Nonradiative Decay Rate Constants Research Articles

Excitonic Quantum Coherence in Light Emission from CsPbBr3 Metal-Halide Perovskite Nanocrystals.

The decay of excited states via radiative and nonradiative paths is well understood in molecules and bulk semiconductors but less so in nanocrystals. Here, we perform time-resolved photoluminescence (t-PL) experiments on CsPbBr3 metal-halide perovskite nanocrystals, with a time resolution of 3 ps, sufficient to observe the decay of both excitons and biexcitons as a function of temperature. The striking result is that the radiative rate constant of the single exciton increases at low temperatures with an exponential functional form, suggesting quantum coherent effects with dephasing at high temperatures. The opposing directions of the radiative and nonradiative decay rate constants enable enhanced brightening of PL from excitons to biexcitons due to quantum effects, promoting a faster approach to the quantum theoretical limits of light emission. Ab initio quantum dynamics simulations reproduce the experimental observations of radiation controlled by quantum spatial coherence enhanced at low temperatures.

Unveiling Halogen-Bonding Interactions between a Pyridine-Functionalized Fluoroborate Dye and Perfluorohaloarenes with Fluorescence Spectroscopy.

We have studied the halogen-bonding interactions of a pyridine-functionalized fluoroborate dye with perfluorohaloarenes (C6F6, C6F5Cl, C6F5Br, and C6F5I) in the two-component-only liquid phase using fluorescence spectroscopy. Based on the results of spectroscopic measurements and electronic-structure calculations, we have confirmed the stability only for the complex between C6F5I and the emissive dye, and it has been demonstrated that halogen-bonding interactions are accompanied by significant Stokes shifts for the ππ* band. We also provide experimental evidence that for this complex, the emission is quenched due to a simultaneous decrease of radiative and increase of nonradiative decay rate constants upon halogen-bonding interactions.

Delocalization suppresses nonradiative charge recombination in polymer solar cells

Suppressing nonradiative deactivation of charge transfer (CT) states is pivotal to realizing further improvements in the power conversion efficiencies of polymer solar cells (PSCs). According to the energy gap law, the nonradiative decay rate constant knr scales exponentially with decreasing CT state energy ECT; thereby, as long as knr is governed by the energy gap law, a decrease in ECT will inevitably increase nonradiative deactivation of CT states and hence decrease the power conversion efficiency. Here, we report the nonradiative decay dynamics of CT states generated in various nonfullerene-acceptor-based PSCs by using transient absorption spectroscopy. The absence of a strong correlation between knr and ECT indicates that the energy gap law is not valid for these PSCs and that parameters other than ECT contribute significantly to knr. We found that knr decreased with an increase in materials’ crystallinities, indicating that increasing crystallinity leads to CT state delocalization, which in turn mitigates the nonradiative deactivation of CT states. We report the nonradiative decay dynamics of charge transfer (CT) states generated in nonfullerene-acceptor-based polymer solar cells. The nonradiative decay rate constants knr decreased with an increase in the efficiency for dissociation of CT states into free carriers, indicating that nonradiative decay of CT states can be mitigated by increasing the delocalization of the CT state wave function.

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

One-Pot Synthesis of Bright Blue Luminescent N-Doped GQDs: Optical Properties and Cell Imaging.

High fluorescent graphene quantum dots (GQDs) are promising in bioimaging and optoelectronics. In this paper, bright blue fluorescent N-doped GQDs were synthesized using a ultrasonic-assisted hydrothermal method. The morphology, structure, surface chemistry, optical properties, and stability subject to photo-bleaching, temperature, pH and preservation period for the N-GQDs were investigated in detail using various microscopy and spectroscopy techniques. The results showed that the N-GQDs possessed an average size of 2.65 nm, 3.57% N doping, and up to 54% quantum yield (QY). The photoluminescence (PL) spectra of the N-GQDs are excitation dependent when excited in the range of 300–370 nm and excitation independent in the range of 380–500 nm for the core and surface states emission. The N-GQDs showed excellent photo-bleaching resistance and superior photo-stability. At room temperature and in the pH range of 3–8, the fluorescence of the N-GQDs was almost invariable. The N-GQDs can be stably preserved for at least 40 days. The average decay lifetime of the N-GQDs was 2.653 ns, and the radiative and nonradiative decay rate constants were calculated to be 2.04 × 108 s−1 and 1.73 × 108 s−1, respectively. The PL mechanism was qualitatively explained. The N-GQDs was used for cell imaging, and it showed good results, implying great potential applications for bioimaging or biomarking.

