Green and sustainable approaches for the Friedel–Crafts reaction between aldehydes and indoles

• Periklis X. Kolagkis,
• Eirini M. Galathri and
• Christoforos G. Kokotos

Beilstein J. Org. Chem. 2024, 20, 379–426, doi:10.3762/bjoc.20.36

Photochromic derivatives of indigo: historical overview of development, challenges and applications

• Gökhan Kaplan,
• Zeynel Seferoğlu and
• Daria V. Berdnikova

Beilstein J. Org. Chem. 2024, 20, 228–242, doi:10.3762/bjoc.20.23

• photolysis in combination with steady-state measurements [59]. It was found that the E–Z photoisomerization of 9a, 9d, 9g, 9h occurred through a singlet mechanism upon direct excitation. However, the triplet state could be achieved by a sensitized reaction. At room temperature, a transient species that could

• be assigned to a triplet state was also observed for rigid molecules 10 and 16, based on the quenching experiments and the fact that the same transient species could be found under sensitized conditions. Further insight in the excited state properties of derivatives 9a, 9d, 9g, 9h and N,N

Perspectives on push–pull chromophores derived from click-type [2 + 2] cycloaddition–retroelectrocyclization reactions of electron-rich alkynes and electron-deficient alkenes

• Michio Yamada

Beilstein J. Org. Chem. 2024, 20, 125–154, doi:10.3762/bjoc.20.13

• , the racemization was observed at 80 °C. Torres et al. performed detailed theoretical calculations and showed that the racemization observed in 59 and 60 is caused by triplet-state photogeneration, which leads to the rotation around the sterically hindered buta-1,3-diene chiral axis [132]. In fact, the

• estimated energy barrier of ≈37 kcal mol−1 is reduced to ≈15 kcal mol−1 upon the electronic excitation of 59 and 60 to the T1 state. It is reasonable to consider that the phototriggered racemization is inhibited in the presence of O2 because O2 acts as a triplet-state scavenger. Osuka et al. synthesized two

• state was observed; the subsequent internal conversion of S2 into the S1 state was also observed, from which the CS state (MP•−–D•+) emerged. Here, since the triplet state 3MP* is energetically lower than the

Radical chemistry in polymer science: an overview and recent advances

• Zixiao Wang,
• Feichen Cui,
• Yang Sui and
• Jiajun Yan

Beilstein J. Org. Chem. 2023, 19, 1580–1603, doi:10.3762/bjoc.19.116

• radicals on polymer surfaces. Its photoexcitation and subsequent reaction with polymers have been studied for decades [164][165][166]. When irradiated at around 360 nm, benzophenone undergoes excitation to a triplet state with biradical behavior. It then abstracts a hydrogen atom from the polymer resulting

Organic thermally activated delayed fluorescence material with strained benzoguanidine donor

• Alexander C. Brannan,
• Elvie F. P. Beaumont,
• Nguyen Le Phuoc,
• George F. S. Whitehead,
• Mikko Linnolahti and
• Alexander S. Romanov

Beilstein J. Org. Chem. 2023, 19, 1289–1298, doi:10.3762/bjoc.19.95

• : guanidine; organic; photoluminescence; TADF; yellow; Introduction Thermally activated delayed fluorescence (TADF) is a photoluminescence mechanism where excitons undergo thermally-assisted reverse-intersystem crossing from an excited triplet state to a higher-lying in energy singlet state to emit delayed

The effect of dark states on the intersystem crossing and thermally activated delayed fluorescence of naphthalimide-phenothiazine dyads

• Liyuan Cao,
• Xi Liu,
• Xue Zhang,
• Jianzhang Zhao,
• Fabiao Yu and
• Yan Wan

Beilstein J. Org. Chem. 2023, 19, 1028–1046, doi:10.3762/bjoc.19.79

• ; electron donor; intersystem crossing; TADF; triplet state; Introduction Thermally activated delayed fluorescence (TADF) compounds are promising emitters to be used in organic light-emitting diodes (OLED) [1][2][3][4][5][6][7][8][9][10][11][12]. These emitters have the advantage of low cost and high

• may also enhance the rISC in OLED devices, in which the electron–hole recombination produces mainly the triplet state (the theoretical probability is 75%, by following the spin statistic rule) [1]. Compared to the application studies, the investigation of the photophysical mechanism of TADF emitters

