Theoretical insights into the photophysics of an unnatural base Z: A MS-CASPT2 investigation

We have employed the highly accurate multistate complete active space second-order perturbation theory (MS-CASPT2) method to investigate the photoinduced excited state relaxation properties of one unnatural base, namely Z. Upon excitation to the S2 state of Z, the internal conversion to the S1 state would be dominant. From the S1 state, two intersystem crossing paths leading to the T2 and T1 states and one internal conversion path to the S0 state are possible. However, considering the large barrier to access the S1 /S0 conical intersection and the strong spin-orbit coupling between S1 and T2 states (>40 cm-1 ), the intersystem crossing to the triplet manifolds is predicted to be more preferred. Arriving at the T2 state, the internal conversion to the T1 state and the intersystem crossing back to the S1 state are both possible considering the S1 /T2 /T1 three-state intersection near the T2 minimum. Upon arrival at the T1 state, the deactivation to S0 can be efficient after overcoming a small barrier to access T1 /S0 crossing point, where the spin-orbit coupling (SOC) is as large as 39.7 cm-1 . Our present work not only provides in-depth insights into the photoinduced process of unnatural base Z, but can also help the future design of novel unnatural bases with better photostability.

Unveiling the double triplet nature of the 2Ag state in conjugated stilbenoid compounds to achieve efficient singlet fission

In this investigation, the excited-state evolution in a series of all-trans stilbenoid compounds, displaying a low-lying dark singlet state of 2Ag-like symmetry nearly degenerate with the bright 1Bu state, was unveiled by employing advanced ultrafast spectroscopies while probing the effect of solvent polarizability. Together with the dual emission, femtosecond transient absorption and broadband fluorescence up-conversion disclosed the double nature of the 2Ag-like state showing both singlet features, a lifetime typical of a singlet and the ability to emit, and a triplet character, exhibiting a triplet-like absorption spectrum. The ultrafast formation (in hundreds of femtoseconds) from the non-relaxed upper singlet state led to the identification of 2Ag as the correlated triplet pair of singlet fission. The spectral difference obtained by comparison of transient absorption peaks of the 2Ag (1TT) and the triplet states was found to be in remarkable agreement with the observed triplet yield and the 1(TT) separation rate constant. Indeed, this spectral shift provided an experimental method to gain qualitative insight into the ease of separation of the 1(TT) and the relative SF efficiency. The highly conjugated polyene-like structures enable the ultrafast formation of the double triplet, but then the large binding energy prevents the triplet separation and thus the effective completion of singlet fission. Even though thermodynamically feasible for all the investigated stilbenoids according to TD-DFT calculations, singlet fission resulted to occur efficiently in the case of 1-(pyridyl-4-ylethenyl)-4-(p-nitrostyryl)benzene and nitro-styrylfuran with the triplet yield reaching 120% and 140%, respectively, triggered by their greatly enhanced intramolecular charge transfer character relative to the other compounds in the series.

Tuning the Photophysics of Two-Arm Bis[(dimethylamino)styryl]benzene Derivatives by Heterocyclic Substitution

The identification of novel molecular systems with high fluorescence and significant non-linear optical (NLO) properties is a hot topic in the continuous search for new emissive probes. Here, the photobehavior of three two-arm bis[(dimethylamino)styryl]benzene derivatives, where the central benzene was replaced by pyridine, furan, or thiophene, was studied by stationary and time-resolved spectroscopic techniques with ns and fs resolution. The three molecules under investigation all showed positive fluorosolvatochromism, due to intramolecular charge-transfer (ICT) dynamics from the electron-donor dimethylamino groups, and significant fluorescence quantum yields, because of the population of a planar and emissive ICT state stabilized by intramolecular hydrogen-bond-like interactions. The NLO properties (hyperpolarizability coefficient and TPA cross-section) were also measured. The obtained results allowed the role of the central heteroaromatic ring to be disclosed. In particular, the introduction of the thiophene ring guarantees high fluorescent quantum yields irrespective of the polarity of the medium, and the largest hyperpolarizability coefficient because of the increased conjugation. An important and structure-dependent involvement of the triplet state was also highlighted, with the intersystem crossing being competitive with fluorescence, especially in the thiophene derivative, where the triplet was found to significantly sensitize molecular oxygen even in polar environment, leading to possible applications in photodynamic therapy.

