The effect of benzo- and benzocyclobutadieno-annelation on cyclic conjugation  in terrylene and the higher members of the rylene homologous series is  examined. Some peculiar regularities are established, that could not be  observed in the case of perylene (the first member of the rylene series)

Gutman Ivan

Marković Marija

Đurđević Jelena

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 J. Serb. Chem. Soc. 77 (6) 751–759 (2012)  UDC 66–945.3+547.53+677.07:66.0123.3 JSCS–4306 Original  scientific  paper 751 Cyclic conjugation in benzo- and benzocyclobutadieno-annelated terrylenes and higher rylenes MARIJA MARKOVIĆ, JELENA ĐURĐEVIĆ and IVAN GUTMAN*# Faculty of Science, University of Kragujevac, P. O. Box 60, 34000 Kragujevac, Serbia (Received 31 January 2012) Abstract: The effect of benzo- and benzocyclobutadieno-annelation on cyclic conjugation in terrylene and the higher members of the rylene homologous se-ries are examined. Some peculiar regularities are established, which could not be observed in the case of perylene (the first member of the rylene series). Keywords: cyclic conjugation; energy effect (of cyclic conjugation); terrylene; quaterrylene; rylenes. INTRODUCTION In this work, we are concerned with the cyclic conjugation in the benzo- and benzocyclobutadieno-annelated derivatives of the rylene homologous series of benzenoid hydrocarbons, Fig. 1. What is meant under “linear” and “angular” benzo- and benzocyclobutadieno-annelation is clarified in Fig. 2. A method for calculating the energy effect ef of cyclic conjugation in indi-vidual rings of polycyclic conjugated compounds was conceived in the 1970s by one of the present authors.1 Its details and an exhaustive bibliography can be found in the reviews,2,3 whereas the mathematical details by which the quantity ef is computed are outlined in recent articles.4,5 When the ef-method was applied to benzo-annelated perylenes,6,7 a remark-able phenomenon was discovered: Contrary to what standard methods8–10 pre-dict, benzo-annelation significantly influences the intensity of cyclic conjugation in the “empty” central ring of perylene; in some cases it even exceeds the inten-sity of cyclic conjugation in the “full” rings. Characteristic results of this kind are shown in Fig. 3. Eventually, systematic studies11–15 revealed that the first observations made in the case of perylene are general regularities that can be stated as follows: Rule 1. Angular benzo-annelation increases the intensity of cyclic conjugation.                                                                                                                     * Corresponding author. E-mail: gutman@kg.ac.rs; jddjurdjevic@gmail.com # Serbian Chemical Society member. doi: 10.2298/JSC120131012M _______________________________________________________________________________________________________________________ Available online at www.shd.org.rs/JSCS/2012 Copyright (CC) SCS752 MARKOVIĆ, ĐURĐEVIĆ and GUTMAN Rule 2. Linear benzo-annelation decreases the intensity of cyclic conjugation.  Rule 3. The effect of angular benzo-annelation is stronger than the effect of linear benzo-annelation.  Fig. 1. The first three members of the rylene homologous series: perylene (I), terrylene (II), and quaterrylene (III), their general formula (IV), and the labeling of their annelation sites; sites marked by a pertain to angular, and those marked by ℓ to linear annelation, cf. Fig. 2. The “empty” rings are labeled by 1, 2,…, k. According to classical Kekulé-structure-based theories, the “empty” rings are devoid of any cyclic conjugation, implying that rylenes should be viewed as consisting of several independent naphthalene fragments. The present approach shows that this is an oversimplification and that cyclic conjugation in the “empty” rings is far from negligible, and is much influenced by benzo- and benzocyclobutadieno-annelation. Rules 1–3 summarize the results of a large number of calculations. In addi-tion, their general validity was recently confirmed by means of theoretical argu-ments.16 Recalling that in some polycyclic conjugated systems (e.g., in pery-lene6,7), as a consequence of Rules 1–3, the conjugation pattern significantly dif-fers from what previous theories8–10 based on the analysis of Kekulé structures would predict. One could even speak of the “breakdown of the Kekulé-structure model”.17 Another