The electron-donating strengths of donor (D) moieties in thirteen donor-π-acceptor systems (D1-π-A to D13-π-A wherein -π- and A represent butadiene and cyanoacrylic acid units, respectively) have ...been studied using B3LYP/cc-pVDZ level density functional theory (DFT) calculations. The selected D moieties are encountered as a part of an organic sensitizer molecule in dye-sensitized solar cell (DSSC) applications. When the D moiety is joined with π-A, a certain amount of electron donation from D to A occurs leading to an increase in electron density at the A site of D-π-A compared to the A site of π-A. This electron reorganization is quantified in terms of a change in molecular electrostatic potential (MESP) minimum (Δ
V
mA
) at the acceptor site, the CN group of the cyanoacrylic acid. The Δ
V
mA
is always negative, in the range of −11.0 to −2.6 kcal mol
−1
which provides a quick assessment of the rank order of the electron-donating nature of the D moieties in the ground state of D-π-A. The optical and photovoltaic properties of D and D-π-A systems are also determined at the TD-CAM-B3LYP/cc-pVDZ//B3LYP/cc-pVDZ level. An absorption red shift (Δ
λ
max
) in the range of 81-242 nm is observed when D moieties change to D-π-A systems. The ground state property Δ
V
mA
showed a strong linear correlation with the excited state property Δ
λ
max
. Furthermore, Δ
V
mA
is found to be proportional to the open-circuit voltage (
V
OC
). The resemblance of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the D-π-A system with the respective energies of donor and π-A systems shows that the donor tunes the HOMO, while π-A tunes the LUMO. Among the thirteen D-π-A systems,
N
,
N
-dialkylaniline, and julolidine are rated as the best donors for the photovoltaic applications. This study shows that the MESP based assessment of the donating strength of donor systems offers a powerful rational design strategy for the development of efficient dyes for DSSC applications.
Donating strength of typically used donors in dye-sensitized solar cells.
Eighteen electron-rich nitrogen incorporated donors with a butadiene π-spacer and a cyanoacrylic acid acceptor (A) as photosensitizers (D1-π-A to D18-π-A) for dye-sensitized solar cell (DSSC) ...applications have been designed for improving the photovoltaic performance. The significance of the nitrogen centres for revamping the donating strength (Δ
V
mA
) of D-π-A is scrutinized using molecular electrostatic potential (MESP) analysis at the B3LYP/cc-pVDZ level of density functional theory (DFT). During the transformation of a donor (D) to D-π-A, a certain delocalization of electron density from D to π-A has occurred, and the change in the MESP minimum (Δ
V
mA
) observed at the cyano region of D-π-A is related to the donating strength of D. Optical and photovoltaic properties are analyzed at the TD/CAM-B3LYP/cc-pVDZ/SMD//B3LYP/cc-pVDZ level. In D1-π-A to D18-π-A, Δ
V
mA
is in the range −7.0 to −19.0 kcal mol
−1
and the increase in donating strength is found to be proportional to the number of planar nitrogens in the donors. D12-π-A exhibited the most negative Δ
V
mA
(−19.0 kcal mol
−1
), indicating the highest electron-donating strength of D12, whereas the least negative Δ
V
mA
(−7.0 kcal mol
−1
) displayed by D7-π-A is correlated to the weak donating character of D7. By increasing the electron-donating strength of D in D-π-A, a red-shift in the absorption maximum (Δ
λ
max
) by 162 to 294 nm is observed. Further, the open-circuit voltage (e
V
oc
) calculated for the D-π-A systems showed a strong linear relationship with Δ
V
mA
. The LUMO (lowest unoccupied molecular orbital) energy of all the D-π-A systems (−1.79 to −2.79 eV) is observed above the conduction band (CB) energy of TiO
2
(−4.0 eV), which ensured a desirable electron injection efficiency (Δ
G
inject
) for them. The analysis of the adsorption energy (
E
ads
) of the D-π-A systems on the TiO
2
semiconductor (D-π-A/TiO
2
) showed that D12-π-A has the highest adsorption stability. Improving the adsorption stability is better for improving e
V
oc
and the power conversion efficiency (PCE). The maximum absorption wavelength (
λ
max
) of the D-π-A/TiO
2
systems ranges from 513 to 703 nm and all of them display a red-shift with respect to the bare D-π-A systems. The study suggests D12 as the most efficient photosensitizer for DSSC applications. Further, it deepens the understanding of the structure-performance relationship of D-π-A systems as photosensitizers.
Donor modifications, especially through N-annulation, for enhancing the structure-performance relationship of D-π-A systems for DSSC applications.