Iodine-Catalyzed Multicomponent Synthesis of Highly Fluorescent Pyrimidine-Linked Imidazopyridines

Herein, we report a metal-free one-pot three-component reaction of aryl methyl ketones, 2-aminopyridines, and barbituric acids for the synthesis of pyrimidine-linked imidazopyridines using a catalytic amount of molecular iodine in DMSO medium. This process involves a one-pot C-H oxidation, followed by the formation of one C-C and two C-N bonds. A wide variety of aryl methyl ketones and 2-aminopyridines were found to be suitable for this methodology. The UV and fluorescence properties of the synthesized products were studied in water and DMSO media. Most of the synthesized products exhibited very good to excellent fluorescence quantum yield. Among all the products, compounds 4p and 4q showed the maximum fluorescence quantum yield (0.36) in water medium under basic conditions and compound 4c showed the maximum fluorescence quantum yield (0.75) in DMSO medium.

Excited state dynamics and photochemistry of nitroaromatic compounds

Nitrated aromatic molecules have unique photoinduced channels. Due to the presence of oxygen-centered non-bonding orbitals, they can undergo sub-picosecond intersystem crossing showing one of the strongest couplings between the singlet and triplet manifolds among organic molecules. Several nitroaromatic compounds also have a distinctive nitric oxide photodissociation channel which occurs through a complex sequence of atom rearrangements and state changes. These remarkable processes have stimulated the attention of several research groups over the last few years who have applied modern femtosecond spectroscopies and new computational methods to these topics. Nitroaromatic molecules also have demonstrated their value as case-studies, where they can serve to understand the influence of torsional motions between the nitro substituent and the aromatic system in the conversions between states. In this contribution we highlight several of the recent results in this area. Due to the importance of the atmospheric photochemistry of nitrated compounds and their accumulating applications as nitric oxide release agents, continued research about the effects of the different state orderings, substitution patterns, and solvent effects is central to the development of future applications and for a better understanding of their environmental pathways. From this analysis, several pending issues are highlighted, which include the nature of the dominant singlet state involved in intersystem crossing, the role of the formation of charge-transfer states, the yield of the internal conversion channel to the electronic ground state, and a more generalized understanding of the sequence of steps which lead to nitric oxide dissociation.

Charge Transfer as the Key Parameter Affecting the Color Purity of Thermally Activated Delayed Fluorescence Emitters

The key factors determining the emission bandwidth of thermally activated delayed fluorescence (TADF) are investigated by combining computational and experimental approaches. To achieve high internal quantum efficiencies in a metal-free organic light-emitting diode via TADF, the first triplet (T₁) to first singlet (S₁) reverse intersystem crossing is promoted by configuring molecules in an electron donor-acceptor (D-A) alternation with a large dihedral angle, which results in a small energy gap (ΔE_ST) between S₁ and T₁ levels. This allows for effective non-radiative up-conversion of triplet excitons to singlet excitons that fluoresce. However, this traditional molecular design of TADF results in broad emission spectral bands (full-width at half-maximum = 70-100 nm). Despite reports suggesting that suppressing the D-A dihedral rotation narrows the emission band, the origin of emission broadening remains elusive. Indeed, our results suggest that the intrinsic TADF emission bandwidth is primarily determined by the charge transfer character of the molecule, rather than its propensity for rotational motion, which offers a renewed perspective on the rational molecular design of organic emitters exhibiting sharp emission spectra.

Deciphering the unusual fluorescence in weakly coupled bis-nitro-pyrrolo[3,2-b]pyrroles

Electron-deficient π-conjugated functional dyes lie at the heart of organic optoelectronics. Adding nitro groups to aromatic compounds usually quenches their fluorescence via inter-system crossing (ISC) or internal conversion (IC). While strong electronic coupling of the nitro groups with the dyes ensures the benefits from these electron-withdrawing substituents, it also leads to fluorescence quenching. Here, we demonstrate how such electronic coupling affects the photophysics of acceptor-donor-acceptor fluorescent dyes, with nitrophenyl acceptors and a pyrrolo[3,2-b]pyrrole donor. The position of the nitro groups and the donor-acceptor distance strongly affect the fluorescence properties of the bis-nitrotetraphenylpyrrolopyrroles. Concurrently, increasing solvent polarity quenches the emission that recovers upon solidifying the media. Intramolecular charge transfer (CT) and molecular dynamics, therefore, govern the fluorescence of these nitro-aromatics. While balanced donor-acceptor coupling ensures fast radiative deactivation and slow ISC essential for large fluorescence quantum yields, vibronic borrowing accounts for medium dependent IC via back CT. These mechanistic paradigms set important design principles for molecular photonics and electronics.

