Efforts to tune optic responses of molecular aggregates often alter the characteristics and performance of functional materials, revealing that chemical properties largely rely on molecular stacking and interactions. Although the intrinsic nature of materials is primarily determined by single-molecule structures, molecular aggregation behavior that determines material property resembles the architectural style of a building in which the bricks themselves could be less important. While the establishment of surface-enhanced Raman spectroscopy (SERS) inspired numerous research interest for trace analysis up to single-molecule level, Raman spectroscopy is also recognized for its importance in solving several issues relating to molecule aggregates owing to the capability of non-destructive detection and spectral fingerprints by which chemical structures and aggregation states can be identified. Raman spectroscopy is not only applied to identify chemicals at the gas phase, liquid phase and solid state and to monitor in-situ reactions of materials at reduced sizes but also to probe gas-to-particle conversion in aerosols, microcrystal magnetization and phase transition at aggregated states, which are believed to attract uprising research interest in the near future.
- molecular aggregates
- Raman spectroscopy
- aggregation-induced resonance
Aggregation structures such as those of hydrophobic and hydrophilic surfactant molecules widely exist in aqueous solutions, micelles, liquid crystals, various membranes, and biological systems and are important for understanding physical and chemical properties and functions. Extensive investigations have demonstrated that optic responses of molecular aggregates often determine the property and performance of optical functional materials [1–3]. Also found was that, selective controlling of excitonic states of molecular aggregates profits to engineer optical properties of promising photonic materials described in terms of the model of Frenkel excitons [4–6]. In poor solvents, self-assembly of organic molecules in the form of weakly coupled aggregates display significantly different spectroscopic behavior compared to their monomers . From UV-vis spectral analysis based on Kasha exciton theory [8–10], typical aggregation behavior can be determined by checking out a tilt angle of molecular stacking [11–13]. In addition to UV-vis absorption, fluorescence of aggregates has also been meticulously investigated, where the fluorescence intensity of organic molecules often diminishes upon aggregation due to intermolecular interactions [14–18], but allowing for interesting exceptions such as those demonstrated as aggregation-induced emission enhancement (AIEE), revealing relationships between molecular structures/molecular arrangements and emission properties [2, 3, 19–21]. While numerous UV-vis and fluorescence investigations help determine the aggregation behavior, it is important to utilize vibrational spectroscopic fingerprints of molecular aggregates to identify the components and structures, phase transition, likely isomers, and conformation transition of chemicals at reduced sizes [22, 23].
Raman effect arises when light impinges upon a molecule or molecule aggregates and interacts with the electron cloud and chemical bonds. A fascinating world of Raman spectroscopy toward both bulk materials and single molecules has been fully demonstrated (i.e., the two ends), but for molecule aggregates, there are relatively less Raman spectroscopic studies in reported publications so far. A few Raman investigations have shed light on ionic surfactants [24–28] and photogenerating reactions [29–31] of organic molecule assemblies; and in-situ Raman techniques have been utilized to monitor real-time reactions and catalysis [32–36], as well as photo-induced polymerization and magnetization . Recently, it was reported that small organic molecules could form uniform assembly with head-to-tail J-aggregation along the inner walls of the pores of an anodic aluminum oxide (AAO) template, giving rise to an interesting topic for surface-enhanced Raman spectroscopy [11, 13, 38]. Also, there were a few surface-enhanced Raman spectroscopy (SERS) investigations endeavoring to determine the plasmonic property of metal clusters and structural information of molecule aggregates. Raman spectroscopic studies of atomic/molecular clusters and aggregates are expected to become a significant solution to identify chemical structures at primary state of nucleation and growth of materials, gas-to-particle conversion mechanism of aerosols, as well as the aggregation states in liquid crystals, micelles, bilayers/monolayers, and biomembranes. These efforts also help understand the fundamentals in various fields such as catalysis, optics, magnetism, and medicine, etc.
