Intramolecular Charge Transfer Nature Research Articles

Blue Luminescent Boron Complexes Based on N,N‐Type Imidazo[1,5‐a]pyridine Ligand for Mitochondrial Imaging

AbstractWe have synthesized two boron complexes based on N,N‐type bidentate imidazo[1,5‐a]pyridine ligands exhibiting blue emission with good quantum yields in solution. The transient absorption and theoretical studies of the boron complexes indicated the intramolecular charge transfer nature. In addition, one specific complex, 1 a was found to work as a mitochondrial imaging agent.

Supramolecular Polymerization as a Tool to Reveal the Magnetic Transition Dipole Moment of Heptazines.

Heptazine derivatives have attracted significant interest due to their small S1-T1 gap, which contributes to their unique electronic and optical properties. However, the nature of the lowest excited state remains ambiguous. In the present study, we characterize the lowest optical transition of heptazine by its magnetic transition dipole moment. To measure the magnetic transition dipole moment, the flat heptazine must be chiroptically active, which is difficult to achieve for single heptazine molecules. Therefore, we used supramolecular polymerization as an approach to make homochiral stacks of heptazine derivatives. Upon formation of the supramolecular polymers, the preferred helical stacking of heptazine introduces circular polarization of absorption and fluorescence. The magnetic transition dipole moments for the S1 ← S0 and S1 → S0 are determined to be 0.35 and 0.36 Bohr magneton, respectively. These high values of magnetic transition dipole moments support the intramolecular charge transfer nature of the lowest excited state from nitrogen to carbon in heptazine and further confirm the degeneracy of S1 and T1.

Intramolecular interactions and optical properties of indole-containing alkaloids extracted from Arundo donax L.

Three optically active natural alkaloids were computationally investigated. The electronic ground state was found to be characterised by numerous intramolecular interactions, among which a hydrogen bond. These interactions are responsible for the different rate constants of the kinetics of conversion from one atropisomer to the other. The complex molecular electrostatic potential reveals a strong polar nature of these compounds, which strengthens when the electronic excited states are populated. The absorption spectra were interpreted and the intramolecular charge transfer nature of the Sn ← S0 transitions was revealed. The ECD spectra for the different atropisomers of these compounds indicate that the assignment proposed in the literature for 1 should be revised. The geometry of the excited state shows dramatic changes with respect to the ground state; some intramolecular interactions get lost. The fluorescence spectrum, proposed for the first time, shows de-excitation energies in the blue-green region (488 nm to 518 nm). Internal Conversion and intramolecular Electronic Energy Transfer may be fluorescence quenching channels.

The protein environment restricts the intramolecular charge transfer character of the luciferine/luciferase complex.

The electronic characterization of the luciferine/luciferase complex is fundamental to tune its photophysical properties and develop more efficient devices based on this luminiscent system. Here, we apply molecular dynamics simulations, hybrid quantum mechanics/molecular mechanics (QM/MM) calculations and transition density analysis to compute the absorption and emission spectra of luciferine/luciferase and analyze the nature of the relevant electronic state and its behaviour with the intramolecular and intermolecular degrees of freedom. It is found that the torsional motion of the chromophore is hampered by the presence of the enzyme, reducing the intramolecular charge transfer nature of the absorbing and emitting state. In addition, such a reduced charge transfer character does not correlate in a strong way neither with the intramolecular motion of the chromophore nor with the chromophore/amino-acid distances. However, the presence of a polar environment around the oxygen atom of the thiazole ring of the oxyluciferin, coming from both the protein and the solvent, enhances the charge transfer character of the emitting state.

Tetraphenylethene-Modified Colorimetric and Fluorescent Chemosensor for Hg2+ With Aggregation-Induced Emission Enhancement, Solvatochromic, and Mechanochromic Fluorescence Features.

A tetraphenylethene (TPE)-modified rhodanine derivative was successfully designed and prepared, and this luminophor showed intramolecular charge transfer nature from the TPE unit to the rhodanine-3-acetic acid unit. Interestingly, this luminogen not only exhibited typical aggregation-induced emission enhancement (AIEE) behavior but also showed good cell imaging performance. Remarkably, this AIEE-active TPE-containing rhodanine derivative possessed noticeable solvatochromic fluorescence effect involving multiple fluorescent colors of green, yellow-green, yellow, orange, and red. Meanwhile, this fluorescigenic compound displayed reversible mechanochromic fluorescence behavior based on the mutual transformation of between stable crystalline and metastable amorphous states. On the other hand, this multifunctional fluorophor could selectively and sensitively detect Hg2+ in an acetonitrile solution. Furthermore, this chemosensor could also be used to detect Hg2+ on test paper strips.