Fluorescent Chromophores Containing the Nitro Group: Relatively Unexplored Emissive Properties

Apart from numerous applications, for example in azo dye precursors, explosives, and industrial processes, the nitro group (-NO₂ ) appears on countless molecules in photochemical research owing to its unique characteristics such as a strong electron-withdrawing ability and facile conversion to the reduced substituent. Although it is well known as a fluorescence quencher, fluorescent chromophores that contain the nitro group have also emerged, with 3-nitrophenothiazine being recently reported to have 100 % emission quantum yield in nonpolar solvents. The diverse characters of nitro-containing chromophores motivated us to systematically review those chromophores with nitro substituents, their associated photophysical properties, and applications. In this Review, we succinctly elaborate the advance of the fluorescent nitro chromophores in fields of intramolecular charge transfer, fluorescent probes and nonlinear properties. Special attention is paid to the rationalization of the associated emission spectroscopy, so that the readers can gain insights into the structure-photophysics relationship and hence gain insights for the strategic design of nitro chromophores.

Competition between fluorescence and triplet production ruled by nitro groups in one-arm and two-arm styrylbenzene heteroanalogues

One-arm nitro-stilbenoids shows high triplet yield, appealing for optoelectronic and photovoltaic devices while double-arm analogues, showing appreciable TPA, are candidates as emitting probes and traceable photosensitizers for photodynamic therapy.

Anthracene derivatives as broadband nonlinear optical materials: nonlinear absorption and excited-state dynamics analysis

Two anthracene derivatives, AN-1 and AN-2, with different π-bridge lengths were designed and synthesized to investigate their optical nonlinearities. The nonlinear absorption (NLA) properties of both derivatives were measured via the femtosecond Z-scan technique with the wavelength range from 532 nm to 800 nm. The reverse saturable absorption (RSA) of both compounds results from two-photon absorption induced excited-state absorption (TPA-ESA). At all wavelengths, the reverse saturable absorption of AN-2 is superior to that of AN-1 due to a better molecular planarity for AN-2. Compared with the results of AN-1, the two-photon absorption coefficient of AN-2 can be increased by nearly 8 times (from 0.182 × 10⁻² cm GW⁻¹ for AN-1 to 1.42 × 10⁻² cm GW⁻¹ for AN-2) at 600 nm by extending the π-bridge. The evolution of femtosecond transient absorption (TA) spectra reveals the relaxation process from the singlet local excited-state (LES) to charge transfer state (CTS) for both compounds. The results imply that anthracene derivatives may be potential candidates for applications in future laser photonics.

Expanding the π-bridge to adjust the molecular planarity via increasing the amount of ethylene can modulate the nonlinear optical response.

Intramolecular charge transfer and solvation dynamics of push-pull dyes with different π-conjugated linkers

The solvation-dependent excited state dynamics of two push-pull fluorophores with donor-π-acceptor (D-π-A) structures were investigated using steady-state and ultrafast transient absorption (TA) spectroscopy, backed by theoretical calculations. Identical D and A groups were present in both dyes, which differed only in the structure of their central π-conjugated linkers. Dye 1 features a p-phenylenediethynyl linker, while dye 2 contains a 2,5-diethynylthiophene linker. From the steady-state spectra, no appreciable shifts in absorption bands were observed, whereas large red-shifts in emission were seen with increasing solvent polarity, which indicated that the excited states were more polar than the ground state. Theoretical calculations support charge transfer from the triphenylamine (TPA) donor to the pentafluorosulfanyl (SF₅) acceptor viaπ-conjugated linkers to form an intramolecular charge transfer (ICT) state. TA spectra revealed that a solvation-stabilized conformationally relaxed intramolecular charge transfer (ICT') state was formed in polar solvents, but only an ICT state was observed in nonpolar solvent. The SE band was quenched within 1 ps in high-polarity solvent, which corresponds to the low fluorescence quantum yield. It can be concluded that the dye with the p-phenylenediethynyl π-linker (i.e., dye 1) exhibits a larger degree of ICT than the thiophene analogue (i.e., dye 2). These findings demonstrate how solvation can fine-tune the photophysical properties of push-pull dyes, and this study highlights the importance of π-conjugated linkers in the excited state ICT process.

Effect of the size of polycyclic aryl groups on the competition between adiabatic/diabatic photoisomerization mechanisms of cis-styrylarenes

Adiabatic photoisomerization from ¹Z* to ¹E* has been experimentally evidenced for several styrylarenes. Its weight on the overall cis–trans photoisomerization has been found to increase upon increasing the chromophore size.

Smart, chiral, and nonconjugated cyclohexane-based bis-salicylaldehyde hydrazides: multi-stimuli-responsive, turn-on, ratiometric, and thermochromic fluorescence, single-crystal structures via DFT calculations

Multidentate and environmentally sensitive dyes show turn-on, ratiometric, and thermochromic fluorescence.

