Obituary Prof. Dr. Gerd Heppke

We mourn the loss of Professor Dr. Gerd Heppke, who passed away on 4 June 2022 at the age of 82. Gerd Heppke was a distinguished researcher and a brilliant teacher, who significantly influenced the progress of liquid crystal research. In this article, some of his achievements are reviewed in recognition of Gerd Heppke’s outstanding merits.

Being a physicist by training, Gerd Heppke (Figure 1) received his Ph. D. in 1971 and his habilitation in 1975, became Professor at the Technical University of Berlin (TUB), founded a large research group in the division of physical chemistry, initiated and led an interdisciplinary research programme on liquid crystals at TUB, and established the special research area of ‘Anisotropic Fluids’ in Berlin. Heppke’s ability to guide interdisciplinary work in both physics and chemistry enabled the synthesis and characterisation of many new liquid crystalline compounds, in particular cholesteric liquid crystals and mesogenic chiral additives, ferroelectric smectic liquid crystals, low-molar-mass glass-forming liquid crystals, and bent-core mesogens. Through his extraordinary creativity, his continuous readiness to challenge state-of-the-art mainstream assumptions, his outstanding managing capabilities and hard work, Heppke facilitated the explanation of some surprising experimental observations and the discovery of new effects in the fields of re-entrant smectic phases, helix inversion of cholesteric phases, electric field effects in blue phases, ferroelectricity, antiferroelectricity and ferrielectricity of smectic C-phases, higher ordered smectic phases and phases of bent-core mesogens, optical storage effects, and optical nonlinearities (Table 1) [1–79].

Figure 1. Prof. Gerd Heppke 1991 (Foto: Inge Kundel-Saro).

Table 1. Examples of various mesophases and their physical properties studied by Gerd Heppke, his students, and scientists collaborating with him. (N nematic; BP blue phase; Sm smectic; TGB twist grain boundary phase; Col columnar; P pyramidal; an asterix ‘*’ indicates the respective chiral modification, suffixes mean: ‘1’ monolayer, ‘d’ partial bilayer, ‘2’ bilayer, ‘s’ synclinic, ‘a’ anticlinic, ‘F’ ferroelectric, and ‘A’ antiferroelectric smectic ordering; the suffix ‘D’ indicates a respective phase composed of disc-like molecules; B₂, B₃, B₄ and B₇ are traditional abbreviations for mesophases composed of bent-core molecules). The electro-optic effects include (i) scattering → transparent transitions, (ii) reorientation of the optical axis, (iii) dichroism, (iv) colour changes, (v) Kerr effect, (vi) field-induced phase transitions, or (-) no switching observed in a higher ordered chiral smectic phase.

Class of compounds	Meso-phase	Polymorphism, phase diagram and/or phase structure	Helical pitch and/or selective reflection	Electric conductivity, dielectric permittivity and/or spontaneous polarisation	Electro-optic effects	Dynamic behaviour (e.g. viscosity and/or switching speed)	Optical nonlinearity including second harmonic generation(SHG)	Vitrified, glass-like state
Thermotropic: calamitic	N	[1,5–8,11]	-	[3,4]	[4] (i), [10] (iii)	[2]
	N*		[12,13,19–21,32]		[9] (vi)
	Blue Phases BPI- III, BPH		[14–17]		[14] (iv, vi), [15–17] (iv), [18] (v)			[72]
	TGB	[45,47]
	SmA1, SmAd, SmA2	[5–8,78]
	SmC1, SmCd, SmC2	[79]
	SmCa	[38]
	SmO (=SmCa)	[38]
	SmA*	[22,26,27,39,40]		[24]	[28,33] (ii), [24,29,30] (vi)			[73]
	De Vries SmA*	[48]
	SmB*				[33] (ii)
	SmC*	[22–27,40,42]	[32,35,46]	[22,23,31,35,40,42,47]	[22,23,31,42,44,47] (ii)	[22,23,35,[42]]	[74]	[73,74]
	SmCα*	[40,45–47]		[40]	[44] (ii)
	SmCβ*	[46]
	SmCβ*	[40,45–47]		[40]	[44] (ii)
	SmCA*	[38–40,42,45,46]	[46]	[40–42,46,47]	[40,42,44] (ii)	[41,42]
	SmE*	[33]			[33] (ii)
	SmF*	[36]
	SmG*	[30,36]			[36] (-)
	SmH*	[36]			[36] (-)
	SmI*	[36,40]
	SmIA*	[40,42,46]		[40,42]	[42] (ii)
	SmJ*	[36,34,37]		[36]	[36] (ii)
	SmM* or SmM’*	[34–37]	[35]	[35–37]	[37] (ii)	[35]
	SmO* (=SmCa*)	[38]
	SmQ*	[39,43,[45]]
Thermotropic: discotic	ND							[75]
	N D*	[49–51]	[49–51]		[51] (iv)			[49]
	BPDI, BPDII, BPDIII	[49,54,76]	[54]					[49,76]
	Col*	[52,54]		[52,54]	[52,54] (ii)	[52–54]
Thermotropic: pyramidal molecules	P	[55]				[55]
Thermotropic: bent-shaped molecules	B2: SmCsPF, SmCsPA, SmCaPF, SmCaPA	[56–60,63]		[58,77]	[58] (ii) [61,77] (i)	[58,61,[63],77]	[64–67,77]	[77]
	B3	[58,60]
	B4: HNF	[60,69]					[66,67]	[69]
	B7: SmCG*	[59,60,62]		[59,60]	[60] (ii),
	M1: SmCG*	[63]		[63]	[63] (ii)	[63]
Lyotropic: discotic molecules in dodecane	NCol*	[70]	[70]		[70] (ii)			[71]
	TGBCol ?	[70]
	Col* (‘D1’),Col* (‘D2’)	[70]			[70] (ii)

