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. 2026 Mar 16;11(12):19406–19414. doi: 10.1021/acsomega.5c12992

Roles of π‑Conjugation and Intermolecular Interactions in Molecular Raman, Fluorescence, and Lifetime Spectroscopy

Qiyuan Yu †, Sibing Chen ‡, Yansong Liu †, Chunlong Wu †, Yexin Shi †, Xing Chen ‡,*, Hong-Ying Gao †,*
PMCID: PMC13044622  PMID: 41939403

Abstract

Understanding the relationship between molecular structure at the microscale and spectral behavior is essential for designing functional materials. Here, we report how the chemical structure, π-conjugation, and intermolecular interactions jointly modulate Raman spectroscopy, fluorescence, and excited-state lifetimes of seven molecules. The charge-coupled device (CCD) camera and field-emission scanning electron microscopy (FE-SEM) image the powders of these molecules to reveal the microscale particle crystal morphology. In contrast, a scanning tunneling microscope (STM) images the molecules to reveal their intermolecular interactions (hydrogen bonding, halogen bonding, and ionic interaction) during crystallization. The Stokes and anti-Stokes Raman spectra of 2,6-naphthalenedicarboxylic acid, biphenyl-4,4′-dicarboxylic acid, and dodecanedioic acid powders all exhibit distinct Raman signatures of their carbon skeletons. With increasing the laser excitation power, these Stokes Raman peaks show a linear power relationship, while the anti-Stokes Raman peaks show a second-order polynomial (nonlinear) power dependency. In comparison, 2,2′:6′,2″-terpyridine-4,4′,4′′-tricarboxylic acid does not show any Stokes or anti-Stokes Raman peaks, while 6,6′′-dibromo-2,2′:6′,2′′-terpyridine (DT) shows obvious Stokes Raman peaks. The DT’s Raman peaks show a second-order polynomial (nonlinear) power dependence. Furthermore, sodium acrylate shows a series of Stokes and anti-Stokes Raman peaks, with predominantly linear power dependency. However, sodium propiolate shows much weaker Stokes and anti-Stokes Raman spectra. In general, density functional theory (DFT)-calculated Raman spectra of the seven molecules show good agreement with the experimental Raman spectra. Further, the fluorescence and lifetime spectra of these seven molecules are measured, which could also reflect the properties of their chemical structure, π-conjugation, and intermolecular interactions. Moreover, the attenuated total reflection Fourier transform infrared spectroscopies (ATR-FTIR) of these seven molecules also verify the carboxylic acid or carboxylate fingerprints, agreeing well with Raman spectra. This work provides an integrated Raman, fluorescence, ATR-FTIR, CCD, FE-SEM, and STM study of organic molecules to reveal their chemical structure, π-conjugation, and intermolecular interaction differences.

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Introduction

The photophysical responses of molecules are governed by their electronic structures and intermolecular interactions. Understanding these structure-dependent spectral properties is essential for the rational design of organic materials in sensing, photonics, and electronics. , Both Raman and fluorescence spectroscopies are highly sensitive and widespread to subtle variations in molecular geometry, , π-conjugation degree, − and intermolecular forces, − providing a powerful spectroscopic tool to probe structure–property relationships. Generally, aromatic compounds with extended π-conjugated systems typically exhibit enhanced Raman scattering , alongside distinct fluorescence characteristics such as red-shift and narrowband emission. , In contrast, flexible aliphatic chains often yield weakened optical signals due to the increased vibrational degrees of freedom and nonradiative decay pathways. , However, systematic comparisons across closely related moleculeswhere π-framework characteristics, flexibility, and substituents are variedremain challenging, making it difficult to understand how these parameters cooperatively determine the optical behaviors of molecules in condensed powders. ,,

To explore molecular interactions, we combine molecular-scale STM imaging with field-emission scanning electron microscopy (FE-SEM) morphology as characterization approaches. STM visualizes the self-assembly structure of molecules at the subnanometer scale, which provides insights into the dominant intermolecular interactions. In contrast, FE-SEM provides a micrometer-scale morphology of the molecular powder, which may reveal its crystalline morphology.

