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3 November 2025

Hydrogen-Bond Engineering for Highly Efficient Room-Temperature Phosphorescence with Tunable Multi-Color Emission

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1
Hubei Engineering Technology Research Center of Spectrum and Imaging Instrument, School of Electronic Information, Wuhan University, Wuhan 430072, China
2
Wuhan Institute of Quantum Technology, Wuhan 430206, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Spectrosc. J.2025, 3(4), 28;https://doi.org/10.3390/spectroscj3040028
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Abstract

Achieving long-lived room-temperature phosphorescence (RTP) with high quantum efficiency is of significant interest for applications in anti-counterfeiting, flexible optoelectronic displays, and multi-level information encryption. Here, we presented a hydrogen-bond engineering strategy to enhance RTP performance by progressively increasing the number of hydrogen-bonding sites within a polyvinyl alcohol (PVA) matrix. A series of carbazole-based chromophores (Cz, ICz and 2ICz) were embedded into the PVA network, and their photophysical properties were systematically characterized using steady-state photoluminescence spectra, time-decay spectra, Fourier-transform infrared (FTIR), and Raman and X-ray photoelectron spectroscopy (XPS). Spectroscopic analysis revealed that the increased number of N-H groups significantly strengthened hydrogen-bonding interactions, effectively suppressing non-radiative decay pathways and stabilizing triplet excitons. As a result, the phosphorescence lifetime was prolonged up to 1.68 s with a quantum yield of 38.63%. Furthermore, leveraging the spectral overlap integral between the phosphorescent emission and dye absorption, efficient Förster resonance energy transfer (FRET) was realized, enabling tunable multi-color afterglow emissions. This study establishes a design strategy validated by spectroscopy for high-performance RTP materials and highlights their promising potential in advanced optical encryption and flexible photonic applications.

1. Introduction

Room-temperature phosphorescence (RTP) has attracted considerable attention for applications such as anti-counterfeiting [], optoelectronic displays [], and bioimaging [] due to its unique photoluminescent properties. It has become an exciting research area within photophysics and materials science. Unlike fluorescence, which typically involves rapid radiative relaxation from the singlet excited state, RTP arises from the radiative decay of long-lived triplet excitons. Achieving RTP in organic materials is more challenging due to the weak spin–orbit coupling (SOC) and fast non-radiative decay pathways in organic molecules, which often hinder efficient intersystem crossing (ISC) and triplet-state stabilization [,,]. These limitations often lead to short phosphorescence lifetimes and low quantum yields, creating a persistent bottleneck for developing high-performance organic RTP (ORTP) materials. In recent years, long-lived RTP materials have attracted growing attention due to their remarkable potential for information security and low-power display technologies. On one hand, the long-lived RTP materials enable controllable luminescence differences in the time domain. In particular, by employing materials with distinct lifetimes, a “lifetime-encoded” information recognition system can be constructed. This encoding strategy significantly enhances the security and complexity of anti-counterfeiting technologies. On the other hand, long-lifetime RTP materials can release the absorbed energy gradually in the form of persistent luminescence, maintaining emission without continuous external excitation. Such long afterglow behavior not only reduces energy consumption but also provides stable and enduring visual displays, making these materials highly valuable for low-power afterglow applications such as night-vision displays and emergency signage [,,,,].
Recent advances have demonstrated that precise molecular engineering combined with matrix strategies can effectively overcome these limitations [,,,,]. Incorporating heavy atoms [,,], carbonyl groups [,,,], or aromatic molecules [,,,] can enhance ISC, while using rigid matrices [,,], crystalline structures [,], or hydrogen-bond networks [,,,] could suppress non-radiative pathways and stabilize triplet excitons. For example, Yan et al. [] achieved ultralong organic phosphorescence emission with an RTP efficiency of up to 77.5% at room temperature by modulating the number of carboxylic acid groups on triphenylamine units, creating strong repulsive interactions between aromatic hydrogens on the chromophore and the polymer chains. Gao et al. [] enhanced the lifetime from 14.3 μs to 256.5 ms and achieved a quantum yield of 16.04% using a three-level confinement strategy. Zhou et al. [] realized an ultralong lifetime of 3.26 s using a planarization-locking approach. Despite these remarkable achievements, designing organic RTP materials with both long lifetimes and high quantum efficiency still remains a significant challenge.
To address this issue, we propose a hydrogen-bond engineering strategy to enhance RTP performance by progressively increasing hydrogen-bonding sites within a polyvinyl alcohol (PVA) matrix. Carbazole possesses a high triplet energy level and excellent film-forming and crystallization abilities, which effectively suppress non-radiative decay when incorporated into polymeric or organic host–guest matrices. In addition, its nitrogen-containing heteroaromatic structure facilitates efficient RTP. A series of carbazole-based chromophores (Cz, ICz, and 2ICz) were embedded into the PVA network, where their N-H groups act as anchoring points for intermolecular hydrogen bonds. Systematic spectroscopic characterization, including steady-state photoluminescence spectra, time-decay spectra, Fourier-transform infrared (FTIR), and Raman and X-ray photoelectron spectroscopy (XPS), revealed that enriched hydrogen bonding effectively suppresses non-radiative transitions and stabilizes triplet excitons. As a result, the phosphorescence lifetime was significantly prolonged to 1.68 s with a quantum yield of 38.63%, accompanied by tunable multi-color afterglow emissions achieved via efficient Förster resonance energy transfer (FRET). These findings established a spectroscopically validated approach for designing high-performance RTP materials and expanded their potential in advanced optical encryption, flexible photonic displays, and multi-level information security systems.

