The properties of photo-generated reactive species, holes and electrons in bulk TiO(2) (anatase) film and nano-sized TiO(2) were studied and their effects towards decomposing pollutant dye methyl ...orange (MO) were compared by transient absorption spectroscopies. The recombination of holes and electrons in nano-sized TiO(2) was found to be on the microsecond time scale consistent with previous reports in the literature. However, in bulk TiO(2) film, the holes and electrons were found to be on the order of picoseconds due to ultra fast free electrons. The time-correlated single-photon counting (TCSPC) technique combined with confocal fluorescence microscopy revealed that the fluorescence intensity of MO is at first enhanced noticeably by TiO(2) under UV excitation and soon afterwards weakened dramatically, with the lifetime prolonged. Photo-generated holes in nano-sized TiO(2) can directly oxidize MO on the time scale of nanoseconds, while free electrons photo-generated in bulk TiO(2) film can directly inject into MO on the order of picoseconds. Through cyclic voltammetry measurements, it was found that MO can be reduced at -0.28 V and oxidized at 1.4 V (vs. SCE) and this provides thermodynamic evidence for MO to be degraded by electrons and holes in TiO(2). Through comparison of the hole-scavenging effect of MO and water, it was found that in polluted water when MO is above 1.6 × 10(-4) M, the degradation is mainly due to a direct hole oxidation process, while below 1.6 × 10(-4) M, hydroxyl oxidation competes strongly and might exceed the hole oxidation.
High‐valent iron‐nitrido species are nitrogen analogues of iron‐oxo species which are versatile reagents for C−H oxidation. Nonetheless, C−H activation by iron‐nitrido species has been scarcely ...explored, as this is often hampered by their instability and short lifetime in solutions. Herein, the hydrogen atom transfer (HAT) reactivity of an Fe porphyrin nitrido species (2 c) toward C−H substrates was studied in solutions at room temperature, which was achieved by nanosecond laser flash photolysis (LFP) of its FeIII‐azido precursor (1 c) supported by a bulky bis‐pocket porphyrin ligand. C−H bonds with bond dissociation enthalpies (BDEs) of up to ≈84 kcal mol−1 could be activated, and the second‐order rate constants (k2) are on the order of 102–104 s−1 m−1. The Fe‐amido product formed after HAT could further release ammonia upon protonation.
A reactivity study on the hydrogen atom transfer (HAT) reaction between C−H substrates and an Fe‐nitrido porphyrin species was achieved by employing nanosecond laser flash photolysis and by using a bis‐pocket porphyrin ligand. Thermodynamic and kinetic parameters obtained in this study are fundamentally important for designing C−H amination and N2 fixation reactions involving Fe‐nitrido intermediates.
Benzoin is one of the most commonly used photoinitiators to induce free radical polymerization. Here, improved benzoin properties could be accomplished by the introduction of two methoxy ...substituents, leading to the formation of 3',5'-dimethoxybenzoin (DMB) which has a higher photo-cleavage quantum yield (0.54) than benzoin (0.35). To elucidate the underlying reaction mechanisms of DMB and obtain direct information of the transient species involved, femtosecond transient absorption (fs-TA) and nanosecond transient absorption (ns-TA) spectroscopic experiments in conjunction with density functional theory/time-dependent density functional theory (DFT/TD-DFT) calculations were performed. It was found that the photo-induced α-cleavage (Norrish Type I reaction) of DMB occurred from the nπ* triplet state after a rapid intersystem crossing (ISC) process (7.6 ps), leading to the generation of phenyl radicals on the picosecond time scale. Compared with Benzoin, DMB possesses two methoxy groups which are able to stabilize the alcohol radical and thus result in a stronger driving force for cleavage and a higher quantum yield of photodissociation. Two stable conformations (
-DMB and
-DMB) at ground state were found via DFT calculations. The influence of the intramolecular hydrogen bond on the α-cleavage of DMB was elaborated.