• transient species of the dyads formed upon photoexcitation, the ns-TA spectra of NI-PTZ-F in HEX were studied (Figure 8a). Upon pulsed laser excitation, a positive absorption band centered at ca. 430 nm was observed, which is inconsistent with the characteristic triplet state absorption of the NI unit (460

Naphthalimide-phenothiazine dyads: effect of conformational flexibility and matching of the energy of the charge-transfer state and the localized triplet excited state on the thermally activated delayed fluorescence

• Kaiyue Ye,
• Liyuan Cao,
• Davita M. E. van Raamsdonk,
• Zhijia Wang,
• Jianzhang Zhao,
• Daniel Escudero and
• Denis Jacquemin

Beilstein J. Org. Chem. 2022, 18, 1435–1453, doi:10.3762/bjoc.18.149

• can be controlled. The singlet oxygen quantum yield (ΦΔ) of NI-PTZ is moderate in n-hexane (HEX, ΦΔ = 19%). TADF was observed for the dyads, the biexponential luminescence lifetime are 16.0 ns (99.9%)/14.4 μs (0.1%) for the dyad and 7.2 ns (99.6%)/2.0 μs (0.4%) for the triad. Triplet state was

• : charge-transfer; electron donor; intersystem crossing; TADF; triplet state; Introduction Thermally activated delayed fluorescence (TADF) has attracted much attention in recent years, not only for its application in organic light emitting diodes (OLED) [1][2][3] but also as a mean for studying of charge

• interaction-driven ISC is slow. Recently, it was proposed that an intermediate localized triplet state (3LE) is essential to enhance the ISC and rISC, through the so-called spin-vibronic coupling effect [8][16][17][18][19]. However, the effect of the molecular geometry on the ISC and rlSC is complicated, and

Ionic multiresonant thermally activated delayed fluorescence emitters for light emitting electrochemical cells

• Merve Karaman,
• Abhishek Kumar Gupta,
• Subeesh Madayanad Suresh,
• Tomas Matulaitis,
• Lorenzo Mardegan,
• Daniel Tordera,
• Henk J. Bolink,
• Sen Wu,
• Stuart Warriner,
• Ifor D. Samuel and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2022, 18, 1311–1321, doi:10.3762/bjoc.18.136

• stabilized to 3.07 eV and 2.83 eV, respectively. The ΔEST decreases to 0.24 eV and there is no intermediate triplet state. The trend of stabilized S1 and T1 states when a donor group decorates the DiKTa core (S1 = 3.45 eV, T1 = 3.18 eV) has been previously observed in reported emitters such as Cz-DiKTa (S1

Thermally activated delayed fluorescence (TADF) emitters: sensing and boosting spin-flipping by aggregation

• Ashish Kumar Mazumdar,
• Gyana Prakash Nanda,
• Nisha Yadav,
• Upasana Deori,
• Upasha Acharyya,
• Bahadur Sk and
• Pachaiyappan Rajamalli

Beilstein J. Org. Chem. 2022, 18, 1177–1187, doi:10.3762/bjoc.18.122

• -lying triplet state T1 to the singlet excited state S1, we recorded the fluorescence and phosphorescence spectra of aggregates (90 vol % water/THF) at room temperature and 77 K, respectively (Figure 4). The structural phosphorescence band was obtained from the aggregates in 90 vol % water/THF mixtures

• (Figure 4). Additionally, the phosphorescence spectra of both compounds were recorded in THF at 77 K (Figure S3 in Supporting Information File 1). The retention of the structural phosphorescence band in THF indicates its origin from the locally excited triplet state 3LE. The ΔEST values calculated based

Comparative study of thermally activated delayed fluorescent properties of donor–acceptor and donor–acceptor–donor architectures based on phenoxazine and dibenzo[a,j]phenazine

• Saika Izumi,
• Prasannamani Govindharaj,
• Anna Drewniak,
• Paola Zimmermann Crocomo,
• Satoshi Minakata,
• Leonardo Evaristo de Sousa,
• Piotr de Silva,
• Przemyslaw Data and
• Youhei Takeda