Multinuclear 2-(Quinolin-2-yl)quinoxaline-Coordinated Iridium(III) Complexes Tethered by Carbazole Derivatives: Synthesis and Photophysics.

Five mono/di/trinuclear iridium(III) complexes (1-5) bearing the carbazole-derivative-tethered 2-(quinolin-2-yl)quinoxaline (quqo) diimine (N^N) ligand were synthesized and characterized. The photophysical properties of these complexes and their corresponding diimine ligands were systematically studied via UV-vis absorption, emission, and transient absorption (TA) spectroscopy and simulated by time-dependent density functional theory. All complexes possessed strong well-resolved absorption bands at <400 nm that have predominant ligand-based 1π,π* transitions and broad structureless charge-transfer (1CT) absorption bands at 400-700 nm. The energies or intensities of these 1CT bands varied pronouncedly when the number of tethered Ir(quqo)(piq)2+ (piq refers to 1-phenylisoquinoline) units, π conjugation of the carbazole derivative linker, or attachment positions on the carbazole linker were altered. All complexes were emissive at room temperature, with 1-3 showing near-IR (NIR) 3MLCT (metal-to-ligand charge-transfer)/3LLCT (ligand-to-ligand charge-transfer) emission at ∼710 nm and 4 and 5 exhibiting red or NIR 3ILCT (intraligand charge-transfer)/3LMCT (ligand-to-metal charge-transfer) emission in CH2Cl2. In CH3CN, 1-3 displayed an additional emission band at ca. 590 nm (3ILCT/3LMCT/3MLCT/3π,π* in nature) in addition to the 710 nm band. The different natures of the emitting states of 1-3 versus those of 4 and 5 also gave rise to different spectral features in their triplet TA spectra. It appears that the parentage and characteristics of the lowest triplet excited states in these complexes are mainly impacted by the π systems of the bridging carbazole derivatives and essentially no interactions among the Ir(quqo)(piq)2+ units. In addition, all of the diimine ligands tethered by the carbazole derivatives displayed a dramatic solvatochromic effect in their emission due to the predominant intramolecular charge-transfer nature of their emitting states. Aggregation-enhanced emission was also observed from the mixed CH2Cl2/ethyl acetate or CH2Cl2/hexane solutions of these ligands.

D–π–A‐Type Triazatruxene‐Based Dopant‐Free Hole Transporting Materials for Efficient and Stable Perovskite Solar Cells

Three donor–π‐bridge–acceptor (D–π–A)‐type organic small molecules coded CI‐B1, CI‐B2, and CI‐B3 are designed, synthesized, and used as dopant‐free hole transporting materials (HTMs) for perovskite solar cells (PSCs). The strong electron‐donating triazatruxene central core (D), terthiophene conjugated arms (π), and three different strong electron‐accepting units (A) provide high intramolecular charge transfer nature and eliminate the need of dopants during the fabrication of PSCs. HTMs are investigated to understand the effect of terminal functional groups on the PSC performance. Interestingly, due to the change of end‐capping, three different organizations of self‐assembly with π–π stacking are observed in the solid thin films. Dopant‐free CI‐B1, CI‐B2, CI‐B3, and spiro‐OMeTAD with dopants are used with triple cation perovskite composition Cs0.1(MA0.15FA0.85)0.9Pb(I0.85Br0.15)3 (MA: CH3NH3+, FA: NHCHNH3+) in n‐i‐p architecture. The cells prepared with CI‐B3 not only exhibits a comparable power conversion efficiency (PCE) of 17.54% to the state‐of‐art of spiro‐OMeTAD with dopants (18.02%), but also demonstrates improved long‐term stability, maintaining 88% of its original PCE after 1000 h of illumination. The superior photovoltaic performance, synthetic simplicity, dopant‐free nature, high durability, and edge‐on molecular orientation of CI‐B3 show its great promise as a HTM candidate for efficient and stable PSCs.

