporphyrin

Persistence of vibrational coherence in electronic transition has been noted especially in biochemical systems. Here, we report the dynamics between electronic excited states in free base tetraphenylporphyrin (H₂TPP) by time-resolved fluorescence with high time resolution. Following the photoexcitation of the B state, ultrafast internal conversion occurs to the Q_<i>x</i> state directly as well as via the Q_<i>y</i> state. Unique and distinct coherent nuclear wave packet motions in the Q_<i>x</i> and Q_<i>y</i> states are observed through the modulation of the fluorescence intensity in time. The instant, serial internal conversions from the B to the Q_<i>y</i> and Q_<i>x</i> states generate the coherent wave packets. Theory and experiment show that the observed vibrational modes involve the out-of-plane vibrations of the porphyrin ring that are strongly coupled to the internal conversion of H₂TPP

Excited State Intramolecular Proton Transfer Dynamics of 1‑Hydroxy-2-acetonaphthone

Excited state intramolecular proton transfer (ESIPT) of 1-hydroxy-2-acetonaphthone (HAN) has been in controversy, mainly because its Stokes shift is small compared to those of typical ESIPT molecules. We have investigated excited state dynamics of HAN by time-resolved fluorescence with a resolution high enough to record the nuclear wave packet motions in the excited state. Population dynamics of both the normal and tautomer forms were recorded together with the wave packet motions of the tautomer in the excited state, which confirm the ESIPT of HAN. The population dynamics of the normal and tautomer forms imply that the ESIPT dynamics is biphasic with two time constants <25 and 80 fs. Theoretical analysis of the vibrational modes of the tautomer excited impulsively reveals that major part of the change for the ESIPT reaction is on the naphthalene ring

Active Role of Proton in Excited State Intramolecular Proton Transfer Reaction

Proton transfer is one of the most important elementary reactions in chemistry and biology. The role of proton in the course of proton transfer, whether it is active or passive, has been the subject of intense investigations. Here we demonstrate the active role of proton in the excited state intramolecular proton transfer (ESIPT) of 10-hydroxybenzo­[<i>h</i>]­quinoline (HBQ). The ESIPT of HBQ proceeds in 12 ± 6 fs, and the rate is slowed down to 25 ± 5 fs for DBQ where the reactive hydrogen is replaced by deuterium. The results are consistent with the ballistic proton wave packet transfer within the experimental uncertainty. This ultrafast proton transfer leads to the coherent excitation of the vibrational modes of the product state. In contrast, ESIPT of 2-(2′-hydroxyphenyl)­benzothiazole (HBT) is much slower at 62 fs and shows no isotope dependence implying complete passive role of the proton

Multifaceted Ultrafast Intramolecular Charge Transfer Dynamics of 4‑(Dimethylamino)benzonitrile (DMABN)

Intramolecular charge transfer (ICT) of DMABN has been the subject of extensive investigations. Through the measurements of highly time-resolved fluorescence spectra (TRFS) over the whole emission region, we have examined the ICT dynamics of DMABN in acetonitrile free from the solvation dynamics and vibronic relaxation. The ICT dynamics was found to be characterized by a broad range of time scales; nearly instantaneous (<30 fs), 160 fs, and 3.3 ps. TRFS revealed that an ICT state with partially twisted geometry, ICT­(P), is formed within a few hundred femtoseconds either directly from the initial photoexcited state or via the locally excited (LE) state. The ICT­(P) state undergoes further relaxation along the intramolecular nuclear coordinate to reach the twisted ICT (TICT) state with the time constant of 4.8 ps. A conformational diversity along the rotation of the dimethylamino group was speculated to account for the observed diffusive dynamics

Coherent and Homogeneous Intramolecular Charge-Transfer Dynamics of 1-<i>tert</i>-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6), a Rigid Analogue of DMABN

We report the intramolecular charge-transfer (ICT) dynamics of 1-<i>tert</i>-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6), a planar analogue of 4-(dimethylamino)­benzonitrile (DMABN), by using time-resolved fluorescence (TRF) and TRF spectra (TRFS). TRFS allow accurate determination of the ICT dynamics free from the spectral relaxation caused by the solvation and vibronic relaxation. For NTC6 in tetrahydrofuran (THF), the locally excited (LE) state is populated exclusively presumably via a conical intersection from the initial photoexcited S₂ (L_a) state, and the LE state undergoes ICT single exponentially with a time constant of 1.8 ± 0.2 ps. In acetonitrile, however, both LE (22%) and ICT (78%) states are populated from the S₂ state, and the population in the LE state undergoes ICT in 800 ± 100 fs. The ICT state undergoes further relaxation in 1.2 ps along the solvation and the intramolecular nuclear coordinates involving the rotation of the amino group to form a twisted ICT state. Coherent nuclear wave packet motions of 130 cm^–1, which can be assigned to the −CN group bending mode, were observed in the TRF of the reactant (LE) and product (ICT) states, indicating that the ICT reaction is partially coherent. Compared with DMABN, the ICT dynamics of NTC6 are quite homogeneous, and we speculated on the narrow conformational distribution of NTC6 in the ground state along the rotation of the amino group due to its rigid structure