2. Materials and Methods

2.1. Materials

Carbazole (Cz) (97%), 5,12-Dihydroindolo[3,2-a]carbazole (ICz) (97%), and 10,15-Dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (2ICz) (97%) were purchased from Aladdin Reagent (Shanghai, China). Polyvinyl alcohol (PVA) was purchased from Macklin (MW ≈ 20,000, 99% hydrolysis). All chemical reagents were used without further purification. Deionized water (18.2 MPa) was obtained from an ultrapure water system for laboratory purposes.

2.1.1. Preparation of Cz@PVA

Prepare a Cz@PVA film by dispersing 5 mg of Cz powder in 5 mL of deionized water, followed by the addition of 3 mL of PVA aqueous solution. Stir the mixture at 85 °C for 20 min and then use a syringe to drop 3 mL of the resulting aqueous solution onto a PTFE plate. Heat the plate until the water evaporates, yielding a Cz@PVA film.

2.1.2. Preparation of ICz@PVA

Prepare a ICz@PVA film by dispersing 5 mg of ICz powder in 5 mL of deionized water, followed by the addition of 3 mL of PVA aqueous solution. Stir the mixture at 85 °C for 20 min and then use a syringe to drop 3 mL of the resulting aqueous solution onto a PTFE plate. Heat the plate until the water evaporates, yielding a ICz@PVA film.

2.1.3. Preparation of 2ICz@PVA

Prepare a 2ICz@PVA film by dispersing 5 mg of 2ICz powder in 5 mL of deionized water, followed by the addition of 3 mL of PVA aqueous solution. Stir the mixture at 85 °C for 20 min, and then, use a syringe to drop 3 mL of the resulting aqueous solution onto a PTFE plate. Heat the plate until the water evaporates, yielding a 2ICz@PVA film.