Theoretical investigation of excited-state proton transfer (ESPT) for 2,5-bis(2-benzothiazolyl)hydroquinone: single or double?

ABSTRACT In this work, the single and double proton transfer dynamic process in the first excited state of 2,5-bis(2-benzothiazolyl)hydroquinone (BBTHQ) were theoretically investigated based on the time-dependent density functional theory (TD-DFT) method. The calculations of primary bond lengths, bond angles, IR vibrational spectra and NCI analysis reveal that two intramolecular hydrogen bonds of BBTHQ are strengthened in the first excited state, which provides a driving force for excited state proton transfer. The frontier molecular orbitals (FMOs) indicate the nature of intramolecular charge transfer. In addition, the potential energy surfaces (PESs) of the ground state and first excite state were also constructed to further elucidate the mechanism of intramolecular proton transfer of BBTHQ.

High-performance ambipolar benzodifurandione-based donor-acceptor copolymer with balanced hole and electron mobility

Well-balanced ambipolar benzodifurandione (BDO) donor–acceptor copolymers were synthesized by tuning its intramolecular charge transfer nature by expanding the volumetric portion of the donor moiety, i.e., by using thiophene copolymerized with thiophene vinylene thiophene (TVT) or selenophene vinylene selenophene (SVS). Photophysical analyses revealed that BDO-SVS formed highly aggregated states in the solution state compared with BDO-TVT, facilitating a well-ordered self-assembled crystalline structure in the solid state. Using two-dimensional grazing incident X-ray diffraction and quantitative pole figure analyses, we established that the molecular orientation and degree of crystallinity was higher in BDO-SVS than in BDO-TVT. The morphological features identified by atomic force microscopy also showed a higher crystalline nature of SVS derivative, implying that SVS can strengthen intermolecular charge transport. This improved environment for charge transport, was revealed by the performance of organic field-effect transistors: BDO-SVS had superior performance than that of BDO-TVT for both hole and electron mobilities. This research shows that BDO, with a strong acceptor moiety, renders high-performance ambipolar characteristics when copolymerized with an expanded and aggregating donor moiety, such as SVS.

Synthesis of new 2-((5-(4-alkyl-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)thiophen-2-yl)methylene)malononitrile: Dopant free hole transporting materials for perovskite solar cells with high power conversion efficiency

Two multi-chromophoric hybrid small molecule organic materials (ICTH1 and ICTH2) with D-π-A architecture were synthesized and their applicability as hole transporting materials for perovskite solar cells was explored. The strong electron-donating dithieno[3,2-b:2′,3′-d]pyrrole (D) and electron-accepting dicyanovinylene (A) moieties impart high intramolecular charge transfer nature to these materials and eliminates the need of additives or dopants during the fabrication of solar cell devices. Both ICTH1 and ICTH2 have deep HOMO level compared to spiro-OMeTAD and is advantageous to produce high Voc values. Hole transporting ability of ICTH1 and ICTH2 is ascertained by fabricating OFETs. Compared to spiro-OMeTAD, both ICTH1 and ICTH2 showed one order higher hole mobility. Perovskite solar cells (PSCs) with conventional architecture of FTO/Meso-TiO2/MAPbI3/HTM/Ag are fabricated using ICTH1 and ICTH2 as hole transporting and the simplest MAPbI3 as perovskite materials. PSCs with ICTH2 and ICTH1 HTMs, showed power conversion efficiency (PCE) of 18.75% and 17.91% with active area of 2.04 and 2.18 cm2, respectively. Under the similar conditions, PSCs with spiro-OMeTAD HTM showed 14.74% PCE with active area of 1.84 cm2. Even after 40 days, solar cell devices with ICTH2 showed superior performance (PCE of 16.08%) over the spiro-OMeTAD devices (PCE of 6.77%). Apart from the superior device performance, other prominent factors like synthetic simplicity, dopant-free nature and low production cost make ICTH1 and ICTH2 as potential alternatives to spiro-OMeTAD for using as HTM in PSCs.

Peridinin Torsional Distortion and Bond-Length Alternation Introduce Intramolecular Charge-Transfer and Correlated Triplet Pair Intermediate Excited States.