Beilstein J. Org. Chem. 2022, 18, 459–468, doi:10.3762/bjoc.18.48

• ) occurs in the singlet excited state (1CT). An efficient rISC can be mediated by mixing the 1CT state with a locally excited triplet state on the donor (3LED) or the acceptor (3LEA) through spin–vibronic coupling [14] or non-Condon effects [15][16]. In 2016, we developed a twisted D–A–D compound POZ-DBPHZ

• compound POZ-DBPHZ, transitions to T1 display 48% probability whereas fluorescence has around 4%. The remaining probabilities are mostly distributed between transfers to T2 and T3 with about 20% each. Transfers to higher lying triplet states may end up relaxing to the lowest triplet state by means of

Effect of a twin-emitter design strategy on a previously reported thermally activated delayed fluorescence organic light-emitting diode

• Ettore Crovini,
• Zhen Zhang,
• Yu Kusakabe,
• Yongxia Ren,
• Yoshimasa Wada,
• Bilal A. Naqvi,
• Prakhar Sahay,
• Tomas Matulaitis,
• Stefan Diesing,
• Ifor D. W. Samuel,
• Wolfgang Brütting,
• Katsuaki Suzuki,
• Hironori Kaji,
• Stefan Bräse and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2021, 17, 2894–2905, doi:10.3762/bjoc.17.197

• and T1 states is different, for example by additionally involving a third (local) triplet state with different symmetry, because otherwise SOC vanishes when the orbital types for these two states are the same, according to El-Sayed’s rule [9]. Designing a molecule able to achieve RISC and the desired

Visible-light-mediated copper photocatalysis for organic syntheses

• Yajing Zhang,
• Qian Wang,
• Zongsheng Yan,
• Donglai Ma and
• Yuguang Zheng

Beilstein J. Org. Chem. 2021, 17, 2520–2542, doi:10.3762/bjoc.17.169

• to its triplet state CuICN*, in which the fluoroalkyl iodides were reduced to Rf• and I−. Subsequently, the radical Rf• attacks the alkene forming a new alkyl radical species. This radical species is then trapped by CuII(CN)n to generate a CuIII intermediate, which undergoes reductive elimination to

• homolysis to produce CuI species and radical intermediates. These intermediates can initiate productive organic transformations [39]. 2.1 Visible-light-mediated Cu(I) catalytic cycle Upon the absorption of a photon (Scheme 4), CuILn forms a singlet MLCT state, which subsequently yields the excited triplet

• state CuI*Ln via rapid ISC. The excited CuI*Ln species has a lifetime to finish the chemical processes. A radical mechanism is proposed in Scheme 4. In path a (a ligand transfer cycle), CuI* is oxidized by an electrophilic reagent (haloalkane) to form CuII and the radical species R• in a single-electron

Photoredox catalysis in nickel-catalyzed C–H functionalization

• Lusina Mantry,
• Rajaram Maayuri,
• Vikash Kumar and
• Parthasarathy Gandeepan

Beilstein J. Org. Chem. 2021, 17, 2209–2259, doi:10.3762/bjoc.17.143

• the triplet-state diradical 23-I. A HAT process between 23-I and the alkane substrate generates the desired carbon-centered radical 23-II with concomitant formation of ketyl radical species 23-III. The thus formed alkyl radical 23-II undergoes Giese addition to alkene 94 resulting in the radical

An initiator- and catalyst-free hydrogel coating process for 3D printed medical-grade poly(ε-caprolactone)

• Jochen Löblein,
• Thomas Lorson,
• Miriam Komma,
• Tobias Kielholz,
• Maike Windbergs,
• Paul D. Dalton and
• Robert Luxenhofer

Beilstein J. Org. Chem. 2021, 17, 2095–2101, doi:10.3762/bjoc.17.136

• exposed PCL fibers in the PHEMA-filled MEW scaffolds. Schematic representation of the self-initiated photografting and photopolymerization (SIPGP) of 2-hydroxyethyl methacrylate (HEMA) including intersystem crossing from singlet to triplet state which is mandatory for the formation of a biradical (left

Exploring the possibility of using fluorine-involved non-conjugated electron-withdrawing groups for thermally activated delayed fluorescence emitters by TD-DFT calculation