way to attach a six-membered ring to a conjugated system is the benzocyclobutadieno-annelation, Fig. 2. The effect of such an annelation was also studied in due detail.18–23 In many cases, but not all,22,23 the following re-gularities could be envisaged: Rule 4. Angular benzocyclobutadieno-annelation decreases the intensity of cyclic conjugation. Rule 5. Linear benzocyclobutadieno-annelation increases the intensity of cyclic conjugation. Rule 6. The effect of linear benzocyclobutadieno-annelation is stronger than the effect of angular benzocyclobutadieno-annelation. _______________________________________________________________________________________________________________________ Available online at www.shd.org.rs/JSCS/2012 Copyright (CC) SCS  CYCLIC CONJUGATION IN ANNELATED TERRYLENES AND HIGHER RYLENES  753 Fig. 2. Modes of benzo- and benzo-cyclobutadieno-annelation with regard to the ring R.  Fig. 3. Energy effects of the central “empty” ring (1) of perylene and its angularly annelated benzo-derivatives. The greater is the number of angular benzo-annelations, the greater is ef. In tetra-angularly annelated perylene, each of the four six-membered rings adjacent to ring 1 has ef = 0.0437, which is smaller than ef(1) = 0.0522. This contradicts the predictions of classical theories,8-10 according to which there is no cyclic conjugation in the “empty” ring. A remarkable violation of Rule 4 was found in the case of benzocyclobu-tadieno-annelated perylenes.23 Surprisingly, in these polycyclic systems, angu-larly annelated benzocyclobutadieno-fragments have practically no effect on the ef-value of the “empty” central ring. Characteristic results of this kind are shown in Fig. 4. The evident question that could be asked at this point is whether analogous regularities also exist in the higher members of the rylene series, in particular in terrylene and quaterrylene, cf. Fig. 1. The importance of knowing the answer to this question lies in the fact that oligorylenes (the first few, experimentally avail-able, members of the rylene series) possess interesting and non-standard optical and optoelectronic properties.24–26 These are primarily caused by the π-electrons, the degree of delocalization of which is significantly influenced by conjugation in the “empty” rings. Thus, by means of appropriately combined benzo- and ben-zocyclobutadieno-annelation, a fine-tuning of the optical and optoelectronic pro-_______________________________________________________________________________________________________________________ Available online at www.shd.org.rs/JSCS/2012 Copyright (CC) SCS754 MARKOVIĆ, ĐURĐEVIĆ and GUTMAN perties of these compounds could be achieved, which would be of evident value for their practical (technical) applications.  Fig. 4. Energy effects ef of the central ring (1) of various angularly benzocyclobutadieno-an-nelated perylenes. Practically, angular benzocyclobutadieno-annelation has an influence on the value of ef. These results should be compared with those in Fig. 6. In what follows, some partial answers to the above question are offered. NUMERICAL WORK The energy effects, ef, of the six-membered rings were calculated by means of a several times previously described1-7 graph-theoretical method, using in-house software. As usual, the ef values are given in the units of the HMO carbon–carbon resonance integral β. Thus, posi-tive ef values indicate thermodynamic stabilization caused by cyclic conjugation. The greater is ef, the greater is the intensity of cyclic conjugation in the underlying ring. There are 3 monobenzo-, 14 dibenzo-, 24 tribenzo-, 32 tetrabenzo-, 18 pentabenzo- and 7 hexabenzo-annelated terrylenes, i.e., a total of 98 benzo-annelated congeners. Exactly the same counts apply to benzocyclobutadieno-annelated terrylenes. All calculated ef-values of the six-membered rings of these derivatives of terrylene are available from the authors (M.M.) upon request. The number of terrylene derivatives possessing both benzo- and benzocyclobutadieno- -annelations is several hundred. Their ef values were not calculated. CONJUGATION IN BENZO-ANNELATED TERRYLENE AND HIGHER RYLENES The ef-values of benzo-annelated terrylenes and higher rylenes are found to obey fully the Rules 1–3. A few characteristic examples are depicted in Fig. 5. A structural feature that does not exist in perylene is the sites