2. Raman theory of aggregated molecules
According to the molecular exciton theory developed by Davydov  and Kasha [8, 10], the aggregation of molecules alters their absorption spectrum, reflecting hypsochromic (i.e., blue-shift) or bathochromic (i.e., red shift) absorption bands, corresponding to H-aggregate (face-to-face, or side-by-side) or J-aggregate (head-to-tail, or linear herring bone) absorptions, respectively. Assuming that different accumulations of molecules have different tilt angles (defined as the angle between the transition dipole and the molecular axis of the aggregate), typical H-aggregates bear a tilt angle value greater than 54.7° and exhibit a broader, blue-shifted absorption band, while J-aggregates bear a tilt angle smaller than 54.7° characterized by a red-shift in the UV-vis spectrum relative to the monomer [11–13]. From UV-vis spectral analysis based on Kasha exciton theory [8–10], simply the approximate tilt angle for accumulation of N molecules can be calculated according to the following equation,
where Δν is the spectral shift from the monomer absorption;
Further, in light of the molecular exciton theory, Akins  reported a study on Raman scattering enhancement theory for a finite aggregate structure consisting of N molecules, assuming the formation of molecular vibro-excitonic levels. The quantum mechanical Hamiltonian describing the internal system of the N molecules can mutually interact through a potential term V, which was given by ,
3. Plasmon-free Raman scattering of molecule aggregates
Raman scattering of molecule aggregates, held together by dispersion and electrostatic forces, has been found different from that of non-aggregated monomers and bulk crystals. Moreover, the Raman bands observed for the aggregates could shift from those of the isolated monomers depending on the intermolecular interactions (i.e., strong or weak; covalent or non-covalently coupled interactions). This characteristic enables Raman spectroscopic studies of molecule aggregates to determine phase transition and photo-assisted polymerization. It is worth noting that, upon resonant excitation, the excited states often bring forth new Raman bands associated with lattice motions, e.g., typical motions in the aggregate formation direction. In this section, we emphasize on a few examples of normal Raman investigations of molecule aggregates.
3.1. To determine phase transition
It has been widely recognized that the assembly of target molecules can result in novel responses of Raman spectroscopy. For example, when examining Raman spectra of barium dialkyl phosphates at various chain lengths, Okabayashi et al.  found that the PO2− symmetric stretching mode of dipentyl phosphate appears at 1106 cm−1 for the liquid crystal state formed at room temperature, which differs from that in the aqueous solution (ca. 1075 cm−1). Also found was that, the relative intensities of the Raman spectral lines changed sharply at the phase-transition temperature but were found to be a constant below and above the transition point. By fully examining the Raman intensities and Raman shifts of the PO2− symmetric-stretching modes of dipentyl phosphate (1075 cm−1), diester O-P-O and dibutyl phosphate (at 1090 and 1068 cm−1), phase transition and the coexistence of two types of aggregation structures were determined. These Raman investigations illustrated how cation-phosphate interactions are important to form aggregation structures and affect the phase transition in liquid crystals of dialkyl phosphates.
3.2. Aggregation-enhanced Raman scattering (AERS)
A few meticulous investigations dealing with relative Raman intensities and selection rules for aggregates have demonstrated a concept termed “aggregation-enhanced Raman scattering” (AERS) , which was proposed to represent a concept solely for studies of aggregates, which differs from a mechanism based on SERS, resonance Raman scattering, and Mie scattering since these Raman effects ignore the impact of aggregation of molecules. The aggregation of molecules in their ground state may result in enhanced polarizability compared to monomeric species and hence intensified radiation according to the basic principle for a dipole moment
3.3. Resonance Raman effect from aggregation
Resonant Raman spectroscopy (RRS) has been known as a main enhancement strategy to solve the sensitivity issue and to derive Raman labels for applications. For example, Zajac et al.  reported an interesting study on aggregation-induced resonance Raman optical activity (AIRROA) of astaxanthin (3,3′-dihydroxy-β,β-carotene-4,4′-dione, AXT), a chiral xanthophyll that was known to bear high antioxidant potency beneficial to cardiovascular, inflammatory, immune and neurodegenerative diseases. Along with striking differences of UV-vis spectra of the monomer and aggregates, Raman spectra (Figure 1) demonstrated spectral changes (e.g., 5 cm−1 blue-shift of the C═C stretching mode, and the appearance of a new band at ~280 cm−1) due to J-aggregation of astaxanthin along with a quenched fluorescence background. Considering the fact of low sensitivity determined by the main limitation of Raman optical activity (c.a., approximately one photon in a billion), resonance effect and aggregation enhancement is important to further advances of AIRROA investigations.