Exciton Scattering Mechanism in a Single Semiconducting MgZnO Nanorod

Excitonic phenomena, such as excitonic absorption and emission, have been used in many photonic and optoelectronic semiconductor device applications. As the sizes of these nanoscale materials have approached to exciton diffusion lengths in semiconductors, a fundamental understanding of exciton transport in semiconductors has become imperative. We present exciton transport in a single MgZnO nanorod in the spatiotemporal regime with several nanometer-scale spatial resolution and several tens of picosecond temporal resolution. This study was performed using temperature-dependent cathodoluminescence and time-resolved photoluminescence spectroscopies. The exciton diffusion length in the MgZnO nanorod decreased from 100 to 70 nm with increasing temperature in the range of 5 and 80 K. The results obtained for the temperature dependence of exciton diffusion length and luminescence lifetime revealed that the dominant exciton scattering mechanism in MgZnO nanorod is exciton–phonon assisted piezoelectric field scattering

Coherent Nuclear Wave Packets Generated by Ultrafast Intramolecular Charge-Transfer Reaction

Intramolecular charge-transfer (ICT) dynamics, including reaction coordinates, structural changes, and reaction rate, has been noted experimentally and theoretically. Here we report the ICT dynamics of laurdan investigated by time-resolved fluorescence at extreme time resolution of 30 fs. A single high-frequency coherent nuclear wave-packet motion on the product potential surface is observed through the modulation of the fluorescence intensity in time. Theory and experiment show that this vibrational mode involves large displacement of the carbon atoms in the naphthalene backbone, which indicates that the naphthalene backbone coordinates are strongly coupled to the ICT reaction of laurdan, not the twisting or planarization of the dimethylamino group

Effects of Gold-Nanoparticle Surface and Vertical Coverage by Conducting Polymer between Indium Tin Oxide and the Hole Transport Layer on Organic Light-Emitting Diodes

The effect of varying degrees of surface and vertical coverage of gold nanoparticles (Au-NPs) by poly­(styrenesulfonate)-doped poly­(3,4-ethylenedioxythiophene) (PEDOT:PSS), which was used as a capping layer between indium tin oxide (ITO) and a hole transport layer (HTL) on small-molecule fluorescent organic light-emitting diodes (OLEDs), was systemically investigated. With respect to the Au-NP loading amount and size, the resultant current densities influenced the charge balance and, therefore, the OLED device performance. When the capping layer consisted of ITO/Au-NPs/PEDOT:PSS+Au-NPs, superior device performance was obtained with 10-nm Au-NPs through increased surface coverage in comparison to other Au-NP PEDOT:PSS coverage conditions. Furthermore, the Au-NP size determined the vertical coverage of the capping layer. The current densities of OLEDs containing small Au-NPs (less than 30 nm, small vertical coverage) covered by PEDOT:PSS decreased because of the suppression of the hole carriers by the Au-NP trapping sites. However, the current densities of the devices with large Au-NPs (over 30 nm, large vertical coverage) increased. The increased electromagnetic fields observed around relatively large Au-NPs under electrical bias were attributed to increased current densities in the OLEDs, as confirmed by the finite-difference time-domain simulation. These results show that the coverage conditions of the Au-NPs by the PEDOT:PSS clearly influenced the OLED current density and efficiency

Fluorescence from Multiple Chromophore Hydrogen-Bonding States in the Far-Red Protein TagRFP675

Far-red fluorescent proteins are critical for in vivo imaging applications, but the relative importance of structure versus dynamics in generating large Stokes-shifted emission is unclear. The unusually red-shifted emission of TagRFP675, a derivative of mKate, has been attributed to the multiple hydrogen bonds with the chromophore <i>N</i>-acylimine carbonyl. We characterized TagRFP675 and point mutants designed to perturb these hydrogen bonds with spectrally resolved transient grating and time-resolved fluorescence (TRF) spectroscopies supported by molecular dynamics simulations. TRF results for TagRFP675 and the mKate/M41Q variant show picosecond time scale red-shifts followed by nanosecond time blue-shifts. Global analysis of the TRF spectra reveals spectrally distinct emitting states that do not interconvert during the S₁ lifetime. These dynamics originate from photoexcitation of a mixed ground-state population of acylimine hydrogen bond conformers. Strategically tuning the chromophore environment in TagRFP675 might stabilize the most red-shifted conformation and result in a variant with a larger Stokes shift

Fluorescence from Multiple Chromophore Hydrogen-Bonding States in the Far-Red Protein TagRFP675

Far-red fluorescent proteins are critical for in vivo imaging applications, but the relative importance of structure versus dynamics in generating large Stokes-shifted emission is unclear. The unusually red-shifted emission of TagRFP675, a derivative of mKate, has been attributed to the multiple hydrogen bonds with the chromophore <i>N</i>-acylimine carbonyl. We characterized TagRFP675 and point mutants designed to perturb these hydrogen bonds with spectrally resolved transient grating and time-resolved fluorescence (TRF) spectroscopies supported by molecular dynamics simulations. TRF results for TagRFP675 and the mKate/M41Q variant show picosecond time scale red-shifts followed by nanosecond time blue-shifts. Global analysis of the TRF spectra reveals spectrally distinct emitting states that do not interconvert during the S₁ lifetime. These dynamics originate from photoexcitation of a mixed ground-state population of acylimine hydrogen bond conformers. Strategically tuning the chromophore environment in TagRFP675 might stabilize the most red-shifted conformation and result in a variant with a larger Stokes shift