Investigating phosphorescence capability of halogen-substituted metal-free organic molecules: A theoretical study

The radiative and non-radiative decay processes of five compounds are investigated through a comprehensive computational approach, for the aim of investigating the effect of different halogen substituents to the phosphorescent emission. Their optimal configurations at the ground (S0) and lowest triplet excited (T1) states are obtained and the calculated phosphorescent emission spectra are comparable with the experimental values. For 1,4-di(9H-carbazol-9-yl)benzene (PDCz), the electronic transition is between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), while for the four halides, the electronic transitions are attributed to several molecular orbitals. According to calculations, 9,9′-(2,5-diiodo-1,4-phenylene)bis(9H-carbazole) (PDICz) possesses the largest radiative decay rate constant (kr) and non-radiative decay rate constant (knr), which can be attributed to the strong spin-orbital coupling from the heavy iodine atom. However, the phosphorescent quantum efficiency (Φ) of PDICz is lower than that of 9,9′-(2,5-dibromo-1,4-phenylene)bis(9H-carbazole) (PDBCz), implying that a comprehensive consideration is necessary. Furthermore, by analyzing the vibrational mode, we have confirmed that the reorganization energies are also influenced by the different halogen atoms. While the dominated factor that determines the kr and knr comes from the spin-orbital coupling. We expect that our research findings will be beneficial to the newly designed organic phosphorescent materials in the future.

Investigating the Nonradiative Decay Pathway in the Excited State of Silepin Derivatives: A Study with Second-Order Multireference Perturbation Wavefunction Theory.

The fluorescence quantum yield for fluorescent organic molecules is an important molecular property, and tuning it up is desired for various applications. For the computational estimation of the fluorescence quantum yield, the theoretical prediction of the nonradiative decay rate constant has become an attractive subject of study. The rate constant of thermally activated nonradiative decay is related to the activation energy in the photoreaction; thus, the accuracy and reliability of the excited-state potential energies in the quantum chemical computation are critical. In this study, we employed a second-order multireference perturbation wavefunction theory for studying the thermally activated decay via conical intersection (CI) of 1,1-dimethyldibenzo[b,f]silepin derivatives. The correlation between the computed activation energy to reach the CI geometry in the S1 state and the experimentally determined fluorescence quantum yield implied that silepins nonradiatively decay via the CI triggered by the twisting of the central C-C bond. Geometry optimization of the transition state using multireference perturbation theory drastically reduced the estimated activation energy. Our computation gave reasonable predictions of the activation free energies of photoexcited 1,1-dimethyldibenzo[b,f]silepin. The energy profiles and geometry optimizations using proper quantum chemical methods played a critical role in reliable estimation of the rate constant and fluorescence quantum yield.

Understanding the photophysics of stercobilin-Zn(II) and urobilin-Zn(II) complexes towards faecal pigment analysis

A detailed photophysical study of two faecal pigments (FPs), Urobilin (UB) and Stercobilin (SB), and their zinc complexes [FP-Zn(II)] was carried out. The enhancement of UB and SB fluorescence resulting from the formation of their Zn(II) complexes was attributed to the complexation-induced rigidity of the chromophoric units, and the corresponding decrease of nonradiative decay rate constants of the excited singlet states (knr). The effect of various physicochemical environments was also studied in detail in order to understand the fluorescence behaviour of the Zn(II) complexes. FP-Zn(II) complexes have a lower solubility in water that results in the formation of molecular aggregates. The aggregation-induced loss of fluorescence of FP-Zn(II) complexes could be overcome by using the appropriate mixture of ethanol and water (70:30). Molecular orbital calculations on the FP-Zn(II) complexes provided a good idea of the geometry of the complexes and helped rationalise the enhancement of fluorescence after complexation. This study could pave the way towards developing a convenient non-extraction aqueous phase analytical procedure for detection of FPs using Zn(II) complexation method.

Density Functional Studies on Photophysical Properties of Boron-Pyridyl-Imino-Isoindoline Dyes: Effect of the Fusion.