Recently, Gaufrès et al.  reported an interesting study on encapsulated and aggregated dye molecules inside single-walled carbon nanotubes , where giant Raman scattering effect was discovered. Raman measurements for the rod-like dyes (α-sexithiophene and β-carotene) assembled in single-walled carbon nanotubes exhibit highly-polarizable J-aggregates, as shown in Figure 1, giving an enhanced resonant Raman cross-section above that required for detecting individual aggregates. It was found that the shielding of carbon nanotube enables fluorescence-free background and photobleaching-free Raman signals, allowing the giant Raman effect used as functionalized nanoprobe labels for Raman imaging with robust detection using multispectral analysis. Beside this, there are also a few other interesting research papers dealing with Raman scattering by encapsulated molecules in carbon nanotubes, where the formation of aggregates give rise to well-resolved Raman spectra due to interaction and charge transfer within the carbon nanotubes [56, 57].
3.4. Magnetic field-trapped Raman scattering
The exploration of magnets and magnetism is associated with human history. Recently, Luo et al.  reported an interesting photo-assisted method to magnetize microcrystal fullerene C60 at room temperature by exciting C60 molecules to triplet states via proper laser radiation and then trapping the spin-polarized states under a strong magnetic field (Figure 2). Raman spectroscopy was found an operative probe due to its fingerprint spectra regarding energy levels and molecular states, and the crystalline form of C60 molecule aggregates is held together by van der Waals forces allowing the conversion to polymeric phase under proper laser radiation . As results, novel changes on Raman scattering of micro-crystal solid C60 were discovered in the presence and absence of the magnetic field; also found was that the Raman spectra exhibited “hysteresis” phenomenon when the external magnetic field was removed. Together with first-principles calculations which well reproduced the Raman activities of C60 on different states , as seen in Figure 2, photo-assisted magnetization (PAM) of the fullerenes and magnetic-field trapped Raman spectroscopy (MFTRS) were proposed . The PAM strategy with MFTRS verification opens a new approach and, as a general protocol, enables the magnetization of common materials that consist of only light elements. The importance of spin-spin and spin-orbit interactions was also demonstrated in nano-graphene fragments ; Raman spectroscopy also plays an important role in identifying single- and few-layer graphene [60, 61].