a5, a6,… (see Fig. 1). Benzo-annelation on these sites also obeys Rules 1 and 3, but the effect is somewhat weaker than at the terminal sites a1–a4. Examples: For a1- and (a1,a2)- -annelation ef = 0.0293 and ef = 0.0367, whereas for a5- and (a5,a6)-annelation ef _______________________________________________________________________________________________________________________ Available online at www.shd.org.rs/JSCS/2012 Copyright (CC) SCS  CYCLIC CONJUGATION IN ANNELATED TERRYLENES AND HIGHER RYLENES  755 = 0.0287 and ef = 0.0352. Consistently, the ef values for (a1,a5)- and (a1,a6)-an-nelation are 0.0355 and 0.0358, which lie between 0.0287 and 0.0367.  Fig. 5. Energy effects, ef, of the two “empty” rings in benzo-annelated terrylenes; the numbers below the structures are ef(1) and ef(2), in that order. Rules 1–3 are satisfied in all cases. In the higher rylenes, there are several “empty” rings and annelation influ-ences all their energy effects. This can be seen from the examples depicted in Fig. 5. In order to show how fast this “transmission” of annelation effect de-creases, the sixth member of the rylene series (k=6, see Fig. 1) is considered. For the non-annelated species, ef(1) = ef(6) = 0.0252, ef(2) = ef(5) = 0.0282 and ef(3) =  = ef(4) = 0.0290. For the (a1,a2)-dibenzo derivative, ef(1) = 0.0396, ef(2) = = 0.0305 , ef(3) = 0.0301, ef(4) = 0.0296, ef(5) = 0.0285 and ef(6) = 0.0253. For the (ℓ1,ℓ2)-dibenzo derivative, the attenuation of the annelation effect is even faster: ef(1) = 0.0216, ef(2) = 0.0279, ef(3) = 0.0289, ef(4) = 0.0289, ef(5) =  = 0.0281 and ef(6) = 0.0252. CONJUGATION IN BENZOCYCLOBUTADIENO-ANNELATED TERRYLENE AND HIGHER RYLENES In the case of benzocyclobutadieno-annelation, the first point to be examined is whether violation of Rule 4 also occurs at the higher members of the rylene series. The findings are surprising: the same as in the case of perylene, angular benzocyclobutadieno-annelation in the terminal positions a1–a4 has a negligible influence on cyclic conjugation in the “empty” rings of terrylene; examples are _______________________________________________________________________________________________________________________ Available online at www.shd.org.rs/JSCS/2012 Copyright (CC) SCS756 MARKOVIĆ, ĐURĐEVIĆ and GUTMAN given in Fig. 6. On the other hand, angular benzocyclobutadieno-annelation in the lateral positions a5, a6,… diminishes the ef values, thus obeying Rule 4; examples are given in Fig. 7. From the examples given in Fig. 7, it is seen that the same regularity also applies to benzocyclobutadieno-annelated quaterrylenes.  Fig. 6. Energy effects, ef, of the “empty” rings of benzocyclobutadieno-derivatives of terrylene, annelated in terminal positions a1, a2, a3 and a4. The numbers below the structures are ef(1) and ef(2), in that order. Practically, such angular benzocyclobutadieno-annelation has an influence on the value of ef. These results should be compared with those in Fig. 4. The present calculations confirmed that Rules 5 and 6 apply without a single exception. A few illustrative examples are provided in Fig. 8. The results of the present work indicate that there exists an annelation mode (or, more precisely, an angular benzocyclobutadieno-annelation mode) that leaves the pattern of cyclic conjugation in the members of the rylene homologous series essentially unchanged. The theoretical explanation of why this is so remains a task for the future. Other annelation modes obey the relatively “reasonable” regu-larities stated here as Rules 1–6. By employment of these rules, it would be pos-sible to design oligorylenes with the desired conjugation patterns, thus possessing optical and optoelectronic properties required for technical applications. _______________________________________________________________________________________________________________________ Available online at www.shd.org.rs/JSCS/2012 Copyright (CC) SCS  CYCLIC CONJUGATION IN ANNELATED TERRYLENES AND HIGHER RYLENES  757  Fig. 7. The above three examples illustrate that contrary to angular benzocyclobutadieno-an-nelation in terminal positions, annelation in lateral positions a5, a6,… obeys Rule 4 and diminishes the intensity of cyclic conjugation in the “empty” rings of terrylene. The three examples below show that the same regularities also hold in the case of higher members of the rylene homologous series. The numbers below the structures are ef(1), ef(2), and, where applicable, ef(3), in that order.  