Although STM images represent interfacial two-dimensional monolayer assembly rather than three-dimensional crystal packing, they should reflect the same intermolecular interactions. , In parallel, powder optical spectroscopies report ensemble-averaged responses of microcrystalline powders. Together, these complementary views enable a multiscale rationalization of the molecules. For clarity, this work “π-framework/π-delocalization” refers to electron delocalization within aromatic rings or unsaturated π fragments (e.g., CC or CC units) that directly contribute to its optical response and corresponding electronic transitions.

In this work, we investigate seven structurally related organic molecules, which systematically vary in (i) π-framework characteristics and backbone rigidity/flexibility (primarily tuned in the diacid series), (ii) the substituents, and (iii) the transition from carboxylic acids to two carboxylate salts. The exact molecular chemical structures and abbreviations are listed in Figure . 2,6-Naphthalenedicarboxylic acid (NDCA), biphenyl-4,4′-dicarboxylic acid (BDA), and dodecanedioic acid (DDA) carry the same terminal carboxylic groups but have different aromaticity and conformational flexibility (naphthalene vs biphenyl vs aliphatic chain). − 2,2′:6′,2″-Terpyridine-4,4′,4′′-tricarboxylic acid (TATP) and 6,6″-dibromo-2,2′:6′,2′′-terpyridine (DT) carry the same π-conjugated carbon backbones but differ in functional substituents (carboxyl vs Br), enabling a comparison of substituent effects. Sodium acrylate (SA) and sodium propiolate (SP) carry the same Na+ ions but differ in their unsaturated backbone (CC versus CC). Through a comparative analysis of these molecules, we elucidate how functional substituents, π conjugation, and intermolecular interactions act in concert to modulate their optical behaviors.

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Molecular structures of the compounds investigated in this study. (a) NDCA, BDA, and DDA; (b) TATP and DT; and (c) SA and SP.

Materials and Methods

All chemicals were purchased from commercial suppliers. Detailed information about the molecular materials can be found in the Supporting Information. CCD images of seven molecular powder samples were acquired by a commercial CCD camera. SEM characterization of the powder’s morphology was performed on a field-emission scanning electron microscope (Hitachi Regulus 8100, Japan) using secondary-electron imaging. Powder samples were mounted on conductive carbon tape (sputter-coated with a thin Pt layer to mitigate charging) and imaged at an accelerating voltage of 3.0 kV. All STM measurements were conducted by a ScientaOmicron low-temperature UHV STM instrument at 78 K with a base pressure of 1 × 10–10 mbar. Tungsten tips were prepared by electrochemical etching. STM images were acquired in the constant-current mode, with the tip grounded and a bias voltage applied to the sample. The Au(111) and Cu(111) surfaces were cleaned by multiple cycles of argon-ion sputtering (1 kV, 10 mA, 15 min) and thermal annealing (630 °C, 15 min for Au(111) and 530 °C, 15 min for Cu(111), respectively). The annealing process was monitored by both infrared and thermal coupling detectors. The NDCA, BDA, DDA, TATP, and DT molecules were deposited onto clean Au(111) surfaces by an organic molecule evaporation source (Quad Cluster Source, QCS), and their sublimation temperatures are 543 K, 393 K, 323 K, 429 K, and 353 K, respectively. The SA and SP molecules were evaporated from a homemade funnel , at room temperature onto the Cu(111) and Au(111) surfaces held at 214 and 258 K, respectively. The cold surfaces were maintained by cooling the STM manipulator head with a flow of cold nitrogen gas.

All spectral data in this study were measured by a home-built optical measurement system, as shown in Figure S1. Raman measurements were performed with continuous-wave (CW) lasers of 532 and 633 nm (Cobolt). The laser beam was guided onto the sample surface by a reflecting mirror, a cube beam splitter (CSMH-25–550, Sigma), and an objective lens. The backscattered Raman signals were collected by an optical fiber through the same beam splitter, a notch filter (532 nm, Sigmakoki or 633 nm, Chroma), and a plano-convex lens. The Raman signal was then guided into a spectrograph (SpectraPro HRS-500, Princeton Instruments) equipped with a liquid-nitrogen-cooled CCD camera (PYL-400BRX, Princeton Instruments) for spectral analysis. To improve the signal-to-noise ratio and spectral accuracy, all Raman spectra were manually baseline-corrected. Detailed procedures are provided in the Supporting Information.