In this work, to make out the aryl-fusion effect on the photophysical properties of boron-pyridyl-imino-isoindoline dyes, compounds 1–5 were theoretically studied through analyses of their geometric and electronic structures, optical properties, transport abilities, and radiative (kr) and non-radiative decay rate (knr) constants. The highest occupied molecular orbitals of aryl-fused compounds 2–5 are higher owing to the extended conjugation. Interestingly, aryl fusion in pyridyl increases the lowest unoccupied molecular orbital (LUMO) level, while isoindoline decreases the LUMO level; thus, 4 and 5 with aryl fusion both in pyridyl and isoindoline exhibit a similar LUMO to 1. Compounds 4 and 5 show relatively low ionization potentials and high electron affinities, suggesting a better ability to inject holes and electrons. Importantly, the aryl fusion is conducive to the decrease of kIC. The designed compound 5 exhibits a red-shifted emission maximum, low λh, and low kIC, which endow it with great potential for applications in organic electronics. Our investigation provides an in-depth understanding of the aryl-fusion effect on boron-pyridyl-imino-isoindoline dyes at molecular levels and demonstrates that it is achievable.

Efficient structural modification of electron‐withdrawing substituents on Pt(II) complexes for red emitters: A theoretical study

In this study, the electronic structures and optical properties of a cyclometalated Pt(II) complex (M1) and a series of derivatives (M1–F, M1–CF3, and M1–CN) with electron‐withdrawing substituents (–F, –CF3, and –CN) at the carbazole moiety were theoretically investigated by density functional theory and time‐dependent density functional theory. The calculation results reveal that these Pt complexes display deep red phosphorescence emission above Λ = 640 nm. When the 3MLCT/π → π* to triplet metal‐centered 3MC/d–d state decay mechanism is taken into consideration, the nonradiative decay rate constant (knr) decreased in the order M1 > M1–CF3 > M1–F > M1–CN. The <T1|HSOC|Sm> and kr values of M1‐F are similar with those of M1, however the Knr rate ofM1‐F is larger than that of M1. M1–F is expected to have improved quantum yields. Moreover, through the analyses of the HOMO/LUMO level and triplet energy, it is found that the introduction of –F and –CN substituents in M1 results in efficient energy transfer from the host material 4,4′‐N,N′‐dicarbazole‐biphenyl to these complexes. In view of the electroluminescent applications in organic light‐emitting diodes, M1–F can serve as efficient deep‐red guest materials with improved electron injection and transport ability.

Interplay of Radiative and Non Radiative Rate Constants in the Photophysics of Fluorescent Proteins

Genetically encodable fluorophores like Fluorescent Proteins (FPs) have been engineered to provide better contrast, localization and specificity. Red-FPs (RFPs) are known to have an extended chromophore through an acylimine moiety with respect to their blue shifted and generally brighter counterparts. The molecular brightness is based on the product of the extinction coefficient (EC) and fluorescence quantum yield (QY). Development of brighter FPs has focused on increasing the QY, which is directly proportional to fluorescence lifetime (FL), a time constant corresponding to the inverse sum of the ensemble radiative and non-radiative decay rate constants. We have performed directed evolution of RFPs through random and site-directed mutagenesis on a multi-parametric microfluidic sorting system capable of FL and brightness selections. We also applied site directed mutagenesis on RFPs based on structural analysis to investigate the structural effects on FL and QY. The selected mutants derived from mCherry and FusionRed show that the improvement in FL brings about an expected linear increase in QY (∼3.2 fold for mCherry and ∼2.3 fold for FusionRed). This can be attributed to significant decrease in the non-radiative rate constant, and a modest improvement in radiative rate constant that is supported by hypsochromic shifts and changes in absorption profiles. Both approaches reveal that the improvement in molecular brightness with increased FL is mainly governed by a decrease in the non-radiative rate constant in conjunction with improved absorption (narrower absorption spectrum and higher ECmax). This strategy forms the basis of the best photophysical route to engineer brighter RFPs.

