1 Introduction

Without doubt, advances in polymer science have brought unprecedented developments to our society. Consequently, research in polymer science prospers. A class of polymers that is of continuing interest is dendritic polymers, which are subclassed into dendrimers, dendrons, hyperbranched polymers, and dendrigraft polymers [1]. The three-dimensional (3D) topology of dendritic polymers, in addition to their good solubility and low viscosity, which enhances processability, position these polymers in a unique landscape in fundamental and applied polymer research [2–5]. Furthermore, dendritic polymers have a molecular, polymeric or supramolecular core, branches and multiple peripheral termini, which can be synthetically tailored to tune properties such as glass transition temperature [6, 7], and to impart electronic [8–13] and magnetic functions [14–18], as well as bioactivity [19–23] and photoactivity [24–28].

Photoactive dendritic polymers are accessed through incorporation of chromophoric and/or luminophoric molecules within the dendritic framework; ultimately leading to photoinduced processes that have far-reaching fundamental and practical implications [29]. Evidently, the dendritic architecture is an attractive platform for the design of photoactive materials giving its inherent properties such as improved solubility as well as the most often observed dendritic effects [1]. Indeed, photoactive dendritic polymers are increasingly used in diverse applications such as in solar energy harvesting systems, biomedicine, photonics, and nanotechnology. Accordingly, a review of photoactive dendritic polymers is worthwhile and we expect this comprehensive review to increase research interests along this direction. Most reviews on dendrimers [3,9,30–38] examine different functions; however, this review focuses exclusively on photoactive dendritic polymers. Prior to examining these polymers, a short historical overview of dendritic polymers is presented.

2 From Cascade Molecules to Photoactive Dendritic Polymers: A Historical Overview

In their 1978 paper, ‘Cascade and “nonskid-chain-like” syntheses of molecular cavity topologies’ Buhleier et al. reported the synthesis of the first dendritic polymer, a low molecular weight amine, obtained via iterative steps [39]. The present euphoria about dendritic polymers is influenced by the 1985 seminal reports of Newkome et al. [40] and Tomalia et al. [41] on the syntheses of “monocascade spheres” and “starburst-dendritic macromolecules”, respectively. These early dendrimers were obtained via a divergent growth strategy [42] in which the dendrimer is grown outward from a central core molecule via iterative coupling of the building block (dendron or branch) to the core in the zeroth generation and to the peripheral termini in higher generations [43]. The convergent approach, first reported in 1990 by the group of Fréchet [44, 45], builds the dendrimer “inward” by first synthesizing the dendron, and subsequently coupling it to the core molecule [43]. These strategies, which represent the traditional synthetic approaches to dendrimers and dendrons, are time consuming and resource intensive as they involve iterative stepwise growth that, in most cases, involves deprotection/activation steps and tedious purification protocols. Accelerated and orthogonal synthetic strategies that mitigate these shortcomings are actively researched in dendrimer synthesis [46–50] and were the focus of recent reviews [51, 52].

Although branched polymers are known since the late 19th century [3, 53], Kim and Webster first used the term “hyperbranched polymer” in 1990 in their report of a water-soluble hyperbranched polyphenylene [54]. Since this report, increasing interest has been directed towards this subclass of dendritic polymers, as their synthetic methodology is less challenging compared to dendrimers and dendrons. Conventional single-monomer polymerization techniques such as polycondensation, proton-transfer polymerization, self-condensing vinyl or ring-opening polymerizations, as well as double-monomer polymerization techniques are the common synthetic routes to hyperbranched polymers [3, 43]. A recent addition to the class of dendritic polymers is the dendrigraft polymers that combined features of dendrimers, dendrons or hyperbranched polymers with those of linear polymers. First reported in 1991 by Tomalia et al. [55], this subclass is distinct from the other dendritic polymers by their assembly from uniform polymeric building blocks and their high molecular weights, which is attained in few synthetic steps [56].

Less than a decade after Newkome’ and Tomalia’s seminal paper, the unique topology of dendritic polymers was exploited in designing photoactive materials by incorporating chromophores or luminophores within the dendritic framework. As an example, in the early 1990s, Campagna et al. reported a number of dendritic, luminescent polynuclear Ru(II) and Os(II) complexes [57–64]. So far, various chromophores and luminophores that include transition metal complexes, nanoparticles, organic dyes and fluorophores as well as organic molecules with delocalized pi-electrons have been utilized as building blocks of the core, branches and/or peripheral termini in these photoactive polymers. The remaining part of the review will focus on photoactive dendritic polymers.