ATR-FTIR spectra of the solid powders were collected using a Fourier transform infrared spectrometer (Thermo Fisher Scientific, Nicolet iS50). The FTIR data are provided in the Supporting Information.

Fluorescence and lifetime spectroscopy measurements were conducted with a pulsed diode laser of 402.9 nm (PicoQuant) as the excitation source. To suppress the scattering background, a 405 nm notch filter (ZET405NF, Chroma) and a 409 nm long-pass filter (LOPF-25C-409, Zolix) were inserted into the collection optical path. Note that the lifetime spectroscopy measurements were performed by directing the fluorescence with an additional mirror and a plano-convex lens onto a single-photon avalanche diode (SPAD) detector. Then, the fluorescence was analyzed by a MultiHarp 150 (4P, PicoQuant) with a time-correlated single-photon counting (TCSPC) technique, enabling precise lifetime measurement.

Results and Discussion

CCD Images of Molecular Powders

The powder samples of seven molecules were acquired with a commercial CCD camera, as shown in Figure a–g. These images provide a millimeter-scale view of molecular powders for subsequent Raman, photoluminescence, fluorescence lifetime, and ATR-FTIR measurements. Note that the NDCA, BDA, DDA, DT, and SA molecules are white powders; in contrast, the TATP and SP molecules show obviously dark (TATP) and light brown (SP) colors. Figure h illustrates the sample preparation procedures before CCD imaging. The samples were prepared by a double-glass-slide-pressing method, in which the powder was sandwiched between two glass slides and gently pressed. The top glass slide was then removed, and the sample was imaged with a CCD camera.

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CCD images of the molecular powders on a glass plate (a–g) and a schematic diagram of the sample preparation and imaging setup (h). Note that a white sheet was placed under the TATP glass plate to improve the contrast of imaging due to its dark brown color.

FE-SEM Images of Molecular Powders

To prove the crystal structures of the seven molecular powders, we further conducted FE-SEM measurements. FE-SEM images (Figure and Figure S2) show these powders with distinct crystal particle features (layered plate-like structures). The NDCA powder consists of many crystal particles with a layered, plate-like structure (typically several tens of micrometers). In contrast, the BDA powder shows much smaller particles but still with a layered, plate-like structure. DDA powder exhibits thick lamellar blocks with smooth facets. TATP powder appears as irregular fractured particles with a broad size distribution (still showing some layered plate-like structures, as marked by the white dashed circle), while DT powder shows large flat plates. The carboxylate salts SA and SP mainly present layered, stacked, plate-like aggregates at the tens of micrometers scale. Overall, FE-SEM images of these seven molecules support their crystal powder morphologies, complementing optical images and optical spectroscopic measurements.

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FE-SEM images of the powder morphologies of the seven molecules: (a) NDCA, (b) BDA, (c) DDA, (d) TATP, (e) DT, (f) SA, and (g) SP. Scale bar: 3 μm.

STM Images of the Submonolayer Self-Assembly Structures of Molecules

To explore the intermolecular interactions at the nanoscale, we continuously measured STM images of the seven molecules on metal surfaces. Their corresponding STM images are shown in Figure and SI-Figure S3, with the overlaid molecular chemical structures and the proposed self-assembly arrangements. Note that the SA molecules could not be grown on the cold Au (111) surface but can be grown on the cold Cu(111) surface, as shown in Figure f.

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STM images of molecular self-assemblies on metal surfaces. NDCA (a), BDA (b), DDA (c), TATP (d), DT (e), and SP (g) were deposited on the Au(111) surface, while SA (f) was deposited on the Cu(111) surface. The periodic unit cells in (a-e) are marked by the vectors a and b and angle θ. The scan sizes are 5 × 5 nm2 for (a-e), (f-ii), and (g-ii), and 20 × 20 nm2 for (f-i) and (g-i). The corresponding imaging parameters are: a, −1.5 V, 30 pA; b, −1.5 V, 50 pA; c, −0.5 V, 200 pA; d, −2.0 V, 50 pA; e, −1.0 V, 100 pA; f-i, −1.5 V, 100 pA; f-ii, −1.0 V, 100 pA; g-i, −2.0 V, 10 pA; g-ii, −2.0 V, 10 pA. The white, gray, red, brown, blue, and purple balls represent H, C, O, Br, and N atoms, and the Na+ ion, respectively.

