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In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/materialsC Page 1 of 27 Journal of Materials Chemistry C Journal Name RSC Publishing ARTICLE π-Expanded Coumarins: Synthesis, Optical Properties and Applications Cite this: DOI: 10.1039/x0xx00000x Mariusz Tasior, a Dokyoung Kim, b† Subhankar Singha, b† Maciej Krzeszewski, a Kyo Han Ahn,* b and Daniel T. Gryko* ac Manuscript Received 00th January 2014, Accepted 00th January 2014 Coumarins fused with other aromatic units have recently emerged as a hot topic of DOI: 10.1039/x0xx00000x research. Their synthesis is partly based on classical methodologies such as www.rsc.org/ Pechmann reaction or Knoevenagel condensation, but it also sparked the discovery of completely new pathways. In very recent years so-called vertically-expanded coumarins were synthesized, effectively expanding the portfolio of existing architectures. A subtle relationship exists between the structure of fused coumarins and their optical properties. Although absorption of UV-radiation and light is Accepted unifying theme among these π-expanded coumarins, the fluorescence properties C strongly depend on the structure. The mode of fusion, the type of additional ring and the presence of electron-donating and electron-withdrawing substituents all influence the photophysical parameters. Recent advances made it possible to modulate their absorption from 300 nm to 550 nm, resulting in new coumarins emitting orange light. This review serves as a guide through both synthesis strategies and structure-property relationship nuances. Strong intramolecular charge-transfer character made it possible to reach suitable values of two-photon absorption cross- section. Photophysical advantages of π-expanded coumarins have been already Chemistry utilized in fluorescence probes and two-photon excited fluorescence microscopy. 1. Introduction of old and recent literature. We will also present, compare and analyze linear optical properties of these compounds. Finally, Coumarins fascinated humans from the very beginning. From we will describe the current and possible applications. In the the smell of freshly cut grass through laser dyes to fluorescent following sections we will present representative examples of probes – we are surrounded by them. 1 More than that, we carry the linear and angular benzocoumarins (Chapter 2), them on our shoulders as optical brighteners. Although π- biscoumarins (Chapter 3), vertically expanded coumarins expanded coumarins, i.e. coumarins possessing additional (Chapter 4), optical properties (Chapter 5) and applications benzene or heterocyclic rings fused in different ways with Materials (Chapter 6). Coumarins possessing conjugated C-C double and chromen-2-one, were synthesized for the first time by 2 triple bonds are outside of the scope of this review for space Pechmann in 1884, (and some types of them are ubiquitous in of reasons. nature), it is not until very recently that they make their way towards modern photonic-oriented applications. Even more 2. Coumarins fused with one additional aromatic ring: recently entirely new skeletons possessing one or two coumarin benzocoumarins and their analogues moieties expanded or combined with one another were discovered. Since this topic was never described before, we 2.1 Structure of benzocoumarins undertook a systematic journey through these molecules. The Benzocoumarin derivatives can be classified into four types review will cover synthetic developments, focusing on the most depending on the position of the fused benzene ring: important examples, with special emphasis given to comparison Journal This journal is © The Royal Society of Chemistry 2014 J. Name ., 2014, 00 , 1-3 | 1 Journal of Materials Chemistry C Page 2 of 27 Journal Name ARTICLE benzo[ c]coumarin, benzo[ f]coumarin, benzo[ g]coumarin, and benzo[ h]coumarin types (Fig. 1). 