2.2. Characterization

X-ray diffraction (XRD) was measured by a MiniFlex-600 (Rigaku, Tokyo, Japan) scanned at 5° min−1 in the 2θ range from 10° to 30°. Fourier-transform infrared spectroscopy (FTIR) spectra in the range of 4000–500 cm−1 were collected using a Nicolet-5700 (Thermo Nicolet, Waltham, MA, USA). The X-ray photoelectron spectroscopy (XPS) of the samples was tested using a SCIENTIFIC ESCALAB 250Xi (Thermo Fisher, Waltham, MA, USA). Raman spectra in the range of 2500–3500 cm−1 were measured by a Confocal Raman Microscope (HORIBA Jobin Yvon, Palaiseau, France). Photoluminescence spectra and phosphorescence spectra were measured by an F-4700 (Hitachi, Japan). Phosphorescence lifetime and absolute photoluminescence quantum yields were measured by an FLS-1000 (Edinburgh, UK). Photos were taken with a Redmi Note 11Pro.

2.3. Theoretical Calculation

The calculations were performed using the Gaussian 16 Revision C.01 package. Structural optimization and frequency calculations were carried out at the B3LYP/6-31G(d) level of theory along with Grimme’s DFT-D3 dispersion correction employing BJ damping. The total complexation energy was calculated by symmetry-adapted perturbation theory (SAPT) under the SAPT0/jun-cc-pVDZ level of corresponding compounds with a segment from polyvinyl alcohol in the Psi4 software package. The total complexation energy can be decomposed into electrostatic, induction, dispersion, and repulsion energies by energy decomposition analysis. All the preparation procedures are shown in the Figure 1.
Figure 1. (a) Preparation of the phosphorescent materials. (b) Phosphorescence photos of Cz@PVA, ICz@PVA, and 2ICz@PVA after UV excitation.
To evaluate the RTP performance of the prepared materials, we systematically characterized their optical and spectroscopic properties under ambient conditions. First, steady-state photoluminescence and phosphorescence spectra (Figure 2a–c) showed prompt emission peaks at 361, 382, and 380 nm and phosphorescence peaks centered at 438, 461, and 455 nm for Cz@PVA, ICz@PVA, and 2ICz@PVA, respectively (Figure S1). Next, excitation–emission mapping (Figure 2d–f) revealed the optimal excitation wavelengths to be 260 nm, 280 nm, and 300 nm for Cz@PVA, ICz@PVA, and 2ICz@PVA, respectively. Time-decay measurements (Figure 2g) further indicated progressively extended lifetimes of 0.62 s, 1.37 s, and 1.68 s for the three systems, accompanied by phosphorescence quantum yields of 9.34%, 10.23%, and 11.31% (Table S1), consistent with the naked-eye afterglow durations. Using 2ICz@PVA as a representative, RTP spectra collected at different doping levels (Figure 2h) showed that emission intensity generally increased from 0.1 wt% to 1 wt%, while further increasing the loading to >1–2 wt% led to decreases in phosphorescence lifetime and afterglow brightness due to molecular aggregation and enhanced triplet–triplet annihilation (TTA); thus an emitter content of ~1 wt% was selected as the optimal composition. Finally, all three systems exhibited deep-blue fluorescence and blue RTP, as visualized on the CIE chromaticity diagram (Figure 2i), confirming the successful implementation of our hydrogen-bond engineering strategy.
Figure 2. (a–c) The prompt spectra and phosphorescence emission spectra of Cz@PVA (a), ICz@PVA (b) and 2ICz@PVA (c). (The prompt emission is dominated by the fluorescence component, whereas the delayed emission predominantly arises from phosphorescence, delay time =10 ms.) (d–f) Excitation-phosphorescence emission mapping of Cz@PVA (d), ICz@PVA (e), and 2ICz@PVA (f) under ambient conditions. (g) Time-decay spectra. (h) Phosphorescence emission intensity of 2ICz@PVA for different doping ratios. (i) CIE coordinates of phosphorescence spectra.