In contrast to the common intramolecular charge transfer (ICT) emission, planarized intramolecular charge transfer (PLICT) based materials usually possess higher proportion radiative decay for their ...flat and rigid conformation in excited states. Herein, a strategy for designing PLICT‐based emitters by the excited state quinone‐conformation induced planarization is proposed. By virtue of RIR mechanism on TPP (tetraphenylpyrazinyl) unit, the newcomers named as TPP‐PPI (1‐phenyl‐2‐(4′‐(3,5,6‐triphenylpyrazin‐2‐yl)‐1,1′‐biphenyl‐4‐yl)‐1H‐phenanthro9,10‐d imidazole) and TPP‐PI (1‐phenyl‐2‐(4‐(3,5,6‐triphenylpyrazin‐2‐yl)phenyl)‐1H‐phenanthro9,10‐dimidazole) exhibit aggregation‐induced emission (AIE) characteristics. TPP‐PPI and TPP‐PI have obvious PLICT properties via series of spectral measurements. Employing theoretical calculation in ground and excited states in different solvents, their PLICT process is confirmed further, and TPP's contribution on PLICT formation becomes clear. In non‐doped organic light‐emitting diodes, these two emitters with AIE and PLICT characteristic exhibit good performance with external quantum efficiency (4.85% and 4.36%) as blue emitters.
Unlike the donor–acceptor type organic light‐emitting diode (OLED) emitters with common intramolecular charge transfer process, the planarized intramolecular charge transfer‐based materials have strong emission in high polar surrounding. A novel and feasible strategy is developed by utilizing excited state quinone‐conformation induced planarization of tetraphenylpyrazine (TPP). Two derivatives of TPP exhibited good OLED performance with high efficiency and low efficiency roll‐off.
Halide perovskite narrowband photodetectors based on a charge collection narrowing mechanism have emerged as a new class of optoelectronic devices for monochromatic imaging. However, improving the ...figures‐of‐merit of such narrowband photodetectors remains challenging due to the inability to manipulate the major material players in the elusive photoresponse process. Here, a novel approach of manipulating ion migration to enhance the narrowband photoresponse of self‐driven p‐i‐n type photodetectors is taken by intentionally adding mobile ions into the formamidine and methylamine mixed cation perovskite layer. The excess mobile ions reduce the activation energy of ion migration, and this facilitated migration orchestrates the ions in the perovskite layer to re‐engineer the energy band, and thus modulates the charge separation and collection energetics and kinetics, leading to an unprecedented boost of the narrowband photoresponse. The photodetector based on this approach achieved a peak responsivity of as high as 112.41 mA W−1 at 820 nm at zero bias with a full‐width at half maximum of only 22 nm and an over 3‐fold improvement in the spectral rejection ratio, making it highly promising for the next‐generation color imaging devices.
Excess ions are intentionally added to the perovskite to enhance the ion redistribution, which modulates the photodetector's energy band structure and external quantum efficiency (EQE). The modified photodetector shows an improved spectral rejection ratio of 15 and EQE of 17% at 820 nm. The narrow full‐width at half maximum of 22 nm indicates its high wavelength selectivity and promising application in next‐generation color imaging devices.
The excited‐state dynamics of cyanide‐bridged trinuclear carbonyl rhenium(I) complexes with general formula of RepRecRep {Rec=(−NC)ReI(CO)2(phen)(CN−); Rep=−ReI(CO)2(L L)(La), L L= diimine or ...diphosphine, La=triphenylphosphine or carbonyl ligand} and the monomeric peripheral control complexes have been elucidated through a study with ultrafast transient absorption spectroscopy. The time constants for the energy transfer between the MLCT excited states of the peripheral and central Re(I) chromophores (ca. 0.56–0.78 ps) are typical for other cyanide‐bridged polynuclear transition metal complexes. The emissive excited states and the absorption transient have been characterized by nanosecond transient Raman spectroscopy.
The excited state dynamics, including energy transfer processes between different MLCT excited states, of cyanide‐bridged trinuclear rhenium(I) complexes have been elucidated with resonance Raman, transient Raman, and ultrafast transient absorption spectroscopy.
Double‐bond photoswitching molecules typically can undergo trans→cis photoisomerization, but their photoisomerization mechanisms in the solid state have been rarely reported. Herein, the ...excited‐state evolutions of trans‐azobenzene (AB), trans‐stilbene (TS), and N‐benzylideneaniline (NBA) are studied to unveil the relationship between the double‐bond photoswitching molecules and the photoisomerization kinetics in the solution phase and in nanocrystal suspensions (NCS). The photoisomerization rate of NBA is the fastest among these three molecules in the solution phase due to its most apparent single‐bond feature of the central double bond in the S1 state. The free space ordered crystal configuration boosts the photoisomerization rates of AB and TS in NCS to be faster than that of the solution phase because the hindrance of the wrapping solvent is eliminated in the crystals. In contrast, when NBA is prepared into NCS, it becomes distorted into a nonplanar structure because of the asymmetrical C‐H···π interactions, resulting in NBA having a large spin‐orbital coupling (SOC) value which opens the intersystem crossing channel to generate the triplet state instead of undergoing photoisomerization in solution. Therefore, this work reveals that the crystal configuration and the molecular structure may critically determine the photophysical properties of photoswitching materials.