Effects of β-bromine substitution and core protonation on photosensitizing properties of porphyrins: Long wavelength photosensitizers

In this study, a series of β-brominated meso-tetraphenylporphyrins, H2TPPBrx (x = 2, 4, 6 and 8) with remarkably red shifted absorption bands towards the longer wavelengths of the visible light known as therapeutic window and their diprotonated analogues were used as photosensitizers in the aerobic oxidation of cyclooctene to investigate the influence of degree of β bromination on the photocatalytic activity and oxidative stability of H2TPP. Furthermore, the β brominated porphyrins showed significantly decreased fluorescence quantum yields (φF) and radiative decay rate constants but increased non-radiative decay rate constants. The highest catalytic activity was observed in the case of the partially brominated porphyrins, H2TPPBr2 and H2TPPBr4. While, H2TPPBr2 and H2TPPBr4 were as stable as H2TPP, the oxidative degradation of H2TPPBr6 and H2TPPBr8 was remarkably higher than that of the non-brominated porphyrin. A good correlation was observed between the photocatalytic activity of H2TPPBrx and their diprotonated species and the singlet oxygen quantum yields (ϕΔ = 0.02–0.21 and 0.12 to 0.59, respectively), determined chemically through the reaction of singlet oxygen with 1,3-diphenylisobenzofuran (DPBF). In order to examine the effects of steric hindrance on the photocatalytic properties of the brominated porphyrins, β tetra-brominated derivative of meso-tetra(2-methylphenyl)porphyrin, H2T(2-Me)PPBr4 with bulky methyl substituents at the ortho positions was prepared and compared with H2TPPBr4. The steric hindrance caused by the presence of four methyl groups could improve the oxidative stability of the photosensitizer by 30 to 50% at the cost of significant loss of photocatalytic activity. On the other hand, diprotonation of H2TPPBrx with weak and strong acids (CH2ClCOOH and CF3COOH) was found to be an efficient approach to overcome the extensive oxidative degradation of these photosensitizers, with concomitant red shift of the absorption bands to 451–490 and 674–743 nm, in the Soret and Q band regions, respectively.

Experimental and theoretical insights on the effect of solvent polarity on the photophysical properties of a benzanthrone dye.

Benzanthrone derivatives show interesting solvent dependent photophysical properties. Understanding of their photophysical properties is essential for developing the fluorescence probes based on benzanthrone derivatives. The photophysical properties of 3-(N'-chlorophenyl)piperazino-7H-benzo[de]anthracen-7-one [ClPh-PBA] molecule are reported in different solvents and solvent mixtures. The change in Stokes shift, quantum yield, fluorescence life time and radiative rate constants as a function of solvent polarity shows that the Intermolecular Charge Transfer (ICT) is affected by solvent polarity and hydrogen bonding. The quantum yield and fluorescence life time values decrease and the nonradiative decay rate constant (knr) values are observed to be higher in polar solvents. The weak emission of ClPh-PBA in polar solvents is primarily due to the non-radiative torsional motion of the chlorophenyl group around benzanthrone moiety. The torsional motion of chlorophenyl group at the remote nitrogen around benzanthrone moiety is also evident from TDDFT calculations performed using B3LYP/6-311+ G (d, p) basis set. The ground state and excited state dipole moments, absorption and emission maxima (nm) along with other quantum chemical parameters are obtained using B3LYP/6-311+ G (d, p) basis set. The experimental and theoretical results follow the similar trends.

Cyano substitution effect on the emission quantum efficiency in stilbene derivatives: A computational study

Abstract A series of cyano-substituted stilbene derivatives exhibit quenched fluorescence in solution, in sharp contrast to their highly emissive parent counterparts. By employing the polarizable continuum model (PCM) method coupled with thermal vibration correlation function (TVCF) formalism, we comparatively investigated the electronic structures and excited-state dynamics of cyano-substituted para-distyrylbenzene (CN-DSB), 1,2-bis(pyridylphenyl)ethane (CN-BPPE) and the corresponding non-substituted DSB and BPPE lack of the cyano groups at their vinylene moieties in solution. The results show that the non-radiative decay rate constants of the CN-substituents are greatly enhanced by more than 4 orders of magnitude, while the radiative decay rate constants experience slight reduction, resulting in the very tiny fluorescence quantum efficiency. By analyzing the reorganization energies projected onto both normal modes and internal coordinates, the non-radiative consumption ways via molecular torsional motions of the single even double bonds adjacent to the CN group are active upon CN-substitution. Our study presents a rational explanation for the experimental CN-substitution induced quenching phenomena in stilbene derivatives, which is beneficial for the design of new fluorescent emitters and probes.