To further explore the impact of the material structure on the gradual increase in lifetime, we conducted a series of characterizations to analyze the structure of the three composite materials. XRD pattern results revealed that all three phosphorescent materials exhibited an amorphous nature (Figure 3b). Taking 2ICz@PVA as an example, we analyzed its chemical composition using X-ray photoelectron spectroscopy (XPS). The C1s, N1s, and O1s XPS spectra of 2ICz@PVA were deconvoluted for further analysis. The C1s spectrum of 2ICz@PVA displayed three peaks at 284.84 eV, 286.31 eV, and 287.78 eV, which were attributed to C–C/C=C, C–O, and C–N bonds, respectively (Figure 3d). The N1s spectrum (Figure 3e) could be fitted with a single peak centered at 400.22 eV, corresponding to the N–H group on the chromophore. The O1s spectrum also showed a single peak at 532.39 eV (Figure 3f), which was attributed to the abundant –OH groups on the PVA chains, further confirming the successful incorporation of the chromophore. FTIR spectroscopy (Figure 3a) indicated that when the guest molecules were doped into the PVA matrix, the O–H stretching vibration at 3500 cm−1 gradually broadened and shifted to a shorter wavenumber. Raman spectroscopy also confirmed this observation (Figure 3c). In particular, the RTP materials doped with guest molecules showed a significant peak broadening at around 3220 cm−1 compared to pure PVA without guest molecules, indicating strong hydrogen bonding between the guest molecules and the PVA matrix. These abundant hydrogen bond interactions greatly stabilized the triplet excitons of the chromophores and suppressed non-radiative transitions, leading to the gradual enhancement of phosphorescent lifetimes.
Figure 3. (a) The FTIR spectrum of PVA, Cz@PVA, ICz@PVA, and 2ICz@PVA. (b) The XRD patterns of PVA, Cz@PVA, ICz@PVA, and 2ICz@PVA. (c) The Raman spectra of PVA, Cz@PVA, ICz@PVA, and 2ICz@PVA. (d) C1s, (e) N1s, (f) O1s spectra of 2ICz@PVA.
In more detail, we conducted a qualitative analysis of the interactions between Cz, ICz, 2ICz, and the PVA chains using the molecular planarity binding independent gradient model based on Hirshfeld partition (IGMH) []. The sign(λ2)ρ colored IGMH δginter isosurface maps for these three systems are presented in Figure 4a–c. The elliptic δginter clearly illustrates the presence of hydrogen-bonding interactions between the chromophores and the PVA matrix. Notably, as we moved from Cz to ICz and then to 2ICz, the hydrogen-bonding interactions significantly increased with the increasing number of -NH sites, demonstrating the enhanced interaction with the PVA chains. Additionally, we employed the wavefunction-based quantum chemistry technique of symmetry-adapted perturbation theory (SAPT) [] coupled with the basis set of jun-cc-pVDZ to quantitatively analyze the binding energies (Ecomplex) between Cz, ICz, and 2ICz chromophores and the PVA matrix. In general, a higher Ecomplex value indicates stronger intermolecular interactions. The interaction between the PVA chain and the guest molecules was decomposed into physically meaningful energy components, including electrostatics, induction, dispersion, and exchange energies. As shown in Figure 4d,e, the total complexation energies of Cz@PVA, ICz@PVA, and 2ICz@PVA were −13.24, −20.46, and −23.30 kcal/mol, respectively. The calculated results indicated that the trend in Ecomplex values correlates positively with the room-temperature phosphorescent lifetimes of the doped PVA systems (Figure S2). This further demonstrated that the progressively enriched hydrogen-bonding interactions played a crucial role in enhancing the luminescent properties.
Figure 4. (a–c) Sign(λ2) ρ colored isosurfaces of δginter corresponding to IGMH analyses for Cz@PVA (a), ICz@PVA, (b) and 2ICz@PVA (c). (d–f) Calculated complexation energies between Cz@PVA (d), ICz@PVA (e), 2ICz@PVA, (f) and PVA matrix.
Compared to monochromatic RTP, multi-color RTP is more appealing. Fluorescence resonance energy transfer (FRET) is a non-radiative energy transfer process in which the excitation energy migrates between molecules via long-range dipole–dipole coupling. When the emission spectrum of a donor significantly overlaps with the absorption spectrum of an acceptor, the excited donor can transfer its energy to a nearby acceptor without photon emission, leading to the excitation of the acceptor. This process enables a precise modulation of emission color, the broadening of emission bandwidths, and the realization of cascaded energy-level transfer, thereby offering a powerful strategy for designing advanced photofunctional materials. Since the phosphorescent emission of 2ICz@PVA overlapped significantly with the absorption spectra of the dyes sodium fluorescein (Fluc) and rhodamine B (RhB) (Figure 5a,b), this blue