The nature of intramolecular charge transfer (ICT) and the mechanism of intramolecular singlet fission (SF) in peridinin remain open research questions. Obtaining an understanding of the population evolution from the bright state to dark state following a photoinduced electronic transition is critical. Unambiguously describing this evolution in peridinin, and light-harvesting carotenoids in general, has proven elusive experimentally and computationally. To offer a balanced description of the bright- and dark-state electronic structures, we here apply ab initio multireference perturbation theory quantum chemistry-the density matrix renormalization group self-consistent field and complete-active-space self-consistent field with second-order N-electron valence perturbation theory. At traditional bright- (S2) and dark-state (S1) optimized geometries, we find that an additional correlated triplet pair state and ICT state are derived from the canonical polyene Bu (S3) and 3Ag (S4) dark singlet excited states, respectively. Whereas the S3 state's physical properties are insensitive to peridinin's allene-tail donor and lactone ring acceptor functionalization, the S4 state exhibits a markedly enhanced oscillator strength and highest occupied molecular orbital-lowest unoccupied molecular orbital transition density. These changes suggest that the ICT character stems from mixing between the bright S2 and putatively dark S4.

Dipole Moment and Polarizability of Tunable Intramolecular Charge Transfer States in Heterocyclic π-Conjugated Molecular Dyads Determined by Computational and Stark Spectroscopic Study.

The annulation of two redox-active molecules into a compact and planar structure paves the way toward a new class of electronically versatile materials whose physical properties can be tuned via a substitution of one of the constituting moieties. Specifically, we present tetrathiafulvalene-benzothiadiazole donor-acceptor molecules. The critical role played by the dielectric properties of these molecules is evident by the large spectral shifts of the ground-state absorption spectra in a range of solvents. Stark spectroscopy is performed to determine experimentally dipole and polarizability change over transitions in the visible range with particular attention to the transition from the highest-occupied molecular orbital (HOMO) to the lowest-unoccupied molecular orbital (LUMO). The experimental results are compared to the results of time-dependent density functional theory calculations, and we reciprocally validate results from calculation and experiment. This allows us to filter out effective models and reveal important insights. The calculations are initially performed in the gas phase and subsequently a polarizable continuum model is adopted to probe the influence of the solvent on the molecular dielectric properties. The results show a large charge displacement from the HOMO to the LUMO and confirm the intramolecular charge transfer nature of the lowest-energy transition. Substitution of the acceptor moiety with electron-withdrawing groups results in changes to the experimentally determined molecular properties consistent with the effects predicted by computational results. The dominant contribution to the electroabsorption signal is due to the change in dipole moment, which is measured to be roughly 20 D for all samples and forms a small angle with the transition dipole moment in a toluene solvent environment.

Pyrenylpyrazole-based donor/acceptor fluorescent dyes: Synthesis and photophysical properties

The reaction of 1-acetylpyrene phenylhydrazone with Vilsmeier reagent (DMF-POCl3) afforded 1-phenyl-3-(pyren-1-yl)-1H-pyrazole-4-carbaldehyde in good yield. Condensation of this compound with a series of active methylene compounds (methyl ethyl ketone, diethyl ketone, malononitrile, 1,3-indandione and N,N-diethythiolbarbituric acid) gave a series of donor/acceptor dyes exhibiting fluorescence in the almost whole range of the visible spectrum. The electronic structure of the dyes was studied theoretically by the (TD) DFT method. Calculations elucidated the nature of intramolecular charge transfer (ICT) processes leading to emissive excited states of the synthesised compounds. It turned out that the electron acceptor ability influences the direction of ICT. For compounds with relatively weak acceptor groups, ICT from the pyrene moiety toward the remaining part of the molecule prevailed, whereas for those having stronger acceptor groups the main ICT pathway became that from the phenylpyrazole group to the acceptor group.

Solvent Effects on the Intramolecular Charge Transfer Character of N,N-Diaryl Dihydrophenazine Catalysts for Organocatalyzed Atom Transfer Radical Polymerization.

The nature of intramolecular charge transfer of N,N-diaryl dihydrophenazine photocatalysts (PCs) in different solvents is explored in context of their performance in organocatalyzed atom transfer radical polymerization (O-ATRP). PCs having a computationally predicted lowest energy excited state exhibiting charge transfer (CT) character can operate a highly controlled O-ATRP in a wide range of solvent polarities, from non-polar hexanes to highly polar N,N-dimethylacetamide. For PCs having a computationally predicted lowest energy excited state not possessing CT character, their ability to operate a controlled O-ATRP is decreased. This study confirms the importance of CT character in the excited state for N,N-diaryl dihydrophenazine PCs, and a deeper understanding of the activity of CT PCs has enabled the synthesis of polymers of low dispersity ( < 1.10) in a controlled fashion.