Influence of restricted rotation of small-sized substituent on phosphorescence efficiency for Pt(II) complexes: A theoretical investigation

Abstract Herein, the radiative and non-radiative decay processes of six carborane-based Pt(II) complexes were explored with the help of the density functional theory and time-dependent density functional theory. In order to interpret the influence of non-metal heavy atoms on the phosphorescent quantum yields, the determined factors, i.e., the ZFS (zero-point filed splitting) parameters, radiative decay rate constants and photodeactivation mechanisms, were unveiled. The results indicate that the spin-orbital coupling effects can be slightly regulated via introducing the non-metal heavy atoms into the carborane. Also, the non-metal heavy atoms substituents could cause slightly decrease of the energy barriers between the 3ES and 3MC excited states, and thus, for Pt-6, a fast non-radiative decay process may occur. Moreover, compared with the Pt-1, the rotation of methyl in Pt-2 ∼ 5, can be effectively restricted with the aid of hydrogen bonding and halogen bonding, which is beneficial for obtaining small non-radiative decay rate constants. Accordingly, it provides a strategy for realizing the restriction of small group rotation to achieve the high-efficient phosphorescent emitters.

The luminescence properties of [Re(CO)2(P^P)(N^N)]+ complexes: Comparison with their [Re(CO)3(N^N)(Cl)] analogues

The luminescence properties of the heteroleptic [Re(CO)2(P^P)(N^N)]+ complexes with the bidentate P^P – cis-1,2-bis(diphenylphosphino)ethene and different N^N – α-diimines ligands have been studied in acetonitrile solutions at room temperature and in methanol/ethanol (1: 1) glasses at 77 K. The investigated complexes exhibit the metal to ligand charge-transfer (MLCT) phosphorescence with the emission characteristics strongly affected by the nature of coordinated α-diimine N^N. Depending on the α-diimine attached to the [Re(CO)2(P^P)]+ core, the room-temperature values of emission wavelength λem, quantum yield ϕem, and life-time τem vary in the range of 580–630 nm, 0.022–0.23, and 0.25–22 μs, respectively. The lowering of the measurement temperature leads to hypsochromic shift of the emission maxima (by 30–60 nm) and lengthening the emission life-times (up to 52 μs). The observed changes in the λem, ϕem, and τem values have been explained within the Mulliken-Hush formalism by invoking the electronic interactions between 3*LC (lowest excited triplet of N^N ligand) and 3*MLCT states as well as the spin-orbit interactions between 3*MLCT and 1*MLCT species, respectively. All quantities necessary for the description have been accounted from the analysis of the absorption 1 *MLCT ← S0 and emission 3*MLCT → S0 charge transfer processes. It is also demonstrated that the radiative kr and non-radiative knr decay rate constants of the excited 3 *MLCT states can be interpreted within the same set of parameters. The investigated complexes have been found to be similar to their [Re(CO)3(N^N)(Cl)] analogues but some intrinsic differences distinguish two compared series.

Extraction of Radiative and Nonradiative Rate Constants of Super-Radiant J-Aggregates from Emission Spectra.

Equations relating the radiative and nonradiative decay rate constants of singlet excitons in super-radiant aggregates to the relative intensities of the 0-0 and 0-1 peaks in the emission spectrum were derived and tested against experimental data. In many instances, these equations eliminate the need for a time-dependent fluorescence decay measurement. They can also be useful in determining the fluorescence quantum yield.

A potential strategy used for controlling the phosphorescence quantum yield of cyclometalated (CˆC*) platinum(II) NHC complexes: The theoretical insight

Effectively adjusting the phosphorescent quantum efficiency of transition metal complexes is of great significance for the development of new energy-saving organic light-emitting diodes devices. In this article, density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods are employed to investigate the effects of electronegativity caused by introduction of azole moieties on the phosphorescence properties, radiative and nonradiative decay rate constants and photodeactivation mechanism of cyclometalated (CˆC*) platinum(II) NHC complexes. According to calculated results, by decreasing the electronegativity of NHC ligand, the maximum wavelength of the complex appears blue shift; the electron transition density of the lowest triplet state is concentrated to the Pt(II) atom, the singlet-triplet splitting energy and the spin-orbital coupling (SOC) matrix elements are obviously increased, leading to a significant increase of radiative decay rate constant. However, the decrease of ligand electronegativity results in a slight decrease in molecular rigidity and the SOC matrix elements between the ground state and the lowest triplet excited state increase, the energy barriers in photodeactivation pathway decrease, which eventually leads to the increase of nonradiative decay rate constant. Considering its prominent effect on the enhancement of the radiative decay rate constant, this method may still effectively improve the phosphorescence quantum yield of the complexes. This work has a certain theoretical reference value for the design of highly efficient phosphorescent complexes.