• Dongyang Chen and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2021, 17, 210–223, doi:10.3762/bjoc.17.21

• -dependent density functional theory calculations indicate that when only two ortho-disposed carbazole donors are used (Type I molecules), the lowest-lying triplet state possesses locally excited (LE) character while the lowest-lying singlet state possesses charge-transfer character. When five carbazole

• ]. We analysed the higher excited states of the fluorinated acceptor-containing emitters in both Type I and Type II structures. For 2CzCF3, the T1 is locally excited; further, T2 (3.46 eV) to T6 (3.58 eV) all exhibited significant LE character. The lowest triplet state that exhibits charge transfer

Insight into functionalized-macrocycles-guided supramolecular photocatalysis

• Minzan Zuo,
• Krishnasamy Velmurugan,
• Kaiya Wang,
• Xueqi Tian and
• Xiao-Yu Hu

Beilstein J. Org. Chem. 2021, 17, 139–155, doi:10.3762/bjoc.17.15

• /22 ratio of 1:2, which revealed that the singlet spin-state of coumarin could be involved in the photodimerization process within the CB[8] cavity and that the triplet state could be shielded from external quenchers (O2 as a triplet quencher). These results also indicated that photodimerization could

Recent developments in enantioselective photocatalysis

• Callum Prentice,
• James Morrisson,
• Andrew D. Smith and
• Eli Zysman-Colman

Beilstein J. Org. Chem. 2020, 16, 2363–2441, doi:10.3762/bjoc.16.197

• acid so that a Dexter energy transfer process is possible from the photocatalyst to the substrate to promote 155 into its triplet state, which can then cyclise with 156 to give enantioenriched cycloaddition products 158. The selectivity of this reaction is generally low (6 examples, up to 93:7 er, 67

• by Bach to the deracemisation of allenes rac-194 (Scheme 29) [85]. The proposed mechanism proceeds through configurationally isomeric hydrogen bonding complexes 195 and 195’, with subsequent photoexcitation of the thioxanthone chromophore leading to racemisation of the allene through a triplet state

• enantioselectivity via triplet state intermediate 200, rather than proceeding via an enantioselective rearrangement. Bifunctional hydrogen bonding photocatalysts have been developed by other groups as well. Sivaguru developed an atropisomeric thiourea-based catalyst 201 and used it for the intramolecular

Formation of an exceptionally stable ketene during phototransformations of bicyclo[2.2.2]oct-5-en-2-ones having mixed chromophores

• Asitanga Ghosh

Beilstein J. Org. Chem. 2020, 16, 2297–2303, doi:10.3762/bjoc.16.190

• -dependent absorption spectra [13]. Also no external sensitizer was needed for these transformations. So an efficient intersystem crossing (ISC) may be involved in such systems to reach to the triplet state. A further fascinating aspect is that the two competitive pathways viz. 1,2-AS and photoindiuced 1,5

Photosensitized direct C–H fluorination and trifluoromethylation in organic synthesis

• Shahboz Yakubov and
• Joshua P. Barham

Beilstein J. Org. Chem. 2020, 16, 2151–2192, doi:10.3762/bjoc.16.183

• principle, the photosensitizer is regenerated and therefore functions in a very similar way to a catalyst and can be deemed a photocatalyst. A common example of this concept is triplet photosensitization. The excited photocatalyst has a relatively high triplet-state energy and a long lifetime, such that it

• light and is excited from the ground singlet state S0{PS} to the excited singlet state S1{PS}. Intersystem crossing (ISC) transforms S1{PS} to its more stable triplet state, T1{PS}. ISC is a process in which a nonradiative transition of a singlet excited electronic state to a triplet excited state takes

• place at a point where the potential energy curves of these two states interset, which requires spin–orbit coupling [117]. In the absence of spin–orbit coupling, it is a forbidden transition according to the Pauli exclusion principle. The resulting triplet state, T1{PS} is in an excited vibrational

Clickable azide-functionalized bromoarylaldehydes – synthesis and photophysical characterization

• Dominik Göbel,
• Marius Friedrich,
• Enno Lork and
• Boris J. Nachtsheim

Beilstein J. Org. Chem. 2020, 16, 1683–1692, doi:10.3762/bjoc.16.139

• can occur. While the latter decay (fluorescence) takes place without a change in the electron spin, phosphorescence is defined as the radiative transition from the lowest excited triplet state (T1) to the singlet ground state (S0) [18][19][20][21]. Triplet state excitons are generated by the spin