For molecules containing intact carboxylic acid groups (Figure a–d), ordered supramolecular assemblies are observed, which are primarily stabilized by −H···O hydrogen bonding. Additionally, the molecular backbone structures also contribute to the intermolecular interactions. For instance, the NDCA and BDA molecules exhibit π–π stacking interactions of aromatic phenyl rings (Figure a,b), whereas the alkyl chains of DDA interact through van der Waals forces (Figure c). These STM images are consistent with crystallographic reports of the corresponding diacids: NDCA/BDA are organized mainly by directional O–H···O hydrogen bonding between carboxylic acid groups, whereas dodecanedioic acid shows carboxylic acid hydrogen bonding together with van der Waals packing of the flexible aliphatic chains, matching the van der Waals-dominated chain interactions observed for DDA on the surface. ,, For TATP molecules, the formation of O–H···O/N hydrogen bonding between the carboxylic acid group and another carboxylic group/pyridinic nitrogen atom facilitates the stabilization of either an ordered or disordered self-assembly structure (the disordered self-assembly structure is shown in SI Figure S3d). In contrast, the DT molecules, in which the carboxylic acid groups are replaced by two bromine atoms, form well-ordered self-assemblies driven by intermolecular Br···Br halogen bonding. The three-dimensional unit-cell parameters of NDCA, BDA, and DDA have been reported in the literature, ,, and in contrast, NDCA, BDA, DDA, TATP, and DT exhibit periodic monolayer assemblies in our STM measurements that can be described as follows: NDCA (a: 0.69 nm, b: 1.09 nm, θ: 59.5°), BDA (a: 0.74 nm, b: 1.31 nm, θ: 48.6°), DDA (a: 0.46 nm, b: 1.66 nm, θ: 85.3°), TATP (a: 1.41 nm, b: 1.91 nm, θ: 62.4°), and DT (a: 0.93 nm, b: 1.57 nm, θ: 39.2°). Meanwhile, the SA and SP molecules exhibit only disordered molecular clusters on the substrate surface (Figure f,g).

ATR-FTIR Verification of Bonding States (Carboxylic Acids vs Carboxylate Salts)

To verify the bonding states of the carboxyl functionalities in the molecular powders, ATR-FTIR spectra were collected for all seven compounds (Figure S4). The results confirm that NDCA, BDA, DDA, and TATP are present as hydrogen-bonded carboxylic acids, whereas SA and SP are present as carboxylate salts. In contrast, DT shows no carboxyl-related bands. This verification provides the same chemical basis as the Raman and photophysical comparisons discussed below.

Raman Spectra of NDCA, BDA, and DDA

This study first focuses on the influence of π-conjugation extent on the Raman vibrational peaks of carboxyl groups and carbon frameworks. To this end, three dicarboxylic acids were selected: NDCA, BDA, and DDA. Raman spectra of these three compounds were measured by a 532 nm laser. After the manual baseline correction (Figure S5), their characteristic Stokes, anti-Stokes, and DFT-calculated spectra are presented in Figure a–c. Although all three molecules exhibit peaks corresponding to CO vibrations in DFT calculations (blue lines in Figure a–c): NDCA (∼1730 cm–1), BDA (∼1728 cm–1), DDA (∼1752 cm–1), it is noteworthy that no distinct CO vibrational peak was detected in NDCA, BDA, and DDA molecular Raman spectra, consistent with previous research. −

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Experimental and theoretical Raman spectra of (a) NDCA, (b) BDA, and (c) DDA molecules. The experimental spectra are acquired by a 532 nm laser and manually baseline-corrected. DFT calculations were carried out by the B3LYP/6–311++G­(d,p) approach. The Stokes shift, anti-Stokes shift, and DFT-calculated spectra are shown in colored black, red, and blue, respectively. Vertical offsets are applied for clarity. (d, e) Power dependence of the selected Stokes (d) and anti-Stokes (e) Raman peaks at the same integration time is also shown. The power dependencies of Stokes peaks are fitted by a linear function, while those of anti-Stokes peaks are fitted by a second-order polynomial.