Donor-π-acceptor (D-π-A) systems typically used in dye-sensitized solar cells (DSSC) have been studied for assessing the donating strength of six donors (D1-D6) under the influence of substituents ...such as CH
3
, C
5
H
11
, isopropyl,
t
-butyl, OH, OCH
3
, OC
2
H
5
, NH
2
, N(CH
3
)
2
, PhCH
3
, and PhNH
2
along with π-spacer butadiene and acceptor moiety cyanoacrylic acid. The substituent effect enhances electron donation from D to A through the π-spacer. The enhancement in electron density at A has been quantified in terms of the difference in the molecular electrostatic potential (MESP) minimum at the cyano nitrogen (Δ
V
mA
) between π-A and D-π-A. For unsubstituted D-π-A systems, Δ
V
mA
is in the range −0.1 to −5.7 kcal mol
−1
, whereas the substitution enhances the negative character of Δ
V
mA
in the range −0.8 to −8.0 kcal mol
−1
. In alkyls and Ph-CH
3
substituted D-π-A systems, Δ
V
mA
lies in the range −0.8 to −6.7 kcal mol
−1
, whereas the N(CH
3
)
2
substituted systems exhibit more negative Δ
V
mA
(more enhanced donating strength) in the range −5.1 to −8.0 kcal mol
−1
. The more negative value of Δ
V
mA
implies the greater electron-donating ability of the D−π−A system. Optical and photovoltaic parameters (Δ
G
reg
, Δ
G
inject
, e
V
OC
) are analyzed at the TD-CAM-B3LYP/SMD/cc-pVDZ//B3LYP/cc-pVDZ level of DFT. An excellent linear correlation is observed in all six sets between Δ
V
mA
and the absorption maximum (
λ
max
) showing that
λ
max
increases with enhanced donating strength. The higher absorption maximum obtained by N(CH
3
)
2
substituted D-π-A systems lies in the range 430 nm to 490 nm, explaining the outstanding donating ability of N(CH
3
)
2
compared to other substituents. The reduced highest occupied molecular orbital (HOMO) - lowest unoccupied molecular orbital (LUMO) gap (from 3.14 to 2.17 eV) with enhanced donating strength confirms the influence of substituent effects in broadening the absorption maximum. Furthermore, in photovoltaic parameters, a strong influence of the substituent effect is observed. The N(CH
3
)
2
substituted D1-π-A (D1-N(CH
3
)
2
) exhibits the highest e
V
OC
(1.38 eV). The strong linear correlation observed for the ground state property Δ
V
mA
and open-circuit voltage e
V
OC
provides guidelines for developing an effective strategy for designing dye sensitizers for desirable photovoltaic applications.
The significance of electron releasing groups at the donors of D-π-A systems for improving the donating strength and power conversion efficiency of photosensitizers.
The unique four-level photocycle characteristics of excited-state intramolecular proton transfer (ESIPT) materials enable population inversion and large spectral separation between absorption and ...emission through their respective enol and keto forms. This leads to minimal or no self-absorption losses, a favorable feature in acting as an optical gain medium. While conventional ESIPT materials with an enol–keto tautomerism process are widely known, zwitterionic ESIPT materials, particularly those with high photoluminescence, are scarce. Facilitated by the synthesis and characterization of a new family of 2-hydroxyphenyl benzothiazole (HBT) with fluorene substituents, HBT-Fl1 and HBT-Fl2, we herein report the first efficient zwitterionic ESIPT lasing material (HBT-Fl2). The zwitterionic ESIPT HBT-Fl2 not only shows a remarkably low solid-state amplified spontaneous emission (ASE) threshold of 5.3 μJ/cm2 with an ASE peak at 609 nm but also exhibits high ASE photostability. Coupled with its substantially large Stokes shift (≈236 nm ≈10,390 cm–1) and an extremely small overlap of excited-state absorption with ASE emission, comprehensive density functional theory (DFT) and time-dependent DFT studies reveal the zwitterionic characteristics of HBT-Fl2. In opposition to conventional ESIPT with π-delocalized tautomerism as observed in analogue HBT-Fl1 and parent HBT, HBT-Fl2 instead shows charge redistribution in the proton transfer through the fluorene conjugation. This structural motif provides a design tactic in the innovation of new zwitterionic ESIPT materials for efficient light amplification in red and longer-wavelength emission.
The transmission of substituent effect through a variety of spacers, that is to say, alkyl, alkenyl, alkynyl, phenyl, thiophenyl, and polyacene has been studied by modeling Y‐G‐X type molecular ...systems (Y: reaction center; G: spacer moiety; X: substituent) using B3LYP/6‐31G(d,p) density functional theory calculations. The reaction center is always kept as a C=C double bond and the molecular electrostatic potential (MESP) minimum (Vmin) observed for this bond showed subtle variation with respect to the changes in the spacer unit and the nature of substituent. Strong linear correlations are observed between Hammett substituent constants (σI and σp) and Vmin, which recommend the aptness of Vmin as an electronic descriptor to quantify the substituent effect. Since Vmin offers an alternative measure of substituent effect, the correlation between Vmin and σp has been used for assessing the transmission of substituent effect through a variety of spacer moieties. The highest transmission coefficient (γ) is always observed for smaller spacer length. Among all the spacers, alkenyl showed the highest and alkyl showed the lowest transmission power. The study recommends the use of short chains of C=C double, C≡C triple or a combination of both as spacers for the effective transmission of substituent effect to the reaction center.