3.5. Raman probes for aerosols
Raman spectroscopy is useful in characterizing atmospheric aerosols profiting from the development of portable Raman instrument in recent years [62–77]. For example, Aggarwal et al.  developed a Raman spectrometer using 532-nm continuous wave laser and used for detecting and identifying chemical aerosols of a low-concentration in atmospheric air. As results, they demonstrated the successful application of Raman for trace detection and analysis of iso-vanillin aerosols up to a mass concentration of 1.8 ng/cm3 with the signal-to-noise ratio at about 19 in 30s for the 1116-cm−1 mode with a decent Raman cross section of 3.3 × 10−28 cm2 at the use of 8-W double-pass laser power. Among others, Batonneau et al.  reported an interesting study on heterogeneous chemistry of aerosol particles utilizing Raman mapping and spectroscopic method, which was found in agreement with elemental images obtained by X-ray-mapping. An et al.  conducted a systematic study to identify a few typical organic compounds (isoprene, terpenoids, pinenes etc.) which are known as the main sources of organic aerosols (OAs) particle matter in air pollution. Raman and IR spectra of isoprene, terpenoids, pinenes and their mixtures were examined showing distinguishable vibrational spectroscopic fingerprints of the three components respectively. It was noted that, in a certain case such as β-pinene, a dimer model reproduces the experimental results other than single molecule modeling, indicating nonneglectable intermolecular interactions and aggregation states for aerosols challenging the present mechanisms based on single molecule theory. Further, Raman spectra from an ambient sample can be analyzed using a hierarchical clustering method to check out whether the spectra of aerosols in consistence with relating organic compounds. In particular, analysis on time-resolved aerosol Raman spectra over the course of several hours, simply by checking the D-G bands of amorphous carbon plotted vs time (e.g., a half-hour intervals), enables to monitor and judge the increase/decrease of related pollution in atmosphere [78, 80, 81].
Recently, Davies and Wilson  employed an aerosol optical tweezer technique for contactless levitation of single droplets (e.g., 3–6 μm in radius) and then for Raman investigations, as shown in Figure 3. Flexible environmental control system allows for rapid exchange of the gas-phase humidity source between H2O and D2O (Figure 3) to monitor the progression of the droplet composition using Raman spectroscopy. Utilizing a model describing diffusion in a sphere (i.e., solution to Fick’s second law), they analyzed the data by varying diffusion coefficients (
4. SERS systems involving molecule aggregates
Besides normal Raman, abundant SERS investigations have been undertaken by employing thin films of analytes on functional substrates, such as the extensive investigations of Raman and SERS from Langmuir-Blodgett (LB) films which are often associated with intermolecular interactions of aggregates [82–89]. One of the advantages is that the uniform sampling of thin films allows for better signal-to-noise of the SERS spectra from the analytes [82, 86]. Besides extensive investigations of such 2D assemblies of “analytes + nanoparticles” into thin films, previous publications also addressed 1D assemblies of molecular aggregates (analytes) and metal nanoparticles (signal amplifier).
4.1. Template-based uniform assembly
Considering that Raman/SERS measurements at different positions of the samples could take on diversity due to molecule orientation and disorder degree, location and/or “hot-spots” dependence, the uniformity of molecular aggregates or SERS substrate is largely desired in order to get a better averaged collection of Raman signal. Anodic aluminum oxide (AAO) membrane is widely utilized as a versatile template to prepare 1D rodding/tubing and 2D nanoarray ordered structures, [90–95] both of which have found applications to SERS investigations. On one hand, AAO templates are ideal sublayers to filtrate and support noble metal nanoparticles hence forming highly SERS-active systems [96–98]. For another, AAO templates were also utilized to assemble organic molecules (e.g., perylene) for SERS investigations , as shown in Figure 4(left), where highly-ordered arrays of core-shell nano-pillars of Ag-perylene were fabricated simply by preparing perylene nanotubes utilizing the versatile AAO template , and followed by an electrochemical deposition of Ag . Well-resolved Raman spectra with very good signal-to-noise background were obtained for the perylene (originally a large fluorescence yield) at an UV-vis excitation, profiting from the uniform assembly of perylene molecules. Based on the aforementioned theory and Eqs. (1) and (2), it was estimated that the molecular tilt angle is less than 54.7° indicating a head-to-tail J-aggregation of the perylene molecules along the inner walls of the AAO pores . Similarly, high-quality SERS spectra of fullerene C60/C70 were also obtained from ordered arrays of core-shell nano-pillars of Au@C60/C70, as shown in Figure 4(right) . These results evidenced that coincident and uniform assembly of fullerene molecules along the Au nano-rods leads to fluorescence quenching, and the ordered arrays of nano-pillars generate enhanced LSPR and hence remarkable SERS effect up to 10 times of signal amplification compared to the usual SERS results.