Next, the Stokes (anti-Stokes) Raman peaks of carbon frameworks are also identified, as shown in Figure a–c (black spectra). The NDCA molecules exhibit two distinct and intense peaks at 1386 cm–1 and 1636 cm–1 (anti-Stokes −1394 cm–1 and −1642 cm–1), primarily attributed to aromatic C–C stretching and C–H bending vibrations. Additionally, the C–H in-plane bending modes are observed at 1117, 1237, and 1300 cm–1, supporting the vibrational activity of the aromatic framework. , In contrast, the BDA molecules display a strong peak at 1611 cm–1 (anti-Stokes, −1615 cm–1, the C–C stretching) and a peak at 1302 cm–1 (the inter-ring C–C stretching), along with characteristic peaks at 1143 cm–1 and 1199 cm–1 (C–H in-plane bending). ,, Overall, both π-conjugated NDCA and BDA molecules exhibit well-defined C–C skeletal vibrations, consistent with their DFT-calculated Raman spectra. In contrast, the DDA molecules display typical aliphatic C–C stretching vibrations at 1069 and 1116 cm–1 (anti-Stokes −1064 and −1111 cm–1). At lower wavenumbers (<1000 cm–1), the NDCA molecules show skeletal C–C–C vibrations at 525 and 770 cm–1 (anti-Stokes: −520 and −766 cm–1), consistent with their rigid π-conjugated structure. The BDA molecules show an out-of-plane skeletal vibration of the phenyl ring at 413 cm–1 (anti-Stokes: −406 cm–1). Meanwhile, the DDA molecules show weak C–C–C bending modes at 443 cm–1 (anti-Stokes at −436 cm–1) and a characteristic polyethylene-type peak at 914 cm–1 (anti-Stokes at −907 cm–1). Importantly, the DDA molecules show aliphatic-specific peaks of CH2 twisting vibration at 1302 cm–1, scissoring vibration at 1406 cm–1, 1428 cm–1, and symmetric stretching in the range 2849 cm–1, 2887 cm–1, and 2913 cm–1. Additionally, for the NDCA and BDA molecules, weak C–H stretching vibrations of aromatic rings are observed at 3064 and 3090 cm–1 (NDCA) and 3080 cm–1 (weak, BDA), respectively. Note that the nondistinct Raman peaks of NDCA, BDA, and DDA molecules are not discussed and are shown in Figure a–c (black and red spectra). Furthermore, the Stokes and anti-Stokes Raman spectra of NDCA, BDA, and DDA molecules by the 632 nm laser reveal similar peaks (Figure S6).

To explore the power-dependency of Raman peaks, the representative skeletal C–C modes are selectively analyzed, as shown in Figure d,e. The Stokes peaks exhibit a linear dependence on the laser’s power (Figure d), consistent with the theoretical expectation under nonsaturating conditions.3 Notably, molecules with stronger π-conjugated structures (e.g., NDCA and BDA) exhibit a more pronounced intensity growth gradient along with the excitation power increase, which can be attributed to their significantly enhanced Raman scattering cross sections compared to the nonconjugated DDA. In detail, the power-dependency slopes of the 1386 and 1636 cm–1 peaks of NDCA, and the 1302 and 1611 cm–1 peaks of BDA, are greater than those of DDA. In contrast, the anti-Stokes peaks show a second-order polynomial power dependence (Figure e). Figure S7 shows more details on the power dependence of the NDCA, BDA, and DDA Raman peaks.

These results indicate that aromatic π-delocalization in the carbon framework plays an important role in the Raman response of the organic molecules. The degree of π-conjugation not only influences the vibrational frequencies and mode distributions of carbon backbones but also mediates the sensitivity of the Raman signal to excitation power.

Raman Spectra of DT and TATP

To explore how the substituent modulates Raman behaviors, two terpyridine-based molecules, DT and TATP, were measured and compared. To our surprise, the TATP molecules do not show any detectable Raman peaks in either the Stokes or anti-Stokes regions. In contrast, the DT molecules show only well-defined Raman peaks in the Stokes region, as seen in Figure , and no peaks in the anti-Stokes region. The effective suppression of Raman response for TATP is likely due to enhanced hydrogen bonding between neighboring carboxyl groups as well as between carboxyl groups and pyridine N atoms. They may reduce the vibrational freedom and thereby reduce the Raman response. The DFT-calculated TATP spectra are shown in Figure S8, which supports the idea that intermolecular interactions do influence the Raman spectra.