Through space(rs): The molecular electrostatic potential (MESP) minimum (Vmin) at the terminal C=C double bond is used to quantify the transmission power of substituent effect through various spacers.
Ensuring the security and safety of passengers and cargo through effective baggage screening presents a critical challenge in high-traffic environments like airports, where traditional manual ...processes are affected by high error rates, fatigue among security personnel, and privacy concerns. These issues underscore the urgent need for sophisticated automated systems capable of accurately detecting concealed threats. Advancements in deep learning have introduced more refined approaches to baggage screening, yet they struggle with capturing the global context of threats and challenges presented by clutter, concealment, and occlusion. Addressing these issues, this paper presents the Multi-Scale Hierarchical Transformer for X-ray Detection (MHT-X), a novel framework that surpasses conventional limitations by integrating a multi-scale contour mapping technique with a hierarchical feature extraction process using visual Transformers. MHT-X is distinctive in its ability to generate multi-level contour maps that unveil complex features of threat objects, facilitating their accurate identification. It employs a top-down approach for hierarchical feature processing, capturing vital spatial characteristics essential for precise threat localization through an efficient encoder-decoder mechanism. MHT-X represents a significant advancement in the detection of concealed and obstructed threat objects in X-ray baggage scans. By combining multi-scale contour maps with a hierarchical feature extraction process, it significantly enhances detection accuracy and outperforms traditional Convolutional Neural Network (CNN) models by effectively capturing the global context of illegal items with advanced feature representation. The proposed framework is rigorously validated on two public datasets, dubbed CLCXray, and PIDray, where it outperforms the state-of-the-art (SOTA) methods by achieving the mean average precision score of 65.26%, and 78.19%, respectively.
In this paper, we consider two single server queueing systems to which customers of two distinct priorities (P1 and P2) arrive according to a Marked Markovian arrival process (MMAP). They are served ...according to two distinct phase type distributions. The probability of a P1 customer to feedback is θ on completion of his service. The feedback (P1) customers, as well as P2 customers, join the low priority queue. Low priority (P2) customers are taken for service from the head of the line whenever the P1 queue is found to be empty at the service completion epoch. We assume a finite waiting space for P1 customers and infinite waiting space for P2 customers. Two models are discussed in this paper. In model I, we assume that the service of P2 customers is according to a non-preemptive service discipline and in model II, the P2 customers service follow a preemptive policy. No feedback is permitted to customers in the P2 line. In the steady state these two models are compared through numerical experiments which reveal their respective performance characteristics.
Color purity is a critical prerequisite for full color displays. Creation of deep-red phosphorescent materials with high PLQYs is particularly challenging because of the “energy gap law”. ...Simultaneously achieving high yielding solution processable Pt(II) complexes further complicates this challenge. In this report, we developed a high-yielding synthetic route to a solution processable/deep-red Pt(II) complex with a rigid tetradentate structure, in which we identified an octahedral Pt(IV) complex as a major side product formed under the standard complexation conditions. We managed to effectively transform the octahedral Pt(IV) species into a highly luminescent deep-red square-planar Pt(II) complex through a base-promoted reduction. The Pt(II) complex was found to exhibit high solution and blend film PLQYs. X-ray crystal structure and DFT calculations of the Pt(II) complex showed that perpendicular orientation of molecular dipoles enhanced the luminescence properties. In neat films, there was no luminescence enhancement due to interdigitation of the attached hexyloxy tails, preventing strong Pt···Pt interactions in the solid state. Solution-processed OLEDs based on the Pt(II) complex showed a low turn-on voltage of 3.3 V (at 1 cd/m2) with a maximum brightness of 2000 cd/m2 and a maximum EQE of ≈6% (4% at 100 cd/m2). A narrow electroluminescence with a full width at half-maximum of ≈50 nm was observed with a peak at 623 nm and deep-red emission with 1931 CIE coordinates of (0.65, 0.35). Transient electroluminescence measurements were used to investigate the EQE roll-off of the OLEDs.
X-ray imagery systems have enabled security personnel to identify potential threats contained within the baggage and cargo since the early 1970s. However, the manual process of screening the ...threatening items is time-consuming and vulnerable to human error. Hence, researchers have utilized recent advancements in computer vision techniques, revolutionized by machine learning models, to aid in baggage security threat identification via 2D X-ray and 3D CT imagery. However, the performance of these approaches is severely affected by heavy occlusion, class imbalance, and limited labeled data, further complicated by ingeniously concealed emerging threats. Hence, the research community must devise suitable approaches by leveraging the findings from existing literature to move in new directions. Towards that goal, we present a structured survey providing systematic insight into state-of-the-art advances in baggage threat detection. Furthermore, we also present a comprehensible understanding of X-ray-based imaging systems and the challenges faced within the threat identification domain. We include a taxonomy to classify the approaches proposed within the context of 2D and 3D CT X-ray-based baggage security threat screening and provide a comparative analysis of the performance of the methods evaluated on four benchmarks. Besides, we also discuss current open challenges and potential future research avenues.
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