Heterogeneous photocatalysis in flow chemical reactors

• Christopher G. Thomson,
• Ai-Lan Lee and
• Filipe Vilela

Beilstein J. Org. Chem. 2020, 16, 1495–1549, doi:10.3762/bjoc.16.125

Photocatalysis with organic dyes: facile access to reactive intermediates for synthesis

• Stephanie G. E. Amos,
• Marion Garreau,
• Luca Buzzetti and
• Jerome Waser

Beilstein J. Org. Chem. 2020, 16, 1163–1187, doi:10.3762/bjoc.16.103

• case (Scheme 1, box 2), the excited photocatalyst can engage in an EnT mechanism. Upon the excitation and intersystem crossing, the triplet state photocatalyst 3PC* can interact directly with a ground state species Sub and transfer the triplet energy to generate an excited state Sub* and the ground

• state PC. In most cases, Sub* is in the triplet state if it is an organic molecule. A common exception to this is molecular oxygen, which upon excitation attains a more reactive singlet state. In the third mode of activation, atom transfer (AT, Scheme 1, box 3), the excited state photocatalyst PC* can

• compared to other methods for radical generation [56]. Within this field, organic dyes can act as competent photocatalysts for direct HAT processes. Specifically, upon light excitation, photoactive carbonyl compounds, such as benzophenone and its derivatives, reach an electronically excited triplet state

Recent applications of porphyrins as photocatalysts in organic synthesis: batch and continuous flow approaches

• Rodrigo Costa e Silva,
• Luely Oliveira da Silva,
• Aloisio de Andrade Bartolomeu,
• Timothy John Brocksom and
• Kleber Thiago de Oliveira

Beilstein J. Org. Chem. 2020, 16, 917–955, doi:10.3762/bjoc.16.83

• photocatalysts. Under light irradiation, one electron from the ground state (S0) is promoted to the excited singlet state (S1) which has a short lifetime (10−9 s). Therefore, fast intersystem-crossing of one electron gives the excited triplet state (T1) with a relatively longer lifetime (10−6 s). While the

• molecular oxygen (triplet state). In this process, well-known singlet oxygen (1O2) is generated. Singlet oxygen can be considered a very versatile reagent in organic synthesis since it promotes many mild oxidation processes instead of combustion [23][57][58][59][60]. This excited state form of molecular

Aldehydes as powerful initiators for photochemical transformations

• Maria A. Theodoropoulou,
• Nikolaos F. Nikitas and
• Christoforos G. Kokotos

Beilstein J. Org. Chem. 2020, 16, 833–857, doi:10.3762/bjoc.16.76

• capable to induce other types of reactions, such as hydrogen atom abstraction (HAT) processes or triplet state energy transfer processes (EnT). Carbonyl compounds, especially diaryl ketones, have shown great potential as far as their catalytic scope is concerned. Benzophenone or acetophenone (64) and

• a nonbonding electron on the oxygen atom to the first excited singlet state. This singlet state can lead to the corresponding triplet state T1 (n,π*) through intersystem crossing. This triplet state can be quenched through various chemical transformations, like Norrish type I and Norrish type II

• dissociations (Scheme 1). However, when a possible energy acceptor is present, the triplet state energy can be transferred from the carbonyl compound to the acceptor, returning the carbonyl compound to the ground electronic state. For example, in reaction (1) of Scheme 1, the β-bond dissociation of methyl n

Regioselectively α- and β-alkynylated BODIPY dyes via gold(I)-catalyzed direct C–H functionalization and their photophysical properties

• Takahide Shimada,
• Shigeki Mori,
• Masatoshi Ishida and
• Hiroyuki Furuta

Beilstein J. Org. Chem. 2020, 16, 587–595, doi:10.3762/bjoc.16.53

• triplet state can be another pathway to understand the lower emission quantum yield of the BODIPYs. However, reactive singlet oxygen species (1O2) was not observed under the aerobic conditions probed by the near-infrared photoluminescence at 1270 nm. The above spectral features were likewise shown with