No abstract

The photocatalytic reduction of water to form hydrogen gas (H 2 ) is a promising approach to collect, convert, and store solar energy. Typically, ruthenium tris(bipyridine) and its many derivatives are used as photosensitizers (PSs) in a variety of photocatalytic conditions. The bis(terpyridine) analogues, however, have only recently gained attention for this application because of their poor photophysical properties. Yet, by the introduction of electron-donating or -withdrawing groups on the terpyridine ligands, the photophysical and electrochemical properties can be considerably improved. In this study, we report a series of nonsymmetric 2,6-di(pyridin-2yl)pyrimidine ligands with peripheral pyridine substituents in different positions and their corresponding ruthenium(II) complexes. The presence of the pyrimidine ring stabilizes the lowest unoccupied molecular orbital, leading to a red-shifted emission and prolonged excited-state lifetimes as well as higher luminescence quantum yields compared to analogous terpyridine complexes. Furthermore, all complexes are easier to reduce than the previously reported bis(terpyridine) complexes used as PSs. Interestingly, the pyridine substituent in the 4-pyrimidine position has a greater impact on both the photophysical and electrochemical properties. This correlation between the substitution pattern and properties of the complexes is further investigated by using time-dependent density functional theory. In hydrogen evolution experiments under blue-and red-light irradiation, all investigated complexes exhibit much higher activity compared to the previously reported ruthenium(II) bis(terpyridine) complexes, but none of the complexes are as stable as the literature compounds, presumably because of an additional decomposition pathway of the reduced PS competing with electron transfer from the reduced PS to the catalyst.

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Braun-Blanquet classification of the piedmont and mountain broad-leaved forests (formed by Carpinus betulus, Quercus petraea/hartwissiana/robur, Fraxinus excelsior) was done basing on 224 relevés collected in 2014–2018 in the North-Western Caucasus (N 43,5–44,8°, E 38,5–41,5°; Fig. 2). DCA-ordination of the data corresponds to their correlation with environment variables (absolute elevation, geographical coordinates, tree canopy density) in Landolt’s ecological scales was carried out. Suballiance Tamo communis–Carpinenion betuli suball. nov. prov. and new lower syntaxa are proposed (Table 1) within the alliance Crataego–Carpinion caucasicae Passarge 1981. Nomenclature type of the suballiance is the ass. Tamo communis–Carpinetum betuli ass. nov. (Table 3, holotypus is relevé 4: author’s number 83, author N. E. Shevchenko, 19.07.2016, N 44.257°, E 39.760°, 352 m above sea level, slope 5°NE) with three variants: typica ­(Table 3: 1–15), Staphylea colchica (Table 3: 16–27) and Festuca drymeja (Table 3: 28–38). Also, there are ass. Aro macu­lati–Carpinetum betuli (Table 4: 1–15; holotypus is relevé 5: author’s number 427, author N. E. Shevchenko, 12.05.2017, N 44.471°, E 40.516°, 455 m above sea level, slope 6°NE) as well as Abies nordmanniana–Carpinus betulus community (Table 4: 16–27) in new suballiance. Not too common in the North-Western Caucasus are ash (Fraxinus excelsior) forests which have no suffiicient floristic peculiarity and are considered as facies in associations Tamo communis–Carpinetum betuli var. typica and Aro maculati–Carpinetum betuli. The specificity of the studied forests in comparison with hornbeam and oak forests in the Central Caucasus (Georgia), North Turkey, the Balkans and the Crimea (Passarge, 1981a; Korzhenevskiy, 1982; Didukh, 1996; Korkmaz et al., 2008; Košir et al. 2013; Çoban, Willner, 2019; Novak et al., 2019) is that the North-­Western Caucasus forest flora includes (Table­ 2), besides European species of temperate broad-leaved forests (Acer campestre, Euonymus europaea, Carex sylvatica, Convallaria majalis, Rubus caesius), also southern European species of thermophilous broad-leaved forests (Acer tataricum, Cornus mas, Ligustrum vulgare, Lonicera caprifolium, Tamus communis, Vincetoxicum scandens, Hedera helix, Festuca drymeja), and reduced set of species which are character for Euxinian and ­Caucasian ­forests (Quercus­­ ­hartwissiana, Tilia begoniifolia, Rhododendron luteum, Daphne caucasica, Staphylea colchica, Smilax excelsa, Paris incompleta, Polygonatum orientale, Lathyrus roseus, Campanula alliariifolia, but without Daphne pontica, Epimedium pubigerum, Erica arborea, Ostrya carpinifolia, Salvia forskahlei, Vaccinium arctostaphylos). DCA-ordination (Fig. 8) showed that the differences in species composition of the broad-leaved ­forest syntaxa are due to both absolute elevation (vector Elev in Fig. 7) and geographic longitude (vector E) of the relevés. So, forests of ass. Aro maculati–Carpinetum betuli and community Abies nordmanniana–Carpinus betulus are situated, in general, at higher positions than forests of ass. Tamo communis–Carpinetum betuli, and the first syntaxon is situated east of the two last ones. Floristic difference between these syntaxa corresponds with parameters assessed by values of Landolt’s scales: soil aeration (vector D), climate continentality (vector K) and light regime (vector L). The lowest α-diversity is in ass. Tamo communis–Carpinetum betuli, and the highest is in the ass. Aro maculati–Carpinetum betuli and community Abies nordmanniana–Carpinus betulus it (Fig. 9). Associations Carpino betuli–Quercetum petraeae Grebenshchikov et al. 1990 and Rhododendro lutei–Quercetum petraeae Grebenshchikov et al. 1990, earlier described on small sets of relevés in the North-Western Caucasus (Grebenshchikov et al. 1990) within the alliance Carpino betuli–Quercion petraeae Grebenshchikov et al. 1990 (now invalid due to absence of stated holotypus), are very close to the new ass. Tamo communis–Carpinetum betuli. Recently described (also on small sets of data) five associations (Akatova, Ermakov, 2020), within the alliance Crataego–Carpinion caucasicae Passarge 1981, are valid. Therefore, further clarification and optimization of the North-Western Caucasus oak-hornbeam forest classification is required, having in mind the final decision on the alliance/suballiance names and diagnosis. Their belonging to the order (Carpinetalia betuli or Rhododendro pontici–Fagetalia orientalis) is also a debatable question, because researchers working in such forests on Balkans, in the Crimea and the North Turkey have come to different conclusions. The analysis of species with constancy 60–80 % in 224 relevés from the studied area reveals 8 diagnostic ones of the order Carpinetalia betuli vs. only 3 diagnostic ones of the order Rhododendro pontici–Fagetalia orientalis, that allows to assign these oak-hornbeam forests to the first order. The Abies nordmanniana–Carpinus betulus community is intermediate between these two orders but after trees from shade-tolerant fir undergrowth become, the canopy mature will be mixed that is character for forests of the order Rhododendro pontici–Fagetalia orientalis.