3 Photoactive Dendritic Polymers

3.1 Nanoparticle-Based Photoactive Dendritic Polymers

Some inorganic nanoparticles, especially gold, are reputed for remarkable optical properties due to their localized plasmonic resonant absorption in the visible and/or near infrared spectra region [65, 66]. Through covalent or supramolecular interactions, these nanoparticles are stabilized by dendritic polymers, yielding nanoparticle-cored dendritic polymer, dendritic polymer-encapsulated, or -stabilized nanoparticle [67, 68]. Taking advantage of these specific interactions, material scientists have designed photoactive dendritic polymeric/inorganic nanocomposite. For instance, a gold nanorod (GNR)- cored polyethylene glycol-modified poly(amidoamine) (PEG-PAMAM) dendrimer (1), obtained via timed addition of PEG-PAMAM dendrimers with a cystamine core to growing GNR (Scheme 1), exhibits near infrared (NIR) light-induced surface plasmon resonance [69]. In vivo, the third generation of 1 generates heat under NIR irradiation, increasing the temperature of tumour tissues and ultimately suppressing tumour growth in mice (Fig. 1). This property portrays the dendrimer as an excellent candidate for photothermal therapy.

Scheme 1
scheme 1

Synthesis of gold nanorods-cored dendrimer 1. Reproduced with permission from Ref. [69]. Copyright @ 2014 Royal Society of Chemistry

Full size image
Fig. 1
figure 1

Photothermal treatment of mice with colon carcinoma 26 tumors using NIR (λ = 808 nm, 0.24 W cm−2) irradiated gold nanorods-cored dendrimer 1 or phosphate buffered saline (PBS) for 10 min. a Thermographs of tumors at 0, 1, and 2 min after NIR irradiation; b temperature profile of tumor after injection with dendrimer 1 or PBS under NIR irradiation; c tumor volume after treatment with dendrimer 1 or PBS under NIR irradiation. Reproduced with permission from Ref. [69]. Copyright @ 2014 Royal Society of Chemistry

Full size image

Photothermal property is not restricted to Au nanoparticle-cored dendrimers. Evidently, this property is exhibited by other dendritic polymers stabilized Au nanoparticles. As an example, a PEG-modified PAMAM encapsulated Au nanoparticles feature visible light-induced heat generation property [70, 71]. Other dendritic polymer-stabilized nanoparticles have been shown to be photoactive. PMAM dendrimer-encapsulated CdSe nanoparticles [72], phenyl-cored thiophene dendrimer-CdSe nanoparticle composite [73], oligothiophene dendron-capped CdSe nanoparticles [74], and oligo(phenylene vinylene) dendronized-γ-Fe2O3 nanocrystals [75] are photoactive.

In addition to the light harvesting property of semiconductor nanoparticles, such as CdSe, are their high electron affinities that facilitate exciton dissociation at nanoparticle/polymer interface [73]. Consequently, these properties have been exploited not only in designing solar cell devices but also in enhancing the performance of these devices. A recent contribution highlights the design of a photoelectrochemical biofuel cell based on PAMAM dendrimer-encapsulated CdSe nanoparticles (DEN-CdSe-NP) that absorbs and emits at 500 and 620 nm, respectively (Fig. 2) [72]. The DEN-CdSe-NP functions as a sensitizer in an engineered DEN-CdSe-NP-sensitized titanium dioxide photoanode, extending the photoresponse of the latter into the visible region. After visible light irradiation, photo-induced current is generated as electron–hole separates with the hole consumed by the biomass and the electrons transported to a cathode through an external circuit (Fig. 2).

Fig. 2
figure 2

a Absorption and emission spectra of DEN-CdSe-NP in water; b Photoinduced current response of DEN-CdSe-NP/titanium dioxide photoanode and titanium dioxide drop cast on Toray carbon paper. Reproduced with permission from Ref. [72]. Copyright @ 2014 Royal Society of Chemistry

Full size image

Single wall carbon nanotubes (SWNT), remarkable for their optical properties, have been functionalized with dendritic polymers [76–80], leading to materials with photophysical properties. Through rational design, photoinduced charge separation, a desirable feature of solar energy harvesting devices, is facilitated via integration of the photo processes of SWNTs with those of photoactive molecules, electron donor or acceptor dendrons. Such facilitated charge separation is observed in SWNT–zinc(II) porphyrin (ZnP) electron donor–acceptor conjugates depicted by 2 (Fig. 3), where the selective photoexcitation of the ZnP is accompanied by fast charge separation, leading to the formation of reduced SWNT and oxidized ZnP [79].