phosphorescent emission could be used as an energy donor for efficient FRET [], resulting in the emission of multiple colors from the material. The mechanism has been illustrated in Figure 5g. By adjusting the doping concentrations of Fluc and RhB in the PVA system, we further revealed the energy transfer process between the donor and acceptor during FRET. As shown in Figure 5a, the system doped with 0.1 wt% fluorescein sodium (Fluc) exhibited a visible green afterglow for 10 s, with a lifetime of 0.78 s (Figure S7). The system doped with 1 wt% RhB displayed a pink afterglow lasting 5 s, with a lifetime of 0.57 s (Figure S8). Steady-state and transient fluorescence spectra (Figure 5c,d) showed that the original phosphorescent emission of 2ICz@PVA was significantly absorbed, while the emission from the dyes became more prominent. By varying the doping concentrations of Fluc from 0.1 wt% to 0.3 wt% and 0.5 wt% (Figure 5e), and RhB from 1 wt% to 3 wt% and 5 wt% (Figure 5f), the relative colors of the phosphorescent spectra of these materials are visualized in the CIE chromaticity diagram (Figures S9 and S10). As the fluorescence intensity of the energy donor (Fluc) decreased, the fluorescence intensity of the energy acceptor (RhB) increased (Figures S3–S6). Notably, the delayed fluorescence of the energy acceptor did not diminish but rather was enhanced, further confirming the occurrence of phosphorescent energy transfer from the donor to the acceptor.
Figure 5. (a) Phosphorescence photos of 2ICz-0.1% Fluc@PVA and 2ICz-1% RhB@PVA after UV excitation. (b) Phosphorescence spectrum of 2ICz@PVA (in blue shade) and absorption spectra of the dyes. (c,d) The prompt spectra and phosphorescence emission spectra of 2ICz-0.1% Fluc@PVA (c) and 2ICz-1% RhB@PVA (d). (e) Afterglow emission spectra Fluc-doped 2ICz@PVA films at different doping weight concentrations. (f) Afterglow emission spectra RhB-doped 2ICz@PVA films at different doping weight concentrations. (g) Mechanism of fluorescence and phosphorescence FRET processes.
Based on the excellent luminescent properties of the materials, we explored their potential applications in display and anti-counterfeiting. Due to the high flexibility of the fabricated films, we folded the films of different colors into paper cranes (Figure 6a), which exhibited vibrant red, green, and blue phosphorescent art displays when viewed after the UV light was turned off. In Figure 6b, the logos of Neptune and Wuhan University are screen-printed on paper, and after brief UV excitation, the phosphorescent patterns remained visible for nearly 20 s. Figure 6c illustrates the process of creating anti-counterfeit patterns using RTP materials. The prepared anti-counterfeit ink was screen-printed onto paper and dried. Under daylight, the QR code pattern was nearly invisible to the naked eye. However, upon UV light exposure, the pattern became clearly visible, and after turning off the UV light, the QR code displayed a bright blue afterglow. Scanning the QR code with a smartphone revealed the information “SMD Lab,” which remained visible for 20 s. After 20 s, the QR code became almost invisible and could no longer be scanned, achieving a multi-dimensional encrypted display and erasure of information (Figure 6d). Furthermore, the tunable color and lifetime of the phosphorescent materials were applied in Morse code encryption technology (Figure 6e). The operational rules are as follows: the on and off states of the afterglow were defined as short and long horizontal lines in Morse code. When the UV light was on, all four points emitted light, and the Morse code decryption revealed the wrong password “HHHH.” After turning off the UV light for 5 s, observing the pattern and translating it gave the correct primary password “PHQY.” After 10 s without UV light, the secondary password “PLQY” could be obtained, demonstrating an efficient mode of multi-level information encryption.
Figure 6. (a) Photographs of the persistent emission of the paper cranes folded by films of different colors. (b) Photographs of the persistent emission of the logos of Neptune and Wuhan University. (c) The manufacturing process of the QR code pattern. (d) Photographs of the persistent emission of the QR code pattern. (e) Morse code encryption system.