Effect of Substituents on the Electronic Structure and Degradation Process in Carbazole Derivatives for Blue OLED Host Materials

We investigate the dissociation mechanism of the C–N bond between carbazole and dibenzothiophene in carbazole-dibenzothiophene (Cz-DBT) positional isomers, selected as representative systems for blue host materials in organic light-emitting diodes (OLEDs). The C–N bond dissociation energies, calculated at the density functional theory level, are found to depend strongly on the charge states of the parental molecules. In particular, the anionic C–N bond dissociations resulting in a carbazole anion can have low dissociation energies (∼1.6 eV) with respect to blue emission energy. These low values are attributed to the large electron affinity of the carbazole radical, a feature that importantly can be modulated via substitution. Substitution also impacts the energies of the first excited electronic states of the Cz-DBT molecules since these states have an intramolecular charge-transfer nature due to the spatially localized character of the frontier molecular orbitals within the carbazole moiety (for the HOMO...

Furan and pyridinechalcogenodiazole-based π-conjugated systems via a donor-acceptor approach

Two novel furan and pyridinechalcogenodiazole based monomers, namely 4,7-di(furan-2-yl)-[1, 2, 5]thiadiazolo[3,4-c]pyridine (Fu-S-Fu) and 4,7-di(furan-2-yl)-[1, 2, 5]selenadiazolo [3,4-c]pyridine (Fu-Se-Fu), where a single atom in the pyridinechalcogenodiazole unit is varied from S to Se, were designed and synthesized via a donor-acceptor approach, and then the corresponding polymers, P(Fu-S-Fu) and P(Fu-Se-Fu), were electrosynthesized. Also, structure characterization and optoelectronic properties, including FTIR, SEM, DFT theoretical calculations, intramolecular charge transfer nature, optical and electrochemical behaviors, and electrochromic performance, were systematically investigated and comparatively discussed. The obtained monomers exhibited lower oxidation potential (Fu-S-Fu: 1.12 V; Fu-Se-Fu: 1.09 V), leading to the facile electrodeposition of uniform hybrid polymer films with outstanding electroactivity at low oxidation potentials. Optical spectroscopy of corresponding polymers showed that Se substitution led to a red-shift in the low-energy transition, while the high-energy band remains relatively constant in energy.

A research on excited-state intramolecular proton-transfer mechanism of a new chemosensor

Based on the density functional theory (DFT) and time-dependent density functional theory (TDDFT), the excited-state intramolecular proton-transfer (ESIPT) mechanism of a new reported chromophore by Kim et al. (Sensors Actuators B Chem 206:430–434, 2015) has been investigated theoretically. The calculated results of bond lengths and bond angles of hydrogen bond O–H···N, the infrared vibrational spectra and the hydrogen bonding energies all demonstrated that the intramolecular hydrogen bond is strengthened in the first excited state. It is no denying the fact that our calculated results reproduced the experimental absorbance and fluorescence emission spectra well, which demonstrates that the TDDFT theory we adopted is reasonable and effective. From the analysis of frontier molecular orbitals, it is reasonable to suggest that the intramolecular charge-transfer nature of the excitation prompts the proton transfer giving rise to an ESPT reaction. The constructed potential energy curves of ground state and the first excited state based on keeping the O–H distance fixed from 0.993 to 2.343 A have been used to illustrate the ESIPT process. A relative lower barrier of 4.17 kcal/mol in the first excited-state potential energy curve proved the ESIPT mechanism.

Synthesis and Solvatochromism of Substituted 4-(Nitrostyryl)phenolate Dyes.