Effects of intramolecular hydrogen bonding and sterically forced non-coplanarity on organic donor/acceptor two-photon-absorbing molecules.

Two photon absorption (2PA) is of great interest across many disciplines and there has been a large effort to increase the two-photon cross section (σ2) via synthetic modification, especially by enhancing intramolecular charge-transfer (ICT). This work takes the previously studied (7-benzothiazol-2-yl-9,9-diethylfluoren-2-yl)diphenylamine (AF240), an asymmetric D-π-A chromophore, and intentionally appends a functional group (-OH, AF240-OH or -OCH3, AF240-OMe) to the 6-position of the fluorenyl π-bridge of the new chromophores. Electrochemical results in both dichloromethane and acetonitrile support stabilization of the highest occupied molecular orbital in the derivatives due to inductive electron donating effects of the hydroxy and methoxy groups. The lowest unoccupied molecular orbital is stabilized via intramolecular hydrogen bonding to the benzothiazole moiety in the case of AF240-OH. As previously observed for AF240, the steady-state emission spectra show significant solvatochromism as they broaden and red shift with increasing solvent polarity. The fluorescence lifetimes and quantum yields show that the non-radiative rate constant is increased for AF240-OH in all solvents, especially in nonpolar media. The results suggest there is forced intramolecular hydrogen bonding to the benzothiazole in nonpolar solvents because the solvent poorly solubilizes the hydroxy group. This increases the non-radiative decay rate constant (knr) via additional vibrational decay pathways. While not as dramatic, the increase in knr in polar solvents supports some deactivation via hydrogen bonding to the solvent. Steric effects are also observed in the methoxy derivative, which inhibits planarization of the benzothiazole with the fluorene, increasing the energy of the excited state. Ultrafast transient absorption spectroscopy in tetrahydrofuran solution supports stabilization of the excited state in a few ps as solvent and structural reorganizations occur. In the case of AF240-OH, no evidence of proton transfer is observed. The decrease in emission energies in the case of AF240-OH support increased ICT driven by higher degree of coplanarity and the quinoidal structure in the excited state. However, a moderate increase in the intrinsic 2PA cross-section is resulted. It is likely because of the two possible and competing solvent-stabilized ICT processes (PICT and TICT) in AF240-OH. Nevertheless, the strategic presence of a hydroxide group capable of intramolecular hydrogen bonding in AF240-OH provides a much broader 2PA sensitivity window than AF240.

Synthesis and Optical Properties of Excited-State Intramolecular Proton Transfer Active π-Conjugated Benzimidazole Compounds: Influence of Structural Rigidification by Ring Fusion.

Two excited-state intramolecular proton transfer (ESIPT) active benzimidazole derivatives (1 and 2) were synthesized by acid-catalyzed intramolecular cyclization. The steady-state fluorescence spectrum in THF revealed that ring-fused derivative 1 exhibits a dual emission, namely, the major emission was from the K* (keto) form (ESIPT emission) at 515 nm with a large Stokes shift of 11 100 cm-1 and the minor emission was from the E* (enol) form at below 400 nm. In contrast, the normal emission from the E* form was dominant and the fluorescence quantum yield was very low (Φ ∼ 0.002) for nonfused derivative 2. The time-resolved fluorescence spectroscopy of 1 suggested that ESIPT effectively occurs due to the restricted conformational transition to the S1-TICT state, and the averaged radiative and nonradiative decay rate constants were estimated as ⟨kf⟩ = 0.15 ns-1 and ⟨knr⟩ = 0.60 ns-1, respectively. The fluorescence emission of 1 was influenced by the measurement conditions, such as solvent polarity and basicity, as well as the presence of Lewis base. The ESIPT process and solvatochromic behavior were nicely reproduced by the DFT/TDDFT calculation using the PCM model. In the single-crystal fluorescent spectra, the ESIPT emissions were exclusively observed for both fused and nonfused compounds as a result of hydrogen-bonding interactions.