Fig. 3
figure 3

a An illustration of single wall carbon nanotube–zinc(II) porphyrin (ZnP) dendrimer (2). Reproduced with permission from Ref. [79]. Copyright @ 2009 American Chemical Society; b An illustration of dendritic polypyridine complex, Os[(dpp)Ru(bpy{pyrene})2] 8+3 , (3). Reproduced with permission from Ref. [81]. Copyright @ 2007 Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim

Full size image

3.2 Organometallic Complex-Based Photoactive Dendritic Polymers

The ability of d-block metal complexes to absorb electromagnetic radiation results in their characteristic colors. Absorption can result from electronic transitions between metal and ligand-centered molecular orbitals (charge transfer band). Material scientists have exploited these photoinduced transitions in designing photoactive dendritic polymers by incorporating d-block metal complexes within dendritic frameworks. For instance, the d-metal polypyridyl-type complexes, where the presence of the d-block metal induces strong spin–orbit interaction that result in fast intersystem crossing and ultimately in photoinduced population of the lowest-lying metal-to-ligand charge-transfer triplet state [81], are of fundamental and practical relevance. Specifically, the metal-to-ligand charge transfer band as well as the relatively long lived and luminescent excited states of Ru(II) and Os(II) polypyridine complexes allow fundamental investigations of the photophysical properties of d-metal polypyridine complexes-based dendritic polymers [81, 82]. As an example, in a fundamental study of the dendritic polypyridine complex (3), Os[(dpp)Ru(bpy{pyrene})2] 8+3 where dpp = 2,3-bis(2,3-bis(2′-pyridyl)pyrazine and bpy = 2,2′-bipyridine, simultaneous energy transfer from the peripheral pyrenes and an initially excited Ru-bpy ligand to the Os triplet metal–ligand charge-transfer state was observed using a broadband visible femtosecond probe [81]. Additionally, the presence of the peripheral pyrene ligands instead of an additional layer of Ru(bpy)2 complex, facilitates a unidirectional energy transfer to the Os core as well as enhances the UV and visible absorption spectra regions of 3. The studies implicate 3 as solar energy harvesting antennae as the absorption covers the UV and visible regions and energy transfer is exclusively channelled towards the Os core.

Artificial platforms for solar energy harvesting are intensively researched and d-metal polypyridyl-type complexes are prominent in this quest. When incorporated as part of dendritic frameworks, these complexes can perform better than the free complexes as solar energy harvesting motifs. As an illustration, the metal-to-ligand charge-transfer band of ruthenium(II) pyridyl-cored, polythiophene dendrimers (4–7) (Fig. 4) are broader, more intense, and red-shifted compared with the non-dendritic ruthenium ligands, [Ru(bpy)2(phen)]2+ and [Ru(phen)3]2+, where bpy is bipyridine and phen is phenanthroline (Fig. 5). These findings portray these dendrimers as better solar energy antennae than the non-dendritic ligand [83]. A broad absorption that spans the entire visible spectra region (250–750 nm) was also observed in the UV–vis absorption spectra of films or CHCl3 solutions of a series of mono-, bis-, and tris-ruthenium terpyridine dendritic complexes [84]. Additionally, these series of ruthenium-based dendrimers absorb beyond their metal-to-ligand charge-transfer band resulting in small optical bandgap (1.51–1.86 eV) (Fig. 6) .