4.2. Assemblies of “analytes + nanoparticles”
By utilizing porous polymer monoliths functionalized with Ag nanoparticles as media, Liu et al.  showed a SERS system with assembly of the analyte molecules, as shown in Figure 5. The polymer monoliths composed of porous 3D-structured organic materials were prepared from monomers with unsaturated vinyl groups . The monoliths could have μm- to nm- sized tortuous fluidic channel networks which enable the convection flow for rapid mass transfer while shorten the characteristic diffusion length. Compared to usual colloidal SERS systems, the monolith was demonstrated to concentrate the embedded metal nanoparticles and present a tremendous amount of surface area and more interaction between the analyte and Ag nanoparticles .
The one-dimensional assembly of “analytes + nanoparticles” may not need any supports or templates. For example, Z. Luo found that, micro-fiber assembly of organic molecules such as 2,2′-bipyridyl (22BPY) can be formed by directly injecting saturated solution of the target molecules into ice-cold Ag colloid . This strategy was demonstrated to resemble a reprecipitation method (or named as microfluidic technique) which was widely used to prepare size-controlled organic nanostructures . As shown in Figure 6, the high-quality spectrum suggested that the microfiber assemblies of 22BPY combined with Ag nanoparticles are a highly SERS-active systems differing from SERS of individuals .
4.3. On-the-specimen method
As a powerful technique for trace analysis and detection due to the extremely high sensitivity and rich structural information that it can offer, SERS has been extensively investigated not only on the primal three classes of SERS systems (i.e., metal colloids, electrodes, and island films), but also non-traditional substrates [103–105]. For example, SERS studies involving surface coatings of Ag/Au nanoparticles, as named
Recently, Tian et al. have further pullulated this method in help of shell-isolated nanoparticle-enhanced Raman scattering (SHINERS) technique [113, 114], as sketched in Figure 8. For a typical SHINERS system, Au nanoparticles were coated with ultrathin silica shells and sowing on probed surfaces, where the Au core provides SERS signal enhancement while the silica shell shields the metal core from direct contact with analyte molecules (i.e., prevents the contamination of the chemical system under study) , which differs from general SERS sampling method, simply by adding analytes onto SERS-active substrates or directly mixing the target solution with metal colloids [115–118]. The SHIERS ‘smart dust’ on the analyte surfaces was demonstrated of practical use in a few interdisciplinary research fields, such as inspecting pesticide residues on food and fruit, examining drug security and environment protection accurately and rapidly, and characterizing biological structures.
5. Raman investigations of cluster systems
5.1. Raman scattering of gas-phase clusters
Raman spectroscopy can also provide essential vibrational information of clusters, although only a few investigations have been published so far. For this study, coherent anti-Stokes Raman spectroscopy (CARS) takes advantages of its excellent sensitivity as applied to investigations of isolated water clusters [119–121], but the nonlinearity of CARS experiments remains a challenge to the assignment of cluster sizes and structure identification. It is worth mentioning that Raman activity of gas-phase clusters is associated with free-rotating especially at high temperature, thus the internal rotational motions could largely differ from that obtained by matrix-assisted vibrational spectroscopic measurements where the molecule/clusters could be partially or wholly clamped. Among others, Otto et al.  reported a study of cold water oligomers utilizing spontaneous Raman scattering (SRS) technique and revealed the vibrational dynamics of water molecules, as shown in Figure 9. Analysis of Raman and IR activities of the hydrogen bond-mediated water clusters seeded in rare-gas expansion molecule beam provides insights into the excitonic OH oscillator coupling, as well as vivid information of ultrafast intermolecular energy transfer (which was often suppressed in femtosecond experiments for the condensed phase due to isotopic dilution). Such investigations from gas-phase chemistry enable to determine weakened or intensified Fermi resonance between OH-stretching and OH-bending motion of hydroxide radicals.