Fig. 8. Examples confirming that Rules 5 and 6 hold in the case of benzocyclobutadieno-an-nelated terrylenes. The numbers below the structures are ef(1) and ef(2), in that order. Note that the ef-values in the ℓ1- and (ℓ1,a2)-derivatives are practically identical; this is the consequence of the non-validity of Rule 4, cf. Fig. 6. _______________________________________________________________________________________________________________________ Available online at www.shd.org.rs/JSCS/2012 Copyright (CC) SCS758 MARKOVIĆ, ĐURĐEVIĆ and GUTMAN Acknowledgement. The authors thank the Ministry of Education and Science of the Re-public of Serbia for support (Grant No. 174033). ИЗВОД ЦИКЛИЧНА КОНЈУГАЦИЈА У БЕНЗО- И БЕНЗОЦИКЛОБУТАДИЕНО-АНЕЛИРАНИМ ТЕРИЛЕНИМА И ВИШИМ РИЛЕНИМА МАРИЈА МАРКОВИЋ, ЈЕЛЕНА ЂУРЂЕВИЋ и ИВАН ГУТМАН Prirodno–matemati~ki fakultet Univerziteta u Kragujevcu Испитиван је утицај бензо- и бензоциклобутадиенске анелације на цикличну конјуга-цију у терилену и вишим члановима риленског хомологног низа. Установљене су извесне неочекиване правилности, које нису могле бити уочене у случају перилена (првог члана ри-ленског низа). (Примљено 31. јануара 2012) REFERENCES 1.  I. Gutman, S. Bosanac, Tetrahedron 33 (1977) 1809 2.  I. Gutman, Monatsh. Chem. 136 (2005) 1055 3.  I. Gutman, in: A. Graovac, I. Gutman, D. Vukičević (Eds.), Mathematical Methods and Modelling for Students of Chemistry and Biology, Hum, Zagreb, 2009, pp. 13–27 4.  I. Gutman, Int. J. Chem. Model. 2 (2010) 335 5.  I. Gutman, G. V. Milovanović, Bull. Acad. Serb. Sci. Arts (Cl. Sci. Math. Natur.) 143 (2011) 1 6.  I. Gutman, N. Turković, J. Jovičić, Monatsh. Chem. 135 (2004) 1389 7.  I. Gutman, B. Furtula, J. Đurđević, R. Kovačević, S. Stanković, J. Serb. Chem. Soc. 70 (2005) 1023 8.  I. Gutman, S. J. Cyvin, Introduction to the Theory of Benzenoid Hydrocarbons, Springer-Verlag, Berlin, 1989 9.  M. Randić, Chem. Rev. 103 (2003) 3449 10.  A. T. Balaban, Polycycl. Arom. Comp. 24 (2004) 83 11.  I. Gutman, J. Đurđević, A. T. Balaban, Polycycl. Arom. Comp. 29 (2009) 3 12.  J. Đurđević, I. Gutman, J. Terzić, A. T. Balaban, Polycycl. Arom. Comp. 29 (2009) 90 13.  I. Gutman, J. Đurđević, J. Serb. Chem. Soc. 74 (2009) 765 14.  S. Jeremić, S. Radenković, I. Gutman, J. Serb. Chem. Soc. 75 (2010) 943 15.  A. T. Balaban, I. Gutman, S. Jeremić, J. Đurđević, Monatsh. Chem. 142 (2011) 53 16.  I. Gutman, A. T. Balaban, J. Serb. Chem. Soc. 76 (2011) 1505 17.  I. Gutman, S. Marković, S. Jeremić, Polycycl. Arom. Comp. 30 (2010) 240 18.  A. T. Balaban, J. Đurđević, I. Gutman, S. Jeremić, S. Radenković, J. Phys. Chem., A 114 (2010) 5870 19.  A. T. Balaban, I. Gutman, S. Marković, D. Simijonović, J. Đurđević, Polycycl. Arom. Comp. 31 (2011) 339 20.  A. T. Balaban, I. Gutman, S. Marković, D. Simijonović, Monatsh. Chem. 142 (2011) 797 21.  I. Gutman, B. Furtula, A. T. Balaban, J. Serb. Chem. Soc. 76 (2011) 733 22.  I. Gutman, Indian J. Chem., A 50 (2011) 670 23.  I. Gutman, B. Stojanovska, Maced. J. Chem. Chem. Eng. 30 (2011) 235 _______________________________________________________________________________________________________________________ Available online at www.shd.org.rs/JSCS/2012 Copyright (CC) SCS  CYCLIC CONJUGATION IN ANNELATED TERRYLENES AND HIGHER RYLENES  759 24.  Electronic Materials: The Oligomer Approach, K. Müllen, G. Wegner, Eds., Wiley-VCH, Weinheim, 1998 25.  J. Wu, A. Pisula, K. Müllen, Chem. Rev. 107 (2007) 718 26.  T. M. Figueira-Dharte, K. Müllen, Chem. Rev. 111 (2011) 7260. _______________________________________________________________________________________________________________________ Available online at www.shd.org.rs/JSCS/2012 Copyright (CC) SCS

Journal of the Serbian Chemical Society

Cyclic conjugation in benzo- and benzocyclobutadieno-annelated terrylenes and higher rylenes

The effect of benzo- and benzocyclobutadieno-annelation on cyclic conjugation
   in terrylene and the higher members of the rylene homologous series is
   examined. Some peculiar regularities are established, that could not be
   observed in the case of perylene (the first member of the rylene series).</jats:p

Cyclic conjugation in benzo- and benzocyclobutadieno-annelated terrylenes and higher rylenes

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