Weak Donor-/Strong Acceptor-Linked Anthracenyl π-Conjugates as Solvato(fluoro)chromophore and AEEgens: Contrast between Nitro and Cyano Functionality

Steady development on photophysical behaviors for a variety of organic fluorophores inspired us to generate anthracene-based fluorescent molecules with a strong acceptor and a significantly weak donor through a π-spacer. Such molecules are found to have substantial twisted conformational orientations in the solid state and enhanced apolar character because of the attachment of tolyl or mesityl group with an anthracenyl core. Upon exposure to a variety of solvents, strong solvatochromism was noticed for 4-nitro compound (84 nm red shift) in contrast to the cyano analogue (18 nm red shift). Both these probes were highly emissive in apolar solvents while nitro-analogue, in particular, could discriminate the solvents of E _T(30) (a measure of microscopic solvent polarity) ranging from 31 to 37. Thus, 4-nitro compounds can be successfully employed to detect and differentiate the apolar solvents. On the contrary, the 2-nitro analogue is almost nonemissive for the same range of solvents perhaps because of favorable excited-state intramolecular proton-transfer process. The fundamental understanding of solvatochromic properties through the formation of twisted intramolecular charge-transfer (TICT) state is experimentally analyzed by synthesizing and studying the π-conjugates linked to only benzene in place of nitro or cyanobenzene, which exhibits no solvatochromism and that helped finding the possible emission, originated from the locally excited state. Moreover, the molecular structures for these compounds are determined by the single-crystal X-ray diffraction studies to examine the change in emission properties with molecular packing and alignment in the aggregated state. The measurement of dihedral angles between the substituents and anthracenyl core was helpful in finding the possible extent of electronic conjugations within the system to decipher both solvatochromism and aggregation enhanced emission (AEE)-behavior. The cyano analogue exhibited prominent AEE-behavior, whereas nitro analogues showed the aggregation-caused quenching effect. The reason behind such dissimilarity in solvatochromism and AEE-behavior between cyano- and nitro-linked anthracenyl π-conjugates are also addressed through experimental outcomes.

Comparative photophysics and ultrafast dynamics of dimethylaminochalcone and a structurally rigid derivative: experimental identification of TICT coordinate

Restriction of torsional relaxation and solvent dependent competing photophysical dynamics of free and rigid dimethylaminochalcones were investigated by time resolved spectroscopy.

An Ionic 1,4-Bis(styryl)benzene-Based Fluorescent Probe for Mercury(II) Detection in Water via Deprotection of the Thioacetal Group

Highly sensitive and selective mercury detection in aqueous media is urgently needed because mercury poisoning usually results from exposure to water-soluble forms of mercury by inhalation and/or ingesting. An ionic conjugated oligoelectrolye (M1Q) based on 1,4-bis(styryl)benzene was synthesized as a fluorescent mercury(II) probe. The thioacetal moiety and quaternized ammonium group were incorporated for Hg²⁺ recognition and water solubility. A neutral Hg²⁺ probe (M1) was also prepared based on the same molecular backbone, and their sensor characteristics were investigated in a mixture of acetonitrile/water and in water. In the presence of Hg²⁺, the thioacetal group was converted to aldehyde functionality, and the resulting photoluminescence intensity decreased. In water, M1Q successfully demonstrated highly sensitive detection, showing a binding toward Hg²⁺ that was ~15 times stronger and a signal on/off ratio twice as high, compared to M1 in acetonitrile/water. The thioacetal deprotection by Hg²⁺ ions was substantially facilitated in water without an organic cosolvent. The limit of detection was measured to be 7 nM with a detection range of 10–180 nM in 100% aqueous medium.

Ultrafast Investigation of Intramolecular Charge Transfer and Solvation Dynamics of Tetrahydro[5]-helicene-Based Imide Derivatives

We report the excited-state intramolecular charge transfer (ICT) characteristics of four tetrahydro[5] helicene-based imide (THHBI) derivatives with various electron-donating substitutes in different polarity of solvents using steady-state, time-resolved transient absorption (TA) spectroscopy. It is found that, the small bathochromic-shift of the absorption spectra but large red shift of the emission spectra for all dyes with increasing solvent polarity indicates the larger dipole moment of the excited state compared to ground state. The results of theoretical calculations exhibit the charge transfer from the terminal donors to helical backbone, which accounts for the degrees of red shift of the emission spectra from different extent of ICT nature. Time-resolved TA spectra recorded as a function of electron-donating substitutes and solvent polarity show the dye with stronger donors (THHBI-PhNPh2) in more polar solvent behaves faster excited-state ICT relaxation, leading to the formation of solvent-stabilized ICT state (ICT' state) from the excited ICT state; The dyes (THHBI-Ph, THHBI-PhCF3 and THHBI-PhOMe) with relative weaker donors show weaker dependence on solvent polarity, and instead of that intersystem crossing (ISC) becomes possible from ICT state to triplet state.