5.2. Raman scattering from monolayer-protected clusters
While expanded Raman investigations of gas-phase clusters remain a reasonable challenge, there are several publications addressing the Raman activities of monolayer-protected clusters (MPCs). Considering that the mid infrared region of the spectrum mainly reveals information about the ligand structures (e.g., C─H, N─H, O─H bond etc.) and their interactions with the metal core [123, 124], Raman spectroscopy actually has its advantages to identify low frequency vibrational modes such as S-Au-S stretching, wagging, scissoring, rocking, and twisting. These vibrations are expected to be weak in IR spectrum due to the low polarity (i.e., IR non-active) but likely prominent in the Raman spectrum (i.e., Raman active) [125, 126]. Figure 10 presents a typical example of Au38 and Au25 MPCs, where the vibrations of two clusters containing monomeric (SR-Au-SR) and dimeric (SR-Au-SR-Au-SR) gold-thiolate staples in the metal-ligand interface are addressed. Raman activities of these clusters at different charge state with different protection ligands illustrated influences of cluster sizes and composition with respect to the monomeric and dimeric moieties .
In addition to the small number of experimental Raman investigations of clusters, there are vast theoretical studies relating to cluster systems [128–136]. For example, utilizing time-dependent density functional theory (DFT) calculations, Chen et al.  conducted a detailed Raman study of pyridine adsorbed on M@Au12 and M@Ag12 (M = Mo, W) clusters. They found that, the calculated Raman intensity of pyridine on M@Ag12 at charge transfer (CT) transition excitations were twice as that for pyridine on M@Au12, as shown in Figure 11, and the energies used for SERS excitations (in the region of 1.63–2.10 eV) were largely different from each other. Calculated interactions between the core and shell produced varying and strong CT transitions from the metal clusters to pyridine, which was demonstrated to be responsible for the altered optical properties. Also found was that, the complexes of pyridine on silver-caged clusters are largely tunable with the core compared to gold-caged clusters, providing insights to the silver and gold clusters even at the same sizes.
Regarding to the interesting CT of silver cluster with small organic molecules, recently Chen et al.  have given a study to the interactions between tetracyanoquinodimethane (TCNQ) and two typical silver clusters Ag13 and Ag20, as shown in Figure 12. It was found that charge transfer from silver clusters to TCNQ molecules initiates the Ag─N bond formation at selective sites giving rise to different isomers of the Ag13-TCNQ and Ag20-TCNQ complexes. From a spectroscopic analysis for the two CT complexes mainly on Raman and infrared activities, vivid illustration of electron cloud interactions and the behavior of TCNQ adsorbed on silver clusters was comprehensively demonstrated, along with frontier molecular orbital (FMO) and natural bond orbital (NBO) patterns. The calculated Raman activity for a TCNQ molecule of Ag20 was found consistent with experimental Raman measurement of TCNQ molecules on single-crystal Ag(1 1 1) surface. Further efforts in this field regarding to clusters and complex molecular aggregates are expected to clarify the charge-transfer interactions within building blocks of granular materials [138–140].
In this chapter, Raman spectroscopy is demonstrated of importance in solving scientific issues relating to molecule aggregates and cluster systems profiting from its spectral fingerprints by which aggregation states, phase transition and cluster structures can be identified. Raman theory for aggregated molecules is simply introduced based on molecular exciton theory and Raman scattering enhancement at the formation of vibro-excitonic levels. Next, we summarize the research advances toward both plasmon-free Raman and SERS systems, such as aggregation-enhanced Raman scattering (AERS), resonance Raman (RR) effect from aggregation, magnetic-field trapped Raman scattering (MFTRS), shell-isolated nanoparticle-enhanced Raman scattering (SHINERS), and Raman probes for aerosols, etc. With the development of scientific instrumentation, the importance of Raman spectroscopy toward precise-sized molecule aggregates and cluster systems will be more clearly embodied, enabling to step toward interdisciplines of cluster science, molecular science, material science and surface science.
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