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(a) The experimental and theoretical Raman spectra of DT molecules. The Raman spectra are acquired by a 532 nm laser and manually baseline-corrected. DFT calculations are obtained by the B3LYP/6–311++G­(d,p) approach. The Stokes Raman spectrum and DFT-calculated spectrum are shown in black and blue, respectively. Vertical offsets are applied for clarity. (b) Power-dependence plots for selected Raman peaks at the same integration time are fitted by a second-order polynomial function.

In detail, the DT Raman peak at 656 cm–1 corresponds to the ring and C–Br stretching modes. The Raman peaks at 990 and 997 cm–1 correspond to the classic “ring breathing” mode of pyridine (∼990 cm–1). The Raman peaks at 1332 and 1458 cm–1 correspond to the C–C, C–N stretching and C–H bending in-plane modes. Meanwhile, two strong Raman peaks at 1565 and 1586 cm–1 are attributed to C–N stretching, C–C stretching, and H–C–C out-of-plane torsion vibrations together.

Further, the power-dependency plots of DT Raman peaks are shown in Figure , which are fitted by a second-order polynomial function. To our surprise, the power-dependency of DT Raman peaks exhibits a sublinear growth trend with increasing laser power, indicating a nonlinear Raman response. In contrast, the superlinear increase of anti-Stokes power-dependency of NDCA, BDA, and DDA molecules may originate from the resonant optical absorption associated with the fluorescence process, which enhances local heating and vibrational population at proper powers. Additionally, the same Raman spectra measurements of DT and TATP were conducted with a 633 nm laser. Unfortunately, neither DT nor TATP molecules exhibit Raman responses at 633 nm excitation (Figure S9).

Raman Spectra of SA and SP

Considering that the SA and SP are carboxylate salts and are probably polymerized under optical excitation, − the detailed Raman spectra and power-dependence analyses for SA and SP are provided in the Supporting Information (Figures S10–S12). Briefly, SA shows readily detectable Raman features, including a band in the CC stretching region, whereas SP exhibits a much weaker Raman response under identical acquisition conditions.

Molecular Fluorescence and Lifetime Spectra

Subsequently, we systematically measured the fluorescence spectra and lifetimes of these compounds, and the results are shown in Figure . Note that the corresponding data for SA and SP are provided in the Supporting Information (Figure S13). It is believed that the differences in molecular structure and intermolecular interactions will not only influence the Raman spectra but also regulate the fluorescence behaviors of organic molecules. The fluorescence lifetime decay curves were analyzed by the Easy Tau software package. The fitting was performed with the fast Marquardt–Levenberg optimization algorithm based on an exponential tail fitting model. All decay profiles were fitted by a two-component exponential function to obtain the characteristic lifetime components and their corresponding amplitudes. The fluorescence lifetime fitting results are summarized in Table and Figure b and d, while the fluorescence lifetime fitting parameters for SA and SP are summarized in Table S1. The two-component exponential function and the percentages of each component are described as follows:

I(t)=I0+A1e−tτ1+A2e−tτ2
In=Anτn⁣(n=1or2)
Pn=InI1+I2×100%⁣(n=1or2)

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Fluorescence spectra (a, c) and lifetime decay curves (b, d). For better comparison, the fluorescence spectra of DDA in (a) and TATP in (c) are multiplied by 30 and 2, respectively. All fluorescence lifetime curves were normalized to a peak intensity of 10,000. Horizontal offsets were applied to improve clarity and comparison.