In 1970, when Gerd Heppke started to get interested in liquid crystals [1], standard textbooks of physics and chemistry described the latter as being composed of rod-like molecules, which tend to align parallel to each other in certain temperature ranges, thereby forming mesophases, in particular a nematic (N) phase [in which the locally preferred direction of the molecules is uniform and can be described by the unit vector n, the director], a cholesteric (N*) phase [which appears in the presence of chiral molecules. i. e. molecules without mirror symmetry, and is characterised by a helical director field n(r)], or one of different smectic (Sm) phases [where the molecules form layers in addition to their orientational order]. Television and computer screens were heavy, bulky objects with large power consumption, based on cathode ray tubes (CRT), quite different from the flat liquid crystal displays (LCDs) that are ubiquitous, today. In the early 1970s, first wrist watch displays and simple display prototypes based on the dynamic scattering mode [80], dichroic guest-host effect [81], helical unwinding [82] or twisted nematic (TN) cells [83] were gradually appearing. Since then, the research on fundamental liquid crystal properties emerged very rapidly. Gerd Heppke entered the field and promoted its progress with great enthusiasm. Together with Frank Schneider [1–7], he measured the orientational order parameter using ESR [1], studied the dynamics of magnetic field effects [2], explored the anisotropies of conductivity σ_a and dielectric permittivity ε_a [3,4], observed a temperature-induced sign inversion of σ_a (indicating cybotactic groups) in the vicinity of a N-SmA transition [3,4], and looked at interesting phase diagrams, which showed an induced smectic phase in a binary mixture of nematic compounds [5,6], or a re-entrant nematic phase [7]. Together with Kattera A. Suresh and Ranganathan Shashidhar, re-entrant smectic phases were found and characterised [8]. In addition to reporting an ‘inverse scattering mode’ [3,4], Heppke and his co-workers also tried to improve electrooptic effects based on helical unwinding (‘phase change’) [9] and dichroism [10]. The synthetic chemists Klaus Praefcke [11] and Günter Scherowsky [10] supplied novel mesogenic compounds to Gerd Heppke’s group and facilitated studying the relation between molecular structure and macroscopic properties systematically. In the early 1980s, Heppke started to build up his own chemistry lab which became a very productive source of entire series of novel compounds that were synthesised in relatively large amounts with high purity in the very vicinity of his state-of-the-art equipment to characterise their mesophase structures and basic physical properties. Chirality, the absence of mirror symmetry, became a key element in Gerd Heppke’s further work. Many of his more than 220 publications and 19 patents demonstrate impressively to which extent a simple symmetry relation – the lack of mirror symmetry – can induce a helical structure (like in the N* phase mentioned above), or alter the optical properties, cause new electro-optic effects or induce distinct phases – phenomena that will not appear in the racemic mixture, i. e. in a non-chiral mixture containing chiral molecules with opposite handedness in a 1:1 ratio. A method to detect the screw sense of the helical structure of the N* phase (Figure 2(a)) was developed [12,13], a brilliant electro-optic colour effect in blue phases (BPs) (Figure 2(b)) – double twisted short pitch cholesteric modifications with a cubic superstructure that show Bragg reflection – [14–17], an unusually large Kerr effect [18] and field-induced phase transitions in BPs [14] were found. Mesogenic chiral dopants with an extremely large ability to induce a helical structure with small pitch at low dopant concentrations in a nematic host (helical twisting power, HTP) [19], and other dopants with temperature-induced sign inversion of HTP were developed [20,21]. When other scientists discovered ferroelectric switching in chiral tilted smectic SmC* phases [84,85], Gerd Heppke and one of his Ph. D. students were among the first, who found compounds with an extremely large value of the spontaneous polarisation, high tilt angles, a weak temperature dependence of the tilt angles and other unusual properties, which could (at least in part) be attributed to a first order (as opposed to second order) SmC*-SmA* phase transition [22–30]. Purposeful development of new compounds in Heppke’s laboratory facilitated transferring the insights and effects found initially in N*, SmA* and SmC* phases [12–32] to new kinds of liquid crystal phases, such as calamitic higher ordered smectic phases (Figure 2(c)) or ferrielectric and antiferroelectric smectic phases [33–48], thermotropic discotic phases [49–55] (i. e. mesophases composed of disk-shaped molecules [86]), chiral phases made of achiral bent-core molecules [56–69], and even lyotropic discotic phases (solutions of disk-like molecules in an isotropic organic solvent) [70,71]. In particular, chiral dopants with high helical twisting power (HTP) [19,49–51,54,76] or sign-inversion of HTP [20,21,32,50], selective reflection (i. e. visible Bragg reflection) and its application [14–17,35,49–51], blue phases [14–18,49,50,54,76], electroclinic switching [28,33], as well as ferroelectric and antiferroelectric switching [22–24,35-37,40–42,52-54,56–63] were studied in a variety of very different LC systems. The syntheses of chiral mesogenic compounds was in some cases promoted by the use of biological compounds [22,42]. Several studies confirmed that the enantiomeric excess in chiral-racemic mixtures is an important thermodynamic variable of state in LCs, the change of which can induce phase transitions [25,38,39].