2.2. Synthesis of benzocoumarins and their analogues 2.2.1. General synthetic routes to benzocoumarins Synthesis of benzocoumarin derivatives except the benzo[ c]- coumarin series generally follows the synthetic routes established for coumarin and its derivatives 3 including Knoevenagel condensation, Pechmann reaction, and metal Scheme 1 General strategy of benzocoumarin synthesis from o- hydroxynaphthaldehydes. catalyzed cyclization. Therefore, known synthetic routes to benzo[ g]coumarin, benzo[ h]coumarin and benzo[ f]coumarin Alternatively, benzocoumarin derivatives can be directly derivatives are summarized here. Most of these synthetic synthesized through electrophilic substitution of naphthols with pathways start from two major classes of compounds, either o- Manuscript β-keto esters followed by cyclization (the upper route in hydroxy-naphthaldehyde or naphthols. Scheme 2). The reaction occurs with appropriate electrophiles in the presence of strong Brønsted acid or Lewis acid. In a modified way, the metal-catalyzed aryl C-H functionalization of alkynoates is also feasible. Here also the formation of different types of benzocoumarin derivatives depends on the position of hydroxyl group of commercially available starting naphthols (2-naphthol or 1-naphthol). From 2-naphthol, two different isomers also form in substantial amounts, which could be separated in some cases, due to their polarity differences. Accepted R1 R1 X C R R 2 direct condensation 2 R3 OH R3 O O R4 R4 X R1 R2 Fig. 1 Structures and numbering of chromen-2-one (coumarin) and R3 O O X = Substituent benzochromenones (benzocoumarins). R4 via alkynoates From o-hydroxynaphthaldehyde, the synthesis proceeds R1 R2 R3 R4 Product through initial Knoevenagel condensation with a malonate ester -H -CH=CH-CH=CH- -H Benzo[g]coumarin or its analogue followed by intramolecular cyclization with the -H-H -CH=CH-CH=CH- Benzo[h]coumarin Chemistry hydroxyl group to yield the 2 H-chromen-2-one moiety (Scheme -CH=CH-CH=CH--H -H Benzo[f]coumarin 1). Formation of various types of benzocoumarin derivatives Scheme 2 Direct transformations of phenols into benzocoumarins. thus depends on the relative positions of formyl and hydroxyl groups on the starting o-hydroxynaphthaldehydes (2-hydroxy- On the basis of above mentioned two main strategies, several 3-naphthaldehyde or 2-hydroxy-1-naphthaldehyde or 1- synthetic routes have been developed for benzo[ g], benzo[ h] hydroxy-2-naphthaldehyde). Among the o-hydroxynaphth- and benzo[ f]coumarins as described in the following. Synthesis aldehydes, 1-hydroxy-2-naphthaldehyde and 2-hydroxy-1- of benzo[ c]coumarins follows rather dissimilar approaches, and naphthaldehyde can be readily synthesized from corresponding accordingly those are mentioned later separately. naphthol derivatives by formylation through the Vilsmeier– Psoralene (furanocoumarin) derivatives as well as Materials Haack or Duff reaction, Reimer–Tiemann reaction, or MgCl 2- pyrrolocoumarins are generally synthesized through different or TiCl 4-mediated reaction. On the other hand, 2-hydroxy-3- strategies from those used for benzocoumarins. Hence, their naphthaldehyde derivatives can only be obtained by synthetic routes are also separately discussed. of formylation through directed lithiation of methoxy or MOM (methoxymethyl ether) protected 2-naphthol followed by 2.2.2. Synthesis through the Knoevenagel condensation 4,5 deprotection of the hydroxyl group. Although the formylation Benzocoumarins can be synthesized from o-hydroxynaphth- through the directed lithiation produces one isomer as the major aldehydes through tandem Knoevenagel condensation and product, a regioisomer that forms in a substantial amount needs intramolecular cyclization with ethyl cyanoacetate, ethyl to be removed by column chromatography which sometimes acetoacetate and dialkyl malonates that contain an active causes an isolation problem due to their very similar polarity. methylene group in the presence of a catalytic amount of Journal piperidine in dry ethanol or methanol (Scheme 3). 3 This This journal is © The Royal Society of Chemistry 2014 J. Name ., 2014, 00 , 1-3 | 2 Page 3 of 27 Journal of Materials Chemistry C Journal Name ARTICLE synthetic strategy is preferred for the formation of a specific R2 type of benzocoumarins, as the regioselectivity of the ring CHO O active methylene compound R1 OH formation depends on the position of the formyl and hydroxyl R1 O groups on the naphthalene ring of the starting o-hydroxy- catalyst naphthaldehydes. As demonstrated, benzo[ g]coumarins can be synthesized from 2-hydroxy-3-naphthaldehyde (Scheme 3), Active methylene compound Catalyst Product benzo[

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