The nanocrystals of double‐bond photoswitching molecules exhibit totally different trans‐cis photoisomerization excited state evolution processes compared to the solutions phase of these molecules due to the crystal configuration and molecular structure critically determining the photophysical properties of photoswitching materials.
Nitrenium ions are important reactive intermediates in chemistry and biology. In this work, femtosecond and nanosecond transient absorption (fs-TA and ns-TA) along with nanosecond time-resolved ...resonance Raman (ns-TR³) experiments were employed to examine the photochemical pathways of
-(4,4'-dibromodiphenylamino)-2,4,6-trimethylpyridinium BF₄
(salt (DN) from just absorption of a photon of light to the production of the important
,
-di(4-bromophenyl)nitrenium ion
. In acetonitrile (MeCN), the formation of halogenated diarylnitrenium ion
was observed within 4 ps, showing the vibrational spectra with strong intensity. The nucleophilic adduct reaction of ion
with H₂O was also examined in aqueous solutions. The direct detection of the unique ortho adduct intermediate
shows that there is an efficient and exclusive reaction pathway for
with H₂O. The results shown in this paper give new characterization of
, which can be used to design time-resolved spectroscopy investigations of covalent addition reactions of nitrenium ions with other molecules in future studies.
The photochemical reactivity of 2‐hydroxymethylphenol (1) was investigated experimentally by photochemistry under cryogenic conditions, by detecting reactive intermediates by IR spectroscopy, and by ...using nanosecond and femtosecond transient absorption spectroscopic methods in solution at room temperature. In addition, theoretical studies were performed to facilitate the interpretation of the experimental results and also to simulate the reaction pathway to obtain a better understanding of the reaction mechanism. The main finding of this work is that photodehydration of 1 takes place in an ultrafast adiabatic photochemical reaction without any clear intermediate, delivering quinone methide (QM) in the excited state. Upon photoexcitation to a higher vibrational level of the singlet excited state, 1 undergoes vibrational relaxation leading to two photochemical pathways, one by which synchronous elimination of H2O gives QM 2 in its S1 state and the other by which homolytic cleavage of the phenolic O−H bond produces a phenoxyl radical (S0). Both are ultrafast processes that occur within a picosecond. The excited state of QM 2 (S1) probably deactivates to S0 through a conical intersection to give QM 2 (S0), which subsequently delivers benzoxete 4. Elucidation of the reaction mechanisms for the photodehydration of phenols by which QMs are formed is important to tune the reactivity of QMs with DNA and proteins for the potential application of QMs in medicine as therapeutic agents.
Dry run: The photochemical reactivity of 2‐hydroxymethylphenol is investigated experimentally by photochemistry under cryogenic conditions, by detecting reactive intermediates by IR spectroscopy, and by using nanosecond and femtosecond transient absorption spectroscopic methods in solution at room temperature. Theoretical studies are also performed to facilitate the interpretation of the experimental results and also to simulate the reaction pathway.
Nitrenium ions are common reactive intermediates with high activities towards some biological nucleophiles. In this paper, we employed femtosecond transient absorption (fs-TA) and nanosecond ...transient absorption (ns-TA) as well as nanosecond time-resolved resonance Raman (ns-TR
) spectroscopy and density function theory (DFT) calculations to study the spectroscopic properties of the
(4,4'-dibromodiphenylamino)-2,4,6-trimethylpyridinium BF
salt (
) in an acidic aqueous solution. Efficient cleavage of the N-N bond (4 ps) to form the
-di(4-bromophenyl)nitrenium ion (
) was also observed in the acidic aqueous solution. As a result, the dication intermediate
appears more likely to be produced after abstracting a proton for the nitrenium ion
in the acid solution first, followed by an electron abstraction to form the radical cation intermediate
. These new and more extensive time-resolved spectroscopic data will be useful to help to develop an improved understanding of the identity, nature, and properties of nitrenium ions involved in reactions under acidic aqueous conditions.
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