1. Summary of Biexponential Fitting Parameters for the Transient Fluorescence Decay Curves of NDCA/BDA/DDA and TATP/DT.

  NDCA BDA DDA TATP DT
τ1 (ns) 12.27 4.37 4.30 1.72 4.12
τ2 (ns) 1.12 1.15 1.00 0.48 0.96
P 1 83.95% 66.53% 72.46% 37.55% 31.94%
P 2 16.05% 33.47% 27.54% 62.45% 68.06%
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In detail, Figure a,b displays the fluorescence spectra and lifetime decay curves of NDCA, BDA, and DDA, showing a relation between their emission peak and the molecular central π-conjugation. NDCA and BDA molecules exhibit their emission peaks at ∼450 nm and ∼473 nm, respectively. Therefore, a red shift of BDA (∼23 nm) arises compared with NDCA. The rigid and planar naphthalene central core in NDCA restricts intramolecular vibrations, suppressing nonradiative decay and yielding an emission band with the longest fluorescence lifetimes (dominant, τ1 = 12.27 ns, 83.95%, and minor, τ2 = 1.12 ns, 16.05%), as shown in Table . In contrast, BDA’s rotatable C–C bond between phenyl rings introduces conformational flexibility, leading to the emission peak broadening and two weak shoulders at about 465 and 492 nm, with shorter lifetimes (dominant, τ1 = 4.37 ns, 66.53%, and minor, τ2 = 1.15 ns, 33.47%), as shown in Table . DDA, lacking π-conjugation, shows a very weak fluorescence peak at about 548 nm with a redshift (∼98 nm) compared to NDCA. Note that a weak and narrow emission peak of DDA at ∼456 nm is also observed (see Figure S14). Furthermore, DDA’s fluorescence lifetimes (dominant, τ1 = 4.30 ns, 72.46%, and minor, τ2 = 1.00 ns, 27.54%, as shown in Table ) show the shortest lifetimes compared to NDCA and BDA. This overall trendNDCA > BDA > DDA in lifetime τ1is consistent with the combined effects of increased π-delocalization and backbone rigidity, which can suppress nonradiative relaxation channels in the solid state.

The substituents modulate the intermolecular interactions, thereby governing the solid-state fluorescence emission behavior, as shown in Figure c,d. Indeed, DT and TATP molecules show their emission peaks at ∼546 (with a satellite peak at 690 nm) and ∼540 nm, respectively. In DT, the two Br substituents suppress nonradiative relaxation, resulting in a narrower double-peak profile and longer lifetimes (minor, τ1 = 4.12 ns, 31.94%, and dominant, τ2 = 0.96 ns, 68.06%, as shown in Table ). In contrast, the three −COOH groups in TATP form extensive intermolecular hydrogen bonding, which leads to a broad single-peak profile and shorter lifetimes (minor, τ1 = 1.72 ns, 37.55%, and dominant, τ2 = 0.48 ns, 62.45%, as shown in Table ).

In other words, the fluorescence and lifetime behaviors of the compounds are governed by molecular conjugation, rigidity, and intermolecular interactions. Normally, the conjugated systems exhibit distinct emission profiles, sharp peaks, and long lifetimes, while the flexibility results in broad peaks and short lifetimes. Supplementary data for the carboxylate salts (SA/SP) are provided in the Supporting Information and serve as an additional comparison within the broader framework.

Conclusions

In summary, a unified Raman and fluorescence measurement platform was established to elucidate how π-conjugation, substituent effects, and intermolecular interactions collectively influence Raman scattering, fluorescence emission, and excited-state lifetimes. The NDCA, BDA, and DDA experiments show that molecules with extended π-conjugation and higher structural rigidity exhibit stronger Raman and fluorescence signals, greater laser power sensitivity, and longer excited-state lifetimes. The substituent-controlled experiments of TATP and DT further reveal that the two Br or three −COOH substitutions in terpyridine derivatives show different intermolecular interactions, resulting in the Raman signal appearing or being quenched, the fluorescence spectra profiles, and their fluorescence lifetimes. Moreover, the contrasting behaviors between carboxylate salts SA and SP show the critical role of inner-molecule CC or CC bond differences in governing the optical responses of organic molecules. Furthermore, the STM and SEM provide, correspondingly, micrometer and nanometer-scale images on the seven molecules for identifying the intermolecular interactions. ATR-FTIR spectra independently confirm the carboxylic acid versus carboxylate states, as stated in the discussion. Collectively, these results establish an integrated structure–property framework linking molecular geometry to vibrational and photophysical properties, providing fundamental insights for the rational design of functional materials in sensing, imaging, and optoelectronics.

Supplementary Material

ao5c12992_si_001.pdf (2MB, pdf)

Acknowledgments

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China under grant nos. 22372119 and 21972104, the Natural Science Foundation of Tianjin under grant no. 23JCYBJC01660, the “1000-Youth Talents Plan”, the Fundamental Research Funds for the Central Universities, and Haihe Laboratory of Sustainable Chemical Transformations.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c12992.

  • General methods, additional experimental data, and computational details (PDF)

§.

Q.Y. and S.C. contributed equally to this work.

The authors declare no competing financial interest.

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