Fig. 4
figure 4

Structures of dendrimers 4–7. Reproduced with permission from Ref. [83]. Copyright @ 2011 American Chemical Society

Full size image
Fig. 5
figure 5

Metal to Charge transfer band of dendritic 4–7 and non-dendritic, reference ruthenium ligands, [Ru(bpy)2(phen)]2+ and [Ru(phen)3]2+, where bpy is bipyridine and phen is phenanthroline. Reproduced with permission from Ref. [83]. Copyright @ 2011 American Chemical Society

Full size image
Fig. 6
figure 6

Structures of dendrimers 8–13. Reproduced with permissions from Refs. [87, 91]. Ref. [87]: copyright @ 2009, Elsevier B. V. All rights reserved and Ref. [91] copyright @ 2014, American Chemical Society

Full size image

Charge transfer absorptions are not limited to d-metal pyridyl complexes. Other d-metal ligand systems have been used in the design of photoactive dendrimers [85–91]. For instance, d-metal carbozoyl-based dendrimeric emitters are known [85–87, 91]. A dendritic Pt(II) complexes, [(t-BuCzPBI)Pt(acac)] (8) and [(t-BuCzCz-PBI)Pt(acac)] (9) where t-BuCzPBI = 1-(4-(3,6-di-t-butylcarbazol-9-yl))phenylbenzo-imidazole, t-BuCzCzPBI = 1-(4-(3,6-di-(3,6-di-t-butyl-carbazol-9-yl))carbazol-9-yl)phenyl-2-phenyl-benzoimidazole, and acac = acetylacetonato ligand, emits green phosphorescence from the metal-to-ligand charge-transfer excited state at room temperature [87]. The electroluminescene performance of the dendrimers is better than a non-dendritic Pt complex, as the dendritic framework limits aggregates and excimer formation. Clearly, the performance improves significantly by a switch of the ligand from acac (8 and 9) to dipivaloylmethane (dpm) (10 and 11) or from a heteroleptic one-sided dendron (10 and 11) to a homoleptic, two-sided dendron (12 and 13) conformation that further restrain aggregates and excimer formation [91]. For instance, the use of [t-BuCzPBI)Pt(dpm)] (10) as a dopant in organic light emitting diode (OLED) results in a current efficiency (CE) of 24.76 cd/A and a maximum external quantum efficiency (EQE) of 7.77 % that are significantly higher than those obtained with 8. Further, a positive dendritic effect on CE and EQE was found as 12 gives a CE and EQE of 29.31 cd/A and 9.04 %, respectively.

The synthetically less-challenging hyperbranched polymers also offer a macromolecular framework for designing photoactive materials [92, 93]. A ferrocene-functionalized hyperbranched polyphenylene (14) and its cobalt complexes have high refractive indices of 1.75 and 1.69 at 633 nm and high Abbé numbers of 436 and 681, respectively [93]. In addition to this rare characteristic, 14 and its cobalt complex also feature extremely low optical dispersion of 0.0023 and 0.0014, respective, depicting them as candidates for photonic applications (Fig. 7).

Fig. 7
figure 7

a Structure of hyperbranched polymer 14; b refractive indices of films of 14 and its cobalt complex. Reproduced with permission from Ref. [93]. Copyright @ 2010, American Chemical Society

Full size image

Inspired by the fascinating functions of porphyrin and phthalocyanine derivatives in nature, material scientists anticipating similar functions in artificial systems, have designed porphyrin and phthalocyanine-containing dendrimers. Of interest from the photophysical perspective is their red-shifted Q-bands in the UV–vis spectra region, which suggest their possible biomedical applications as photosensitizers in light harvesting devices as well as in photodynamic therapy and diagnosis.

Exploiting the iterative synthetic strategies of dendrimers, porphyrins or phthalocyanines has been precisely built into dendritic core, branches and/or termini. Aiming to replicate the wheel-like array of the natural light-harvesting complex, LH2, hexaarylbenzene-cored polyester dendrimers with 6–36 peripheral zinc porphyrins (15–19) (Fig. 8) was designed and systematically probed for its structure–property relationship [94]. Steady-state absorption measurements revealed positive dendritic effects on the molar extinction coefficients of the Q-bands, suggesting better light-harvesting capacity as the number of porphyrins increases. In these dendrimers, the porphyrin moieties emit individually, a conclusion based on the similarity between fluorescence spectra as well as between quantum yields of the different generations of the dendrimers. The authors inferred that the molecular architecture of the dendrimers strongly influences their photophysical properties.