3. Conclusions

In this work, we developed a hydrogen-bond engineering strategy to realize highly efficient and long-lived room-temperature phosphorescence (RTP) in carbazole-based chromophores embedded within a polyvinyl alcohol (PVA) matrix. By systematically tuning the number of hydrogen-bonding sites, we achieved a significant enhancement in RTP performance, with prolonged phosphorescence lifetimes, improved quantum yields, and tunable multi-color afterglow emissions. Comprehensive spectroscopic analyses confirmed that strengthened hydrogen-bonding interactions effectively stabilize triplet excitons and suppress non-radiative decay pathways, thereby underpinning the observed performance improvements. Moreover, leveraging efficient Förster resonance energy transfer (FRET) further enabled controllable multi-color phosphorescence. This study provided a spectroscopically validated approach for developing highly efficient RTP materials, offering broad potential for applications in optical encryption, anti-counterfeiting, and flexible photonic devices.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/spectroscj3040028/s1, Figure S1: Overlapped Excitation and Emission Spectra; Figure S2: Illustration of the relationship between RTP lifetime and total complexation energy.; Figure S3: Prompt spectra and phosphorescence emission spectra of 2ICz-0.3% Fluc@PVA (excitation wavelength = 300 nm); Figure S4: Prompt spectra and phosphorescence emission spectra of 2ICz-0.5% Fluc@PVA (excitation wavelength = 300 nm); Figure S5: Prompt spectra and phosphorescence emission spectra of 2ICz-3% RhB@PVA (excitation wavelength = 300 nm); Figure S6: Prompt spectra and phosphorescence emission spectra of 2ICz-5% RhB@PVA (excitation wavelength = 300 nm); Figure S7: Phosphorescence lifetime curves of 2ICz-0.1% Fluc@PVA at 545 nm; Figure S8: Phosphorescence lifetime curves of 2ICz-1% RhB@PVA at 610 nm; Figure S9: CIE coordinates of phosphorescence spectra of 2ICz-1% RhB@PVA, 2ICz-3% RhB@PVA, and 2ICz-5% RhB@PVA; Figure S10: CIE coordinates of phosphorescence spectra of 2ICz-0.1% RhB@PVA, 2ICz-0.3% RhB@PVA, and 2ICz-0.5% RhB@PVA; Table S1: Excited wavelength (Ex), photoluminescence wavelength (PL), phosphorescent wavelength (Phos), phosphorescent lifetime (τavg), photoluminescence quantum yield (PLQY), and phosphorescent quantum yield (PhQY) of the materials.; Video S1: Photoluminescent video.

Author Contributions

Conceptualization, L.D. and Z.T.; Methodology, L.D., Z.T. and J.L.; Software, Z.T.; Validation, Z.T.; Formal analysis, Z.T.; Investigation, R.P.; Resources, X.K. and R.W.; Writing—original draft, L.D.; Funding acquisition, X.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key Research and Development Program of China (Grant No. 2023YFE0105500) and the Wuhan University postgraduate research credit course project of Intelligent Packaging and Food Safety (Grant No. 1506/413100017).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/supplementary material. Further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors thank the Core Facility of Wuhan University for the FTIR, XRD, and Raman tests, as well as eceshi (www.eceshi.com) for the XPS test.

Conflicts of Interest

The authors declare no conflicts of interest.

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