The impact of solvents on the singlet and triplet states of selected fluorine corroles – absorption, fluorescence, and optoacoustic studies

This paper examines the influence of aprotic solvents on the spectroscopic properties as well as the energy deactivation of two free-base corrole dyes substituted with C₆F₅ and/or 4-NO₂C₆H₄ groups.

Solvent sensitive intramolecular charge transfer dynamics in the excited states of 4-N,N-dimethylamino-4′-nitrobiphenyl

Solvent sensitive excited state dynamics of DNBP is explored. In polar solvents, the ultrafast barrierless TICT process is the major relaxation pathway, whereas, in nonpolar solvents the excited state undergoes the PICT process, followed by efficient intersystem crossing to the triplet state.

Dipyrrolonaphthyridinediones – structurally unique cross-conjugated dyes

Previously unknown dipyrrolo[1,2-b:1′,2′-g][2,6]naphthyridine-5,11-diones possess intense fluorescence in the range of 520–740 nm.

Pyrrolo[3,2-b]pyrroles-From Unprecedented Solvatofluorochromism to Two-Photon Absorption

A combined experimental and theoretical study of the two-photon absorption (2PA) properties of a series of quadrupolar molecules possessing a highly electron-rich heterocyclic core, pyrrolo[3,2-b]pyrrole, is presented. In agreement with quantum-chemical calculations, large 2PA cross-section values, σ2PA ≈10(2) -10(3)  GM (1 GM=10(50)  cm(4)  s photon(-1) ), are observed at wavelengths of 650-700 nm, which correspond to the two-photon allowed but one-photon forbidden transitions. The calculations also predict that increased planarity of this molecule through removal of two N-substituents leads to further increase in the σ2PA values. Surprisingly, the most quadrupolar pyrrolo[3,2-b]pyrrole derivative, containing two 4-nitrophenyl substituents at positions 2 and 5, demonstrates a very strong solvatofluorochromic effect, with a fluorescence quantum yield as high as 0.96 in cyclohexane, whereas the fluorescence vanishes in DMSO.

Unexpected multiple activated steps in the excited state decay of some bis(phenylethynyl)-fluorenes and -anthracenes

The temperature effect on the photophysical parameters of four acetylene-derivatives [bis(phenylethynyl)-anthracenes and -fluorenes with substituents of different electron acceptor efficiencies] has been investigated by absorption and emission spectroscopy, using stationary and pulsed (ns/fs resolution) techniques. The nature of the central nucleus (anthracene or fluorene) and the peripheral electron-withdrawing group (nitro or formyl) strongly affect the deactivation of the excited states of these push-pull molecules. In some cases the study evidenced an interesting role of two activated steps in the deactivation of the excited singlet state, namely an activated inter-system crossing to an upper triplet state of n,π* nature (previously hypothesized on the basis of TD-DFT calculations) and a sort of activated internal conversion, discussed also on the basis of maximum entropy method analysis of the fluorescence decay data. Nicely, an efficient ISC was found for the fluorene-derivatives where small energy gaps between S1 (π,π*) and Tn (n,π*) states had been calculated while no activated ISC was evidenced in the case of anthryl-derivatives where higher S1-Tn energy gaps are expected. A peculiar temperature effect for a fluorene-derivative was pointed out and also explained on the basis of quantum-mechanical calculations at the DFT level taking into account the solvation effects by means of the conductor-like polarizable continuum model CPCM. The presence of dual emission, at first evidenced by a shoulder in the emission spectrum of the fluorene-derivative featuring a peripheral formyl group in dichloromethane at low temperatures, was nicely confirmed by femtosecond up-conversion measurements at room temperature.

Ultrafast Photogeneration of a Tetrazolinyl Radical

Radicals in solution are crucial for many chemical processes. In this work, we unveil the photoreaction sequence leading to radical formation from tetrazolium salts, which are extensively used in enzyme assays and also exhibit a rich photochemistry. Upon UV irradiation, the tetrazolium ion turns into the tetrazolinyl radical via two intermediates on a nanosecond timescale. The solvent's polarity governs the rate of formation, but the reaction pathway towards the tetrazolinyl radical is identical for aqueous and alcoholic solutions, although the final photoproduct distribution differs. These observations provide new insight into the versatile reactivity of tetrazolium salts and ultrafast radical formation in the liquid phase.