Figure 2. Examples of experimental results and insights. (a) Modified Grandjano-Cano preparation facilitating to determine the handedness of a chiral nematic helical structure (reproduced from Ref [13]. with kind permission by Taylor & Francis). (b) Electric field-induced colour change of the selective reflection of blue phase I (left) and blue phase II (right) as described in Ref [14]. (c) Phase transition observed by polarising optical microscopy in a compound exhibiting the higher ordered smectic SmQ* phase as described in Refs. [39,43]. (d) Surface topography of a discotic blue phase measured by atomic force microscopy (reproduced from Ref [76]. with kind permission by the Royal Society of Chemistry). (e) Sketch explaining how achiral bent-core molecules may form a tilted smectic phase that is chiral, i. e. not identical to its mirror image (reproduced from Ref [56]. with kind permission by the American Association for the Advancement of Science. (f) Intensity of the second harmonic generation (SHG) signal indicating a field-induced transition from an achiral to a chiral state in a mesophase composed of achiral bent-core molecules (reproduced from Ref [65]. with kind permission by the American Physical Society).

Independent of the aspect of chirality, Gerd Heppke was continuously searching for compounds that show a glass-like or vitrified state [49,72–77], in order to utilise this property either for optical nonlinearity and storage effects [73–75,77] or for analysing their phase structure and surface topography (Figure 2(d)) [72,76]. Throughout his professional life, Gerd Heppke was interested in the important role of phase transitions and its theory, their order, tricritical behaviour and critical points, the experimental determination of phenomenological coefficients and critical exponents [26,27,30,78,79], re-entrant phase phenomena or unusual phase sequences [7,8,38,63] and the influence of external electric fields on phase transitions [14,29,30,62].