Fig. 8
figure 8

Structures of dendrimers 15–19. Reproduced with permission form Ref. [94]. Copyright @ 2006, WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim

Full size image

Advancing a similar objective, Aida et al. designed a multiporphyrin array (20) that consists of a free-base porphyrin core with four zinc porphyrin dendrons attached (Fig. 9) [95, 96]. In this dendrimer, the zinc porphyrins interact weakly in the ground state as the Q-band negligibly shifts and hardly broadens. Excitation of the zinc porphyrins at 544 nm results in a strong emission at 658 and 723 nm, attributed to the cored, free-base porphyrin, and weak emissions at 589 and 623 nm, attributed to the zinc porphyrins. These results suggest efficient energy transfer from the zinc(II) to the free-base porphyrin [95]. Other studies have addressed this objective by incorporating porphyrins or phthalocyanines as core in dendritic frameworks [97–103].

Fig. 9
figure 9

a Structure of dendrimer 20; b emission spectra of 20 and non-dendritic porphyrin reference. Reproduced with permission from Ref. [95]. Copyright @ 2001, WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim

Full size image

Clearly, the dendritic architecture affords a unique platform for macromolecular design of light-harvesting antennae. Advances into the biomedical field are rigorously pursued as porphyrins and phthalocyanine dendritic polymer-based photosensitizers are proposed for photodynamic therapy (PDT) and diagnosis. Specifically, the near infrared (NIR) optical properties as well as the photoinduced generation of reactive oxygen species (ROS) of phthalocyanine are applied in non-invasive treatment of cancer cells [104–109]. As an example, water-soluble, photostable phthalocyanine-cored glycodendrimer (21) is a strong photosensitizer, exhibiting excellent in vitro photodynamic efficiency with human bladder cancer cell lines, UM-UC-3 and HT-1376 (Fig. 10) [109]. Within 24 h after PDT, 21 imparts higher photoinduced cytotoxicity in UM-UC-3 cells than in HT-1,376 cells despite similar cellular uptake of 21 by the cells. The authors attributed these findings to a temporary higher antioxidant defense against ROS in HT-1376 than in UM-UC-3 cells. Indeed, massive cell death occurs in HT-1376 after 24 h. Clearly, the results highlight the potential of porphyrin and phthalocyanines in PDT (Fig. 10).

Fig. 10
figure 10

a Structure of dendrimer 21; b percentage of TUNEL-positive cells (dead cells) 24 and 72 h after PDT with 21; c fluorescence micrographs of cell death in HT-1376 and UM-UC-3 24 and 72 h after PDT with 21; green is used to visualize dead cells. Reproduced from Ref. [109]. Copyright @ 2014, Pereira et al. (Color figure online)

Full size image

3.3 Organic Chromophores and Luminophores-Based Photoactive Dendritic Polymers

Over the years, the effort of synthetic chemists has led to a rich library of organic chromophores and luminophores that allows material scientists the flexibility and a range of choices in the design of photoactive dendritic polymer for fundamental studies as well as practical applications. A recent addition to the library is the family of boron-dipyrromethanes (BODIPYs) dyes, known for their small Stokes shift, sharp excitation and emission maxima and environment-independent, high fluorescence quantum yield [110, 111]. Despite these remarkable properties, some BODIPY dyes are insoluble in water; therefore their utilization in aqueous environment is limited. The relative solubility of dendrimer offers an attractive solubility enhancement platform for these dyes. Indeed, aqueous solubility is successfully conferred on a BODIPY dye, Keio Fluor, using a water-soluble polyglycerol dendrimer (22) [112]. In aqueous medium, 22 absorbs and emits at 685 and 705 nm, respectively (Fig. 11), photophysical fingerprints comparable to a free Keio Fluor or its analogues. With molar extinction coefficient and fluorescence quantum yield of 190 500 M−1 cm−1 and 0.57, respectively, 22 is speculated to be the brightest water-soluble far-red emitting organic fluorophore [112]. Of practical relevance is the noticeable photostability, low level of blinking as well as bright, long-lasting fluorescence of 22 observed during a single-molecule imaging experiments, suggesting its possible application in single-molecule imaging.