4-(Nitrostyryl)phenols 2a-9a were synthesized, and by deprotonation in solution, the solvatochromic phenolates 2b-9b were formed. Their absorption bands in the vis region of the spectra are due to π-π* electronic transitions, of an intramolecular charge-transfer nature, from the electron-donor phenolate toward the electron-acceptor nitroarene moiety. The frontier molecular orbitals and natural bond orbitals were analyzed for the protonated and deprotonated forms. The calculated geometries are in agreement with X-ray structures observed for 4a, 6a, and 8a. The HOMO-LUMO energy gaps suggest that, after their deprotonation, an increase in the electron delocalization is observed. In the protonated compounds, the HOMO is primarily localized over the phenol ring and the C═C bridge. After deprotonation, it extends toward the entire molecule, including the NO2 groups. The solvatochromism of each dye was studied in 28 organic solvents, and it was found that all compounds exhibit a reversal in solvatochromism, which is interpreted in terms of the ability of the media to stabilize their electronic ground and excited states to different extents. The Catalán multiparameter equation is used in the interpretation of the solvatochromic data, revealing that the most important contribution to the solute/solvent interaction is the hydrogen-bond donor acidity of the solvent.

Highly Emissive Whole Rainbow Fluorophores Consisting of 1,4-Bis(2-phenylethynyl)benzene Core Skeleton: Design, Synthesis, and Light-Emitting Characteristics.

To create the whole-rainbow-fluorophores (WRF) having the small Δλem (the difference of λem between a given fluorophore and nearest neighboring fluorophore having the same core skeleton) values (<20 nm) in full visible region (λem: 400-650 nm), the high log ε (>4.5), and the high Φf (>0.6), we investigated molecular design, synthesis, and light-emitting characteristics of the π-conjugated molecules (D/A-BPBs) consisting of 1,4-bis(phenylethynyl)benzene (BPB) modified by donor groups (OMe, SMe, NMe2, and NPh2) and an acceptor group (CN). As a result, synthesized 20 D/A-BPBs (1a-5d) were found to be the desired WRF. To get the intense red fluorophore (Φf > 0.7, λem > 610 nm), we synthesized new compounds (5e-5i) and elucidated their photophysical properties in CHCl3 solution. As a result, 5h, in which a 4-cyanophenyl group is introduced to the para-position of two benzene rings in the terminal NPh2 group of 5d, was found to be the desired intense red fluorophore (log ε = 4.56, Φf = 0.76, λem = 611 nm). The intramolecular charge-transfer nature of the S1 state of WRF (1a-5d) was elucidated by the positive linear relationship between optical transition energy (νem) from the S1 state to the S0 state and HOMO(D)-LUMO(A) difference, and the molecular orbitals calculated with the DFT method. It is demonstrated that our concept (Φf = 1/(exp(-Aπ) + 1)) connected with the relationship between Φf and magnitude (Aπ) of π conjugation length in the S1 state can be applied to WRF (1a-5d). It is suggested that the prediction of Φf from a structural model can be achieved by the equation Φf = 1/(exp(-((ν̃a - ν̃f)(1/2) × a(3/2)) + 1), where ν̃a and ν̃f are the wavenumber (cm(-1)) of absorption and fluorescence peaks, respectively, and a is the calculated molecular radius. From the viewpoint of application of WRF to various functional materials, the light-emitting characteristics of 1a-5i in doped polymer films were examined. It was demonstrated that 1a-5i dispersed in two kinds of polymer film (PST and PMMA) emit light at the whole visible region and have the small Δλem values (<20 nm) and the high Φf values (>0.6). Therefore, the present D/A-BPBs can be said to be the desired WRF even in the doped polymer film.

Solvatochromic Fluorescence Emission of an Anthranol Derivative without Typical Donor–Acceptor Structure: An Experimental and Theoretical Study

An anthranol derivative (DTDDP) and an anthraquinone derivative diastereomer (DDDP) without typical donor–acceptor (D–A) structural features were synthesized from 1,4-diaminoanthraquinone via a one-step reaction. The absorption and fluorescence emission properties of DTDDP and DDDP were investigated in common organic solvents with different polarity. In spite of a lack of apparent D–A structure, DTDDP exhibited apparent solvatochromic fluorescence emission with shift >80 nm in aprotic media. The linear correlation of the Stokes shift of DTDDP in a Lippert–Mataga plot was observed, and the change in the dipole moment of DTDDP upon excitation, 10.7 D, was determined. Time-dependent density functional theory (TDDFT) calculation confirmed the marked hole–electron separation of the excited DTDDP molecule as compared to that of the excited DDDP molecule. The experimental results and the theoretical calculation revealed the excited-state intramolecular charge-transfer nature of the emitting state of DTDDP molecu...