In the 1990s, achiral bent-core molecules were reported to show mesophases with unusual switching properties, reminiscent of chiral smectic phases [87,88], which attracted Gerd Heppke’s attention (Figure 2(e)) [56–69]. After international researchers had agreed to classify the novel, unusual mesophases concordantly as B₁, B₂, B₃, B₄, B₅, B₆, and B₇, the sophisticated molecular arrangements in these phases were gradually explored. In this world-wide effort, Gerd Heppke and his students and collaborators (including in particular Antal Jákli) helped exploring the benefits of the B₂ phase, a tilted smectic phase, where two adjacent layers may have the same tilt direction (‘synclinic’, SmC_s) or opposite tilt directions (‘anticlinic’, SmC_a), while the local electric polarisations of neighbouring layers may be either parallel, giving rise to ferroelectric behaviour (SmC_sP_F or SmC_aP_F) or antiparallel, causing antiferroelectric behaviour (SmC_sP_A or SmC_aP_A). Heppke and co-workers developed an alignment method [57], a way of field-induced chiral separation between racemic and homochiral types of the four modifications [58], and an electro-optic switching mode [61], explored optical nonlinearities [64–67] – in particular the switching of second harmonic generation associated with a field-induced nonchiral-chiral transition (Figure 2(f)) –, and the appearance of mesophases in binary mixtures of calamitic liquid crystals and those with bent-core molecules [68]. Gerd Heppke contributed also to experimental evidences that the B₄ phase structure is composed of twisted ribbons [69], while the B₇ phase [62] and the M1 phase [63] observed for symmetric and asymmetric bent-shaped molecules, respectively, were found to exhibit triclinic (C₁) symmetry as expected for a SmC_G* phase (a phase predicted much earlier by Pierre Gilles de Gennes [89]), where biaxial molecules in smectic layers are both tilted and leaning [62,63].

Gerd Heppke was a brilliant university teacher. Owing to his much valued, vivid lectures and textbook chapters, he succeeded to attract many students to join his group, inspired his co-workers constantly by new ideas and deep discussions. He supported his students remarkably. Once they were equipped with basic knowledge and trained in the most important experimental techniques, he encouraged his students to broaden their knowledge by visiting not only conferences but also other scientific institutions – including, for example, the Raman Research Institute in Bangalore (India), the Laboratoire de Physique des Solides in Orsay (France), the Liquid Crystal Institute at Kent State University (OH, USA), or the University of Hawaii at Manoa (HI, USA) and other distinguished institutions. Several of his students’ works were honoured with prizes, including, for example, the Glenn-H.-Brown-Prize or the ILCS Multimedia Award.

Heppke’s outstanding managing capabilities were not restricted to building up his own group, he also encouraged colleagues at the Technical University Berlin (TUB) to apply for funding of an interdisciplinary ‘University Research Focus’ (‘Universitärer Forschungsschwerpunkt’ UF-1) at TUB, which made Berlin internationally visible as a centre of liquid crystal research in the early 1980's and enabled later funding of the Special Research Area ‘Anisotropic Fluids’ by the German Research Foundation (Sfb 335, DFG, 1987–1998). Heppke collaborated with many liquid crystal researchers worldwide, organised several highly efficient workshop-like conferences, including the Conference on Liquid Crystals of One- and Two-Dimensional Order and Their Applications [90] in 1980 (Figure 3(a)), a Bunsen Colloquium on Liquid Crystals in 1983 (Figure 3(b)), an international workshop on blue phases in 1989 [91], and a workshop on liquid crystals composed of ‘banana-shaped’ molecules in 1997. In textbook chapters and in numerous outreach activities, he made liquid crystals known to a broad audience ((Figure 3(c)). For many years during the 1990's, Heppke served for the International Liquid Crystal Society (ILCS) as a regional representative in the ILCS Board of Directors. For several decades, he served also as regional editor for the scientific journal Molecular Crystals and Liquid Crystals. His outstanding scientific achievements and merits were honoured in 2013 with the Alfred-Saupe-Prize bestowed by the Alfred Saupe Foundation and the German Liquid Crystal Society [91].

Figure 3. Examples of books, professional meetings and outreach activities by Prof. Gerd Heppke. (a) Cover of a book [90] published on the occasion of the Liquid Crystal Meeting 1980 at Garmisch-Partenkirchen (Germany). (b) Gerd Heppke and Gerhard Meier (Fraunhofer IAF Freiburg) at the Bunsen-Colloquium 1983, which was organised by Heppke in Berlin. (c) Exhibition at the Hannover Fair 1985, where general properties of liquid crystals, the selective reflection of blue phases, and an optical transmission line of acoustic signals utilising an electro-optic modulator were demonstrated.

Gerd Heppke contributed outstandingly to the development and success of liquid crystals and their applications. In addition, he was always ready to share and discuss his ideas, thereby initiating many works and collaborations on fundamentally new research questions. His former co-workers and colleagues esteem Gerd Heppke to an extremely high degree, and remember also his warm and friendly personality and his sense of humour. Through his enthusiasm, he encouraged many students and young researchers to work on liquid crystals and supported them extraordinarily. The liquid crystal community has lost a bright mind, a famous colleague, a brilliant teacher, and a trusted friend.