Fig. 11
figure 11

a Structure of dendrimer 22; b absorption and emission spectra of 22. Reproduced with permission from Ref. [112]. Copyright @ 2013, rights managed by Nature Publishing Group

Full size image

BODIPY dyes-based photoactive materials are of continuing interest [113], and their dendritic analogues are being designed for use as solar energy harvesting antennae [114–116] and as well as in biomedical applications [117, 118]. For instance, through a rational synthetic approach, a solar energy harvesting dendritic platform consisting of BODIPY dyes at the core, branches, and peripheral is designed and optimized to absorb strongly in the visible region and through efficient energy transfer cascade, emits only from the core [115]. The peripheral, branched, and cored BODIPY moieties absorb at 527, 590, and 655 nm, respectively. Emission and excitation spectra confirm energy transfer from the peripheral BODIPY moieties to that in the core. As evidenced from comparable excitation and emission spectra of this dendrimer and those of a reference that consists of a mixture of BODIPY dyes, most of the emission in the former emanates from the longer wavelength, which corresponds to emission from the cored BODIPY moiety, whereas in the latter, the emission originates from the shorter wavelength. Further, with UV irradiation of the epoxy resin disc containing the reference or the dendrimer, efficient energy transfer is further substantiated as the former disc emits green light and the latter emits bright red [115].

Recently discovered in the group of Tang, aggregation-induced emission (AIE) has emerged as a practically relevant luminophoric phenomenon [119–121]. Simply defined, AIE involves emission due to aggregate formation and is exhibited by a number of organic luminophores. As many photoactive materials are used in the solid state, where aggregates are more likely to be formed, this property is therefore desirable. Tang’s group has designed several hyperbranched polymers that exhibit these properties. For instance, the group reported hyperbranched poly(2,5-silole)s, illustrated by 23 (Fig. 12), that are nonemissive or weakly fluoresces in their good solvents but exhibit high emission when aggregated in their poor solvents [122]. Compared with their monomers, these polymers feature higher fluorescence quantum yields of 3.0 or 5.4 % in THF. With the addition of water to the THF, the quantum yield increases due to AIE. Specifically, at 90 % v/v water fraction, the polymers have quantum yields of 15.4 or 18.4 %. Additionally, these polymers exhibit strong solid-state emission. The emission of the spin-coated polymer films blue shifted from those of their amorphous powders due to the more twisted molecular conformation in the polymers in the film. The addition of an increasing amount of a model explosive, picric acid, to nanoparticle suspension of the dendrimer in THF/water (1:9 v/v) mixture led to a decrease in emission intensity of the polymer (Fig. 12). This finding suggests the potential application of these polymers as explosive detector. Although recently discovered, AIE is exhibited by a number of hyperbranched polymers that include poly(tetraphenylethene)s [123], poly(aroxy-carbonyltriazole)s [124, 125], poly(silylenevinylene)s [126], poly(silylenephenylene)s [122], polytriazines [127], polytriazoles [128–130], poly(aryleneethnylenesilole)s [131] and poly(aryleneethynylene)s [132]. These polymers have been shown to be useful in explosive detection, photopatterning, and optical power limiting as a result of the presence of AIE.

Fig. 12
figure 12

a Illustrated structure of hyperbranched polymers 23; b emission response of THF solution of 23 to various concentrations of picric acid. Reproduced with permission from Ref. [122]. Copyrights @ 2010, American Chemical Society

Full size image

The sensitivity of AIE to steric hindrance facilitates fundamental investigations of dendritic architectures and conformation. Using the AIE phenomenon, the Tang’s group recently probed peripheral crowding in a series of long and short ethylene oxide and tetraphenylethylene decorated dendrimers, depicted by 24 (Fig. 13a) [133]. All the dendrimers emit in the solid-state under UV irradiation. However, in THF, some dendrimers are fully solvated and therefore nonemissive. With the addition of water to the THF solution, aggregation is induced and emission is observed (Fig. 13). Evidently, the critical water fraction in volume percent (f w crit.) needed to induce emission decreases as the dendrimer generation increases due to peripheral crowding that restrains the motion of the tetraphenylethylene moieties as well as a growing intolerance of water at higher generations. As an example, within the short series of dendrimers, the zeroth generation dendrimer only emits at >70 % f w crit., whereas the third generation emits even at 0 % f w crit (Fig. 13). In contrast, the long series of dendrimers has no clear trend as the long ethylene spacer allows rotational freedom. These structure-photophysical property relationships clearly suggest that AIE can probe changes in macromolecular architectures and conformation.

Fig. 13
figure 13

a Structure of 3rd generation, short ethylene oxide-cored and spacers, tetraphenylethylene decorated dendrimers (24); relative emission intensity as a function of water fraction for b short series dendrimers; c long series dendrimers. Reproduced with permission from Ref. [133]. Copyright @ 2014, Royal Society of Chemistry

Full size image

Exploiting the photophysical properties of dendritic polymers to fundamentally investigate their architectures and conformations as well as interactions within dendritic framework is a powerful tool [134–140]. Recently, we established the presence of ground-state as well as excited-state interactions via charge or energy transfer between peripheral, photoactive naphthyl moieties and in-branch redox active organoiron moieties in a new family of redox-, and photoactive dendrimers (25–27) (Fig. 14) [136]. Evidently, the in-branch iron centers distort the spectra shape of the peripheral naphthyl groups and quench the fluorescence intensity; indeed, the removal of the iron centers enhances the emission intensity (Fig. 15). We attributed these findings to the presence of aforementioned interactions. Interestingly, the presence of these interactions suggests that our dendritic architecture is an ideal platform for the design of solar energy harvesting antennae where the peripheral moieties need to transfer energy to energy receptors at the dendritic core or branches.

Fig. 14
figure 14

Structures of dendrimers 25, 26 and 27. Reproduced with permission from Ref. [136]. Copyright @ 2014, WILEY-VCH Verlag GmbH & Co KGaA, Weinheim

Full size image
Fig. 15
figure 15

a Absorption; b emission spectra of dendrimer 25 illustrating the interaction of iron centers with photophysical properties of dendrimers. Reproduced with permission from Ref. [136]. Copyright @ 2014, WILEY-VCH Verlag GmbH & Co KGaA, Weinheim

Full size image

Indeed, various organic chromophores and luminophores such as azobenzenes [24, 141–147], pyrenes [148–150], coumarins [151–153], fullerenes [139,154,155], phosphoramidites [156], and perylenes [157–159] have been used to design dendritic polymers for sensing, solar energy harvesting, optical power limiting, optical storage, and other photonic applications. For instance, we have incorporated azobenzene moieties within macromolecules (27) (Fig. 16) that have dendritic architectures [141]. In this case, the presence of the azobenzene imparts acid sensing properties on these dendritic polymers as the absorption band of 27 at λ max = 435 shift to longer wavelength at λ max = 576 nm in the presence of varying concentrations of HCl. Further, a free-based porphyrin-cored dendrimer (28) (Fig. 17) containing 8 naphthopyranones and 16 coumarins was designed for solar energy harvesting [151]. Excitation of the coumarins in 28 at 335 nm results in no emission from the coumarins, weak emission from the naphthopyranones, and a strong emission from cored porphyrin at 651 and 717 nm. Similarly, excitation of the naphthopyranone at 358 nm leads to predominantly porphyrin emission. These results point to efficient energy transfer from the outermost coumarins to the core porphyrins.

Fig. 16
figure 16

Structure of dendrimer 27. Reproduced with permission from reference Ref. [141]. Copyright @ 2010, Springer Science+Business Media LLC

Full size image
Fig. 17
figure 17

Structure of dendrimer 28. Reproduced with permission from Ref. [151]. Copyright @ 2005, American Chemical Society

Full size image

While a lot of interest is focused on dendrimers, dendrons, and hyperbranched polymers, a number of photoactive dendrigraft polymers have been designed [152, 153]. As an example, coumarins were incorporated into linear-comb and star-comb dendrigraft polybutadienes (29) (Fig. 18) and their photophysical properties investigated in solution as well as in the solid-state [152]. In the solid state, the fluorescence peak at 394–396 nm broadens and additional peaks at 434–437, 470 and 530 nm appear due to emission from excited state dimer and multimer. The dendrigraft architecture enhances the fluorescence properties of the coumarin. Importantly, in addition to enhancing the fluorescence intensity and quantum yield of coumarin, the architecture also affects these properties as the linear-comb exhibits better photophysical properties than the star-comb.

Fig. 18
figure 18

Structure of dendrigraft polymer 29. Reproduced with permission from Ref. [152]. Copyright @ 2013, Royal Society of Chemistry

Full size image

4 Conclusion and Outlook

It is apparent that photoactive dendritic polymers are promising platforms for harnessing light to efficiently do work. Further, the incorporation of photoactive moieties within dendritic structures offers a powerful tool to probe the structure, conformation, and interactions within dendritic polymers. Given the aforementioned, fundamental and applied research in photoactive dendritic polymer continues and will continue to attractive a lot of interest in the future. The majority of the interest is focused on dendrimers, dendrons, and hyperbranched polymers. The dendrigraft architecture is currently underexplored, and so should be studied.